Maintenance Approach for the Six Stonewall Trees on Bonham Road

Study on Stonewall Trees: Maintenance Approach for the Six Stonewall Trees on Slope no. 11SW-A/R577, Bonham Road

C.Y. Jim The University of Hong Kong [email protected]

Highways Department Government of the HKSAR 2012

CONTENTS

LIST OF TABLES 5

LIST OF PHOTOS 6

1. INTRODUCTION 7

1.1 Preamble 7

1.2 Scope of study 7 1.2.1 Background and general concerns 7 1.2.2 Evaluation on the conditions of the stonewall trees 8 1.2.3 Recommendations on the maintenance of the stonewall trees 8

1.3 Deliverables 9 1.3.1 Draft study report 9 1.3.2 Final study report 9 1.3.3 Presentation of the report in a seminar 9

2. BACKGROUND AND GENERAL CONCERNS 10

2.1 Development of stonewall trees on retaining structures 10 2.1.1 Urban development and stone retaining walls 10 2.1.2 Tree flora on stone retaining walls 11 2.1.3 Pre-adaptation of strangler figs to wall habitat 12 2.1.4 Colonization of stone retaining walls by strangler figs 14

2.2 Effect of stonewall trees on retaining structures 17 2.2.1 Past studies of stone retaining walls 17 2.2.2 Historical cases of fatal stone wall failures 19 2.2.3 Historical cases of stone wall failures without casualties 20 2.2.4 Recent tree failures affecting stonewall trees 23 2.2.5 Stonewall tree failures with first-hand field inspection 25 2.2.6 Modes of root-wall interactions 27

2.3 Factors on stability and health of stonewall trees 31 2.3.1 Intrinsic wall factors 31 2.3.2 Quality of wall environs 34 2.3.3 Extrinsic impacts on walls and environs 36 2.3.4 Stonewall tree factor 40

3. TREE RISK ASSESSMENT AND ARBORICULTURAL 43 RECOMMENDATIONS

3.1 Risk assessment of the stonewall trees 43 3.1.1 Site and general tree conditions 43 3.1.2 Assessment of T1 and arboricultural recommendations 45 3.1.3 Assessment of T2 and arboricultural recommendations 46 3.1.4 Assessment of T3 and arboricultural recommendations 61 3.1.5 Assessment of T4 and arboricultural recommendations 66 3.1.6 Assessment of T5 and arboricultural recommendations 76 3.1.7 Assessment of T6 and arboricultural recommendations 86

3.2 Tree values, potential risks and preservation 91 3.2.1 Value of the stonewall trees 91 3.2.2 Risk of the stonewall trees 92 3.2.3 Potentials and limitations in tree preservation 94

4. RECOMMENDATIONS ON STONEWALL TREE MAINTENANCE 97

4.1 Short-term practical maintenance measures for 97 individual trees 4.1.1 Maintenance of T1 97 4.1.2 Maintenance of T2 97 4.1.3 Maintenance of T3 98 4.1.4 Maintenance of T4 99 4.1.5 Maintenance of T5 99 4.1.6 Maintenance of T6 100

4.2 Long-term practical tree maintenance measures 101 4.2.1 Install soil strips at wall crest and toe 101 4.2.2 Remove joint seals to permit new root penetration 102 4.2.3 Prevent wedging effect on tree stability 102 4.2.4 Stop repeated removal of lower branches 103 4.2.5 Stop unprofessional and unnecessary pruning practice 103

4.3 Preventing grave impacts of excavation 105

4.4 Guidelines to inspect and monitor tree structural stability 107 and health

5. EXECUTIVE SUMMARY 109

5.1 Preamble 109 5.2 Study objectives 109 5.3 Background and general concerns 110 5.4 Effect of stonewall trees on retaining structures 111 5.5 Factors on tree stability and health 111 5.6 Tree values, potential risks and preservation 112 5.7 Tree risk assessment and arboricultural recommendations 112 5.8 Long-term practical tree maintenance measures 113

REFERENCES 115

Appendix Resistograph micro-drilling to estimate wood density (conducted by a contractor on 31 October 2012)

Tables 1 to 7

Photo slides 1 to 201

LIST OF TABLES

Table 1. Stonewall tree assessment results of T1 Table 2. Stonewall tree assessment results of T2

Table 3. Stonewall tree assessment results of T3 Table 4. Stonewall tree assessment results of T4 Table 5. Stonewall tree assessment results of T5 Table 6. Stonewall tree assessment results of T6 Table 7. The microdrilling points of T2, T4 and T5 and respective photo references.

LIST OF PHOTOS

THE SITE Photo T0-1 to T0-9 Slides 2 to 11

TREE T1 Photo T1-1 to T1-8 Slides 12 to 20

TREE T2 Photo T2-1 to T2-65 Slides 21 to 90

TREE T3 Photo T3-1 to T3-20 Slides 91 to 115

TREE T4 Photo T4-1 to T4-32 Slides 116 to 154

TREE T5 Photo T5-1 to T5-20 Slides 155 to 182

TREE T6 Photo T6-1 to T6-13 Slides 183 to 201

1. INTRODUCTION

1.1 Preamble

The Highways Department (HyD) is responsible for maintenance of vegetation growing on registered slopes assigned to it. These registered slopes include, among others, stone walls as retaining structures along roadsides. Some of these stone walls have trees growing directly on the structures, with roots anchoring on the walls or penetrating through the walls to the soil lying behind (the ‘aft-soil”) to capture water and nutrients and to secure anchorage.

These stonewall trees are unique landscape assets, but under some circumstances they may create risks to nearby residents or road users. They demand special attention or treatment in order to preserve them in a safe and healthy condition. With reference to the six stonewall trees on slope no. 11SW-A/R577 along Bonham Road (Photos T01 and T02), the main aims of the study are to identify the factors affecting the health and stability of stonewall trees, formulate suitable maintenance measures for their sustainable growth, and advise on mitigation measures to minimize the risk of tree failure. The maintenance responsibility of wall trees on this SIMAR slope was assigned to HyD in 2004.

1.2 Scope of study

1.2.1 Background and general concerns

(a) Review in general the development of stonewall trees on retaining structures in Hong Kong

 This includes a brief account on the local stonewall construction that enables stonewall tree development and an account of suitable tree species adapted to stonewall habitat.

(b) Analyze the effect of stonewall trees on the retaining structures and their stability on walls based on available stonewall tree failure records

 This involves an analysis, with reference to available typical tree failure records, on the physical and biological impact of stonewall trees on the retaining structures supporting them and hence the effect on the stability of the trees on these structures.

 This involves literature review and site observation to identify and elaborate on factors affecting the stability and health conditions of stonewall trees.

1.2.2 Evaluation on the conditions of the stonewall trees

(a) Examine and conduct tree risk assessment for the existing six stonewall trees

 This includes site inspection (and aerial inspection, if necessary) to examine the conditions of the trees. Observation with adequate illustrations and coloured photos and proper annotations shall be provided. Where necessary, testing for defective areas with appropriate tools and analysis on the test results shall be provided.

(b) Appraise the value and assess the potential risks of these stonewall trees to the local community

 This involves the appreciation of the value of the stonewall trees to the local community and an assessment of the potential risks to nearby residents and other road users, as well as after taking into consideration of the value of these trees and the potential risks, the provision of balanced and impartial advice on the preservation of these trees.

 This consists of identification of the limitations and site constraints in preserving the trees. Formulation of practical proposals to enhance their preservation with risk mitigated. Illustrations shall be provided to clearly present the proposals.

1.2.3 Recommendations on the maintenance of the stonewall trees

(a) Formulate short and long term practical maintenance measures to effectively enhance the sustainability of these stonewall trees

 This includes recommendations on maintenance measures to be carried out during routine maintenance and special operation/checking as required to enhance the sustainability of these trees and minimize the risk of failure.

(b) Prepare guidelines to inspect and monitor the structural stability and health conditions of these stonewall trees

 This involves the preparation of maintenance guidelines to clearly list out the aspects to inspect, monitoring procedures and maintenance operation shall be provided. Where appropriate, methodology of conducting the inspection and monitoring works with illustrations shall be included.

1.3 Deliverables

1.3.1 Draft study report

The report is structured mainly according to the above scope of study. Three coloured hard copies and 1 CD containing the draft report to be submitted on a date as agreed by the Government Representative.

1.3.2 Final study report

Three coloured hard copies and 1 CD containing the final report to be submitted within 14 days upon comments on the draft report are issued by the Government Representative.

1.3.3 Presentation of the report in a seminar

The findings of the report shall be presented in a two-hour seminar in 2012 for government officials, including address enquiries as raised in the seminar. HyD will organize the seminar and arrange a venue in Hong Kong. The date and time of the seminar is to be mutually agreed.

2. BACKGROUND AND GENERAL CONCERNS

2.1 Development of stonewall trees on retaining structures

2.1.1 Urban development and stone retaining walls

(a) Terrain constraints to city growth

In the course urban expansion in Hong Kong since the 1840s, the grave shortage of easily developable land was felt right at the incipient stage. Beginning with the City of Victoria founded at the north coast of Hong Kong Island, the rugged topography has steep hills plunging into the harbour to leave little flat land for buildings and roads. Strenuous efforts were made to create land to accommodate the fast city growth. The quest for developable land adopted two concurrent approaches, namely reclamation from the sea and terracing the hillslopes. Both methods incur substantial costs in terms of time and funding.

(b) Hillslope terracing and retaining structures

In the course of cutting the slopes into a series of terraces that resemble a flight of giant steps climbing up the hill, attempts were made to maximize the usable flat surfaces for urban development. Instead of leaving a strip of slope along the contour between an upper and a lower terrace, a vertical cut face was formed. A retaining structure, often a masonry retaining wall, has to be constructed to stabilize the disturbed and oversteepened slope and to prevent its geotechnical failure (Jim, 1998, 2010). Buildings were often constructed close to the retaining structures at both the toe and the crest positions. The traditional Hakka masons were widely recruited to build the stone retaining walls based on traditional Chinese design (Lo, 1971).

The government has recorded over 2500 retaining structure in Hong Kong, of which about 1700 are stone retaining walls of different materials, dimensions and designs built over the years (Chan 1996). They play a key role in providing for and sustaining the continued growth of the city that literally scale up the hillslopes. Concentrated mainly in the north parts of Hong Kong Island, they are particularly common in the old districts of Western, Central, Wanchai and Happy Valley. The walls are distributed from a low footslope level to the Midlevels, with some found on the Peak.

In recent years, stone walls have been replaced by reinforced concrete walls, hence the old and fine masonry structures embedded in our city denote an important but declining cultural heritage of the community.

(d) Stone wall design and plant life

A properly designed and constructed old stone wall has an earth core and stone blocks on the external and internal vertical faces (Chan, 1996). The joints between the individual masonry blocks may be mortared, but the older ones tend to be left unfilled, in the dry stone wall tradition. The presence of numerous gaps between the stones and soil in the wall core and behind the wall (the “aft-soil”) offered opportunities for plant life (Jim, 1998; Jim and Chen, 2011). Some retaining walls were constructed without the core and interior stone face (Chan 1996). The vertical habitats look apparently hostile or even inhospitable to plant growth. Yet a small cohort of plants has the ability to colonize spontaneously the artificial cliffs in the city to usher nature into the heart of the city. In a similar manner, some trees would colonize the external walls of old buildings (Jim and Chen, 2011).

2.1.2 Tree flora on stone retaining walls

(a) Keystone stonewall tree species

The humid subtropical climate Hong Kong permits the existence of a rich plant biodiversity. A special group of woody plants called Banyans (genus Ficus) of the Mulberry family (Moraceae) contains members with pre- adapted characteristics to thrive on the seemingly rather precarious vertical and harsh sites (Jim, 1990, 2006). The most prominent members include trees that can attain large dimensions exceeding 20 m height and crown spread. The botanical nomenclature adopts the Flora of Hong Kong (Agriculture, Fisheries and Conservation Department, 2007, 2008, 2009, 2011). The English common names adopt the Check List of Hong Kong Plants 2004 (Agriculture, Fisheries and Conservation Department, 2004). Most wall trees are dominated by native species, with few exotic ones (* marked with an asterisk). They are represented mainly by (Jim, 1998, 2008a, 2010; Jim and Chen, 2010):

 Chinese Banyan (Ficus microcarpa).  Big-leaved Fig (Ficus virens var. sublanceolata).

(b) Secondary Ficus species

Other less common stonewall tree species in the same genus include:

 Common Red-stem Fig (Ficus variegata var. chlorocarpa).  Japanese Superb Fig (Ficus subpisocarpa; previously known as Ficus superba).  Opposite-leaved Fig (Ficus hispida).  Indian-rubber Tree (Ficus elastica)*.  Mock Bodh Tree (Ficus rumphii)*.

Two close relatives of Ficus trees, in the same Mulberry family can sometimes dwell successfully as ruderals (semi-natural members of the plants of a given place) on stone walls:

 Paper Mulberry (Broussonetia papyrifera).  White Mulberry (Morus alba)*.

(d) Accidental stonewall tree species

Occasionally, some non-Ficus trees with strong rooting habit could accidentally establish on stone walls as garden escapees, such as:

 Chinese Hackberry (Celtis sinensis).  Elephant’s Ear (Macaranga tanarius).  Autumn Maple (Bischofia javanica).  Tree Cotton (Bombax ceiba)*.

2.1.3 Pre-adaptation of strangler figs to wall habitat

(a) Strangler figs in natural tropical forests

The Ficus trees that grow particularly well on stone walls tend to be stranglers in the natural tropical forest habitat. Such trees are pre-adapted to grow on the artificial cliffs mainly due to the vigorous and unusual root growth habit. Their precarious life begins as tiny seeds from parent Banyan trees that are eaten by birds. Some birds would perch on the tropical forest trees during which droppings could be deposited. The droppings contain Banyan seeds which can survive the vigorous digestion process. If by chance the droppings land on the upper surface of branches

or branch forks, a tiny proportion of the lodged seeds may germinate successfully. The lodging sites can be situated up to 30 m above the forest floor.

(b) Early epiphytic and precarious existence

The initial growth form from the seedling to the sapling stages is an epiphyte, which is a plant that grows on the surface of another plant (the host tree). The role of the host during the crucial germinal period is providing a substrate for the attached Banyan (Figueiredo et al., 1995; Jim, 2006; GCW, 2011; IFLA, 2011). At this epiphytic stage, the young Banyan has to overcome multiple stresses that can easily terminate its life. Its roots are located far away from the soil on the forest floor. Day in and day out, it has to struggle to acquire sufficient sustenance to support itself. Nutrient supply has to come from the meagre amount of dropping and accumulated organic debris on the branch surface or fork. Water supply relies on rainfall or dew condensation absorbed by the organic debris and the bark of the host tree, plus some stem flow. Anchorage depends on the limited amount of roots sent out by the feeble plant to grip the host branch.

The young epiphytic Banyan would soon develop a modest amount of branches and leaves to conduct photosynthesis and manufacture its own food. It would divert a notable amount of its hard-earned energy to form aerial roots that hang downwards. As the epiphyte grows gradually bigger, it can devote more energy to elongate the aerial roots and send out more. These soft and rope-like roots in due course will extend further down towards the forest floor. It may take some years for the aerial roots to eventually reach the soil. At this point, the epiphyte is given literally a new lease of life. From a struggling and weak epiphyte, it will henceforth be transformed into a strong and rigorous strangler fig.

(d) Aerial roots reaching the forest floor soil

Upon reaching the forest floor, the aerial roots will forthwith take root in the soil and develop an extensive normal root system in the soil. This ground root system can tap a significantly larger amount of nutrients and water to feed the tree. With notably enhanced sustenance, involving multiple folds of increase, the Banyan strangler can grow quickly. Its crown can expand and often cover up if not overwhelm that of the host tree to capture more sunshine. More importantly, its multiple strands of aerial roots that dangle around the trunk of the host tree can undergo a

drastic changes. They gradually thicken and become woody through the process of lignification.

(e) Strangulation of the host tree

Neighbouring aerial roots that establish physical contact can fuse together in a process of self-grafting. In time, a basket-like mesh of lignified aerial roots is formed wrapping around the trunk of the host tree. The continued secondary thickening of the host trunk exerts a force that pushes outwards away from the trunk centre. The continued thickening of lignified aerial roots exerts a force that pushes towards the trunk centre. The two opposing forces join hands to apply an exceptionally strong force to literally strangle the host tree trunk. The vascular phloem lying below the bark of the host trunk can be squeezed tightly and eventually cut off. The host tree, deprived of water and nutrient supply from the roots to the leaves, and food from the leaves to the roots, will be strangled until it dies.

(f) New lease of life for the strangler

Once dead, it will take about one to two years for the host tree, now a victim, to decompose completely without leaving a trace. Meanwhile, the Banyan can take advantage of the nutrients released by the decomposing tissues of the victim tree. The niche original occupied by the host tree has now been taken over by the Banyan. The strangler fig from then on will continue to grow as though it has begun its life on the ground. Beginning its growth well above the ground and growing initially downwards, it has been transformed into a normal upward growing tree. Starting off as a feeble epiphyte, it is now developing into a giant ground-hugging tree of the forest.

2.1.4 Colonization of stone retaining walls by strangler figs

(a) Stone wall as surrogate host tree

The strangler fig would treat the stone wall in the city as the host tree. It begins as a seed deposited together with bird droppings in a suitable microsite on the stone wall, such as an unfilled crevice between stones, a protruding pointing between stones, uneven stone surface, wall ledge, wall coping, and wall toe. Of the many seeds that manage to land on microsites, only a tiny proportion can escape subsequent dislodgement by natural forces of wind, water, or gravity before they have the chance to germinate. Many will perish by desiccation due to the scarcity of water in landing microsites. Of the seeds that can stay and remain alive on the wall, only a 14

small proportion can germinate successfully due to the paucity of nutrients and water. Of the seedlings that can manage to grow up, only a small proportion can reach the sapling stage, because of prolonged lack of nutrients or water, or dislodgement due to natural forces or advertent or inadvertent removal by humans (Jim, 1998, 2008a, 2010; Jim and Chen, 2010).

(b) Early epiphytic and perilous existence

Beginning as an epiphyte of a microsite, the early growth of a wall tree can only be sluggish and precarious. The forces of attrition and stresses will continue to take their toll. Of the many seedlings that manage to reach the sapling stage, only a small proportion can soldier on to become mature stonewall trees. Making use of its natural strangler capability, the wall tree can gain a strong foothold on the vertical habitat. The unusual and exceptionally vigorous growth habit of its roots plays the key role in its successful colonization of the artificial vertical habitat. The roots can grip the wall face and penetrate the gaps between masonry blocks to reach the aft-soil. Once it is established, it tends to hold fast to the vertical habitat to realize their potential biological size under the right combination of conditions.

Once the aft-soil is reached by the roots, a new lease of life for the hitherto struggling tree can be triggered (Jim, 1998, 2008a, 2010; Jim and Chen, 2010). A large amount of normal roots can spread out and capture nutrients and water from a large aft-soil catchment. The spreading root system in the soil also helps to reinforce the tree’s anchorage to allow the tree to reach sizeable dimensions. The roots that establish physical contact with each other can develop sturdy wood fusion by self-grafting. The resulting root system can form a tightly knitted network on the wall face. The wood fusion between lignified aerial roots and the tree’s own branches and trunks can form root stands and props to reinforce the biomass structure. Overall, the strangler fig is pre-adapted to thrive on stone walls. Humans provide the stone walls and nature provides the strangler figs. They form a good match to generate a unique urban ecological feature in our city.

(d) History of stone wall colonization by trees

With an urban history that has extended over 160 years, Hong Kong has built many stone retaining walls that offer a rather long duration for Banyan trees to establish on them. Some stonewall trees are older than a 15 century and have reached a huge size, often bigger than their ground- growing counterparts. They provide living landmarks in our old districts, some of which would be treeless but for the presence of the hanging trees (Jim, 2008b). The number, dimension, distribution, condition and ecological and cultural values of our stonewall trees are second to none in the whole wide world. They qualify as a unique world-class ecological gem of our city, and deserve to be protected as the natural-cum-cultural heritage of our community.

2.2 Effect of stonewall trees on retaining structures

The successful growth of stonewall trees demands the fulfilment of a host of fundamental conditions. The presence of the right kind of microsites for seeds to lodge on the wall surface constitutes a critical starting point. Thereafter, it is the ability to overcome the negative impacts and chronic stresses that would make or mar a wall tree’s continued growth.

In the course of growth on the stone wall, under some special circumstances, the condition of the wall could be modified or disturbed. Thus far, few detailed scientific studies have been conducted on the specific impact of stonewall trees on wall structure. Such detailed studies lie outside the purview of the present project. Previous studies mainly rely on the examination of documentary records. This interpretation is based on a review of the relevant literature, mainly recent government- commissioned studies on stonewall trees, and supplemented by the author’s past research experience on stonewall trees in Hong Kong.

2.2.1 Past studies of stone retaining walls

(a) Two main government studies

The cases of past wall and tree failures associated with stone retaining walls have been summarized in two government publications:

 Chan, Y.C. (1996) Study of Old Masonry Retaining Walls in Hong Kong. GEO Report No. 31, Geotechnical Engineering Office, Civil Engineering and Development Department, Government of the HKSAR.

 GEO (2011a) Study on Masonry Walls with Trees. GEO Report No. 257, CM Wong & Associates, Geotechnical Engineering Office, Civil Engineering and Development Department, Government of the HKSAR.

(b) Inclusion of previous study findings

The above two recent studies have embodied the key findings of three previous government studies:

 The 1977 study by Binnie & Partners under the ambit of the Public Works Department.

 The 1980 study by the Geotechnical Control Branch (GCB) of the Building Ordinance Office.

 The 1980 Geotechnical Control Office (GCO) study on old masonry retaining walls.

The first report (Chan, 1996) focused squarely on the engineering aspect of the masonry structure with some assessment of trees that grow on them. The second report (GEO, 2011a) aimed at clarifying the relationship between stone retaining structures and the associated stonewall trees. Both studies include retrospective analysis of some failed walls based on official file records.

These government documents denote an official source of information containing the interpretation of incident reports pertaining to masonry retaining walls filed by relevant departments. It is believed that no other similar systematic information is available. They embody records and analyses that tend to focus on wall failure cases per se. It is likely that the tree failure cases that were not associated with wall failure, especially those that did not involve large trees, were not reported to the GEO or its predecessor GCO. Thus this source of official information is likely to omit a notable number of stonewall tree failure incidents. The corollary is that the wall failure cases in the files may not provide a representative sample to reflect the nature of tree failure on stone retaining walls.

The other issue concerns the objectives of the incident reports which tend to concentrate on the physical fabric of the wall and the interpretation of the mode and possible causes of wall failure. The assessors who conducted the field evaluations and filled the forms were often not trained in tree science and tree failure appraisal, and hence they may not be able to decipher the fundamental and immediate causes of the tree failure. As a result, the incident reports contain little direct or indirect information on the mode and causes of tree failure.

As such, these records cannot be relied upon to distil high-quality scientific assessment of tree failures at masonry retaining wall sites. The corollary is that it may not be advisable to use such information to establish practices or management measures to manage stonewall trees. Moreover, the nature and information contents of the records do not lend themselves to retrospective interpretation of the causes of tree failure.

Any attempt to do so would invariably incur speculative elements that are not based on objective and unambiguous scientific evidence.

2.2.2 Historical cases of fatal stone wall failures

The following case studies of stone retaining wall failures are extracted from the government reports. They denote the more notable incidents with a reasonable amount of documentary records. The more prominent wall failures involve fatalities, which were followed by court inquiries or scientific investigations, contain more details.

(a) Case A: St Joseph Terrace 1917

The failure of old stone walls is not a recent phenomenon. Back on 16 July 1917, a stone retaining wall at St Joseph Terrace (above Caine Road and adjacent to the Catholic Cathedral) collapsed (Chan, 1996; South China Morning Post, 17 July 1917). The unfortunate incident destroyed the rear part of two tenement blocks at 10 and 12 Caine Road, and claimed six lives. The failure of the dry rubble wall, built in 1911, was mainly explained by its inadequate thickness. The failure was preceded by the widening of a crack at the corner of the wall. The collapse was triggered by the repaving work at the wall-crest platform used by the St Joseph College as a playground, causing saturation of the retained soil. About half of the old paving at the platform was removed at the time of the incident, exposing the underlying soil directly to the heavy antecedent rains. The record did not mention the presence of trees on the wall.

(b) Case B: Po Hing Fong 1925

The most catastrophic wall failure happened in the early history of our city’s development on 17 July 1925 at Po Hing Fong in Sheung Wan. It happened after a prolonged period of heavy rains. The en masse collapse of a wall built in 1860 destroyed seven brick tenement buildings lying in front of the wall, incurring a loss of 75 lives. The historical record did not mention whether trees were present on the wall (Chan, 1996; South China Morning Post, 18 July 1925). The redevelopment of the No. 8 Police Station at the wall-crest platform involving excavation for the foundation of a new building could have contributed to the failure. A subsequent court of inquiry recommended improvement in the design, workmanship and maintenance of masonry retaining walls to avoid recurrence of similar mishaps (South China Morning Post, 29-31 July 1925, 8 August 1925, 3 and 5 September 1925).

A fatal wall collapse happened on a more recently built stone retaining wall (Morgenstern and Geotechnical Engineering Office, 2000) in 1994 at Kwun Lung Lau, Western district. It resulted in five fatalities. As no trees were found on the wall, they did not play a role in the wall breakage or collapse. A subsequent scientific study was conducted with the help of an external geotechnical expert to probe into the causes of the wall collapse. Fundamental weaknesses in wall design and pipe leakage behind the wall were found to be the contributory factors.

2.2.3 Historical cases of stone wall failures without casualties

Other notable cases of wall failure which did not incur casualties have been analyzed and reported in Chan (1996):

(a) Case D: Castle Road 1970

A short section (about 9 m long) of the wall failed after days of heavy rainfall at 10 Castle Road on 19 June 1970. It was found to be of poor quality mass concrete and disturbed prior to collapse by recent excavation by the gas company. No casualty and no wall tree were reported. It can be interpreted that the failure was not related to stonewall trees.

(b) Case E: May Road 1973

Almost the entire 40 m length of the retaining wall at Thorp Manor, 1 May Road, collapsed on 2 September 1973. It was then under the influence of Typhoon Ellen. The Thorp Manor was being demolished at that time. Trees and shrubs were found lying on the debris, and some vegetation was present on the small remaining parts of the wall. Whether the vegetation contributed to the failure was not interpreted, and its role cannot be ascertained from the limited amount and quality of visual and other records.

The wall at Caine Lane behind the U-Lam Terrace failed on 25 August 1976. It has been reported that the site suffered from high ground water table, and horizontal drains were installed to lower it. Little other information is available to interpret the fundamental and immediate causes of the wall collapse. The role of trees in stressing the wall was not mentioned in the report. 20

(d) Case G: Circular Pathway 1977

The wall at 3-7 Circular Pathway was found to be critically unstable in August 1977, which was associated with Typhoon Vera and a low pressure trough bringing plenty of rains. Shortly before the incident, the old buildings in front of the wall were demolished together with the removal of arches between the wall and the buildings. Longitudinal cracks and land subsidence were detected at the wall-crest platform along Circular Pathway, and the cracks were widening and extending. A bulge was formed on the wall which was attributed to an unknown source of water exerting pressure on the structure.

The wall had to be stabilized by constructing a free draining embankment and the demolition of buildings at 24A-25A Circular Pathway at the crest platform to reduce loading on the stressed wall. These geotechnical measures were able to stop the wall from further movements. The records did not mention the presence or the role of trees in the destabilization of the wall.

(e) Case H: Old Peak Road 1978

The wall at 22 Old Peak Road was found to display a series of distress symptoms on 11 May 1978. They include bulge, cracked concrete parapet wall at the crest, and crack parallel to the wall along the middle of the road at wall crest. The cracked areas also suffered from land subsidence. These symptoms were apparently associated with a recently reinstated trench for telephone line. The old wall was subsequently stabilized by constructing a new concrete wall in front of it. The record was silent concerning the presence or role of stonewall trees as an agent of wall destabilization.

(f) Case I: Fat Hing Street 1978

At 14-16 Fat Hing Street adjacent to 48-56 Queen’s Road West, a stone wall failed on 29 July 1978. The timing was associated with Typhoon Agnes bringing abundant rainfalls. The old buildings in front of the wall were demolished prior to the incident. Shortly before yielding of the wall, a 0.8 m deep trench was opened for water pipe installation at the wall crest platform subparallel to and near the wall. The sheet piling at the wall toe completed at least six months before the incident could have contributed to the failure. The immediate cause of failure was attributed to the build-up of water pressure behind the wall. Besides heavy rains, the trench at the wall crest would have play part in increasing the water 21

pressure on the wall. Trees were not mentioned in the records as a cause of wall destabilization.

(g) Case J: Wing Wa Terrace 1978

The subject wall was found at 1-10 Wing Wa Terrace near Hospital Road. It lent support to the Wing Wa Terrace at its crest. It deviates from other cases in that the wall failed rather uncommonly outside the wet season on 13 November 1978. It was not associated with particularly heavy rains or a large amount of accumulated rainfalls prior to the collapse. The wall presented a host of distress signals prior to the failure. They included bulge at the failed position, and steepening of the wall batter, failure of a steel strut, movement of masonry blocks at several positions, and broken steps adjacent to the wall.

The owner was required by the government to implement remedial strengthening works which were initiated in September 1978. They included installation of horizontal drains to draw down the ground water, concrete counterweight at the crest to prevent overturning, and sheetpiles to stabilize the wall toe. The vibration of the sheetpile driving caused crack development parallel to and extending for half of the wall length at the crest platform.

The immediate causes of failure were attributed to the sheetpiling getting too close to the wall generating vibration to damage the masonry structure, and the sheet piles physically breaking the wall’s spread footing. The vibration could also have broken a sewer behind the wall to induce a local rise in groundwater level. Trees did not contribute to the wall destabilization or failure.

(h) Case K: Jewish Recreation Club 1979

On 3 August 1979, associated with Typhoon Hope bringing plenty of rains, a section of the retaining wall at the northern edge of the Jewish Recreation Club near Robinson Road failed. The platform supported by the wall, forming the main grounds of the Club, were unpaved and used for car parking and as a tennis court. The wall had bulged for an unspecified period before the incident. The yielded part situated at the west end adjacent to the tennis court, however, did not bulge as seriously as the remaining part to its east. No tree was involved in the incident.

(i) Case L: Peak Road 2007

A wall failure that fortunately did not result in casualties (GEO, 2010) happened on 3 June 2007 on the hillslope adjacent to 84 Peak Road. The wall is believed to be built in the 1920s, measuring up to 2.2 m height and 14 m long; it is of the squared rubble type of dry wall. The wall failure involved some masonry blocks falling off, sliding down a slope and landed on the Peak Road. Different possible causes for the failure were explored. Uneven ground settlement associated with heavy rainfall at the wall base could have loosened masonry blocks. The roots of a stonewall tree near the failure zone were suspected to have contributed to displacement of the stones probably due to root wedging action. This is thus far the only case that included stonewall tree as one of the causes of wall failure.

2.2.4 Recent tree failures affecting stonewall trees

The details of the case studies M to R of stonewall tree failure can be found in the main text as well and appendices of a recent government wall study report (GEO, 2011). They will not be repeated here. Instead, the possible causes of these tree failures and their impacts on walls could be summarized as follows. Information about case S is mainly obtained from a newspaper report which contained photographs. Additional information about this case was acquired from the web-based government slope information system (CEDD, 2012).

(a) Case M: Hill Road 1997 (11SW-A/FR24)

A tree situated near the wall crest uprooted and fell down, and it caused localized collateral damage to the wall by extracting two masonry blocks. The wall did not fail and the general stability of the wall was not affected by the tree loss.

(b) Case N: Ka Wai Man Road 1999 (11SW-A/R120)

A tree situated on a cut slope above the wall was uprooted under typhoon influence. The subject tree was not a stonewall tree. The wall did not fail and its structure was hardly affected by the tree collapse. Some joints near the wall crest showed signs of having been repaired, but they might not be related to the tree failure incident.

A tree planted on a roadside children’s playground at the wall-crest platform was uprooted and hit the top of the adjacent stone wall. No stonewall tree was involved in the incident, and the structure of the subject wall was not affected.

(d) Case P: Tai Hang Road 2003 (11SE-A/C897, previously filed as 11SE- A/R51)

This case did not involve a stone retaining wall and the subject trees was not a stonewall tree. The feature wall in front of Lai Sing Court at Tai Hang Road is actually stone pitching serving as a surface veneer on a cut slope face. A tree situated on a slope above the stone pitching was uprooted under strong wind. In the course of its fall, some stone blocks were dislodged from the stone pitching.

(e) Case Q: Lei Cheng Uk Swimming Pool 2004 (11NW-B/C321)

A tree fell down from a cut slope adjacent to the Lei Cheng Uk Swimming Pool. The slope was covered by stone pitching. The case did not involve stone retaining wall or stonewall tree.

(f) Case R: Wyndham Street 2005 (11SW-B/R735)

A stonewall tree situated at the crest of the feature wall fell down probably due to strong wind. The structure of the wall was not damaged by the tree failure. No record of tree dimensions was available. The photograph shown in GEO (2011: 25) implies a relatively small tree.

(g) Case S: Tai Hang Road 2011 (11SE-A/C114(2))

A wall tree failure at 50C Tai Hang Road opposite the True Light Middle School was reported in the media. It occurred on 13 May 2011. The Chinese Banyan was about 10 m tall with a trunk diameter around 50 cm m grew in the middle part of the stone retaining wall which measures 9 m tall (CEDD, 2012). The wall is inclined at 70 degree, which is well above the lean of most local stone retaining walls. The tree developed a large amount of surface roots forming a dense mat of self-grafted network on the wall face.

It lost its grip on the wall on the rainy day with amber rainstorm warning. It was found that the tree roots encountered difficulties in their attempt to penetrate the wall which has tightly packed square masonry blocks with 24

well mortared joints. With few roots growing through the joints to spread in the aft-soil, its biomass was unable to hold firmly against overturning. In other words, the tree did not have sufficient roots to provide tensional pull against toppling. The cut face of the remaining trunk base showed healthy sound wood unaffected by decay.

2.2.5 Stonewall tree failures with first-hand field inspection

Two cases of tree collapse were subject to detailed first-hand field evaluation by the author to interpret the causes of failure based on evidence.

(a) Case T: Pokfulam Road 1992 (11SW-A/R346)

A Chinese Banyan (Ficus microcarpa) that grew on the wall at Pokfulam Road near the West Gate of the University of Hong Kong collapsed on 01 February 1992. The following account provides a first-hand field inspection of the subject tree by the author shortly after the incident. The tall stone retaining wall reaches a height of 15 m, above which lies a rather natural vegetated slope with open soil. The trunk base of the failed tree rested on the wall crest, with roots spreading in two main directions. The surface roots moved downwards to cover the wall face profusely and to reach the wall toe. At the wall crest, another group of roots grew backwards and spread extensively in the soil of the slope. As a result, the tree developed an exceptionally strong anchorage that is seldom found in stonewall trees, because the wall-crest roots provide ample tensional pull to hold the tree against toppling or overturning. Few of the surface roots were able to penetrate the mortared joints of this wall with tight and well filled gaps between masonry blocks. Thus the tree stability is dependent heavily on the wall-crest roots dwelling in the slope soil.

Unfortunately, shortly before the tree failure, a utility company opened a trench at the wall-crest slope adjacent and parallel to the wall. As a result, the entire bundle of critical wall-crest roots of the subject tree was completely severed. The sudden loss of the crucial tensional pull left the tree vulnerable to failure. On the event day, the tree literally fell down onto the carriageway of Pokfulam Road, and displayed vividly the wholesale cutting of all the tension roots at the wall crest. Thus this tree failure could be attributed to an improper and massive root cutting due to trenching at the wall crest. It has nothing to do with normal causes of tree risks such as insecure anchorage, weak structure or wood decay. The trunk base and a large portion of the surface roots were detached from the

wall face. The dropping of the tree from the wall did not damage the masonry structure.

(b) Case U: Forbes Street 2012 (11SW-A/R838)

A stonewall tree collapse during the peak of Typhoon Vicente around mid-night on 23 July 2012 at the historical stone wall at Forbes Street in Kennedy Town. The site has been used by the MTRC to build the new Kennedy Town station. The footprint of the station and the associated tunnels have been located away from the wall to avoid disturbing its masonry structure as well as the 20 odd stonewall trees dwelling on it.

The subject tree is a Chinese Banyan (Ficus microcarpa) that grew near the wall crest close to the western and taller end of the long wall. The rather large wall tree measured about 13 m tall, 16-18 m crown spread, and it was supported by a single stout trunk of 1.25 m diameter. Its trunk base perches at about 12 m from the road level and at about 2 m below the wall crest. The tree was marked by a rather complete, balanced and sprawling crown, normal foliage size, colour and density, and few missing limbs or major branches. The wall has no visible signs of distress, indicating that the masonry structure is in a sound condition. The surface roots of the tree spread on the masonry face all the way down to the wall toe, where they penetrated into the soil lying under the pavement.

After its collapse, the author’s detailed site inspection found that the tree was supported mainly from below by the masses of surface roots that were pressed tightly against the wall face. The tension roots that pulled the tree towards the wall, and hence held it against toppling or overturning, unfortunately, were grossly inadequate. Only five torn tension roots were observed on the wall at around the trunk base position. They penetrated the wall joints, except one that explored a weep hole. None of them were thicker than 10 cm in diameter. The torn surfaces of the tension roots exposed fresh and sound wood, indicating that they were unaffected by wood decay.

Overall, the tree failed due to the lack of tension root development, which was severely restricted by the sealing of joints between masonry blocks. It encountered unusually strong gusts during the typhoon that exceeded the strength of the limited tension roots in terms of number and thickness, resulting in breakage of tension roots and tree collapse. The maximum wind speed around the time of tree failure recorded at the nearest weather station at Green Island reached nearly 100 km/h. The detachment of the tree from the wall imposed no damage at all on the strong masonry

structure. No stone was shifted or broken, and no joint was widened as a result of the tree’s downfall.

A clear distinction should be drawn between stone wall failure per se, and stonewall tree failure. The stonewall trees brought down by en masse stone wall failure, strictly speaking, is a collateral damage. Such cases do need to be interpreted as stonewall tree failure. A bona fide stonewall tree failure should be associated with mechanical weaknesses in the root system or the stems, or localized disruption and destabilization of the stone structure due to static or dynamic forces exerted by the tree.

2.2.6 Modes of root-wall interactions

(a) Four main types of interactions

The growth of stonewall trees may interact with the wall structure in the following manners:

 Surface roots growing on stonewall face.  Roots growing through the joints into the wall structure.  Roots growing in the aft-soil lying behind the wall.  Surcharge of tree mass and wind effect on wall.

(b) Surface roots

Stonewall trees often spread their roots on the wall face, and such roots can be labelled as surface roots. Ordinary trees would develop roots in the soil and therefore below the soil surface. Some Banyans, however, can develop and maintain a notable amount of roots outside the soil domain. The most common stonewall tree species, Chinese Banyan (Ficus microcarpa) and Big Leaved Banyan (Ficus virens var. sublanceolata), are particularly adept in generating a large amount of surface roots. This rooting habit vividly expresses the inherent strangler fig trait (cf. Section 2.1.4). Instead of using lignified roots to wrap around the trunk of the host tree in the tropical forest, the lignified roots of stonewall Banyans have found a substitute host by gripping the wall.

Depending on the species and individual tree character, the vertical and horizontal spread, diameter, density, and self-grafting wood union of surface roots can vary significantly from tree to tree. These four surface root traits would influence tree interaction with the wall structure. The surface roots grow more or less parallel to the wall face. They are usually

pressed tightly against the masonry blocks. Surface roots characterized by extensive spread, relatively thick diameter, high density, and high frequency of wood union, can offer a strong living network to protect the wall by literally holding the masonry blocks in place. Such ramified and interconnected living roots have high tensile strength. The effect is tantamount to installing a steel mesh on the wall face. They can help to hold the masonry blocks in place, prevent displacement, and overall stabilize the wall structure. They are effective in checking their displacement especially in the perpendicular direction away from the wall face.

Most of the surface roots are lignified and they do not play the role of water and nutrient absorption. In the root-system hierarchy and associated physiological functions of the tree root system, they are transport roots rather than absorption roots. They help to transport the water and nutrients absorbed by fine roots dwelling in the aft-soil to the leaves to support photosynthesis. Without the potentially disturbing impacts of water and nutrient absorption, the surface roots would not secrete chemical substances to degrade the stones. Thus they do not induce chemical weathering which otherwise may in the long term compromise the mechanical strength and integrity of the wall. Additionally, the surface roots are instrumental in gripping the wall surface to contribute to the tree’s foothold and in supporting the tree’s biomass.

Roots growing through the joints may expand by secondary thickening to induce the wedging action, which may displace masonry blocks from their equilibrium positions. However, stone retaining walls have tightly interlocking stones that are firmly pressed against each other to resist the relatively low pressure exerted by root diameter growth. This action could seldom shift stones and disrupt wall stability.

Field observations of Banyan roots that have penetrated the joints indicate the highly adaptable growth habit. The normal circular cross-section of a root, upon entering a gap, will assume a planar shape to fit the geometry of the joint. After venturing through the gap, the root will grow into the aft-soil by resuming the normal cylindrical form. Thus the chance of Banyan roots entering a joint and pushing stones apart is rather remote.

The example of stone displacement by stonewall tree shown in Plates 8.3 and 8.4 of Chan (1996: 113) occurs at the end of the wall where the stone interlocking effect is relatively weak. The photographs clearly indicate the unusual growth of the trunk of a tree directly in the gap, rather than 28

penetration by tree roots. A small number of cases of stone displacement are associated with roots exploring gaps, for instance at the Bonham Road wall in this study, and at the western end of the Forbes Street wall. In these cases, the impact on wall structure could be considered as local and limited. At the wall crest, the roots may enter the gap under the coping to lift it. Similar to the wall end, the limited interlocking effect has permitted roots to widen the horizontal gap at the wall top.

(d) Roots in aft-soil

The Banyan roots have a natural propensity to grow away from light into the gaps between masonry blocks, in line with the physiological response of negative phototropism. Roots tend to ramify in the aft-soil to anchor the tree and strengthen the soil. They offer additional reinforcement to the wall by providing tensional pull, and raise the frictional forces between the wall and the retained soil. For the older generation of loosely packed and dry walls with plenty of gaps, more roots can penetrate and spread in the aft-soil to provide notable wall stabilization. For the later tightly packed and mortared wall, the limited amount of aft-soil exploration by roots will correspondingly reduce this soil strengthening effect.

(e) Root decay in aft-soil

The aft-soil, like most soils, contains pores of different sizes which depend mainly on texture, structure and bulk density (degree of compaction). Tree roots that grow in the aft-soil could die and leave behind cylindrical pores. Small roots tend to have shorter life span and their continual death and replacement by new roots would only leave small pores with limited impact on soil water movement. If the dead roots are large, notable cylindrical pores could be formed to facilitate the transmission of soil moisture. If such pores could concentrate subsurface water flow, pressure could be imposed locally on the stone wall. Most trees tend to keep their large roots and hence their death and formation of notable cylindrical pores will be limited. The invasion by wood-decay fungi and consumption by termites, however, could kill vulnerable large roots to leave channels for water transmission.

(f) Surcharge of tree on stone wall

A tree hanging on a retaining wall, with a certain biomass and wind resistance and affected by gravity and wind, denotes an additional surcharge to the structure. A review of the relevant literature on the analysis of the relevant forces is given in GEO (2011). The load can increase the overturning moment and apply an extra toe pressure to the 29 wall. The size of the tree in terms of total aboveground biomass, trunk diameter, the sail effect of the crown, crown porosity and permeability to wind, and bending flexibility of branches (involving the changing frontal area and drag coefficient in response to wind velocity), could determine the magnitude of the surcharge. Part of the wind load acting on a stonewall tree could be transmitted to the wall.

If a tree has an inherently weak biomass structure or has its mechanical strength compromised by decay or cracking, it may break under strong wind. If the root anchorage is inadequate, or its root system has been damaged by natural or artificial forces, the tree may be uprooted under strong wind. Overall, three modes of failure due to wind could be enumerated: branch breakage, trunk breakage, and uprooting. Before failure, the tree under the influence of strong wind would transmit part of the energy to the wall structure. The root system per se will be strained together with the interfaces between roots and masonry blocks, and between roots and aft-soil. Of the few cases of stonewall tree failures, it has been found that the masonry structure was hardly affected (GEO, 2011).

2.3 Factors on stability and health of stonewall trees

2.3.1 Intrinsic wall factors

(a) Gaps between masonry blocks

Dry random rubble walls with wide and plentiful gaps between masonry blocks can permit rather free passage of air and water into and out of the aft-soil. The same micro-niches could facilitate seed lodging and seedling germination. The gaps offer avenues for roots to penetrate into the wall core and aft-soil. In comparison, dry squared rubble walls have narrower and less plentiful gaps and hence they are less amenable to colonization by stonewall trees. Modern stone walls with squared and coursed masonry blocks and joints completely filled by mortar offer significantly fewer chances for roots to penetrate. They are less conducive or unfriendly to the growth of stonewall trees.

(b) Stone size and length of joints

A wall with smaller masonry blocks contains a longer total length of joints. Joints offer critical microsites for soil and debris accumulation, and moisture and nutrient supply. They are conducive to seed lodging, seed germination and growth and root penetration. More joints are therefore favourable to stonewall tree growth and establishment. They help to sustain their continued development and stability.

A wall composed of mainly rectangular masonry blocks with a long horizontal axis can provide more horizontal joints in comparison with vertical ones. Horizontally-oriented joints are more receptive to seed lodging and seedling growth. They offer more secure microsites for seeds and seedlings to stay on the wall and resist dislodgement by gravity, wind or running water.

(d) Wall inclination

Most walls are not vertical. Instead, they tend to be slightly inclined up to about 10 degrees from the plumb. A more tilted wall provides somewhat less harsh microsites that are more receptive to the lodging of seeds, seedling growth and tree establishment.

(e) Wall height

Birds that bring the seeds to the wall joints via their droppings would avoid getting too close to humans. A tall wall offers a larger proportion of its wall face situated away human disturbance which occurs mainly at the wall toe. Seedlings that grow up at the wall toe and at the lower part of the wall are more likely to be disturbed by humans. They are often removed to provide clearance to pedestrian movement along the narrow footpaths or lanes in front of the wall. Thus low walls are less conducive to wall tree establishment and long-term existence. For a similar wall length, a tall wall provides a large surface area to receive seeds and to permit tree growth.

(f) Original joint filling

The treatment of the wall joints at the time of construction is known as joint filling. Dry walls have their joint left un-mortared. The ashlar walls with squared or rectangular stones and mortared joints are not too amenable to tree establishment. They lack the necessary micro-niches for seeds to lodge, for seedling growth, root penetration, tree establishment, and continued tree growth.

(g) Cement sealing of dry wall joints

Originally dry walls tend to be sealed recently in the course of overzealous and improper wall maintenance practices. Often, a cement plus sand mortar is used to fill the open joints. Some old dry walls have all the joints effectively seals in this manner. In some cases, the filling has resulted in a wide band of mortar along the joints in an undesirable and unsightly practice labelled as buttering.

The newly applied mortar often wraps around existing roots and girdles them as they continue to grow in diameter in the process of secondary thickening. The girdled roots will have its phloem or sapwood restricted or cut off, thus curtailing the ability of the affected tree to acquire water and nutrients, and to distribute the food manufactured by photosynthesis to reach the root system. Girdled roots tend to develop wounds that could be invaded by wood-decay fungi and other natural enemies. Existing roots in the aft-soil will also suffer from the lack of aeration, with inadequate exchange of soil air with outside air. Root respiration will be dampened due to the lack of oxygen supply, and root functions could be dampened by the accumulation of carbon dioxide due to inadequate dissipation.

Meanwhile, new roots will have no avenue to enter the wall gaps and acquire new sources of sustenance from the aft-soil. Overall, tree growth can be stifled, leading to gradual and premature decline. The weakened tree could lose its ability to ward off natural enemies, which may lead to decay and other growth problems. Thus joint sealing can serve as a trigger factor leading to deleterious snow-balling effects on tree health and safety. Wall management should refrain from sealing open joints.

Measures could be adopted to ensure that wall maintenance works can remain conservative and appropriate. They should be friendly to the present cohort of stonewall trees, and permit the unimpeded establishment of the future generation of stonewall trees. For outstanding walls with heritage value, it is important that the maintenance should adhere faithfully to the authenticity principles.

(h) Weathering state of masonry blocks

More weathered stones can release more nutrients in dissolved form from the chemical breakdown of their constituent minerals. The modified materials can retain some water for use by some adventitious roots that happen to grow on the masonry surface.

(i) Seepage on wall face

Long-term seepage on the wall face could indicate a relatively high groundwater table, providing a ready source of water for absorption by adventitious roots resting on the surface of masonry blocks. Moreover, it also indicates that the aft-soil has a rather regular supply of water to feed the roots growing there. However, if the water table stays at a high level for a long duration in relation to the wall, it could imply the application of hydraulic pressure on the wall.

The sudden increase in seepage should be interpreted with caution, as it could be an ominous sign. The source of water should be traced, as it could originate from the breakage of potable water mains, or stormwater drain, or sewage pipe. Such unexpected and unnatural release of a large amount of water could exert inordinate pressure on the wall and it may induce wall failure. The excessive soil moisture could drive out air to harm the roots. If the leakage comes from a sewage pipe, the chemical constituents which may include pollutants could harm the trees.

(j) Weep holes

Not all retaining wall have built-in weep holes. For dry walls with plenty of gaps between masonry blocks, it is not necessary to provide them. For mortared walls, they may or may not be present. Weep holes can offer microsites for seed lodging and seedling growth, but they alone do not offer sufficient conditions to permit the establishment of large wall trees. Weep holes can provide water with dissolved nutrients to nourish surface adventitious roots, and this role is more important in the early stage of a wall tree’s tenure when few roots have spread in the aft-soil. For well- established large wall trees, this function may not be important.

2.3.2 Quality of wall environs

(a) Moisture supply of aft-soil

Above and behind the wall crest, the site is often occupied by a formed platform usually with a building and paved land surface. Such site conditions would limit the supply of water to the aft-soil via infiltration of rainwater. The wall trees will have to depend on groundwater. If the groundwater table behind the retaining wall is dammed and stays at a high level, it may keep the aft-soil too wet and be unfavourable to tree growth. The gaps between masonry blocks and weep holes, however, could discharge the groundwater and keep the water table low relative to the wall. If the groundwater table is occasionally high and not too low most of the time, the tree roots can take up water from the capillary fringe of the ground water.

(b) Volume of aft-soil

The volume of aft-soil, similar to the case of ground-growing trees, would play a key role in supporting the growth of stonewall trees. A larger soil volume can permit extensive spread of roots and capture more moisture and nutrients to satisfy tree requirements. More soil could contribute to a firm tree anchorage by the root system. The volume of aft-soil could be restricted by various reasons. The presence of hard rock, partly weather rock, building foundation and other subterranean installations could limit the amount of aft-soil that can be explored by roots.

Besides the volume of soil accessible to root growth, the quality of soil would regulate the physical and chemical conditions for root growth and 34

functions. Soil conditions that are unfavourable to roots include: soil compaction, high stone content, excessively coarse texture (too sandy), excessively fine texture (too clayey), poor structure, low porosity, insufficient or excessive moisture, restricted aeration, low organic matter content, inadequate available nutrients, and presence of soil pollutants.

(d) Wall-crest slope with unsealed soil

At the wall crest, the presence of unsealed soil covered by vegetation is conducive to wall tree colonization and development. Proximity to natural vegetation has been found to facilitate stonewall tree growth and establishment. In particular, the presence of natural woodland at the crest slope could enhance the number and vigour of stonewall trees. Other types of companion wall vegetation, such as herbaceous and non-vascular (moss and lichen) are also favoured by the nearby natural vegetation cover.

(e) Wind exposure

A stone wall at an exposed and windy site is hostile to seed lodging and seedling growth. The birds or bats that could bring seeds along with their droppings may shun the unduly windy locations. They are less likely to build their nests on the stonewall trees at such locations, or visit the site to seek food or shelter, or to rest. The seeds that manage to lodge in the crevices could be easily blown away by strong wind or washed away by rain in conjunction with wind. Similarly, seed germination and seedling growth could be dampened by strong wind. Under extreme weather conditions, seedlings that are usually rather precariously attached to the wall could be literally plucked and swept away. Even medium or large trees could yield to exceptionally ferocious gusts, resulting in branch loss, stem breakage or uprooting failure.

(f) Solar access

Most stonewall tree species prefer to have plenty of sunshine to allow them to flourish. Sites that are sandwiched between tall buildings could be heavily shaded to suppress photosynthesis and food production. The amount of solar energy that can reach the stonewall trees could be evaluated by the sky view factor. Most stone wall sites are situated close to buildings or narrow roads with buildings nearby. Thus they are unable to enjoy unimpeded sunshine. The lack of solar access is closely related to the lack of above-ground growth space to accommodate normal crown spread. For sites with open space at the wall crest platform, the wall trees tend to be stronger and can reach larger dimensions.

(g) Air quality

Most stone retaining walls occur in the Mid-levels areas where roads are narrow and capacity for traffic is limited. Some of the main roads, however, can have heavy vehicular flow during the peak hours to contribute to air pollution. The narrow roads bounded by high-rise buildings could trap air pollutant emitted by vehicles, and dampen the growth of stonewall trees. As they usually perch on an elevated position, the air pollution impact could be somewhat relieved.

(h) Proximity to buildings and roads

Most stone wall sites are rather cramped with close juxtaposition of buildings and roads. The urban form is typical of our compact mode of development. Proximity to buildings would incur conflicts when branches get too close to windows. Branches extending into the carriageway space at a low level will have to be cleared. For trunks or branches that grow at a low level, frequent removal would be required to maintain footpath clearance. Repeated pruning especially of large branches could dampen tree vigour and invite invasion by wood-decay fungi.

2.3.3 Extrinsic impacts on walls and environs

(a) Soil nail installation and grouting damage

Grouting can impose grave physical and chemical harms on stonewall trees. In the course of drilling, the roots in the aft-soil and on the wall face could be injured or severed. The drilling machine may also accidentally hit the trunks and branches and impose injuries and breakage. The hot exhaust emitted by the machines could stress the trees. The introduction of grouting materials under pressure into the drill holes could induce more serious and long-term damages. The fluid is likely to migrate away from the borehole into the surrounding aft-soil. A fluid with low viscosity and a soil with high porosity would facilitate wider migration to affect a larger volume of aft-soil.

The alkaline grouting fluid in contact with living roots could harm them directly. The soil pH could be raised to cause nutrient imbalance especially for micronutrients in the long run. Soil particles coated by grouting materials will not be able to release nutrients into the soil solution for root uptake. Alteration of the soil chemical environment will suppress the activities of soil organisms, including the microbes that are instrumental in decomposition and nutrient cycling. 36

Soil pores could be permanently occupied by solidified grouting materials, thus reducing the internal soil spaces necessary for moisture storage, moisture transmission, drainage, aeration and root growth. Existing tree roots engulfed by the grouting fluid will be deprived of the means to acquire water, nutrient and air. They will decline and may eventually die.

Installation of the soil nail head could damage surface roots and occupy the spaces that could otherwise be used by tree roots. The multiple deleterious impacts of grouting are irreversible, and the affected trees will usually decline gradually over some years. Stone walls that have been treated with grouting often lead to degeneration of the companion stonewall trees.

(b) Stabilization treatment of contiguous wall crest and toe slopes

Stonewall trees situated at or near the wall crest often send their roots into the slope there. Similarly, the surface roots of stonewall trees tend to send their roots into the slope at the wall toe. The presence of unsealed soil at the wall crest and wall toe provides a wonderful opportunity for tree roots to explore and capture water and nutrients. The additional supplies would greatly improve tree health and vigour, and permit the benefited trees to attain a bigger size.

With strong roots growing in the crest slope in the backward direction, the stonewall tree could be firmly anchored and stabilized by the tensional pull to resist the forces of overturning and toppling. Aerial roots that can land on the wall toe slope could lignified and develop into new root stands or secondary trunks to provide additional support to the stonewall tree. The conditions that permit roots to continue to thrive at the wall crest and wall toe slopes should be maintained to sustain the health of the stonewall trees

Some slopes at the crest and toe positions of retaining walls in recently years were subject to stabilization treatments. The soil is often heavily compacted to render it too dense for root growth. Existing roots could be injured by the strong pressure exerted on the soil in the course of compaction. The excavation associated with the slope works would remove, cut or injure tree roots which tend to be concentrated in the upper soil layer. The removal of the ground vegetation would reduce the blanketing effect on soil moisture and temperature, and the related functions of soil organism activities, organic matter decomposition and nutrient recycling.

The collapse of soil pores due to heavy compaction will render the soil unsuitable for root growth. The significant loss of macropores (> 60 μm diameter), in particular, would deprive the soil of the key avenues for infiltration, percolation, aeration and root growth. The degraded soil environment would become unsuitable to the growth of new roots. The loss of old roots can hardly be compensated by new roots, resulting in a net reduction in the root system in terms of both size (length and surface area) and function. In due course, the tree would lose too many roots to trigger an irreversible decline spiral. Root decline in the crest slope is particularly alarming, as they may provide the bulk of the critical tensional pull to hold the hanging tree in place. The loss of anchorage could therefore generate hazard trees that may be prone to collapse.

If the compacted soil at the crest or toe slope is sealed by shotcrete, which is often applied, the damaged will be further aggravated. The impermeable seal will nullify the chance for the recovery of the groundcover vegetation and undergrowth on the treated slope. Water and air will not be able to infiltrate into the soil, thus roots growing there will be deprived of such pertinent supplies. The unvegetated bare shotcrete surface tends to absorb an appreciable amount of solar heat and transmit it downwards to make the soil environment unwelcomed to roots and soil organisms. Under such stifling soil conditions, old roots will gradually decline whilst new roots will find it difficult to develop. The tree decline process triggered by the compaction treatment would be aggravated by the shotcrete sealing, resulting in accelerated tree degradation.

Stonewall trees often send their roots downwards along the wall face to reach the generally paved areas at the wall toe, such as platform or footpath. If soil is accessible at the wall toe, such as an open soil strip or just a narrow gap with unsealed soil, the roots tend to grow into the available soil. Such roots tend to spread under the paving to capture water and nutrients from a larger catchment. Their development is enhanced if the paving is permeable, such as unit pavers. This way, the stonewall trees could be greatly benefited by acquiring additional water, nutrients and anchorage. They are likely to grow stronger and bigger.

The conditions that allow roots to grow into such wall toe soil should be kept to maintain the health of the stonewall trees. The possible disturbances that may degrade, hamper, cut or kill such wall toe roots should be avoided. The volume and quality of the soil and the access of the soil to moisture and air should not be curtailed. The deleterious

impacts include filling the soil gap with concrete or cement, opening a trench or a pit adjacent to the soil strip, or compacting the soil.

If it is feasible, measures could be adopted to improve the soil conditions for root growth at the wall toe. If no soil strip is present, a new one could be created by removing a strip of paving. The width of the soil strip could be adjusted according to the footpath width and the pedestrian traffic volume. Existing soil strips could be improved by widening and adding soil amendments. In treating the soil, care should be taken to avoid damaging existing roots in the strip.

(d) Soil disturbance at contiguous paved areas at wall crest

The wall crest area could be a paved platform in the grounds of a residential, commercial, government, institutional and community development, or a public road. Such paved areas could be disturbed due to excavation in the course of renovation, repaving, utility trenching, and foundation work for building redevelopment. Roots could be removed together with the enveloping soil in the course of digging. Existing soil volume could be occupied by new utility lines, junction boxes and other subterranean installations. Lowering the height of an existing platform could remove some aft-soil. The existing tree roots will be eliminated, and the soil volume will be permanently reduced.

Removing the old paving could increase water infiltration and increase the soil moisture content of the aft-soil. Whereas more water could be available for tree growth, the elevated pressure of the soil water could have implications on wall stability. Replacing the old and cracked paving with new paving could cut off water supply to the aft-soil and dampen the growth of stonewall trees.

(e) Skin wall impact

Building a new reinforced concrete wall in front of an old masonry retaining wall has been adopted as one of the geotechnical methods to stabilize it. Existing stonewall trees would need to be carefully protected to permit their continued growth and to maintain their health and vigour.

The main concerns are the trapping of surface roots in the gap between the skin wall and the old stone wall, and hence the lack of space for continued growth and thickening of the roots. The moisture trapped in the gap may stay there for too long to induce wood decay. Inserting spongy and water- absorbing materials in the gap may not help, as the continued thickening of roots may soon exhaust the available space. The same materials tend to 39

retain water for extended periods to facilitate the growth of wood-decay fungi.

The skin wall would cover up the masonry wall surface to prevent aerial roots from reaching the joints to grow into the aft-soil. The chance for new roots to reach the aft-soil is thus deprived, which may dampen the further development of the stonewall trees.

(f) Trenching at wall crest or wall toe platform

Trenches opened at the wall crest platform could sever the roots of stonewall trees. Trees situated at or near the crest are more seriously affected as more of their roots are found near the surface of the crest platform. Similar impacts would be incurred for roots growing into the wall crest slope. Trenching adjacent to the wall crest behind stonewall trees could cut most roots that may provide the crucial tensional pull to hold the trees. The mistreated trees are very likely to collapse.

Trenching at the wall toe is usually situated close to or adjacent to the wall, because most footpaths or lanes contiguous to the wall are narrow. If the stonewall trees have sent their roots into the soil in the ground, most roots will be severed or seriously damaged due to excavation. The loss of water and nutrient supply will dampen tree health.

2.3.4 Stonewall tree factor

(a) Tree species

The bona fide stonewall trees, represented by the strangler figs (cf. Sections 2.13 and 2.14), have strong and sprawling roots to grip the wall, and penetrate the joints to acquire water, nutrients and anchorage from the aft-soil catchment. Other species may accidentally grow on stone walls, but their ability to capture resources or secure a foothold is limited in comparison with their Banyan companions.

(b) Tree dimensions

Apparently, large stonewall trees are more liable to failure. This is due to the intuitive thinking that they are heavy, more prone to wind damage, and may have accumulated more decay and structural problems over time. In reality, every tree has its individual growth habit and vicissitudes due to both intrinsic genetic and extrinsic factors. A tree can be subject to the whims of different and changing habitat conditions, environmental 40

impacts, interactions with natural enemies and the built-up fabric, and tree management regime. A tree regardless of size or age could remain strong and stable. Conversely, a tree regardless of size of age could become weak and unstable. The failure of a large tree, however, could lead to more serious consequences on the targets.

Like a ground-growing tree, a stonewall tree can be beset by different biomass structural problems. Such problems could predispose a tree to failure. Most of the problems could be identified by a thorough and professional tree risk assessment using the advanced strand of visual tree assessment method. The main structural weaknesses include decay, canker, cavity or crack at trunks, limbs and main branches, compression fork (also known as v-crotch) especially those associated with included bark and internal decay, lack of tension wood or response wood at critical positions, and branches that are too long, heavy or low.

(d) Tree lean

Many ground-growing trees tend to lean slightly, and for them an inclination angle exceeding 20 degree from vertical is considered as the critical threshold. As wall trees cling on vertical habitats, many would incline by more than this limit, which could range from < 10 to nearly 90 degree. A heavy lean does not necessarily imply instability, and the converse is true. The ability to anchor and support the tree biomass and freedom from structural defects are critical in the stability of stone wall trees.

The tree tilting angle would need to be assessed in conjunction with the nature of the anchorage, to reckon if it is adequate to hold the weight of the tree in both static and dynamic terms. The field assessment may not provide all the information necessary to judge, because the amount of roots that have penetrate into the aft-soil and provide tensional pull cannot be ascertained with a high degree of certainty. Especially for trees with a wide and dense development of surface roots on the wall face, the amount and size of roots that have turned backwards to enter the aft-soil cannot be evaluated easily and accurately.

For a trunk with a heavy lean exceeding 35 degree or so, the amount, position and configuration of the tension wood at the base and on the upper side (opposite to the tilting direction) should be assessed in detail. Strong and effective tension response wood of the right size and shape and developed at the right position could help to pull the trunk or limb against 41

failure. The structural defects that could contribute to instability should be thoroughly assessed.

(e) Root growing habit

The development of a dense network of surface roots on the wall face may not be equated with tree stability. On the contrary, the lack of surface root development cannot be construed as a certain sign of instability. Each stonewall tree has to be assessed in detail on an individual basis.

The critical concern is the presence of surface roots with the ability to turn backwards to reach the aft-soil. A sufficient number of such backward- growing roots of sufficient size and spread, with reference to tree size and hence loading, could provide collectively adequate pulling force to counteract the overturning tendency. Surface roots alone could provide support and some water-nutrient absorption capability, but without enough backward-growing roots the anchorage may not stand the test of exceptionally strong wind.

(f) Quality of pruning work and tree maintenance

Many stonewall trees were subject to rather unprofessional treatments in the past when the standard of as well as the concern for tree care was not that high. As a result, a notable proportion of stonewall trees have suffered widely from the legacy of improper pruning. They commonly include branch tipping and heading cuts, which would induce decay and development of long and heavy epicormic branches at or near the decayed cutting positions. The risk susceptibility of some stonewall trees has been pushed to a higher level due to such erroneous treatments and their collateral consequences of unstable and aggravated biomass structure. The present tree managers have inherited a host of such weakened trees beset by structural defects which are difficult to rectify.

3. TREES RISK ASSESSMENT AND ARBORICULTURAL RECOMMENDATIONS

3.1 Risk assessment of the stonewall trees

3.1.1 Site and general tree conditions

The old masonry retaining wall is located on the south side of Bonham Road opposite numbers 29 to 35, near the junction with Park Road (Photo T0-1). The SIMAR slope registration number is 11SW-A/R577. Its management responsibility together with the stonewall trees has been assigned to the Highways Department (Photo T0-2).

Situated at the wall crest and running parallel to the wall is St Stephen’s Lane. This narrow street permits the buildings at the back of the wall to be set back to provide rooms for the stonewall trees to expand southwards with little obstruction by buildings. Based on HyD record, the wall runs for about 61 m long in a roughly east-west direction. Its height varies from the taller west end at 5.23 m to 2.13 m at the east end (Photo T0-3). The wall is believed to be built together with Bonham Road, hence it is reckoned to be about 150 year old. It represents the first generation of stone retaining walls in Hong Kong. As such, the masonry structure has intrinsic historical and heritage value.

The wall is composed of irregularly sized and shaped masonry blocks that are not coursed (Photos T03 and T04). The original design is likely to be a dry random rubble wall with unmortared joints. The present mortar filling all the joints was probably added at an unknown time some decades ago. The stones come from local volcanic rocks. The stone surfaces are hammer finished roughly with little dressing. Some masonry blocks show clear signs of weathering. The wall face is generally dry with no sign of long-term seepage. It is equipped with weep holes which might be added after its construction. At the west end, a short section of the wall displays modern features and is composed of squared granite stones that are well coursed (Photo T0-4). It denotes a reconstructed section installed after WWII.

The surrounding land use is dominated by residential with ground-floor shops. The vehicular traffic along Bonham Road remains heavy most of the day time. The solar access, air quality and wind exposure of the site can be rated as medium in a three-point scale.

Six trees are found on the wall, and they are mainly dwelling at or near the wall crest. They are all Chinese Banyans (Ficus microcarpa). Starting from the west, they are respectively labelled as T1 to T6 in this report (Photos T03 to T07). T3 to T6 are clustered closely together in about one- third of the wall length at the east side. T1 is located at the far west end. T2 stands near the middle facing Centre Street. T1 is the smallest at 7.26 m height and 30 cm DBH, whereas T2 is the largest at 16.68 m height and 146 cm aggregate DBH. Judging from the size and slow growth rate of stonewall trees, T2 is reckoned to have dwelt on the wall for more than a century. It is one of the few stonewall trees in Hong Kong with such a large size, good tree form and health. As such, it could be recognized as a living heritage of the community. T3 to T6 are young at an estimated age of 60 years. T1 is youngest at about 15 years old.

A bus bay occupies a large proportion of the road length in front of the wall (Photos T0-8 ad T0-1). The pavement at the wall toe is narrow except where it widens somewhat towards the east end at the junction with Park Road. Thus double-decker bus could get rather close to the trees. The footpath is covered by recently installed unit pavers to permit some infiltration of water and air into the underlying soil. A shallow U-channel drain has been installed along two stretches of the wall toe (Photos T0-8 and T0-9).

The field survey of the six trees was conducted on 01 to 07 June 2012. The detailed assessment results of the six trees were recorded in dedicated forms designed for stonewall trees in Tables 1 to 6. The annotated photographs referenced in the forms are given in the attached PowerPoint photo album containing 199 slides.

Three trees, namely T2, T4 and T5, were recommended for microdrilling to ascertain the condition of internal decay or cavity. This task was conducted on 31 October 2012 by a contractor engaged by the HyD, and the drilling results conveyed to me on 13 November 2012 has been included in the Appendix. The positions of the 10 drills for T2 and 5 drills each for T4 and T5 are shown respectively in Appendix Figures T2-P0, T4-P0 and T5-P0. Information on the corresponding photographs indicating the proposed drilling positions are summarized in Table 7. This report has subsequently been updated in the light of the microdrilling findings.

3.1.2 Assessment of T1 and arboricultural recommendations

This is a young tree that dwells at the crest of the wall and hangs well above Bonham Road (Photo T1-1). The compact and rounded crown has a limited span of 8 to 9 m (Photos T1-1 and T1-2). It keeps most of its leaves and branches, with no sign of dieback (Photo T1-3). The live crown ratio exceeds 70%, and the tree vigour is rated as good. The leaves are of normal density, size and colour. The small tree leans at about 15 degree towards Bonham Road, and it does not display symptoms concerning instability. The tree does not suffer from notable trunk, crotch or main branch defects.

T1 rests on a renovated section of the retaining wall which is composed of squared granite stones and equipped with two concrete beams (Photo T1-4). It is likely to be constructed after WWII. The masonry blocks at and around the trunk base has remained intact. At the time of field survey, part of the tree was covered by the scaffolding and protective nylon net of the adjacent building site which was under renovation (Photos T1-4 and T1-5). Its large neighbour T2 shields it on the east side (Photo T1-6).

A short single trunk of 1.46 m height and 30 cm diameter supports three limbs at more or less that same height, denoting the crowded branching phenomenon (Photo T1-7). The crotches between the three limbs are of the stronger U-type rather than the inherently weak V-type. Measures could be taken to remove the use of the tree as support for construction- renovation purpose (Photo T1-8). Few surface roots have spread on the wall face.

Tree hazard assessment has a score of 5 which is rated as low (Table 1G). Minor trimming of a crowded upright and closely spaced competing branch and a nearly horizontal and relatively long branch are recommended in Table 1H and explained in Table 1I. The stone wall of modern design has closely packed masonry blocks and well mortared joints that could effectively ward off penetration by tree roots to reach the aft-soil. In the longer term, the lack of access to the soil catchment behind the wall to obtain new sources of water, nutrients and anchorage may suppress its growth rate and reduce its ability to reinforce its foothold.

3.1.3 Assessment of T2 and arboricultural recommendations

(a) Overall tree structure and condition

(i) Tree-wall relationship and tree dimensions

This is one of the most outstanding stonewall trees in Hong Kong by virtue of its height, crown spread, trunk diameter, tree form, health and vigour (Photo T2-1). It is situated in a prominent location that can be appreciated from different angles by many residents and road users. The dense and sprawling tree stands well above the carriageway of Bonham Road with heavy vehicular and pedestrian traffic during the day time (Photo T2-2). The large tree measures 16.68 m tall, and its trunk base is elevated at 3.84 m above the road level.

(ii) Tree quality and OVT designation

The meritorious specimen tree has fine qualities that deserve special attention. It has been designated under the government's Old and Valuable Tree (OVT) Register. It should be kept in the Register. A well- designed and durable information plaque could be installed at St Stephen's Lane which offers a convenient and safe location to appreciate the magnificent stonewall tree. The pavement on the opposite side of Bonham Road would offer a second suitable location to put up a plaque.

(iii) Tree anchorage at wall crest

The tree growth was initiated near the wall crest with the trunk base leaning outwards at 32 degree, but assuming a more upright posture higher up from 9 to 13 degree (Photo T2-3). Besides hanging above Bonham Road, the south side hangs above St Stephen’s Lane to get close to the buildings there (Photos T2-4 and T2-5). From the top of the Centre Street, the sprawling 27 m east-west spread of the crown can be fully appreciated. In the north-south direction, the crown spread is somewhat narrower but it still spans about 22 m (Photo T2-6).

(iv) Multiple and codominant trunks

Three stout codominant trunks collectively support the huge biomass (Photos T2-7, T2-8 and T2-9). Their trunk diameters (DBH) measure respective at 115 cm for trunk A, 51 cm for B and 74 cm for C, with an aggregate of 146 cm. The thickest trunk A is split at a low level into two strong limbs (A1 and A2).

(v) Tree hazard rating

Tree hazard assessment has a score of 5 which is rated as low (Table 2G). The main contribution to tree risk is the breakage of branches that are structurally weak or decayed.

(b) Assessment of surface roots and interface with the stone wall

(i) Limited spread of roots on stone wall face

The spread of surface roots on the wall face is relatively limited in comparison with the huge tree size (Photo T2-10). Most roots that have ventured into the aft-soil are likely to penetrate the joints at and around the trunk base. Some surface roots have descended to the wall toe to enter the soil below the pavement (Photo T2-11). However, the paving and cement filling of the wall toe gap have restricted this mode of root development. Besides obstructing the growth of existing roots at the wall toe, it will be quite impossible for new roots to enter the soil below the pavement.

(ii) Tree anchorage and wall bulge

The large tree has established a firm hold on the wall by sending its roots into the aft-soil near the crest (Photo T2-12). The trunk base points backwards towards the wall crest, and the trunks curve upwards (Photos T2-13 and T2-14). At the east side of the trunk base, the wall face shows a small and slight bulge (Photo T2-15), which could be related to root growth behind the stones or to the pressure exerted by the retained soil on the stones. This symptom needs to be very closely monitored by a geotechnical engineer especially to assess whether the bulge will continue to push outwards and enlarge.

(iii) East side surface roots and masonry

Root spread beyond the vicinity of the trunk base has been limited by the sealing of joints by cement (Photo T2-15). A few masonry blocks on the east side of the trunk base has been displaced mainly outwards away from the wall face (Photo T2-16). The presence of two small holes at the same location adjacent to a thick surface root implies the dislodgement of masonry blocks in the past. The upper hole has been filled with cement, whereas the lower hole remains vacant (Photo T2-16).

(iv) West side surface roots and masonry

Root spread on the west side beyond the trunk base has also been restricted due to the rather thorough sealing of joints by cement displaying the buttering phenomenon (Photo T2-17). Adjacent to the trunk base on the west side, two masonry blocks are protruding from the wall face indicating perpendicular displacement (Photo T2-18). Further away from the trunk base on its west side, the upper one-third of the wall displays an uneven surface, which may indicate the poor workmanship or slight stone displacement by root growth behind the stone façade (Photo T2-19).

(v) Boulder size and shape

It is notable that the boulders in the lower two-third of the wall are larger, whereas those in the upper one-third are smaller (Photo T2-10). This vertical variation in stone size implies that the upper part of the wall has more gaps between stones to permit penetration by tree roots to enter the aft-soil (cf. Section 2.3.1a). The smaller stones near the wall top are less constrained by the confining pressure of interlocking masonry blocks. Thus they are more amenable to be displaced by roots. This wall-tree interaction favours trees but is detrimental to wall integrity. If the exploitation by tree roots is not excessively, it could be tolerated.

(vi) Removing rubbish from surface roots

Measures can be implemented to improve the root growth conditions of the tree. The rubbish that has deposited on the surface roots and the trunk bases and crotches, together with the leaf litter, should be regularly removed with the help of a brush to avoid moisture accumulation which could induce decay (Photo T2-14). Thorough removal of rubbish in the gap is also essential to avoid the wedging effect (cf. Section 4.2.3).

(vii) Removing cement sealing at masonry joints

The limitations to root growth due to the lack of open and penetrable joints between masonry blocks, and the lack of accessible soil at the wall toe and crest positions, could be ameliorated to enhance tree growth and performance (Photos T2-10, T2-17, T2-19). The joints between masonry blocks have been sealed recently by cement, thus stopping their penetration by tree roots. The rigid cement seal also restricts expansion of existing roots and may cause girdling injury as they continue to thicken. Where the roots are physically obstructed, consideration could be given to

localized removal of the cement seal to permit some new roots to grow into the joints and existing roots to expand (cf. Section 4.2.2).

(viii) Installing soil strip at wall toe and crest

An open soil strip filled with a good-quality soil mix could be installed at the wall toe at Bonham Road to allow growth of roots in the soil below the pavement (Photo T2-11). Similarly, an open soil strip filled with a good-quality soil mix could be installed at the wall crest to allow growth of roots in the soil below the pavement at St Stephen's Lane (Photo T2-5) (cf. Section 4.2.1).

(i) Crown spread and condition

The crown spread in the east-west direction is broad and rather symmetrical and balanced (Photo T2-20). The branching and foliage densities are reckoned as high and typical for a healthy Chinese Banyan (Photo T2-21). At different sides of the crown, these indicators of tree health and vigour are well expressed (Photos T2-22, T2-23 and T2-24). At the ends of the branches, the twigs and leaves do not show signs of dieback (Photo T2-25). The foliage density, leaf size and leaf colour fall into the normal category. The crown is beset by some tipped branches. Overall tree vigour is rated as good.

(ii) Excessive removal of low branches

The live crown ratio of the tree stands slightly above 70%, and the twig dieback can be rated as low at < 5% (Photo T2-26). However, the tree has suffered from excessive removal of lower branches (Photo T2-20). This undesirable practice could reduce the ability of the tree to develop proper trunk taper to compromise its stability (cf. Section 4.2.4). The reduction in taper, meaning less wood development in the lower part of the trunks, could jeopardize the tree’s ability to support the bulky crown load. As the tree adds more weight to its crown, the inadequate corresponding thickening of the trunk base could lead to tree failure. The tree care practice henceforth will need to be revamped to help the tree to recover from this improper treatment. The preferential and repeated removal of lower branches must forthwith be stopped. In the future, the emergence of new branches in the lower parts of the trunk with the potential to develop into sizeable and safe branches should be allowed to grow.

(iii) Crown conflict with nearby buildings

The crown arches above Bonham Road and extends towards the residential building on the opposite side (Photos T2-27, T2-28 and T2-29). If the branches get too close to windows, it could block too much natural light. The swinging of branches in strong wind could hit the windows and create a nuisance or a hazard. Such proximal branches could be shortened by the proper reduction cut.

(d) Assessment of trunks and branches

(i) Overall condition

A detailed assessment of the tree’s scaffold can yield telltale signs about the integrity and stability of the biomass structure. The evaluation could follow a logical sequence, beginning with the large members, namely the trunks and limbs, and then move on to the main and smaller branches. The tree has not had a thorough and high-calibre crown cleaning for some years, and it has accumulated a fair amount of decayed and structurally weak or defective branches. It calls for a professional and comprehensive treatment to rectify the arboricultural problems. The following notable issues deserve attention and follow-up actions. Details about the full range of recommended arboricultural treatments and their explanations have been given in Table 2 Parts H and I.

(ii) Photo T2-30: Tree scaffold

Stems A1 and C are found to be constituted by substantially expanded and elongated epicormic branches (Photo T2-30). The contact between the parent stem and the daughter should be monitored to see whether the decay has set in to compromise the strength of the inherently weak attachment. Close visual inspection assisted by a wooden mallet should ascertain the wood condition at the critical junctures.

(iii) Photo T2-31: Stem A1 with large decayed wound (microdrill points 1 to 3)

Stem A1 has a large wound with decay and cavity development, with notable bulgewood formation (Photo T2-31). The wood condition inside the wound and shortly above and below it needs to be evaluated with the help of microdrilling. The positions and directions of recommended drillings are annotated in the photograph. If the recommended microdrilling finds that the decay at the wound has spread to occupy more

50 than one-third of the nominal trunk diameter, the branch loads of the affected stems have to be reduced correspondingly.

The microdrilling results of stem A1 at points 1, 2 and 3 explore the internal decay condition at the large decayed wound (Appendix Figure T2-P1, T2-P2 and T2-P3; Photo T2-31). At point 1 which is situated above the large wound, a small cavity is found near the centre with a diameter of about 4 cm (denoted by the red zone in Appendix Figure T2- P1). This is likely to be an extension of the decay column from the main decay locus at the large wound. From the wall thickness (t) and stem radius (r) as shown in Graph T2-P1, a t/r ratio can be calculated to estimate the residual strength of the stem:

 First wall thickness, t1 = 13 cm  Second wall thickness, t2 = 17 cm  Average wall thickness, t = (13+17)/2 = 15 cm  Stem radius, r = 17 cm  t/r ratio = 15/17 = 0.88

The t/r ratio at point 1 is notably higher than the critical t/r ratio of 0.33, suggesting that the wood shell around the internal cavity should have sufficient wood thickness to hold the stem. The cavity is internal and it does not have an opening, hence the strength of the wood shell has not been compromised. Furthermore, the wood around the cavity shows limited penetration of advanced decay (denoted by the yellow zone in Appendix Figure T2-P1), suggesting that the fungal infection at this location has been compartmentalized. The steep angle of the graph on the two edges of the cavity signifies that wood strength has not been notably reduced at a short distance away from the cavity. In other words, the impact of wood decay has been largely contained within the confine of the cavity. It should be noted that around point 1 the loss of wood strength due to cavity formation has not triggered compensatory development of notable response wood. However, the lignified aerial roots that have grafted with the stem could provide some reinforcement.

At point 2 which drills through the large open wound (Photo T2-31), Appendix Figure T2-P2 shows the changes in wood strength along the drill path. The stem diameter has been thickened due to the development of response wood creating a bulgewood symptom, which compensates for the loss of strength due to wood decay. Three pockets of rather advanced decay have been detected by microdrilling, namely centred at 16.4 cm, 25.5 cm and 29.0 cm positions on the graph, indicating more decay on the lower half of the stem. The decay pattern suggests the development of

51 multiple decay columns. The first decay column is likely to be connected to the cavity detected at point 1. The development of response wood around the wound has provided compensation for the loss of strength due to decay.

At point 3, the wood quality as shown by the microdrilling trace (Appendix Figure T2-P3) is generally good with limited decay. No advanced decay pocket or cavity has been found.

The overall interpretation of microdrilling results of stem A1 at and proximal to the large decayed wound (Photo T2-31) is that there is sufficient sound wood including response wood to support the stem weight. No drastic pruning or stem removal is necessary. As a precautionary measure, about 10% of the end load should be trimmed. The pruning should selectively remove the thinner and weaker branches with a view to leaving an even spread of remaining branches. The decay may continue to deteriorate to a more advanced stage and may spread to affect more wood. This position of weakness needs to be monitored continually in future inspections.

(iv) Photo T2-32: Stem A1 with decayed wound and possible termite attack

An open and irregular wound on the south side of stem A1 has been formed due to breakage loss of a branch some months ago (Photo T2-32). The suspected termite attack should be checked to see if it is still active. If so, apply effective termite extermination using the hormone bait method at the earliest opportunity. The wound with jagged edges and an uneven surface should be very carefully trimmed using a sharp arborist manual saw with the help of a carpenter’s chisel. As far as possible, well- formed callus tissues should be left undisturbed.

(v) Photo T2-33: Stem A1 with tipped end and epicormic cluster

A tipped branch of medium size of stem A1 has developed a cluster of sprouts at or near the wound to serve as replacement branches and compensatory photosynthetic tissues (Photo T2-33). Detailed visual inspection at close quarters with the help of a wooden mallet should clarify the condition and extent of decay at the tipped end of the parent branch. The cluster of sprouts attached to the decayed tip should be reduced in proportion to the amount of remaining sound wood. The load reduction could be achieved by selective trimming of relatively weak and thin sprouts, leaving an even spacing of stronger sprouts.

(vi) Photo T2-34: Stem A1 with crooked section

A branch of stem A1 has a somewhat crooked section (Photo T2-34). It should be inspected visually at close quarters and with the help of a wooden mallet to see if it is suffering from mechanical defects such as cracking or splitting. If so, the weight of the branch should be correspondingly reduced.

(vii) Photo T2-35: Stem A1 with heavy epicormic branch at tipped end

A thick epicormic branch of stem A1 is curving upwards from an old branch tipping wound (Photo T2-35). Inspecting the wound at close quarters should verify the condition and extent of decay at the tipped end. The weight of the branch should be proportionally trimmed according to the amount of remaining sound wood. The rubbish that hangs on the branches should be carefully extricated and removed in the course of pruning with the help of a hydraulic platform.

(viii) Photo T2-36: Stem A2 with truncated branch and decay

A truncated branch wound on stem A2 has been invaded by wood-decay fungi (Photo T2-36). The jagged edges of the wound and its uneven surface should be very carefully trimmed using a sharp arborist manual saw and with the help of a carpenter’s chisel. As far as possible, well- formed callus tissues should be left undisturbed.

(ix) Photo T2-37: Stem B with decayed tipped end and epicormic cluster

A tipped and decayed branch of stem B has developed multiple sprouts at and near the wound to serve as replacement branches and compensatory photosynthetic tissues. (Photo T2-37). Detailed visual inspection at close quarters with the help of a wooden mallet should confirm the condition and extent of decay at the tipped end of the parent branch, and its influence on wood strength. The cluster of sprouts attached to the decayed tip should be reduced in proportion to the amount of remaining sound wood. The load reduction could be achieved by selective trimming of relatively weak and thin sprouts, leaving an even spacing of stronger sprouts.

(x) Photo T2-38: Stem B with fractured and decayed branch end

A fractured and decayed branch of stem B is supporting an expanded epicormic branch (Photo T2-38). Detailed visual inspection at close quarters with the help of a wooden mallet should clarify the condition and 53 extent of decay at the fractured end of the parent branch. The results of inspection should inform whether the daughter epicormic branch should be shortened to reduce its weight. The jagged edges of the wound should be carefully trimmed using a sharp arborist manual saw. Concerning the three parallel branches, trimming of the above epicormic branch should reduce the competition.

(xi) Photo T2-39: Stem B with crowded branches

On stem B, a crowded branching problem is compounded by two parallel branches (Photo T2-39). The sum of the diameters of the three daughter branches is notably larger than that of the parent branch, imposing a heavy burden on it. The problem could be partly relieved by removing one of the two parallel branches.

(xii) Photo T2-40: Stem B with curved and kinked branches

At the tipped end the long, upward curving but relatively slender branch, an epicormic branch has emerged with an elbow joint (Photo T2-40). As such a structure is potentially hazardous. As it carries a limited amount of foliage, the parent together with its daughter branch is recommended for removal.

(xiii) Photo T2-41: Stem B branch with elbow-joint at tipped end

On stem B, a tipped branch end has developed an epicormic branch attached to its parent with an elbow joint (Photo T2-41). Evaluation at close quarters should inform whether the decayed tip has sufficient sound wood to hold the epicormic branch. If not, the daughter branch should be reduced according to the amount of sound wood at the junction.

(xiv) Photo T2-42: Stem B branch with decay, cavity and bulgewood

The open wound on a branch of stem B has developed decay and cavity, and the resulting loss of mechanical strength has induced response wood formation in the form of bulgewood (Photo T2-42). The extent of decay should be investigated at close quarters. The load on the defective branch should be reduced in proportion to the amount of remaining sound wood.

(xv) Photo T2-43: Stem B branch with crowded and decayed sprouts

A branch of stem B has formed a cluster of crowded sprouts at its end with some decayed branchlets (Photo T2-43). The dead branchlets should

54 be removed to prevent the spread of wood-decay fungi to adjacent healthy wood, and to partly relieve the sprout crowding problem.

(xvi) Photo T2-44: Stem A branch with tipped end and no sprout

Stem A has a tipped medium-sized branch leaving a wound with no sprout development (Photo T2-44). Whether the branch is dead or not could be verified. If so, it should be removed to prevent the spread of wood-decay fungi to adjacent healthy wood.

(xvii) Photo T2-45: Tipped branches with sprout clusters above Bonham Road

Some branches hanging above Bonham Road and situated near the buildings on the opposite side have been tipped with sprout clusters emerging from the wounds (Photo T2-45). They should be trimmed by removing the relatively weaker and thinner sprouts and leaving an even spread of stronger branches.

(xviii) Photo T2-46: Crown extension towards buildings at St Stephen’s Lane

The south part of the crown has extended towards the buildings at St Stephen’s Lane (Photo T2-46). To resolve this conflict, the branches that get too close to windows should be trimmed to provide a horizontal clearance of 2 m. Only the standard reduction cut down to a notable fork should be applied in shortening branches. The heading cut is forbidden. A light-weight telescopic pruning pole could be used to prune the target proximal branches from the nearest windows of the affected residential flats. This way, the need to block the vehicular traffic to conduct pruning from an elevated platform could be avoided.

(xix) Photo T2-47 and T2-48:Tipped branch above St Stephen’s lane with epicormic branches

Some branches above St Stephen’s Lane near buildings have been tipped with sprouts emerging from the wounds (Photo T2-47 and T2-48). If the inspection at close quarters finds that the truncated end of the branch has developed decay and structural defects, the attached epicormic branches should be trimmed to reduce the burden on the parent stem.

(xx) Photo T2-49: Tipped branch above St Stephen’s lane with multiple and crowded epicormic branches

A tipped branch above St Stephen’s Lane has developed multiple and crowded sprouts from the wound (Photo T2-49). They should be trimmed by removing the relatively weaker and thinner sprouts and leaving an even spread of stronger branches.

(xxi) Photo T2-50: Crown extension above building podium at St Stephen’s Lane

The south part of the crown has extended above the building podium at St Stephen’s Lane (Photo T2-50). To resolve this building-tree conflict, the branches that get too close to windows should be trimmed to provide a horizontal clearance of 2 m. Only the standard reduction cut down to a notable fork should be applied in shortening branches. The heading cut is forbidden. A light-weight telescopic pruning pole could be used to prune the target proximal branches from the nearest windows of the affected residential flats. This way, the need to block the vehicular traffic to conduct pruning from an elevated platform could be avoided.

(xxii) Photo T2-51: Stem B with large truncated and decayed limb

Stem B has a large truncated limb developing decay at the wound (Photo T2-51). The stubs with decay should be removed using the standard removal cut at the fork with the parent stem.

(xxiii) Photo T2-52: Truncated limb of Stem B with decay and bulgewood

The base of the truncated limb has a decayed and depressed wound with bulgewood formation and a wasp nest (Photo T2-52). The recommended inspection at close quarters should check the condition of the wound. If it has developed advanced decay and structural defect that may compromise its mechanical strength, to such an extent that it may not support its own weight, the relatively long and stout truncated limb should be removed by the proper removal cut at its fork with its parent stem.

(xxiv) Photo T2-53: Stem B with seam and bulgewood (microdrill point 4)

Stem B has a seam with bulgewood formation indicating internal decay (Photo T2-53). The interior wood condition should be ascertained by microdrilling, with the position and direction of drilling indicated on the photograph. If the microdrilling finds that the wound has developed advanced decay and structural defects that may compromise its 56 mechanical strength to such an extent that it may not support its load, the weight of the branches supported by the stem should be trimmed. The amount to be removed should be contingent upon the condition and quantity of the remaining sound wood.

The microdrilling graph (Appendix Figure T2-P4) indicates that the drill bit stop at 40 cm without exiting the stem, because it was too short to penetrate the entire stem diameter. A microdrill with a longer drill bit or an additional drilling entering the stem from the opposite direction could have overcome this inherent instrumental constraint. Thus the results provided by the contractor show only a partial picture of the internal wood condition at this site of potential weakness. Based on the detected portion of the stem, weakened wood due to decay is found from 16 cm to 21.5 cm. There is enough sound wood on the two sides to provide support. The decay may continue to deteriorate to a more advanced stage and may spread to affect more wood. This position of weakness needs to be monitored continually in future inspections.

(xxv) Photo T2-54: Stem B with limb-loss wound and decay

At the lower part of stems B, a limb-loss wound is beset by decay and has developed some sprouts (Photo T2-54). As callus tissues have formed around the edge, the wound should be monitored for signs of continued decay.

(xxvi) Photo T2-55: Stem B branch with open decayed wound (microdrill point 5)

A decayed wound with incomplete callus formation is found on a wavy branch of stem B. The internal condition of wood should be explored using microdrilling with the arrow denoting the drilling position and orientation (Photo T2-55). The load on the defective branch should be reduced in proportion to the amount of remaining sound wood.

The drill bit went through the branch at point 5 (Appendix Figure T2-P5), indicating a stem diameter of about 21.6 cm. No notable pocket of advanced decay, cavity or crack has been detected along the drill path. The decay at the open wound has not penetrated into the interior of the stem. The results do not call for drastic pruning or end-load reduction treatment.

(xxvii) Photo T2-56: Stem A1 with decayed stub (microdrill point 6)

Stem A1 has a decayed stub with bulgewood formation and wasp nest (Photo T2-56). The interior wood condition should be ascertained by microdrilling, with the position and direction of drilling indicated on the photograph. If the microdrilling finds that the wound has developed advanced decay and structural defect that may compromise its mechanical strength to such an extent that it may not support its load, the weight of the branches supported by the stem should be trimmed. The amount to be removed should be contingent upon the condition and quantity of the remaining sound wood. The stub-end wound with jagged edges or an uneven surface should be very carefully trimmed using a sharp arborist manual saw and with the help of a carpenter's chisel.

The drilling results are presented in Appendix Figure T2-P6. The wood from about 9 cm to 17.5 cm has been degraded by decay which is likely to have spread from the decayed stub shown in Photo T2-56. The decay has not reached the advanced stage at this juncture, and together with the development of response wood as expressed by the bulgewood on both sides of the wound, there should be enough wound wood to hold the stem. It is possible that the decay may progress in due course to the more advanced stage and spread to a larger proportion of the stem. This potential weakness should be continually monitored both visually and with the help of instruments.

(xxviii) Photo T2-57: Stem A1 with depressed and decayed wounds and bulgewood (microdrill points 7 and 8)

The south side of stem A1 has two decayed and depressed wounds with wasp nests (Photo T2-57). The internal condition of the wood should be ascertained by two microdrillings, with the positions and directions of drillings annotated on the photograph. If the microdrilling finds that the wound has developed advanced decay and structural defect that may compromise its mechanical strength to such an extent that it may not support its load, the weight of the branches supported by the stem should be trimmed. The amount to be removed should be contingent upon the condition and quantity of the remaining sound wood.

The microdrilling trace shown in Appendix Figure T2-P7 has not drilled through the stem. It stopped inexplicably at 24 cm whereas the stem diameter should be around 40 cm. It is possible that the drilling erroneously followed an off-centre path, hence a shorter trace was recorded. Thus the drilling graph has detected only a part of the stem interior condition. Based on the part that has been drilled, the wood 58 condition does not demonstrate advanced decay, cavity or other structural weakness, hence drastic pruning to reduce loading on the stem is not warranted.

The microdrilling trace shown in Appendix Figure T2-P8 has gone through the stem. It indicates two pockets of decay which have a limited extent each of about 2 cm diameter. The decay at the open wound has penetrated into the interior part of the stem, although the spread at the present stage is limited. There is enough sound wood to support the stem. As a precautionary measure, about 10% of the end load should be trimmed. The pruning should selectively remove the thinner and weaker branches with a view to leaving an even spread of remaining branches. The weak point should be continually monitored to check whether it will further aggravate to a more risky stage.

(xxix) Photo T2-58: Stem C large decayed wound of truncated limb

A large wound with decay is found at the lower part of stem C, left by the truncation of a thick limb (Photo T2-58). The wound should be monitored for signs of continued decay.

(xxx) Photo T2-59: Stem C with decayed and recessed wound

The lower part of stem C has a decayed wound in a recessed niche (Photo T2-59). It could be very carefully trimmed with the help of a carpenter's chisel.

(xxxi) Photo T2-60: Stem C with trapped rubbish

A piece of plastic rubbish is partly engulfed and trapped by wood growth in Stem C (Photo T2-60). It should be pulled out manually, if it is possible to do so. Refrain from using tools or excessive force in the extraction process.

(xxxii) Photo T2-61: Stem C with cracks, decayed wound and crotch (microdrill points 9 and 10)

The middle section of Stem C has developed cracks, seam, bulgewood, decayed wound, and a decayed crotch (Photo T2-61). The adjacent branch removal wound has developed decay. The internal condition of the wood should be ascertained by two microdrillings, with the positions and directions of drillings annotated on the photograph. The load on the defective branch should be reduced in proportion to the amount of remaining sound wood and the condition of the structural defect. 59

Microdrilling at point 9 (Appendix Figure T2-P9) shows a cavity of about 4.5 cm diameter. The t/r ratio at the thinner side of the wood shell is 11/17.5 = 0.63 which is above the critical threshold of 0.33. The almost vertical edges on both sides of the trough imply that the “cavity” could be a gap between the stem and a lignified aerial root. At point 10, drilling was made through the crotch with surficial decay symptoms (Appendix Figure T2-P10). The wood from 0 cm to 6 cm has been partly degraded by decay. The remaining wood is generally sound and has the strength and thickness to hold the stem. Overall, the stem does not require drastic load reduction.

(xxxiii) Photo T2-62: Stem C branch with bulgewood

The bulgewood developed in a branch of Stem C indicates possible internal decay (Photo T2-62). The load on the defective branch should be reduced in proportion to the amount of remaining sound wood and the condition of the structural defect.

(xxxiv) Photo T2-63: Stem C branch stub with decay

The decay of the stub on a branch of stem C may extend into the parent branch (Photo T2-63). The stub should be removed using the standard removal cut at the fork with the parent stem.

(xxxv) Photo T2-64: Stem C tipped branch with decay spread

The tipped branch of stem C is beset by the spread of decay from the wound and dead sprouts (Photo T2-64). The dead branch should be removed using the standard removal cut at the fork with the parent stem.

(xxxvi) Photo T2-65: Stem C branch with crack at crotch

A branch of stem C has developed a crack at the critical crotch position (Photo T2-65). The crack should be assessed at close quarters to find out its length and depth and whether internal decay has developed. The result will determine the amount of wood to be removed from the affected branch. In the extreme case, the entire branch should be removed along the dotted line.

3.1.4 Assessment of T3 and arboricultural recommendations

(a) Overall tree structure and condition

(i) Tree-wall relationship

T3 is one of the four stonewall trees dwelling on the eastern one-third of the subject wall (Photo T3-1). It is situated near the staircase linking Bonham Road to St Stephen’s Lane. Due to close spacing amongst the row of trees, their crown spread tends to be laterally restricted by mutual interference (Photo T3-2). The semi-mature tree perches on the crest of the wall and hangs above the bus stop at Bonham Road (Photo T3-3). It leans towards Bonham Road by about 40 degree at the base, but assumes a more upright posture in the remainder of the trunk which tilts at 8 degree from the vertical.

(ii) Tree dimensions and biomass structure

The tree measures 14.77 m tall. Some large branches have been tipped (Photo T3-4). Three rather thick trunks with DBH respectively at A=30 cm, B=56 cm and C=74 cm support the confined and reduced crown (Photos T3-5 and T3-6). The aggregate DBH reaches 100 cm. Trunk A that tilts heavily towards T4 at its east side has been truncated, and it does not carry any branches. Trunk C has been truncated at a low level and carries no branches. It is connected to trunk B via a bundle of lignified aerial roots. Trunk C’s main residual function is lending support to the limbs and branches of trunk B.

(iii) Trunk B and low branch removal

Trunk B is the only one that has been left largely intact (Photo T3-5). The tree’s crown is built mainly by branches attached to trunk B. However, a notable number of limbs and branches formerly attached to trunk B have been lost. The tree suffers from excessive removal or loss of its lower branches. This defect could suppress the tree’s ability to develop a proper trunk taper to compromise its mechanical strength at the critical trunk base level (cf. Section 4.2.4).

(iv) Tree hazard rating

Tree hazard assessment has a score of 5 which is rated as low (Table 3G). The main contribution to tree risk is the breakage of branches that are structurally weak or decayed.

(b) Assessment of surface roots and interface with the stone wall

(i) Wall structure and surface root spread

The stone wall at T3 stands 3.27 m above the ground. Similar to the condition at T2, the masonry blocks are larger in the lower two-third of the wall, and smaller in the top one-third (Photo T3-7). Root spread on the wall face is somewhat limited laterally and especially vertically. The surface roots are confined mainly to the upper half of the wall. The more or less straight and horizontal lower edge of the surface root mass indicates that the lower half was cut away. Some roots on the east side have intertwined with those of its neighbour T4, with some grafting of the two root systems.

(ii) Root growth at wall toe and trunk base

A U-channel with concrete covers is present at the wall base (Photo T3-7). The excavation associated with its installation would have removed some of the surface roots at the lower half of the wall and roots that have grown into the soil under the pavement. The trunk base is sitting near the wall top (Photos T3-8, T3-9 and T3-10). Most of the tension roots that hold the tree against overturning and take water and nutrients from the aft-soil are believed to be situated underneath and in the vicinity of the trunk base.

(iii) Shift of masonry block

Just above the trunk base, a masonry block has been pushed away from the wall face (Photo T3-10). At the wall top, the interlocking force that applies a strong force against displacement is relatively weak. The shifting of one piece of stone probably indicates the localized pressure exerted by root growth.

(iv) Removing rubbish from surface roots

Measures can be implemented to improve the root growth conditions of the tree. The rubbish that has deposited on the surface roots and the trunk bases and crotches, together with the leaf litter, should be regularly removed with the help of a brush to avoid moisture accumulation which could induce decay (Photo T3-10). More importantly, keeping the gap clean can avoid the potentially risky wedging effect which may destabilize the tree (cf. Section 4.2.3).

(v) Removing cement sealing at masonry joints

The limitations to root growth due to the lack of open and penetrable joints between masonry blocks, and the lack of accessible soil at the wall toe and crest positions, could be ameliorated to enhance tree growth and performance (Photo T3-7). The joints between masonry blocks have been sealed recently by cement, thus stopping their penetration by tree roots. The rigid cement seal also restricts expansion of existing roots and may cause injurious girdling as they continue to thicken. Where the roots are physically obstructed, consideration could be given to localized removal of the cement seal to permit some new roots to grow into the joints and existing roots to expand (cf. Section 4.2.2).

(vi) Installing soil strip at wall toe and crest

An open soil strip filled with a good-quality soil mix could be installed at the wall toe at Bonham Road to allow growth of roots in the soil below the pavement (Photo T3-7). Similarly, an open soil strip filled with a good- quality soil mix could be installed at the wall crest to allow growth of roots in the soil below the pavement at St Stephen's Lane (Photo T3-17) (cf. Section 4.2.1).

Despite the somewhat small crown and a low live crown ratio of 40-70%, the foliage density appears normal for the species (Photo T3-11). The leaf size and colour also follow the norm. The peripheral branchlets and twigs of the crown do not show symptoms of dieback (Photo T3-12). The crown development is somewhat limited with reference to its height and aggregate trunk diameter. The overall crown shape denotes reduction due to a combination of confinement and branch losses (Photos T3-2, T3-3 and T3-4). The tree has lost an appreciable amount of branches and foliage, and suffers from excessive loss of lower branches that can incur instability problems (cf. Section 4.2.4).

(d) Assessment of trunks and branches

(i) Photos T3-13 and T3-14: Removing Stem A

The remnant part of Stem A, which was snapped in the past, has lost all its natural branches and is harbouring only a small number of feeble sprouts mainly at its broken end (Photos T3-13 and T3-14). The tip has an uneven surface and has developed rather advanced decay. The trunk tilts heavily towards the east to approach its neighbour T4. Not playing a functional 63 role in the overall scheme of the tree, trunk A is recommended for removal by cutting at its base, taking care to minimize injury of adjacent trunk B and associated lignified aerial roots.

(ii) Photos T3-15: Trimming Stem B limb-removal wound

The wounds left by limb removal from stem B have developed decay and cavity (Photo T3-15). Their jagged edges or uneven surfaces or decayed wood should be very carefully trimmed using a sharp arborist manual saw with the help of a carpenter’s chisel. However, the well-formed callus tissues should as far as practicable be left undisturbed.

(iii) Photos T3-16: Upward curving limb of Stem B

A low, upward-curving and truncated limb of stem B supports two long ascending epicormic branches (Photo T3-16). For the one on the right- hand side, evaluation of the cut wound at close quarters should inform whether the decayed tip of the parent branch has sufficient sound wood to hold the epicormic branch. If not, the daughter branch should be shortened by a reduction cut. The amount to be removed shall correspond to the condition of the wood around the cut face.

(iii) Photo T3-17: Long epicormic branch with elbow joint of Stem B

A long epicormic branch emerges with an elbow joint from a branch-cut wound (Photo T3-17). Evaluation of the cut face at close quarters should inform whether the decayed tip of the parent branch has sufficient sound wood to hold the epicormic branch. If not, the daughter branch should be shortened by a reduction cut. The amount to be removed shall correspond to the condition of the wood around the cut face.

(iv) Photo T3-18:Tipped branch with long epicormic branch of Stem B

A top branch of stem B was tipped and has developed a series of long and nearly parallel replacement epicormic branches (Photo T3-18). The crowded branching habit could be partly relieved by removing one of the three parallel branches. Evaluation a close quarters should inform whether the decayed tip of the parent branch has sufficient sound wood to hold the epicormic branches. If not, the daughter branches should be shortened by reduction cut. The amount to be removed shall correspond to the condition of the wood around the cut face.

(v) Photo T3-19: Decayed stubs on Stem B

Decayed wounds are found at the stubs of truncated branches at stem B (Photo T3-19). The decay could spread to the parent stem. The stub with advanced decay should be removed using the standard removal cut at the fork with the parent stem.

(vi) Photo T3-20: Stub with advanced decay on Stem C

The truncated stem C has developed advanced decay at the large wound (Photo T3-20). The terminal end of the stump with advanced decay should be cut away, but the lignified aerial root linking stem C to stem B above should not be cut.

3.1.5 Assessment of T4 and arboricultural recommendations

(a) Overall tree structure and condition

(i) Tree location on stone wall

T4 is a member of the group of four stonewalls trees situated at the eastern one-third of the old stone wall (Photo T4-1). They are of similar size and configuration, and are growing close to each other. T4 is relatively stronger than the other three trees (Photo T4-2). Its crown interlocks partly with two neighbour trees T3 and T5 (Photo T4-3).

(ii) Tree dimensions and crown configuration

The twin-stemmed tree has an aggregate DBH of 77 cm. Tree height attains 14.83 m, supporting a crown that spans 15.7 m parallel to the wall alignment, and 18.7 m perpendicular to it. The crown is highly asymmetrical, being restricted on the west and south sides.

(iii) Trunk lean

Trunk A leans at 52 degree towards Bonham Road, which is the most tilted of the six stonewall trees at the site (Photos T4-2 and T4-4). Trunk B leans backwards at 49 degree towards St Stephen’s Lane, which is also the most tilted in the southward direction amongst the six trees. The curved lower part of trunk A is propped by a bundle of lignified aerial roots to lend extra strength to its basal part (Photo T4-5). The bulk of the crown weight is carried by trunk A.

(b) Assessment of surface roots and interface with the stone wall

(i) Surface root growth pattern

The spread of surface roots on the wall face assumes an unusual pattern (Photo T4-6). The east half of the root mass descends down to the wall toe and penetrates into the soil below the pavement. The west half has been truncated at the middle of the wall, as evidenced by the straight and horizontal lower edge of the root mass. Some roots have intertwined with those of T3 to its west and T5 to its east, involving some tissue grafting and fusion.

(ii) Root growth at wall toe

The wall toe is equipped with a U-channel covered by concrete slabs (Photos T4-6 and T4-7). The excavation necessary to install this drainage channel would have severed or injured the roots that penetrated into soil below the pavement. Some roots could grow below the channel to explore the soil lying underneath and to capture more water and nutrients to support tree growth.

(iii) Trunk base detachment from wall face

The trunk base is attached to the top part of the wall face (Photos T4-8 and T4-9). A narrow gap is found between the wall face and the root mass, indicating that the base of the tree has been detached somewhat from the wall (Photos T4-8, T4-9, T4-10, T4-11 and T4-12). The tree anchorage has failed to a certain extent by yielding a little. The remaining roots are holding the tree against overturning and collapse.

(iv) Dense surface root mass

The dense network of surface roots to the east and west of the trunk base embodies some roots that have turned backwards to move into the joints and henceforth into the aft-soil. Together with the penetrated roots below the trunk base, they afford critical tensional pull to hold the tree against overturning (Photos T4-13 and T4-14). The two obsolete iron angle bars situated near the trunk base could be removed. If their removal may harm the surface roots, the exposed part could be sawn away without extracting the embedded part.

(v) Root growth restriction

(vi) Removing cement seal at masonry joints

The joints between masonry blocks have been sealed recently by cement, thus stopping their penetration by tree roots. The rigid cement seal also restricts expansion of existing roots and may cause girdling injury as they continue to thicken. Where the roots are physically obstructed, consideration could be given to localized removal of the cement seal to

permit some new roots to grow into the joints and existing roots to expand (cf. Section 4.2.2).

(vii) Installing soil strip at wall toe

An open soil strip filled with a good-quality soil mix could be installed at the wall toe at Bonham Road, in lieu of the existing drainage u-channel, to allow growth of roots in the soil below the pavement (cf. Section 4.2.1). If the propping method C explained in Section 3.15e is adopted to support the tree at the Bonham Road pavement, the design could be adjusted to accommodate both the propping frame and the soil strip.

(viii) Installing soil strip at wall crest

An open soil strip filled with a good-quality soil mix could be installed at the wall crest to allow growth of roots in the soil below the pavement at St Stephen's Lane. A segment of the existing stone parapet wall will have to be replaced by railing to permit the roots to reach this new soil strip (cf. Section 4.2.1). If the cable bracing method B to pull the tree at the northern edge of St Stephen's Lane explained in Section 3.15e is adopted, the design could be adjusted to accommodate both the steel frame and the soil strip.

(i) Two subcrowns supported by trunks A and B

The crown of T4 is composed of two portions supported respectively by trunk A and trunk B. The bulk of the crown supported by trunk A hangs above Bonham Road (subcrown A), and similarly the bulk of the crown supported by trunk B hangs above St Stephen’s Lane (subcrown B) (Photos T4-15 and T4-1). Subcrown A was subject to tipping in the past, and it has since rebuilt its lost top by an expanded epicormic branch (Photo T4-15). Subcrown A is notably larger and heavier than B.

(ii) Crown configuration and condition

The live crown ratio of the somewhat enfeebled tree stands at a medium level of 40-70%. Foliage density is rated as sparse due to compromised vigour, but the leaf size and colour are judged as normal. Dieback of twigs is gauged as low at < 5%. The crown is not symmetrical, and some main branches have been tipped. Excessive loss of lower branches is obvious, which carries the undesirable consequence of failing to develop proper

trunk taper to compromise tree stability (cf. Section 4.2.4). Overall tree vigour is judged to be average.

(d) Assessment of trunks and branches

(i) Overall condition (microdrill points 1 and 2)

The truncated tip of Stem A is shouldering the heavy load of a large replacement epicormic branch (Photo T4-15). The evaluation at close quarters should inform whether the tip of the parent branch has sufficient sound wood to hold the rather heavy epicormic replacement branch. If not, the end weight of the daughter branch should be reduced by removing some of the smaller and weaker branches. The amount to be removed shall be commensurate with the condition of the wood around the cut face.

Two microdrillings were made by the contractor through Stems A and B respectively (Photo T4-5). Both drillings do not penetrate the whole trunk; they stop at 40 cm which is the length of the rather short drill bit. Thus the results cannot detect the internal wood condition of the whole trunk section. Based on the limited data, Stem A has sound wood (Appendix Figure T4-P1). Stem B displays lowered strength in 0-17 cm, denoting the presence of decay or weak wood in the lignified aerial roots attached to the trunk (Appendix Figure T4-P2). The limited data show that the main trunk has enough sound wood. No drastic pruning is necessary at this stage. As the decay may progress in due course to a more advanced stage and spread to a large volume of wood, the structurally compromised location should be continually monitored.

(ii) Photo T4-16: Tipped branch of Stem A with epicormic branch and elbow joint

A tipped branch of Stem A carries a replacement epicormic branch with an elbow joint (Photo T4-16). The evaluation at close quarters should tell whether the tip of the parent branch has sufficient sound wood to hold the rather heavy epicormic replacement branch. If not, the end weight of the daughter branch should be reduced by removing some of the small branches. The amount to be removed shall be commensurate with the condition of the wood around the cut face.

(iii) Photo T4-17: Hanging rubbish trapped in crown

Hanging plastic rubbish is trapped in the crown (Photo T4-17). It should be carefully extricated and removed in the course of pruning with the help of a hydraulic platform. 69

(iv) Photo T4-18: Stem A branch with basal curvature and decayed crotch (microdrill points 3, 4 and 5)

A branch of stem A has a curved basal section and a decayed wound at crotch (Photo T4-18). The internal wood condition at the critical juncture should be probed by microdrilling, the position and direction of which are shown in the photograph. The results should inform whether the critical crotch position has sufficient sound wood to hold the two thick branches and the thinner and elongated epicormic branch. If not, the epicormic branch should be removed. If the decay has affected over one-third of the wood in the notional cross-section, the end weight of the two thick branches will have to be reduced by trimming some small branches. The amount to be removed shall correspond to the condition of the wood around the cut face.

All three drillings which went through the branch show sound wood throughout (Appendix Figures T4-P3, T4-P24and T4-P5). Thus no pruning or load-reduction action is recommended.

(v) Photo T4-19: Truncated branch of Stem A with epicormic branches and elbow joint

A truncated branch of stem A has a decayed wound and two epicormic branches with elbow joint (Photo T4-19). Evaluation at close quarters should inform whether the broken end of the branch has sufficient sound wood to hold the two closely spaced epicormic branches. If not, the load on the parent stem should be reduced by an amount that is commensurate with the condition and quantity of sound wood around the wound. From slight to severe loss of wood mechanical strength, the following sequence of actions could be adopted: shorten the thin branch, remove the thin branch, shorten the thick branch, and remove the thick branch.

(vi) Photo T4-20: Stem A branch with fractured wound and decayed stub

A branch of stem A has a fractured wound and a nearby decayed stub (Photo T4-20). The rather fresh fractured wound with jagged edges should be carefully trimmed using a sharp arborist manual saw with the help of a carpenter’s chisel. The decayed stub should be removed using the standard removal cut at the fork with the parent stem.

(vii) Photo T4-21: Branch removal wound with decay on stem A

Stem A has a branch-removal wound with decay (Photo T4-21). The wound with jagged edges and uneven surface should be very carefully trimmed using a sharp arborist manual saw with the help of a carpenter’s chisel. However, the well-formed callus tissues should as far as practicable be left undisturbed.

(viii) Photo T4-22: Branch removal wound with decay on stem A

Stem A has a branch-removal wound with decay (Photo T4-22). It should be left alone but be monitored to see if the decay would move to the advanced stage.

(ix) Photo T4-23: Branch removal wound with decay on stem A

Stem A has a branch removal wound with decay (Photo T4-23). The wound with jagged edges and uneven surface should be very carefully trimmed using a sharp arborist manual saw with the help of a carpenter’s chisel. However, the well-formed callus tissues should as far as practicable be left undisturbed.

(x) Photo T4-24: Branch stub with decay on stem A

Stem A has a branch stub with advanced decay (Photo T4-24). It should be removed using a standard removal cut at the fork with the parent stem.

(xi) Photo T4-25: Meandering shape of Stem A

The lower section of the heavily leaned trunk A assumes a meandering shape (Photo T4-25). The curved parts should be monitored for possible development of cracks.

(xii) Photo T4-26: Stem B curving backwards towards St Stephen’s Lane

The heavily leaned trunk B assumes a meandering shape (Photo T4-26). The curved parts should be monitored for possible development of cracks.

(xiii) Photo T4-27: Stem B stub with advanced decay

A stub of stem B with an irregular tip has developed advanced decay (Photo T4-27). It should be removed using a standard removal cut at the fork with the parent stem.

(xiv) Photo T4-28: Stem B stub with decay

A branch stub of stem B has developed decay (Photo T4-28). It should be removed using a standard removal cut at the fork with the parent stem.

(xv) Photo T4-28: Existing Cobra bracing using Stem B to hold Stem A

An existing Cobra cable bracing has been installed using Stem B which points backwards towards St Stephen’s Lane to hold Stem A that leans and hangs above Bonham Road. Stem B may not have the strength to provide support to Stem A. Please see the alternative support systems explained below (Section 3.1.5e).

(xvi) Prevention of wedging effect and decay at trunk base

The rubbish that has deposited on the surface roots and the adjacent trunk base, together with the leaf litter, should be regularly removed with the help of a brush to avoid moisture accumulation which could induce decay at the critical trunk base position (Photos T4-11 and T4-12). More importantly, objects trapped in the wedge gap caused by the detachment of the tree from the wall could aggravate the detachment process due to the wedging effect. They must be diligently and thoroughly removed from the gap to prevent aggravation of the detachment risk (cf. Section 4.2.3).

(e) Proposal for tree support systems for T4

(i) Tree hazard status

Tree hazard assessment has a score of 9 plus an extra score due to detachment of the trunk base from the wall face. The total hazard score of 10 has pushed the tree from top of the medium risk category into the base of the high risk category (Table 4G). Of the six stonewall trees at the site, T4 has been accorded the highest hazard score. The tensional roots that should pull the tree against overturning have yielded a little to create a gap between the tree base and the wall face (Photos T4-8, T4-9, T4-10, T4-11 and T4-12). The main contribution to tree risk is the potential for the whole tree to be uprooted from the wall. If the tree were to fail, it is likely that the whole tree will collapse on Bonham Road.

(ii) Three tailor-made tree support systems

It is pertinent to provide artificial, effective and long-lasting support to the tree at the earliest opportunity to prevent tree failure. Due to the severe 72 site constraints at both Bonham Road and St Stephen’s Lane, innovative methods have to be tailor-made to stabilize the tree without obstructing pedestrian or vehicular traffic. It could be realized in two modes, namely pulling it towards the south by cable bracing, or propping it from the north. Three options in order of preference are proposed below.

(iii) Method A: Cable bracing

The tree's roots have partly detached from the stonewall face, leaving a gap between the wall and the trunk base. Trunk A is heavily tilted at 52 degree from the vertical towards Bonham Road. It carries the bulk of the tree's mass. As the tree hangs above the pavement and the carriageways, a long-term and effective solution to secure it must be implemented at the earliest opportunity to forestall further detachment, and to abate the risk of tree failure. The present cable bracing shown in Photo T4-29 using trunk B to hold trunk A has doubtful capability to prevent tree failure. A more direct, effective and relatively less costly approach is strongly recommended, by way of a cable bracing system to hold both stems. The cable should have sufficient elasticity to permit the stems to move within a certain limit.

The cables can be anchored on the columns of the nearest building to the south at St Stephen's Lane at an elevated level to permit vehicular and pedestrian headroom clearance (Photo T4-30). The Highways Department with the help of the District Council and Home Affairs Bureau could negotiate with the property owners and building management to obtain the permission to do so. The relevant property owners could be persuaded in the interest of preserving the tree and the associated amenities which could benefit them in the long term. The maintenance of the cable bracing system and the affected parts of the columns or beams should be shouldered by the government. Please see Photo T4-30 to illustrate the concept design of this cable bracing system. The detailed design should be elaborated in conjunction with a structural engineer.

(iv) Method B: Cable bracing

If permission could not be obtained in implement the Method A cable bracing system, the less desirable alternative is to install a strong steel frame near and parallel to the parapet wall at St Stephen's Lane to hold the tree (Photo T4-31). The frame should be firmly anchored in the ground. Cables can then be installed to link the frame to the two trunks. Due to the low bracing position relative to tree height, the amount of swing in the wind would be rather limited. Thus the cable should have a correspondingly lower degree of elasticity. 73

As the Lane is used for vehicular access, the steel frame has to be positioned as near as possible to the parapet wall to maintain sufficient vehicular lateral clearance. It implies that excavation will have to be conducted near the tree which may harm the roots that have penetrated the soil lying below the Lane. To minimize this impact, the positions of the two anchors for the frame could be placed respectively between T4 and T3, and T4 and T5. Moreover, the impact could also be reduced by developing a technique with the help of an engineer to install the anchors with a minimum excavation limit. Please see the Photo T4-31 which illustrates the concept design of this alternative cable bracing system. The detailed design should be elaborated in conjunction with a structural engineer.

(v) Method C: Propping

The two cable bracing methods explained above are preferred to stabilize the tree and to abate the tree failure hazard. Propping the tree in front of the stone wall is inordinately difficult due to the narrow pavement and its use also as a bus stop. If both cable bracing methods cannot be implemented, the least desirable propping option will have to be enlisted. In view of the severe site constraints, this option has to be considered as a compromise that is less effective in holding the tree. A strong steel frame will have to be constructed at the pavement near the wall face to minimize disturbance to pedestrian flow and to reduce risk to buses and other vehicles getting too close to the frame (Photo T4-32). To leave sufficient wide berth for pedestrian flow, the frame below the normal pedestrian vertical clearance will have to abut onto the wall face.

The vertical support column has to be firmly anchored in the ground and the wall to hold the load of the tree and to prevent overturning. A strong cantilever arm will extend from the top of the frame to bear the weight of the tree. The interface between the arm and the trunk should be properly cushioned to prevent bruising injuries. The exact positions of the anchors should be carefully chosen to minimize impacts on surface and penetrated roots. To avoid injuries to the tree, great care in conjunction with physical protection and other precautionary measures should be adopted in installing the steel frame. Please see Photo T4-32 which illustrates the concept design of the proposed propping system. The detailed design should be elaborated in conjunction with a structural engineer.

(vi) Method D: Short-term contingency measure to abate tree risk

Three long-term options in order of preference have been proposed in the above sections to abate the risk of T4 on targets. It is envisaged that one 74 of them will be adopted in due course. As the implementation may incur a lead time to overcome the obstacles in terms of space, traffic and property rights, in the meantime it is deemed necessary to take a short-term measure to reduce the risk of tree failure. Drastic reduction of branches has to be ruled out, because it could impose irreparable damages on the tree and may dampen its vigour and health to render it more hazardous to targets. The critical weakness is the incipient detachment of the trunk base from the wall face (Photos T4-8 to T4-12; Section 3.1.5biii). A temporary cable system could be installed to be anchored in the ground near the parapet wall at St Stephen’s Lane to hold the trunk base and prevent further progression of detachment. Holes can be drilled in the parapet wall to allow cables to extend from the anchor to the trunk base to provide tensional pull to hold the tree against failure. A structural engineer could provide professional input on the design of this ground anchor method. A schematic drawing of this temporary cable bracing method is shown in Photo T4-33.

(vii) Alternative to long-term cable bracing or propping

In the unlikely and undesirable event that none of the three long-term methods (Methods A, B or C) to stabilize the tree can be implemented, the alternative will have to be drastic pruning to reduce the load and hence the risk level. Such an approach will disfigure the tree and reduce its vigour, and may drive the tree towards an irreversible decline spiral to render it more unstable and hazardous. Furthermore, the load-reduction effect of drastic pruning may not last long, as the tree in the post-treatment years will struggle and send out a notable amount of sprouts and epicormic branches in its attempt to regain some of the lost photosynthetic capability. Thus the tree will have to be heavily pruned repeatedly for some years, a process that is tantamount to a protracted death sentence. As there are feasible ways to stabilize the tree, drastic and repeated pruning cannot be recommended.

3.1.6 Assessment of T5 and arboricultural recommendations

(a) Overall tree structure and condition

(i) Tree location on stone wall

T5 is one of the four trees that are clustered at the eastern one-third of the stone wall (Photo T5-1). It stands at 14.97 m tall, which is similar to its three companions. The tree sits near the top of the wall. The height of the first branch is elevated well above the ground at 5.18 m from the trunk base. The tree leans towards Bonham Road at 21 degree.

(ii) Tree dimensions and biomass structure

Unlike its three stronger and multiple-stemmed partners, it has a single trunk with DBH of 41 cm, which is rather slender in comparison with its height (Photos T5-2 and T5-3). The height to DBH ratio of the trunk is as high as 36.4, which signifies an inherently unstable shape. The crown is also unusually narrow especially in the direction parallel to the wall alignment at merely 9.3 m. Perpendicular to the wall, the crown is a little wider at 11.9 m. Some branches interlock with its two neighbours, namely T4 at its west and T8 at its east.

(b) Assessment of surface roots and interface with the stone wall

(i) Surface root distribution on wall face

The wall gets shorter towards the east end, and at T4 it is 2.54 m above the pavement level (Photo T5-4). The surface roots extend from the trunk base to the wall toe, with some penetration into the soil below the pavement. One surface root in the middle of the root mass is particularly thick and it lends important support to the trunk.

(ii) Interface between trunk base and wall face

The contact interface between the trunk base and the wall face is somewhat limited in comparison with its partners on the same wall segment (Photos T5-5 and T5-6). Organic litter and rubbish have accumulated in the gap between the trunk base and the wall face (Photo T5-7). They should be regularly removed to avoid the wedging effect and wood decay (cf. Section 4.2.3).

(iii) Root growth restriction

(iv) Removing cement sealing from masonry joints

(v) Installing soil strip at wall crest

An open soil strip filled with a good-quality soil mix could be installed at the wall crest to allow growth of roots in the soil below the pavement at St Stephen's Lane. A segment of the existing stone parapet wall will have to be replaced by railing to permit the roots to reach this new soil strip (cf. Section 4.2.1). If the cable bracing Method B explained in Section 3.1.6f is adopted, the design could be adapted to accommodate both the steel frame and the soil strip.

(vi) Installing soil strip at wall toe

An open soil strip filled with a good-quality soil mix could be installed at the wall toe at Bonham Road, in lieu of the existing drainage U-channel and pavement covered with unit pavers, to allow growth of roots in the soil below the pavement (cf. Section 4.2.1). If the propping method C explained in Section 3.16f is adopted to support the tree at the Bonham Road pavement, the design could be adjusted to accommodate both the propping frame and the soil strip.

(i) Photos T5-8 and T5-9: Large cavity at trunk base with decay

As the large basal cavity is closely associated with root development and hence compensatory reinforcement, the topic is discussed in this dedicated section. It assumes a typical inverted V-shape of a basal cavity, and is 77

nearly as wide as the trunk base itself (Photos T5-8 and T5-9). The internal surface of the cavity is covered with brownish decayed wood, some of which has a loose consistence that can be removed with the hand.

The thickness of the residual wall around the cavity and especially the amount of remaining sound wood should be ascertained by microdrilling. Three drilling positions have been annotated on the photographs (Photos T5-8 and T5-9). The results can inform the amount of sound wood in the residual wall and whether it is adequate to support the tree’s biomass. Field inspection indicates that the residual wall is rather thin.

The edges on both sides of the cavity has developed prominent strips of response wood to compensate for the loss of wood strength due to cavity formation at the critical position of the trunk (Photos T5-8 and T5-9). The response wood tends to have a higher density, higher mechanical strength and higher resistance to fungal decay. They contribute significantly to reinforcing the trunk base against snapping failure. At the opposite side of the trunk lean and at the basal position, the response wood affords critical tensional pull. Moreover, the response wood development has extended above the cavity to the lower-middle part of the trunk to offer notable ribs of tension wood to pull and stabilize the leaning tree (Photo T5-10).

(ii) Photos T5-11 to T5-13: Trunk reinforcement by lignified aerial roots, thickened root pro, and response wood

In addition to the reinforcement adjoining to the cavity, the tree has developed two rather strong and upright lignified aerial root stands adjacent to the trunk base (Photos T5-11 and T5-12). They provide supplementary support to the tree at the critical load-bearing position, and are instrumental in securing the tree against breakage at the hollowed trunk base. The root stands have assumed a distinctive flattish shape perpendicular to the wall face to maximize the tensional pull strength against the overturning moment (Photo T5-13).

(d) Assessment of tree crown

(i) Narrow and confined crown development

The narrow crown has developed few branches (Photo T5-14). Two major limbs follow a rather upright growth habit. One of the limbs displays evidence of past tipping which has triggered the development of epicormic branches from the wound. The two neighbour trees have literally sandwiched and trapped the relatively weaker T5 in the intervening space.

The foliage density is rated as sparse, although leaf size and colour are normal. Twig dieback is not evident, with less than 5%.

(ii) Competition with neighbour trees and loss of low branches

The tree has lost a good deal of its branches, and branch growth has been confined by proximity and competition with neighbour trees which are situated too close to it. The live crown ratio is put in the lowest category of < 40%. Excessive loss of lower branches is noted, which can suppress the tree’s ability to develop a proper trunk taper and compromise its stability (cf. Section 4.2.4). Overall tree vigour can be reckoned as average.

(e) Assessment of trunks and branches

(i) Photos T5-8 and T5-9: Large cavity at trunk base with decay (microdrill points 1, 2, 3 and 4)

The loose decayed wood in the large basal cavity could be cleaned to reduce the chance of moisture accumulation and hence to dampen fungal invasion of the remaining wood in the residual wall (Photos T5-8 and T5- 9). Only the badly decayed and partly detached wood in the large wound should be very carefully removed. Do not attempt to excavate or remove woody tissues that have not partially detached. Do not overdo this cleaning exercise. The detached wood debris accumulated at the bottom of the cavity should also be removed, if necessary with the help of a powerful vacuum cleaner. The cavity cleaning could help to dampen fungal growth.

As a precautionary measure, a professional extermination company should be enlisted to check thoroughly whether the tree has been infected by termites. If so, the modern hormone bait method should forthwith be applied to get rid of the wood consuming insects.

Two drillings penetrate the remaining wood shell of the large basal cavity. At point 1, the shell thickness is merely 8 cm with somewhat degraded wood (Appendix Figure T5-P1). Wood with advanced decay has been lost, leaving the partly decayed wood in the shell. Lower down at point 2, some wood with advanced decay still lingers on the cavity wall in 0-8 cm beyond which the wood has indication of incipient decay (Appendix Figure T5-P2). The wood shell measures 13 cm. As the cavity is open on the south side, the t/r ratio is not applicable. The lignified aerial roots and root props at and around the trunk base have reinforced it and compensate for the loss of wood strength due to decay and cavity formation (Photos T5-11 to T5-12). Moreover, the liberal development of response wood on

79 the upper side of the trunk base serves as pertinent tension wood reinforcement to pull and help to stabilize the tilted tree (Photo T5-13).

Two more drillings were made above the basal cavity to explore whether the decay has moved upwards into basal large cavity. A large internal cavity of 15 cm diameter is found which signifies the longitudinal extension of the decay column (Appendix Figure T5-P3). The cavity is found on the west side, with a 33 cm residual wood shell of sound wood concentrated on the east side. Further up at point 4 (Appendix Figure T5- P4), the size of the internal cavity is smaller at 4 cm diameter and again situated on the west side. Combining the results of points 3 and 4, the decay column is notably constrained as it moves up the trunk. The additional lignified roots and thickened root prop (Photo T5-11 to T5-13) provide substitute support above point 3 to compensate for the loss of strength due to cavity formation. The tree has been able to respond to the cavity formation by developing supplementary support.

Overall, the saving grace is that the crown load of the tree is limited due to restricted crown development (Section 3.1.6d). The defective trunk does not need to hold a large and heavy crown, hence the pressure on the trunk is somewhat reduced. Despite the large basal cavity, the tree has been able to support itself by a combination of supplementary supports. The risk level could be contained by installing a support system as explained in Section 3.1.6f.

(ii) Photo T5-10: Removal of limbs from the trunk

The multiple wounds on the trunk were due to removal of limbs in the past (Photo T5-10). Their jagged edges or uneven surface or decayed wood should be very carefully trimmed using a sharp arborist manual saw with the help of a carpenter’s chisel. However, the well-formed callus tissues should as far as practicable be left undisturbed.

(iii) Photos T5-11 to T5-13: Trunk reinforcement by lignified aerial roots, thickened root prop and response wood (microdrill point 4)

The loss of wood strength due to the large cavity at the trunk base has been compensated by the development of lignified aerial roots linking the trunk to the root mass on the stonewall face, thickening of a root prop just below the trunk base, and response woos at the lower section of the trunk.

The microdrilling results at point 4 (Appendix Figure T5-P4), which is related to the large basal cavity, are discussed above together with points 1 to 3 in Section 3.1.6ei. 80

(iv) Photo T5-14: Truncated branch with long epicormic branches and elbow joint

The truncated limb has formed epicormic branches from the wound, and lower down at its middle a long and heavy epicormic branch with an elbow joint (Photo T5-14). Evaluation at close quarters should inform whether the tip of the truncated parent branch has sufficient sound wood to hold the epicormic replacement branches. If not, the end weight of the daughter branch should be reduced by removing some of the small branches. The amount to be removed shall be commensurate with the condition of the wood around the cut face.

In addition, the end weight of the long and heavy epicormic branch in the middle of the parent limb (Photo T5-14) should be reduced by one-third by selectively removing by reduction cut the weaker and thinner branches, with a view to leaving an even spread of remaining branches. The heading cut must not be used to trim the branches.

(v) Photo T5-15: Upright limbs with v-crotch

The narrow V-crotch between the two rather upright limbs should be examined at close quarters to see if it has formed included barks (Photo T5-15). It should be checked for the presence of crack and internal decay at the interface. If so, the two rather upright limbs should be linked by an elastic cable to prevent splitting at the weakened fork.

(vi) Photo T5-16: Branch stub on trunk with decay

A thick branch stub on the trunk has developed decay (Photo T5-16). As callus tissues have formed rather well around the edge of the cut face, and the decay has not reached the advanced state, it could be left alone and monitored.

(vii) Photo T5-17: Seam and bulge on heavy limb (microdrill point 5)

A notable seam with slight bulgewood has formed on the heavy limb, with possible internal decay (Photo T5-17). The wood condition inside and around the seam should be explored by microdrilling, the position of which has been annotated on the photograph. If more than one-third of the notional cross section has been degraded by decay or cavity, the end weight of the long and heavy branch should be reduced. The amount to be removed shall be commensurate with the condition of the wood around the

seam. The removal could aim at the weaker or thinner branches, with a view to leaving an even spread of remaining branches.

A large internal cavity is found by microdrilling in the heavy limb (Appendix Figure T5-P5). It measures about 11.5 cm in diameter, but the wood shell on both sides has sound wood. The t/r ratio for the thin (west) side is 10/15.5=0.64 which is above the critical threshold of 0.33. There is enough sound wood to support the load, hence no drastic pruning is needed. The end weight of the long and heavy branch should be reduced by about one-third. The pruning should aim selectively at the weaker or thinner branches, with a view to leaving an even spread of remaining branches. The decay, however, could continue to move into the sound wood shell to reduce wood strength in due course. The weakness should be continually monitored.

(viii) Photo T5-18: Existing Cobra bracing using T5 to hold T4 Stem A

The existing cobra cable bracing using T5 to pull T4 may not be able to help the latter (Photo T5-18). T5 suffers from the significantly weakened trunk base due to the large basal cavity. Despite the development of response wood and lignified root stands, it is unlikely to have much spare strength to hold T4. If T4 were to fail, it is highly likely that T5 will also be brought down collaterally. It is recommended that this cable should be removed to relieve T5 of a burden that it can hardly afford and the associated risk of collateral failure. Instead, T4 and T5 each should have its own supporting system (cf. Sections 3.1.5e and 3.1.6f).

(ix) Preventing decay at trunk base

The rubbish that has deposited on the surface roots and the adjacent trunk base, together with the leaf litter, should be regularly removed with the help of a brush to avoid moisture accumulation which could induce decay at the critical trunk base position (Photo T5-7). More importantly, objects trapped in the gap could induce the wedging effect to detach the trunk base from the wall face. Such materials must be diligently and thoroughly removed from the gap to prevent the detachment risk (cf. Section 4.2.3).

(f) Proposal for tree support systems for T5

(i) Tree hazard status

Tree hazard assessment has a score of 8 which can be slotted at the middle of the medium risk category (Table 5G). Moreover, it has been assigned an extra score due to the alarmingly large basal cavity. Of the six stonewall 82 trees at the site, T5 has been accorded the second highest hazard score trailing after T4. The main contribution to tree risk is the potential for the trunk to snap near the base due to advanced decay and formation of a large cavity. If the tree were to fail, the whole tree may fall down on Bonham Road.

(ii) Three tailor-made tree support systems

In view of the high risk status, something concrete, effective and long- lasting have to be applied to abate the hazard. Due to the severe site constraints, which are similar to those faced by T4, special non-routine measures will have to be developed to stabilize the tree without obstructing pedestrian or vehicular traffic.

As the tree has a limited biomass due to the limited crown size and amount of branches and foliage, a cable bracing method is proposed to provide the pulling force from the south side. The large decayed wound at the trunk base has degraded into a large cavity. It occurs at the back of the inclined trunk, which is the most critical position where strong tension wood should have developed to hold the tree's weight. The tree has developed supplementary and substitute supporting structures in the form of notable ribs of tension response wood, two lignified aerial root props, and a thickened root prop (Photos T5-11 and T5-12). They have been instrumental in maintaining tree stability despite the significant loss of support at the trunk base.

Three supporting systems are proposed, the design of which has taken into account the rather severe site constraints. They are listed below in order of preference.

(iii) Method A: Cable bracing

The pavement and carriageway under the tree, however, are quite frequently used. The tree manager has to do something to abate the risk of tree failure and possible damage or injury of the targets. The most direct and cost-effective way is to install a cable bracing system to hold the trunk. The cable system should have sufficient elasticity to permit the stems to move within a certain limit. The cable can be anchored on the columns of the nearest building to the south at St Stephen's Lane at an elevated level to permit vehicular and pedestrian headroom clearance (Photo T5-19).

The Highways Department with the help of the District Council and Home Affairs Bureau could negotiate with the property owners and building management to obtain the permission to do so. The relevant property 83 owners could be persuaded in the interest of preserving the tree and the associated amenities which could benefit them in the long term. The maintenance of the cable bracing system and the affected parts of the columns or beams should be shouldered by the government. Please see Photo T5-19 which illustrates the concept design of this cable bracing system. The detailed design should be elaborated in conjunction with a structural engineer.

(iv) Method B: Cable bracing

The second option is to install a strong steel frame near the parapet wall at St Stephen's Lane to hold the tree using a cable-bracing device (Photo T5- 20). The frame should be firmly anchored in the ground. Cables can then be installed to link the frame to the trunk at a position above the cavity. Due to the low bracing position relative to tree height, the amount of swing in the wind would be rather limited. Thus the cable should have a correspondingly lower degree of elasticity.

As the Lane is used for vehicular access, the frame has to be positioned as near as possible to the parapet wall to maintain sufficient vehicular lateral clearance. It implies that excavation will have to be conducted very carefully near the tree to avoid harming the roots that have penetrated the soil lying below the Lane. Moreover, the impact could also be reduced by developing a technique with the help of an engineer to install the anchors with a minimum excavation limit. Please see Photo T5-20 which illustrates the concept design of this cable bracing system. The detailed design should be elaborated in conjunction with a structural engineer.

(v) Method C: Propping

If both cable bracing methods are found to be not feasible, the last resort of mounting a strong steel propping frame at the Bonham Road pavement will have to be considered (Method C). The design concept will be similar to the case of T4, hence the same drawing and description will be applicable (cf. Section 3.1.5e and Photo T4-32).

(vi) Method D: Short-term contingency measure to abate tree risk

Three long-term options in order of preference have been proposed in the above sections to abate the risk of T5 on targets. It is envisaged that one of them will be adopted in due course. As the implementation may incur a lead time to overcome the obstacles in terms of space, traffic and property rights, in the meantime it is deemed necessary to take a short-term measure to reduce the risk of tree failure. Drastic reduction of branches has to be 84 ruled out, because it could impose irreparable damages on the tree and may dampen its vigour and health to render it more hazardous to targets. As the trunk base and lower trunk with a large basal cavity is the weakest part of the tree, it cannot be used for cable attachment. Instead, the cable has to be mounted above the large basal cavity and its extension as internal cavity (cf. Section 3.1.6ei). The make-shift cable bracing can be attached to anchors in the ground just behind the parapet wall at St Stephen’s Lane. Holes can be drilled in the parapet wall to allow cables to extend from the anchor points to the trunk. A structural engineer could provide professional input on the design of this cable bracing method. A schematic drawing of this temporary cable bracing method is shown in Photo T5-21.

(vii) Alternative to long-term cable bracing or propping

In the unlikely and undesirable event that none of the three long-term solutions (Methods A, B or C) to stabilize the tree can be implemented, the alternative will have to be drastic pruning to reduce the load and hence the risk level. Such an approach will disfigure the tree and reduce its vigour, and may drive the tree towards an irreversible decline spiral to render it more unstable and hazardous. Furthermore, the load-reduction effect of drastic pruning may not last long, as the tree in the post-treatment years will struggle and send out a notable amount of sprouts and epicormic branches in its attempt to regain some of the lost photosynthetic capability. Thus the tree will have to be heavily pruned repeatedly for some years, a process that is tantamount to a protracted death sentence. As there are feasible ways to stabilize the tree, drastic and repeated pruning cannot be recommended.

3.1.7 Assessment of T6 and arboricultural recommendations

(a) Overall tree structure and condition

(i) Tree location on stone wall

T6 is located at the far eastern end of the wall (Photo T6-1) close to the junction with Park Road. It is the shortest of the group of four stonewall tress clustered closely at the eastern stretch of the old wall. Standing at 12.37 m, it is tilted at 22 degree towards Bonham Road. It also leans significantly towards the east, which is away from its neighbour T5 (Photo T6-2).

(ii) Tree dimensions and biomass structure

The twin-stemmed tree has a thicker trunk A with 33 cm DBH and trunk B at 21 cm (Photos T6-3 and T6-4). The aggregate DBH is 39 cm. The crown shape is notably asymmetrical. Parallel to the wall face, the crown is mainly developed on the east side. On the west side it is decidedly stifled. Perpendicular to the wall face, the crown development is biased towards the front facing Bonham Road (Photo T6-4). Overall tree vigour is rated as average.

(b) Assessment of surface roots and interface with the stone wall

(i) Surface root distribution on wall face

The wall is at its shortest at the eastern end, measuring only 2.13 m above the adjoining pavement level at T6 (Photo T6-5). Surface root density is higher at the upper one-third, and notably decrease in the lower two-third where the roots tend to descend towards the wall toe in four main clusters. Some roots have entered the gap at the wall toe to explore the soil under the pavement. Unlike other trees along the same wall, the wall toe at T6 is not equipped with a U-channel (Photo T6-6).

(ii) Trunk base interface with wall face

The roots penetrate the joints mainly at the wall crest (Photos T6-5 and T6-7). Organic litter and rubbish have accumulated in the gap between the trunk base and wall face (Photos T6-7 and T6-8). They should be removed diligently on a regular basis to avoid the wedging effect which could contribute to detachment of the trunk base. The accumulation of moisture

86 in the gap, which could facilitate fungal growth, can also be avoided (cf. Section 4.2.3).

(iii) Mortar displacement and trapped litter

The entry of roots into joints is critical for the tensional pull of the tree against overturning (Photo T6-9). Some mortar strips have been detached but not broken by root growth at the west side of the trunk base. The detachment is likely to be triggered by the diameter growth of the root that has penetrated the joint. The adjacent masonry blocks have remained in their original positions. The litter trapped in the surface root mass can be removed on a regular basis.

(iv) Root growth restriction

(v) Removing cement sealing at masonry joints

(vi) Installing soil strip at wall crest

An open soil strip filled with a good-quality soil mix could be installed at the wall crest to allow growth of roots in the soil below the pavement at St Stephen's Lane. A segment of the existing stone parapet wall will have to be replaced by railing to permit the roots to reach this new soil strip (cf. Section 4.2.1). If a steel frame has to be installed at the edge of the Lane (cf. Section 3.1.7e) to hold the cable to pull the tree, the soil strip design can be adjusted accordingly to accommodate both.

(vii) Installing soil strip at wall toe

An open soil strip filled with a good-quality soil mix could be installed at the wall toe at Bonham Road, in lieu of the unit pavers, to allow growth of

roots in the soil below the pavement. Similarly, a soil strip can be installed at the wall crest to permit roots to grow backwards to pull the tree.

(i) Crown configuration and loss of low branches

The crown is somewhat small vis-à-vis its height, with a live crown ratio in the lowest category of < 40% (Photo T6-10). The crown outline is highly asymmetrical with heavy development mainly on the eastern side coupled with heavy leaning towards the east. Excessive loss of lower branches from the trunk is quite obvious, and this may dampen the ability of the tree to develop a normal trunk taper. Without sufficient thickening of the lower trunk, the tree may not have enough strength there to support its biomass (cf. Section 4.2.4).

(ii) Foliage and vigour

The foliage density is rated as sparse, whereas leaf size and leaf colour are considered as normal. The branch dieback is in the low category of < 5%. The overall tree vigour is judged to be average.

(d) Assessment of trunks and branches

(i) Upright epicormic branch

A rather upright epicormic branch has emerged with an elbow joint from the parent stem A (Photo T6-11). It bears resemblance to the tree-on-tree structural defect. In due course, its further expansion will impose an increasingly heavy burden on the parent stem. It should be removed whilst it is still small.

(ii) Loss of low branches

Notable loss of lower branches is recorded in both trunks (Photo T6-12). The impact on tree structure has been discussed in Section 3.17c above (cf. Section 4.2.4). The undesirable pruning practice of preferential removal of lower branches should forthwith be stopped. New epicormic branches or sprouts emerging from the lower part of the trunks should be assessed to keep those with the potential to develop into replacement branches.

(iii) Branch removal wound with decay on Stem B

Stem B has a branch removal wound with decay (Photo T6-13). Its jagged edges, uneven surface and decayed wood should be very carefully trimmed using a sharp arborist manual saw with the help of a carpenter’s chisel. However, the well-formed callus tissues should as far as practicable be left undisturbed.

(e) Proposal for tree support systems for T6

(i) Tree hazard status

The hazard assessment yields a total score of 6 which falls on the top end of the low risk bracket. The main concern is about falling of weakly structured branches, and the coincidence of tree lean with the asymmetrical crown.

(ii) Two tailor-made tree support systems

The tree is heavily tilted toward the road and the east side. A large proportion of its branches and foliage is also concentrated on the east side of the crown. The pavement and carriageway under the tree are quite frequently used. The tree manager has to do something to abate the risk of tree failure and possible damage or injury to the targets. Two cable bracing systems are proposed.

(iii) Method A: Cable bracing

In the preferred Method A, the cable can be anchored on the nearest building to the south at St Stephen's Lane at an elevated level to permit vehicular and pedestrian headroom clearance (Photo T5-19). The Highways Department with the help of the District Council and Home Affairs Bureau could negotiate with the property owners and building management to obtain the permission to do so. The relevant property owners could be persuaded in the interest of preserving the tree and the associated amenities which could benefit them in the long term. The maintenance of the cable bracing system and the affected parts of the columns or beams should be shouldered by the government. Please see Photo T4-30 which illustrates the concept design of this cable bracing system. The detailed design should be elaborated in conjunction with a structural engineer.

(iv) Method B: Cable bracing

If Method A is not feasible, the alternative Method B has to be employed. A strong steel frame is proposed to be installed near the parapet wall at St Stephen's Lane to hold the tree using a cable-bracing device. The frame should be firmly anchored in the ground. Cables can then be installed to link the frame to the trunk at a position above the cavity. Due to the low bracing position relative to tree height, the amount of swing in the wind would be rather limited. Thus the cable should have a correspondingly lower degree of elasticity.

As the Lane is used for vehicular access, the frame has to be positioned as near as possible to the parapet wall to maintain sufficient vehicular lateral clearance. It implies that excavation will have to be conducted very carefully near the tree to avoid harming the roots that have penetrated the soil lying below the Lane. Moreover, the impact could also be reduced by developing a technique with the help of an engineer to install the anchors with a minimum excavation limit. Please see Photo T4-31 which illustrates the concept design of this cable bracing system. The detailed design should be elaborated in conjunction with a structural engineer.

3.2 Tree values, potential risks and preservation

3.2.1 Value of the stonewall trees

(a) World-class and unique urban ecological gem

Stonewall trees denote a special if not unique urban ecological endowment of our city. Very few places in the world have so many stone retaining walls concentrated in a relatively small area, and with so many trees of sizeable biomass and high landscape quality to dwell on them. They represent the product of our difficult terrain working in tandem with our labour and ingenuity in creating a city literally from scratch. The traditional Chinese masonry technology has found pragmatic expressions in the territory. It has lent tremendous help to maximize the amount of developable land on hillslopes. The stonewall trees denote a pleasant by- product of this laborious endeavour.

(b) Spontaneous nature in built-up areas

Beginning with a natural environment, we have created the city artefact. On the artificial cliffs embedded in the city, we have inadvertently facilitated nature to return to ameliorate the excesses of urban growth. We did not plan for nature in the cramped built-up areas, yet nature would come spontaneously to embellish them. The infilling of the vertical stonewall habitats by trees reflects faithfully and emphatically nature’s tenacity and resourcefulness in claiming the seemingly inhospitable sites. In building the stone retaining walls, we aimed squarely and narrowly at winning land for city development. Nature would make good use of the opportunities offered by the vacant niches to flourish in the sea of artefacts. It was serendipity and providence that have steadfastly invited them to settle as permanent residents amidst our homes.

Once they arrive and establish themselves on the stone structures, people would begin to appreciate them. They are welcomed as our beloved friends and benefactors amidst our neighbourhood and close to our homes and offices. As children grew up and adults got older, the trees became bigger and stronger. Despite limitations and stresses, they manage to soldier on against heavy odds to become outstanding ambassadors of nature. In close proximity to people, they generously share their valuable ecosystems services with people. They enhance urban landscape quality and enrich urban biodiversity.

(d) Historical and heritage significance

The collection of a row of six stonewall trees on one of the oldest stone walls in Hong Kong, and along one of the oldest roads that was built about 150 years ago, would connote historical and heritage values. They could be construed as objects to fill the collective memory of the community. Traditional stone retaining walls are no longer built, and many existing ones have been demolished or degraded by unsympathetic modifications. The stonewall trees often suffer collateral damages due to unfriendly treatment of the walls. From time to time, nature will take its toll on the weaker members of the hanging tree cohort.

(e) Diminishing and threatened resource

The stonewall trees denote a diminishing natural-cum-cultural heritage of our city. The fortuitous confluence of events in the past has permitted their existence, but such events are becoming rare if not non-existent. The present generation has fortunately received a wonderful bequest from our forefathers and from Mother Nature. We owe the future generations the duty to preserve them in a healthy state so that they could become a sustainable endowment of our community. We should not allow them to degrade and become more threatened.

3.2.2 Risk of the stonewall trees

(a) Wall instability versus tree instability

The risk of the stonewall trees can be linked to two related aspects, namely the stability of the stone wall that holds them, and the stability of the trees themselves. The wall itself is managed by the Highways Department with technical advice given by the Government’s Geotechnical Engineering Office of the Civil Engineering and Development Department. Hong Kong has accumulated plenty of experience and expertise especially in the last few decades in making slopes including retaining walls safe employing mainly engineering methods. The walls that are at risk are closely monitored by the relevant authorities.

(b) Stone wall failure versus stonewall tree failure

A review of the history of wall failures in Hong Kong (Chan, 1996; GEO, 2011; Sections 2.2.4, 2.2.5 and 2.2.6) indicates that few of the cases involved stonewall trees. The causes of wall failures were often attributed 92

to poor design, poor workmanship, poor maintenance, and sometimes associated with leaking pipes situated behind the wall. Only a small number of the failed walls had trees growing on them. The stonewall trees could sometimes be brought down due to en masse wall collapse. In one case, a stonewall tree was hinted as one of the possible causes of wall failure. For the few cases of stonewall tree failures rather than stone wall failures per se, the wall structure was hardly affected by tree uprooting or collapse.

In two documented cases of recent stonewall tree failures, in the form of uprooting, the causes could be more convincingly ascertained. They involved tree failure without damage to the wall structure. The two stonewall trees that toppled in strong wind associated with typhoon or rainstorm were found to be inadequately anchored in the aft-soil. The lack of tension roots that grow backwards into the joints and hence into the aft- soil is interpreted as the main cause of failure. Without sufficient tensional pull, the tree may overturn under extreme gusts. The lack of joints or avenues for roots to penetrate the wall is considered as the fundamental cause of insecure anchorage.

(d) Tree risk at subject site

The six trees at the subject site are located at a busy road with heavy vehicular and pedestrian traffic during the daytime. The probability of a failed tree hitting a target at the frequently used road is reckoned as rather high. Such a level of road use would add scores to the tree hazard assessment. T4 and T5 and to a lesser extent T6 express symptoms of instability that call for remedial measures to abate their risk. As they are situated right above a busy bus stop where people tend to gather and stay, the risk level has been correspondingly raised. More assured methods have to be implemented to suppress the risk to an acceptable level.

The remaining trees do not display visual clues to indicate that they are unduly unsafe. The structural defects and decay problems of individual trees have been identified and specific arboricultural treatments have been recommended to minimize risk due to breaking or falling branches. Please refer to Section 3.1 for relevant details on individual trees. A condensed summary of tree problems and remedial treatments is given in Section 4.1 below.

3.2.3 Potentials and limitations in tree preservation

(a) Liberal availability of crown growth space

The six stonewall trees are all growing at or near the top of the wall, where they are raised above the level of pedestrians and hence do not obstruct their movement. This is more so for trees on the west side and less so on the east. Although the branches hang above the carriageway of Bonham Road, they are sufficiently elevated to provide clearance to vehicular traffic. The elevated space above Bonham Road at the front and St Stephen’s Lane at the back provide adequate room that is rather free from building and other obstructions for their crown to extend. They do not conflict with traffic and hence do not demand regular pruning to abate this conflict. In comparison, stonewall trees at other sites in Hong Kong are often beset by the lack of growing space especially at the back where buildings often abut onto the wall to restrict crown development on one side.

(b) Proximity of branches to nearby buildings

The proximity of some long branches to buildings on the opposite site of Bonham Road and at the back along St Stephen’s Lane could cause nuisance to the residential units. However, this conflict is not serious enough to consider removing or drastically prune the trees. It can be rectified by periodic and skilful shortening of selected branches. A horizontal clearance of 2 m is recommended to abate the nuisance. The limited amount of pruning conducted periodically will have little impacts on tree health or structure. It is very important to forbid the harmful heading cuts in any attempt to shorten branches.

Stonewall trees that have potential risk of failure demand remedial measures in the form of a supporting system such as cable bracing and propping. Three of the trees, namely T4, T5 and T6 have displayed symptoms of instability which call for human assistance. Propping can provide a strong and assured way to buttress the distressed trees. However, the narrow pavement in front of the wall cannot provide adequate space to accommodate a normal propping frame design.

A specially designed propping system with firm anchorage in the ground and the wall could lend support to a sufficiently sturdy cantilever arm to hold the tree’s weight. A tailor-made design of the propping system has to be developed by a tree specialist in conjunction with a structural 94

engineer. This method may be more difficult to apply in comparison with conventional propping, but it is not impossible. The need to puncture the old stone wall face to install the support frame could be considered as damage to the heritage feature. The installation of the anchors in the stone wall will have to be conduct with great care to minimize the impact.

(d) Tree support by cable bracing anchored on adjacent building

In view of the rather tight site constraints, the alternative cable bracing system is more suitable. The best approach is to anchor the cables on the structural columns or beams of the buildings situated at the back of the wall at St Stephen’s Lane. As the Lane allows vehicular access, the cables have to be mounted at a height to permit sufficient vertical clearance. The property owners concerned may not give consent to have the cables attached to their buildings. The Highways Department could solicit the help of the District Council and the Home Affairs Bureau to convince the owners to offer a public service to help the trees. To allay the worries of increase in maintenance cost due to the installation of cable anchors, the government can take up the responsibility of maintenance for the affected columns and beams.

(e) Tree support by cable bracing attached to steel frame

If the property owners cannot be persuaded to mount the cable anchors on their buildings, an alternative cable bracing method will have to be developed. It has to be fitted to the tight site conditions to leave sufficient berth for both vehicles and pedestrians. Behind each tree in distress, a strong steel frame is proposed to be anchored on the ground near the parapet wall. The frame should be placed as close to the wall as possible, but it must avoid injuring the roots that are likely to spread below the paving near the tree base. The cable can be attached to the top of the frame to hold the tree.

(f) Ecosystem services of stonewall trees to neighbourhood

The installation of the tree support systems can sustain the trees. The associated pleasant and welcomed ecosystem services, including landscape, environmental and amenity benefits provided by the lovely stonewall trees, could thus be preserved to benefit the property owners and residents. The sale and rental values of the adjoining properties can be correspondingly lifted by the high-quality proximal greenery. Without the support systems, the stability state of the trees, particularly T4 and T5, could continue to deteriorate, and in the worst-case scenario they may have to be felled in the interest of public safety. It will be a pity to lose 95

these outstanding members of nature embedded in the densely developed neighbourhood.

(g) Human and social dimensions of tree preservation

The human and social dimensions, often delicate if not sensitive, have to be considered in tree preservation. The stonewall trees have existed in the neighbourhood for a long time. T2 is likely to be the oldest resident who has dwelt on the wall for more than a century. It has spanned notionally four human generations, and witnessed the coming and going of buildings and people in its environs. The residents have literally grown up with the trees, especially the younger members such as T1, and to a lesser extent, T3 to T6. It is natural for them to have developed a fond and probably sentimental attachment and a collective memory to their arboreal friends. They harbour an expectation that the government, represented by the Highways Department, will keep the trees in a healthy and safe state. It will not be easily to convince the people to remove the trees without taking efforts and exhausting the means to help them. On the contrary, it will be easy to gain their support of tree preservation endeavours.

4. RECOMMENDATIONS ON MAINTENANCE OF THE STONEWALL TREES

4.1 Short-term practical maintenance measures for individual trees

For the six stonewall trees at the subject site, the results of the visual tree assessment has provided the scientific basis to develop a short-term and long-term tree care package. The detailed recommendations on arboricultural works for individual trees have been elaborated in Section 3.1. They could be condensed as follows.

4.1.1 Maintenance of T1

A small tree with little structural or decay problem and a low risk level. It is vigorous and robust and relatively free from major problems. Two small branches have been recommended for corrective pruning to pre-empt aggravation into serious structural problems in the future. Thus far, the tree has received little pruning in the past, hence it has escaped from the common damages of improper pruning that have been imposed widely on local trees.

The performance of this tree is contingent upon the quality of future tree care. Unnecessary, indiscriminate and unprofessional pruning should be assiduously avoided. It should be given the chance to grow up of its own accord as a robust wall tree without the undue disturbance of unnecessary tree works and other impacts. The modern design of the wall with joints tightly sealed by mortar would restrict the ability of roots to penetrate into the aft-soil. As a result, the tree’s longer term prospect could be curtailed by the lack of new sources of water, nutrient and anchorage.

4.1.2 Maintenance of T2

This is the largest of the six trees and one of the largest and finest exemplar of stonewall trees in Hong Kong. It signifies the landmark if not the signature specimen tree of the site. It is one of the largest trees in the neighbourhood, even though it finds it footing on an unusual vertical habitat rather than on the ground. The tree is overall in good health and has a well-formed scaffold. The risk level is rated as low. Some branches are suffering from structural weaknesses or decay, most of which were associated with a long period of improper pruning and lack of proper care

in the past. These accumulated defects have been identified and a comprehensive range of arboricultural treatments have been recommended to rectify them (cf. Section 3.1.3).

A particularly alarming improper treatment is the systematic and repeated removal of almost all the lower branches over a considerable period. This type of erroneous and unprofessional pruning is widely practised by local landscape contractors. The loss of lower branches over an extended period would suppress the ability of the tree to develop a normal taper. With inadequate thickness at the trunk base but continual expansion of the crown, the tree could suffer from the risk of snapping. It is regrettable that the damage has been quite thoroughly done.

This defect is not easy to correct and it may take a long time to nurture some new lower branches. It is a somewhat probabilistic venture because one can only choose from the new epicormic growths to identify the potential future new branches. If the tree sends out few epicormics or few qualified epicormics, it may not be possible to accomplish the task in good time. The exercise demands a high level of skill, supervision and lots of patience, and it has to be consistently applied over some years to take effect.

4.1.3 Maintenance of T3

This is a semi-mature Chinese Banyan with a much confined scaffold development. Two of its three trunks have been truncated. The remaining one constitutes the bulk of the tree’s biomass, which has been reinforced by clusters of lignified aerial roots linking between branches and from branches to trunks. The crown development is limited in terms of its spread and branch density. It has lost a notable number of limbs and main branches. The live crown ratio is low, and its vigour is rated as average.

No visible symptoms could be discerned to indicate high risk. The hazard rating is classified as low. The tree can be helped by removing trunk A which has been badly decayed with no branches except some sparse sprouts. The remaining stump of trunk C cannot be removed as it lends crucial support to trunk B via a bundle of lignified aerial roots. The truncation tip of trunk C with advanced decay could be removed. Some wounds left by past limb or main branch removal could be carefully trimmed. The burden on two branch removal wounds could be alleviated by selective pruning of their epicormic daughter branches.

4.1.4 Maintenance of T4

This is the relatively stronger tree amongst the group of four stonewall trees growing at the eastern part of the wall. The twin-stemmed tree has strong trunks well reinforced by lignified aerial roots. No trunk is missing or has been truncated. The crown development, stifled by competition for space with neighbour trees, has been curtailed. A notable number of limbs and main branches have been removed or have broken off. Most branch loss wounds are beset by decay. The foliage density is sparse and the live crown ratio is rather low at less than 50%.

Trunk A is tilted at 52 degree towards Bonham Road and trunk B at 49 degree towards St Stephen’s Lane. Both leaning angles are the highest amongst the six trees. The bulk of the tree’s crown weight is carried by trunk A. It is alarming that the trunk base has been slightly detached from the wall face. It indicates the incipient failure of the root anchorage. This critical defect could aggravate and lead to tree collapse in the worst-case scenario. Something concrete and effective must be implemented as soon as possible to support the tree and stop the drop. A cable bracing system is proposed, preferably to be anchored on the nearest building at St Stephen’s Lane. If this is not feasible, a strong steel frame can be mounted on the ground of St Stephen’s Lane adjacent to the parapet wall. If cable bracing is found not feasible, the last resort is to construct a specially- designed spacing-saving steel frame at the narrow Bonham Road pavement to prop the tree.

The decayed and fractured wounds should be trimmed and cleaned up. The end weight of tipped branches carrying heavy loads of epicormic replacement branches should be appropriately reduced. Stubs should be removed.

4.1.5 Maintenance of T5

This is the feeblest of the four trees found at the eastern part of the wall. The single trunk of the slender tree supports a disproportionally small crown that is squeezed tightly between T4 and T6. Most of the lower limbs and branches have been removed or lost. The cutting wounds have been infected by wood-decay fungi. They need to be cleaned up by careful trimming or remedial pruning. Two specific structural defects demand attention, including the V-crotch between the only two ascending limbs, and a seam with bulgewood on the heavy limb.

The most critical issue is the development of large basal cavity with continued wood decay on its greatly reduced residual wall thickness. The tree has responded to the reduction in wood strength by developing strong response wood on and around the cavity. It has also formed two strong lignified aerial roots around the trunk base and a thickened root prop below the trunk base to provide compensatory reinforcement. Despite these self-help by the tree itself, the high pedestrian vehicular volume at the site demands additional assurances. A cable bracing system is recommended to pull the tree towards the south. The cable preferably should be anchored on the adjacent building at St Stephen’s Lane. If it is not possible, a strong steel frame can be mounted on the edge of the parapet wall at the Lane’s edge to hold the cable.

4.1.6 Maintenance of T6

This is situated at the far eastern end of the wall. The main trunk of the twin-stemmed tree is tilted heavily towards the east and Bonham Road. Akin to its companions on the same wall section, its crown development has been stifled due to the lack of growth space. With freedom from competition, its eastward crown extension is notably better. The resulting crown is highly asymmetrical, with most of the biomass concentrated on the east side. Due to the heavy pedestrian and vehicular traffic at the site, the tree can be rendered safe by installing a simple cable bracing system that is similar to the one recommended for T5. The branch removal wounds with decay and the heavy epicormic growths can be trimmed.

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4.2 Long-term practical tree maintenance measures

After conducting the long overdue remedial tree works, future tree care must follow the best international practice to prevent introducing or inducing new problems. In the long term, measures could be taken to enhance their growth and safety. Some generic measures can be implemented to enhance long-term welfare of the six stonewall trees.

4.2.1 Install soil strips at wall crest and toe

A soil strip could be installed at the crest to promote growth of new roots so as to reinforce tensional pull at the critical position. A strip of the concrete paving at the wall crest adjacent to the trunk base of the six stonewall trees can be removed to expose the underlying soil. If necessary, soil amendments and surface mulches can be added to improve the soil condition. Roots have the tendency to grow backwards into the soil strip to provide a new means of tension pull to strengthen tree stability. The additional supply of water and nutrients will also facilitate tree growth and health.

Similarly, a soil strip can be installed at the wall toe to enlarge the soil catchment volume for additional supply of water and nutrients. It means removal of the present U-channel at the wall toe for T3, T4 and T5. The surface water at present drained away via the U-channel can infiltrate into the soil via the new soil strip. For T1, T2 and T6, the unit pavers laid against the wall toe could be removed to expose the soil and facilitate root entry into the soil. This new soil strip can bring immediate benefits to T2 to T6, because their surface roots have already reached the pavement level and penetrated into the soil under it. More new roots are expected to make use of the new opportunity to grow into the soil strip.

However, for T1 which is still small and does not yet have long surface roots extending to the wall toe, the soil strip installation could be postponed. In the long term, the surface roots of T1 could eventually grow into the wall toe soil strip to furnish a new and important source of sustenance to the tree. As the wall supporting T1 is of modern design with little accessible joints for root penetration, the provision of soil strips at both the wall crest and wall toe can notably enhance its future growth. They can literally provide a new lease of life to the otherwise sequestered tree roots.

To preserve the walkable width at the narrow pavement and to avoid human-foot trampling of soil, a metal grille could be placed on the soil 101

strip. The advice of a geotechnical engineer should be sought in the design of the soil strips. If a continuous soil strip is not feasible, an intermitted strip could be adopted instead.

4.2.2 Remove joint seals to permit new root penetration

The continued entry of roots into joints is crucial to provide a new crop of roots for the tensional pull of the tree against overturning. As the trees grow bigger, they need to have a corresponding increase in roots to hold their biomass and capture more water and nutrients. However, the joints between masonry blocks on the old retaining wall have been sealed thoroughly in recent years by cement, thus stopping their penetration by new tree roots. The rigid cement seal plastered around existing roots also restricts their diameter expansion and may cause girdling injury as they continue to thicken. The wall supporting T1 is an outlier as it is of modern design with joints originally sealed by mortar.

Losing the ability to send new roots into the aft-soil and at the same time losing existing roots could jeopardize tree stability. Where the roots are physically obstructed, localized removal of the cement seal could be considered to permit some new roots to grow into the joints and existing roots to expand. This way, the continued growth and stability of the trees will not be unduly suppressed. Such a relief can release the trees from a stifling if not daunting bondage, and ameliorate a potential risk factor faced by the stonewall trees.

4.2.3 Prevent wedging effect on tree stability

The organic litter and rubbish that tend to accumulate in the gap between the trunk base and wall face could pose a danger to the trees. As a stonewall tree tends to swing slightly in strong wind, the gap could be temporarily widened by a small bit to allow the accumulated debris to drop further down into the crevice. The trapped debris would create the wedging effect to prevent the tree to bounce back to its original position. The process could be repeated, and the cumulative effect of repeated wedging could be alarming.

Gradually, the gap could be widened to jeopardize tree stability. The debris in the gap should be removed diligently and thoroughly on a regular basis to forestall a precipitating cause of stonewall tree detachment which may lead to tree failure. A non-destructive method using a trash picker in conjunction with a brush with long and somewhat 102

firm bristles could be employed to clean the gap on a regular basis. In addition to preventing the wedging effect, the cleaning could prevent the accumulation of moisture in the gap so as to avoid fungal growth at the critical trunk base position.

4.2.4 Stop repeated removal of lower branches

All the trees except the very young T1 have lost an inordinate amount of lower branches from their trunks. This wholesale loss of lower branches is a rather common phenomenon afflicting other trees in Hong Kong. It is unlikely that natural causes such as strong wind could preferentially remove the lower branches so thoroughly on so many trees. Field observation of pruning works in action would reveal the widely-adopted but erroneous pruning practice.

This highly undesirable pruning method is probably based on the mistaken idea that the lower branches are relatively shaded and hence they play little role in photosynthesis and plant food production. It is also possible that lower branches are relatively easy to reach and remove in comparison with the higher-up branches. The common use of the hydraulic platform in pruning work in Hong Kong, instead of employing a tree climber, means that the upper part of the crown of large trees could not be easily reached. This may induce the concentration of pruning in the lower reachable parts of the crown.

The repeated removal of lower branches can reduce the natural ability of trees to develop a proper trunk taper. The base of the trunk, deprived of sufficient and timely increase in diameter, in time may not be able to support the expanding crown with increasing weight. Thus an originally healthy and well-structured tree could be unintentionally and unknowingly transformed over the years into a potential hazard tree by this unprofessional pruning practice. The affected tree is prone to snap at the trunk base. Contractors could be coached the proper pruning methods and be weaned from the harmful approach. Briefing and close supervision of contractors are necessary to forthwith stop this rather pervasive negative impact.

4.2.5 Stop unprofessional and unnecessary pruning practice

Unnecessary removal of non-defective and non-hazardous branches appears to be rather common in tree care works in Hong Kong, including the management of the six stonewall trees. Every pruning task should 103 have a clearly considered and explained objective, which should be based firmly on detailed scientific assessment of the tree in the field. The principal reasons for pruning should be removal of defective branches and abatement of hazard or nuisance. Pruning without a defined objective is akin to shooting without aiming.

A pruning proposal that can result in significant reduction in crown size, such as crown lifting, crown lowering, crown shaping, crown reduction, and substantial truncation and shortening of large branches, must not be approved if they are not supported by scientifically justifiable reasons. A pruning proposal that can incur deformation of the natural crown form, such as wholesale removal of branches in a given part of the crown, must not be approved. Contractors should not be given a free hand to prune without a well justified written pruning plan, which must be approved by a relevant professional officer who is equipped with specialist tree knowledge and practice experience.

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4.3 Preventing grave impacts of excavation

Excavation work at the wall crest platform and wall toe can impose grave harms on tree roots and hence on tree health and stability. This source of potential massive injury which could irreversibly dampen and destabilize stonewall trees must be avoided. If it cannot be avoided, the excavation must follow strictly the following guidelines to minimize its short- and long-term impacts:

 Demarcation of tree protection zone (TPZ) around the affected stonewall tree. A recommended TPZ should cover the wall toe platform, wall face and wall crest platform, with a radius equal to the average crown diameter. It should include the soil below the wall toe platform down to 1 m, and the soil behind the wall face and below the wall crest platform. It should also include the air space above the TPZ to exclude impacts on the crown.

 As far as possible, do not permit excavation in the TPZ. Alternative location, routing and detour for the proposed excavation must be seriously considered.

 Excavation should be located at a maximum possible distance from the TPZ.

 Digging within the TPZ should only be adopted as absolutely the last resort.

 Excavation depth and width should be minimized.

 The duration of opening must be minimized.

 If the work has no choice but to intrude into the TPZ, the no-dig, or trenchless, or micro-tunnelling, or precision directional drilling techniques should be employed in lieu of open trenching within the area defined by the TPZ.

 Within the TPZ, only the required pipe or cable should be installed. Other ancillary but obtrusive installations such as junction boxes which will cut roots and permanently eliminate soil volume for root growth must not be placed inside the TPZ.

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 Assess impact of excavation on tree stability, and if necessary, provide cable bracing or propping to prevent tree failure due to the degradation of root anchorage.

 Install hard, rigid and effective protective shields to prevent mechanical damage to any tree parts, including roots, trunk, branches and foliage. They should be removed as soon as they have served their purpose.

 Prevent rainwater or construction wastewater from flowing into the opened pit or trench. Install water-tight bunds around the TPZ perimeter to prevent flowing of water from the work site into the TPZ.

 Use manual tools for excavation in and near TPZ. Light-weight and low-impact pneumatic digging machines could be used only for breaking the paving, and should be approved before any operation.

 After rainfall, water that stands in the pit or trench (not draining away by gravity) for more than 30 minutes should be pumped out within 30 minutes.

 Take away tall excavated paving materials from the site; do not dump them into the pit or the trench.

 Minimize the cutting of roots encountered in the course of digging.

 Ensure proper protection of roots encountered in the course of digging from mechanical damage and desiccation. All exposed roots should be wrapped by three layers of hessian and should be kept moist all the time by spraying with water.

 Refill the pit or trench with a good quality soil mix.

 Prevent soil degradation and root damage in the course of installing the new paving.

 Depending on the specific conditions of the tree, wall and environ, a dedicated method statement must be developed and approved before any excavation work can start.

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4.4 Guidelines to inspect and monitor tree structural stability and health

The visual tree assessment method, specially developed to cater to stonewall trees, has been adopted in this study (cf. Tables 1 to 6). By applying this dedicated method, stonewall trees could receive a comprehensive, scientific and objective assessment that is commensurate with the unique tree and habitat characteristics. In the course of field assessment, individual trees can be subject to a systematic and thorough evaluation of all the major tree parts. The table is divided into nine sections to organize the field assessment in a logical sequence, and to ensure that all the critical features are covered.

Part A covers the basic attributes of the wall, habitat and environs, as well as recording the pertinent measurements of the wall and tree dimensions and related geometrical properties. Part B prompts the assessor to inspect the main components of the tree, beginning with the surface roots, aerial roots, and ground roots at wall toe. Part C draws the assessor attention to the crown and the main scaffold. Part D tackles the trunk. Part E focuses on the crotch between the trunk and limbs, and Part F the limbs and main branches.

Integrating the assessment results of Parts A to F, Part G conducts a hazard tree assessment to rate the tree and establish its hazard rating score. Part H begins to apply the knowledge acquired thus far to practical tree management. It offers a wide range of specific arboricultural recommendations to ameliorate the identified tree problems and to improve tree growth and safety. Part I is linked to Part H to explain the rationales of the critical recommendations, and elaborate on the treatment methods.

The stonewall tree assessment should be implemented by a tree specialist with conceptual and practical experience in this special type of habitat and their companion trees. All recommended arboricultural works should be implemented by a skilful and experienced arborist. The minimum qualifications of the arborist should be a recognized degree in a relevant field, including arboriculture, forestry, horticulture, ecology or botany. It should be accompanied by not less than three years of relevant experience acquired in Hong Kong or in the humid-tropical Asian region with similar tree species composition and climatic conditions. Besides the above formal training and qualification, a relevant certification by a recognized international or national tree care professional body is required.

The critical issues identified in the baseline survey could be given special attention in the fellow-up monitoring surveys. Thereafter, the stonewall 107 trees can normally be inspected once per year using the dedicated method. For trees beset with high risk factors, the inspection frequency should be raised to half-yearly or quarterly. In other words, the inspection frequency could be increased in tandem with the degree of hazard. New critical issues should be recorded in the course of the monitoring surveys. For trees affected by construction activities, the monitoring frequency should be monthly. The monitoring survey should be conducted as soon as possible after a typhoon strike with signal 8 and above.

As tree risk assessment is dependent very much on clear visual images, the baseline survey should build up a detailed photographic record. Subsequent monitoring surveys should take photographs of existing and new critical issues. All photographs should be of high quality and high resolution of preferably not less than 2M per file and in jpeg format. The photographs should be amply annotated. The relevant items in the assessment report should be liberally linked to the photographs by citing the reference number.

The approach begins with collection of comprehensive data on each tree, from which practical tree care recommendations are inspired or distilled. Officers with a sound tree knowledge base could learn the technique and apply it to stonewall trees. It will be helpful if the officer charged with this duty could learn the concepts and practice in both the classroom and the real-world environment.

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5. EXECUTIVE SUMMARY

5.1 Preamble

Six stonewall trees are dwelling on an old masonry retaining wall at a prominent location at Bonham Road near the Park Road junction. The Highways Department has been assigned the management responsibility of the wall and its companion trees. The road, the wall and at least one of the trees have existed in the neighbourhood for more than a century. With historical and heritage significance, they furnish outstanding landmark features and conjure up collective memory for the community. As prized assets, they deserve special attention and care to ensure their sustainability.

The important stonewall trees have been managed on a regular basis to maintain their health and vigour. As stonewall trees have rather unique growth habits and habitat conditions, general knowledge and practice in tree science are not directly applicable. A detailed and dedicated scientific study could yield useful information and insights to further enhance the quality and effectiveness of management.

As the road under the trees is heavily used by both pedestrians and vehicles, it is necessary to ensure that they are stable and safe. They call for a thorough risk assessment using the state-of-art methodology. From the results and findings of the comprehensive study, specific arboricultural recommendations could be distilled to tackle tree growth and structural issues. They can also provide a firm basis to develop a long-term management strategy.

5.2 Study objectives

This study has been commissioned by the Highways Department to fulfil eight key objectives:

 Review the general development of stonewall trees on retaining structures in Hong Kong.

 Analyse the effect of stonewall trees on retaining structures based on tree failure records.

 Identify the factors affecting the stability and health of stonewall trees based on literature review.

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 Conduct detailed tree risk assessment of the six subject trees.

 Appraise the value and potential risks of the six trees to the local community.

 Review the potentials and limitations in preserving them.

 Formulate short and long term maintenance measures to enhance their sustainability.

 Prepare guidelines to inspect and monitor the structural stability and health of the stonewall trees.

The deliverables include a report and a presentation at a seminar. The main results, findings and recommendations are condensed below.

5.3 Background and general concerns

 The extensive construction of masonry retaining structures echoes a pragmatic response to our urban development history in a difficult terrain.

 Stone retaining walls embedded in built-up areas offer artificial cliff habitats that are colonized by spontaneous vegetation.

 Due to the highly stressful habitat conditions, the probability of successful tree establishment on stone walls is rather low.

 Banyan trees, especially those with strangler fig characters, are pre- adapted to grow on the harsh vertical habitats.

 Stonewall trees constitute an outstanding landscape and ecological endowment of the city.

 No other place in the world has such a rich endowment of stonewall trees, making them particularly valuable as both cultural and natural heritage.

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5.4 Effect of stonewall trees on retaining structures

 Past retaining wall failures as indicated by records were largely attributed to inadequacies in wall design, workmanship and maintenance.

 Most stone wall failures did not involve stonewall trees, and stonewall trees were attributed as one of the possible causes in the collapse of a small wall.

 There is no record of stonewall tree collapse causing notable damage to masonry retaining walls.

 Stonewall tree failure could be induced by human disturbance of cutting of critical tension roots.

 Stonewall trees could collapse in strong wind due to inadequate growth of tension roots through the joints into the soil behind the wall.

 A dense network of surface roots covering the stone wall face may protect and stabilize the masonry structure.

5.5 Factors on tree stability and health

 Intrinsic wall factors include gaps between masonry blocks; stone size and length of joints; stone shape and horizontal joints; wall inclination; wall height; original joint filling; cement sealing of dry wall joints; weathering state of masonry blocks; seepage on wall face; and weep holes.

 The quality of the wall environs exerts its influence through the factors of moisture supply of aft-soil; volume of aft-soil; quality of aft-soil; wall-crest slope with unsealed soil; wind exposure; solar access; air quality; and proximity to buildings and roads.

 The extrinsic impacts on walls and environs includes soil nail installation and grouting damage; stabilization treatment of contiguous wall crest and toe slopes; soil disturbance at contiguous paved areas at wall toe and wall crest; skin wall impact; and trenching damage of tree roots at wall crest and wall toe.

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 The stonewall tree factors include tree species; tree dimensions; tree biomass structure; tree lean; root growth habit; and quality of pruning work and tree maintenance.

5.6 Tree values, potential risks and preservation

 The value of stonewall trees is due to their status as world-class and unique urban ecological gem, spontaneous nature in built-up areas, multiple amenity functions to people, historical and heritage significance, and diminishing and threatened resource.

 The risk to local community is related to wall instability and tree instability; historical records indicate minimal contribution of tree failure to wall failure; in-depth understanding, assessment of tree risk coupled with preventive and ameliorative measures could minimize tree failures and their impacts.

 Tree preservation is favoured by site conditions with little conflicts with surrounding buildings and roads; welcomed ecosystem services bestowed on the neighbourhood; sentimental attachment of local community to the trees and hence support of tree preservation.

 Tree preservation is limited by the lack of space and anchor positions for cable bracing or installation of propping frame, and the need to puncture the heritage wall feature if the propping option is chosen.

5.7 Tree risk assessment and arboricultural recommendations

 T1: Small tree with few structural and decay problems; high live crown ratio; free from legacy of past unprofessional pruning; only requires minor corrective pruning.

 T2: Largest, oldest and most outstanding stonewall tree; significant height, crown spread and trunk diameter; well balanced and quite full crown development; one of the top-ranking stonewall trees in Hong Kong; designated as OVT; well-formed scaffold with three codominant trunks; multiple branch defects and wound decay induced largely by past poor pruning practice; some heavy epicormic load on decayed tipping wounds; excessive removal or loss of lower branches with implications on tree stability; no visible imminent high risk symptoms.

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 T3: Two of three trunks truncated with decayed tip; limited and confined crown development; extensive loss of limbs and branches; excessively loss of lower branches; low live crown ratio; liberal reinforcement of tree structure by bundles of lignified aerial roots; most branch loss wounds beset by decay; two heavy epicormic branches; no visible imminent high risk symptoms.

 T4: Twin stemmed tree with relatively stout trunks; heaviest lean of the six trees; low live crown ratio; confined crown development; wound decay at branch removal wounds; heavy epicormic branches attached to decayed tipping wounds; excessive removal or loss of lower branches; alarming risk due to incipient anchorage failure; needs support by cable bracing or propping.

 T5: Single trunk; high height to DBH ratio; low live crown ratio; narrow and confined crown; only two limbs and few branches; compression fork between two limbs; decayed wounds; alarming risk due to large basal cavity partly compensated by response wood at cavity edge, two lignified root stands and a natural root prop; needs support by cable bracing or propping.

 T6: Twin-stemmed tree; one trunk carrying the bulk of the crown load is heavily tilted; low live crown ratio; excessive removal or loss of lower branches; decayed branch loss wounds; risk due to unbalanced biomass distribution plus heavy lean; needs support by cable bracing.

5.8 Long-term practical tree maintenance measures

 Install soil strips at wall crest and toe to facilitate root growth to stabilize anchorage and increase supply of water and nutrients.

 Remove joint seals at suitable locations to permit new root penetration and existing root expansion, so that the continued increase in tree size and weight could be matched by corresponding increase in new roots.

 Prevent wedging effect on tree stability by regular removal of organic debris and rubbish deposited at the gap between trunk base and wall face.

 Stop repeated removal of lower branches and unprofessional and unnecessary pruning practice by demanding well-justified tree pruning proposals and reinforcing coaching of contractors.

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 Prevent extremely harmful impacts of excavation in the tree protection zone of stonewall trees, and establish strict guidelines on work inside the zone.

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