(榮譽)學位課程 Bachelor of Social Sciences (Hons) in Environment and Resources Management

EVALUATION OF THE ECOLOGICAL ROLE OF Acacia confusa IN URBAN ECOSYSTEMS IN HONG KONG BY LAI TAT WA STUDENT NO.:_14676990_

環境及資源管理社會科學學士 (榮譽)學位課程 BACHELOR OF SOCIAL SCIENCES (HONS) IN ENVIRONMENT AND RESOURCES MANAGEMENT

April/2016

畢業論文 PROJECT

EVALUATION OF THE ECOLOGICAL ROLE OF Acacia confusa IN URBAN ECOSYSTEM IN HONG KONG BY LAI TAT WA STUDENT NO.: 14676990

AN HONOURS PROJECT SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF

BACHELOR OF SOCIAL SCIENCES (HONOURS) IN

ENVIRONMENT AND RESOURCES MANAGEMENT

HONG KONG BAPTIST UNIVERSITY

April/2016

HONG KONG BAPTIST UNIVERSITY

April / 2016

We hereby recommend that the Honours Project by Mr LAI TAT WA entitled "Evaluation of the ecological role in the urban ecosystems in Hong Kong" be accepted in partial fulfilment of the requirements for the Bachelor of Social Sciences (Honours) in Environment and Resources Management.

Dr. K.L. Karen Chow

Chief Adviser Second examiner

Overall Grade :

iii

Acknowledgement

I would like to express my gratitude sincerely to my chief supervisor Dr. K.L.

Chow, Karen, for her supervision, guidance and kindly help. She was very patient and supportive to me in my entire study period these two years.

I wish to thank Dr. H.C. Chiu for his hard work for clarification of the requirement of the Honours Project.

______Student’s signature

Date: ______

Abstract

Acacia confusa was one of the exotic tree species commonly used for plantation in Hong Kong. The aim of this study was to evaluate the ecological role of Acacia confusa in urban ecosystems and impacts of urbanization on it.

9 study sites were selected. A. confusa in urban area did not lead to acidification of soils and not cause any effect on electrical conductivity of soils. The existence of it did not alter the invertebrates’ richness. The soil textural classes of the soil samples were dominated by loamy sands and sandy loam. A. confusa can make the soil to become suitable topsoil for cultivation.

The moisture content of soil increased with the level of urbanization, the most urbanized district (13.49%), the medium urbanized district (17.97%) and the least urbanized district (20.06%). A. confusa was threatened by urbanization.

In addition, there was a decreasing trend of soil temperature from the most urbanized district to the medium urbanized district and an increasing trend from medium urbanized district to the least urbanized district. Urbanization provides more available nutrients to A. confusa. A. confusa were generally aging. Most of the DBH of A. confusa recorded were larger than 24 cm. It ranged from 23.04 cm to 60.51 cm. There was the risk for these A. confusa to be infected by fungi and collapse. Moreover, there was a moderate negative

v correlation between the DBH and the health index of A. confusa.

Finally, Syzygium hancei, Schima superba and Castanopsis fissa were the three common native tree species recommended to replace the aging and aged

A. confusa due to their similar properties with A. confusa. (Total word count:

9735)

Table of Contents

Acknowledgement ...... iv Abstract ...... v Table of Contents ...... vii List of Tables ...... ix List of Figures ...... x Chapter 1: General Introduction ...... 1 1.1 Background of Acacia confusa ...... 1 1.2 Plantation History ...... 3 1.3 Characteristics ...... 4 1.4 Problems facing ...... 5 1.5 Hypothesis ...... 6 1.6 Objectives ...... 7 Chapter 2: Literature Review ...... 7 2.1 Overview ...... 8 2.2 Role of Acacia confusa ...... 8 2.3 Tree survey ...... 11 2.4 Impact of Urbanization on trees ...... 12 2.5 Aging problem ...... 15 Chapter 3: Methodology ...... 16 3.1 Study area ...... 16 3.1.1Study sites description ...... 16 3.2 In-situ inspection ...... 18 3.2.1 Soil sampling ...... 18 3.2.2 Tree analysis ...... 19 3.3 Laboratory analysis ...... 20 3.3.1 Soil texture ...... 20 3.3.2 Soil pH & Electrical conductivity ...... 21 3.3.3 Soil moisture ...... 22 3.4 Statistical analysis ...... 22 Chapter 4: Results and Discussions ...... 23 4.1 Ecological role of Acacia confusa ...... 24 4.1.1Soil properties ...... 24 4.1.1.1 Soil pH ...... 24 4.1.1.2 Soil electrical conductivity ...... 27 4.1.2 Effects of Acacia confusa on soil texture ...... 30 4.1.3 Number of invertebrates’ species ...... 34

vii

4.2 Urbanization problems of and effect on A. confusa ...... 38 4.2.1 Soil moisture ...... 38 4.2.2 Soil temperature ...... 42 4.3 Aging problems ...... 45 4.3.1 Health Index ...... 45 4.3.2 Diameter at breast height (DBH) ...... 47 4.4 The ecological role of A. confusa in urban ecosystems and ...... 50 Chapter 5: Conclusion ...... 53 5.1 Summary of findings ...... 53 5.2 Limitations of the study ...... 54 5.3 Future study recommendations ...... 55 Appendix ...... 57 References ...... 59

viii

List of Tables

Table 1. The general parameters of the A. confusa in different

sampling sites, the height, canopy width and frequency...... 23

Table 2. Data results for soil pH values and EC values of all

sampling points and the standard deviation ...... 29

Table 3. The soil texture classes of the 9 sampling sites and their

relative soil composition percentage ...... 32

Table 4. Data result of number of invertebrates’ species of different

sampling sites and the standard deviation ...... 36

Table 5. Data result of soil moisture of different sampling sites and

the standard deviation ...... 40

Table 6. Data result of soil temperature of different sampling sites

and the standard deviation ...... 44

Table 7. Data result of health index of A. confusa of different

sampling sites and the standard deviation ...... 46

Table 8. Data result of DBH of A. confusa of different sampling sites

and the standard deviation ...... 48

List of Figures

Figure 1. The distribution of the 9 sampling sites in Hong Kong .... 18

Figure 2. The comparison of pH value among the different sites and

the correlation between the frequency of A. confusa and the pH

values of the sites ...... 25

Figure 3. The comparison of EC values among the different sites and

the correlation between the frequency of A. confusa and the EC

values of the sites ...... 28

Figure 4. The comparison of percentage of sand, silt and clay

among all the sampling sites ...... 31

Figure 5. The comparison among the number of invertebrates

species found on A. confusa of different sites and the

correlation between the frequencies of A. confusa and the

number of invertebrates species found on it ...... 35

Figure 6. The comparison of soil moisture among all the sampling

sites and the trend of mean soil moisture content among the

three districts ...... 39

Figure 7. The comparison of soil temperature among all the

sampling sites and the trend of soil temperature from the most

urbanized district to the least urbanized district ...... 42

Figure 8. The comparison of health index of A. confusa among all

the sampling sites ...... 45

Figure 9. The comparison of DBH of A. confusa among all the

sampling sites ...... 47

Figure 10. The correlation between the DBH and the health index of

A. confusa...... 50

Chapter 1: General Introduction

1.1 Background of Acacia confusa

Acacia confusa which was commonly called Tai Wan Acacia was one of the very common exotic species. The species name of it was called Acacia confusa Merr. The explanation for the name, Acacia confusa was that Acacia means sharp point and confusa has the meaning of confusing due to the nature of its leaves. (Society, 2013) It has been deliberately introduced to Hong Kong since 1920s. (Corlett, 1999) It was a kind of evergreen shrub which was originated from Tai Wan and Philippines. For the classification, it belonged to the division of Magnoliophyta (flowering plants), the class of MagnoliopsidaIt, the subclass of Rosidae, the order of Fabales, the family of Fabaceae, the subfamily of Mimosoideae, the tribe of Acacieae, the genus of Acacia. The maximum height that it can grow can up to 15 meters. (Cheung, 2011) The shape of canopy was just like an umbrella. It was wide and thick and thus able to provide shading. Its leaves’ shape was shown to be sickle-shape. But actually, the sickle-shape leaves were not the real leaves of the tree. It was only petiole of the real leaves. So, the sickle-shape leaves were the fake leaves of the tree. The real leaves were pinnate compound leaves which only be

1 found during its seedling period. The real leaves degenerate and become petiole when they grow up. Although the petiole was not the real leaf, it still can carry out photosynthesis. The flowering period of Acacia confusa begins in April every year. During the flowering period, there will be a lot of little spherical yellow flowers hanging on the trees. Its fruiting period starts from

June until September. Its fruits were long rod-shape flattened pods. Inside the pods, 5 to 8 little brown seeds can be found. So, seed dispersal was the way of reproduction of Acacia confusa. The surface of stem of Acacia confusa was brown in colour and relatively smooth and without spines. Its diameter can achieve 1 meter width. The tree trunk of Acacia confusa was very useful for making pit prop, railway sleeper, agricultural instruments or firewood. (TaiBIF,

2001and Cheung 2011) Acacia possesses strong roots. It can hold the soil firmly and can even penetrate the rocks in the soil. (GARDEN, 2007) In seedling stage, it needs frequent irrigation, fertilization and weed control for well growth. Its survival rate of seedling was high even on barren land. As its roots can grow very deeply into the soil, it was very difficult to be transplanted.

(Society, 2013) Acacia confusa prefers tropical to semi-tropical climate. It was not suitable to be grown in prolonged flooding habitats. It prefers habitats with adequate amount of sunlight. So, the understory of Acacia confusa was not a

2 suitable habitat for its seedling to grow as its canopy was thick and wide. The amount of sunlight that can penetrate and reach to the ground was limited.

(GARDEN, 2007)

1.2 Plantation History

For the plantation history of Acacia confusa, after the first introduction in

1920s, it was later cut in a vast amount for making firewood during the occupation of Japan. As there was a lack of resources as well as the reduction of firewood supply from mainland, the Japanese continuously cut down a great amount of trees including Acacia confusa for fuel and timber. After the surrender of Japanese in 1946, the cutting of trees did not stop. The civil war in China led to a shortage of fuel again. There was a large demand for timber to be fuel wood. Until the end of wars, plantation works began again. During

1965 to 1997, the plantation aims of the government were water conservation, suppression and prevention of fire. Acacia confusa was starting to be planted increasingly at that time as firebreak. It was called “three gems” together with the Pinus elliottii and Lophostemon confertus during 1950-1960s as they were widely used for plantation. (Corlett, 1999 and Cheung 2011)

Till nowadays, Acacia confusa can be found easily in roadsides, country parks

3 or country trails in Hong Kong. There are currently around 150,000 Acacia confusa on the roadsides of Hong Kong. (Cheung, 2011) Apart from firebreak, it was widely used in horticulture, slope protection, making clothing and as windbreak as well.

Acacia confusa was pest free, but subject to the invasion of Ganoderma which was a kind of fungi. When there was a cut on the bark, it was more susceptible for it to be infected. Ganoderma was specifically well adapted to attack Acacia confusa. (GARDEN, 2007)

1.3 Characteristics

Due to its characteristics of fast-growing, drought resistant and resilient, it was still currently used for plantation purpose. It was one of the exotic species that was planted well with high degree of success. (Corlett, 1999) Not only this, its roots can convert nitrogen in the air into absorbable nitrogenous compounds which can make the soil more fertile and was thus beneficial to other plants or even animals. It possesses large ecological values. While the advantages provided by Acacia confusa were well-known, focus was put on urban ecosystem because urban area was greatly different from country parks and rural areas. Urban areas were subject to various kinds of interference like air

4 pollutants released from vehicles and blockage of wind by high rise buildings.

Moreover, the humidity of urban areas was very different from rural areas.

Therefore, the environment that Acacia grows in urban areas was different from that grows in rural areas or country parks. (Gregg et al, 2003)

1.4 Problems facing

The planted Acacia confusa were facing the problem of aging as the lifespan of Acacia confusa was only around 50 to 60 years. The first stage planting

Acacia confusa in the period of post-world war II was aging and becoming withered. It may cause the risks of invasion of fungi and collapsing. And the more important thing was that the canopy of exotic plant species was thick and wide that will block the sunlight to reach the native plant species. So, the growth of native species was hindered and thus the plantation area become monotonous and lack of biodiversity. Then, the native faunas were e not able to inhabit. In view of this, according to the Agriculture, Fisheries and

Conservation Department (AFCD), the government plans to carry out the

“Country Park Plantation Enrichment Project” to gradually reduce the usage of exotic trees and eradicate them in long term. Instead, the government uses native trees to enhance the performance of the plantation areas so as to

5 increase the biodiversity of both floras and faunas in that area. Thus, in long term, the project aims to enhance the sustainability and landscape of the plantation area and to reduce the risk of outbreak of pest. To determine a suitable native tree species to replace the exotic one, it needed to consider the environmental parameters of plantation area, such as soil pH, moisture content and texture etc. (AFCD, 2006) Actually, it was the same case in the urban areas. The Acacia confusa of plantation area in the urban areas like roadsides or urban parks a were re threatened by the risk of fungal infection and collapsing which may even cause threats to the live and property of the public.

Replacement of trees was necessary in urban areas. It was important to investigate the environmental factors in the urban areas.

1.5 Hypothesis

In this thesis, it was hypothesized that in the most urbanized district of Hong

Kong, the impacts towards Acacia confusa will be the most significant and serious. And the ecological role of Acacia confusa was plant species which will exert different effect to change of habitats and the species richness will be the lowest in the area of highest frequency of Acacia confusa. Apart from it, it was hypothesized the Acacia confusa in study area are facing the problems of

6 aging and the risks of fungal invasion.

1.6 Objectives

There is a lack of study on the ecological role of Acacia confusa in Hong

Kong. It was vital to know what the role of Acacia will be, and what kind of trees can be used to replace Acacia confusa as suitable native species need to be selected to replace it.

All in all, in a series of investigation, the aims of this dissertation were (1) to figure out the role of Acacia confusa in the urban ecosystem and (2) to investigate the effects of urbanization on Acacia confusa by examining the soil properties and the species richness that can be found on the trees, and (3) to figure out what species can be used to replace the aging Acacia confusa.

Finally, recommendations of some native plant species were provided for references. It can help the government to do the plantation more efficiently when choosing which native species to plant.

Chapter 2: Literature Review

2.1 Overview

There were many different kinds of study related to Acacia confusa have been done. In this part, it was to review some study that was related to this thesis.

Studies about the role of Acacia confusa, tree survey, impacts of Urbanization on trees and aging problem of Acacia confusa were reviewed. However, there are still research gap among these studies. So, this thesis aimed to fill some of the gaps.

2.2 Role of Acacia confusa

According to the paper released by Richard T. Corlett of University of Hong

Kong, Acacia confusa was an exotic species that has been introduced to Hong

Kong since 1928 form Tai Wan. As it was a leguminous tree, the roots of

Acacia confusa were covered with rhizobia, so it possessed the nitrogen-fixing function that made it able to convert the nitrogen in the soil into absorbable form of nitrogenous compound nutrients. Thus, it was a tree which can improve the soil quality by making the soil full of favorable nutrients. It made the poor soil to become fertile. It can grow quickly even on infertile and rough soil. It possessed small leaves and was not burnt easily. So, it can be used as firebreak. (Corlett, 1999 and Au, 2000 and Cheung, 2011)

In the study of Au Pui Sze, Acacia confusa can act as a role of soil acidification. With the higher the plantation age of the trees, the lower the pH value of the soil. (Au, 2000) There were mainly three ways for the soil to be acidified by trees. The first way was the process of metabolism carried out by the roots of trees to release organic acid like carbonic acid. (Kimmins, 1997)

The second way was the removal of the nutrients which were basic in nature.

And the base nutrients were stored in the trees. (Edwards, 1982) The third way was that during the tree litter decomposition, acidic humus was generated.

(Berden, 1987) In Au’s study, totally 6 plantation sites were chosen for investigation. 2 of the selected sites were borrow area, while the remained 4 sites were fire-affected sites. These 6 plantation sites were 6 sites of different plantation age ranged from 5- 35 years. Apart from it, the study also investigated the effect of A. confusa on soil texture of the sampling areas.

According to Agronomy Club of Purdue University, loamy sand, sand and sandy loam were described as coarse to moderately coarse. (Purdue University,

2015) In addition, in accordance with Dawn Pettinelli and Harvey D. Luce of

University of Connecticut, sandy loam and loamy sand were the two soil texture classes suitable for being topsoil. For loamy sands, although it has the drawback of possessing a low water-holding capacity, it was suitable to be

9 used for cultivation. Apart from it, loamy sands were compaction resistant, so it has the advantage on the areas that suffer from soil compaction because of the reasons of traffic or other related causes. For sandy loams, it was said that it was the optimum texture class for being topsoil that can supply adequate aeration to the roots of plants. Other texture classes, such as clays, silty clays, clay loams and sandy clay loams etc. were not suitable to be topsoil as they can provide inadequate aeration to plant roots and were low permeability to water. So, it makes them difficult to be used for cultivation. (Pettinelli and

Luce, 1914)

To fill up the research gap, it was important to study the effect of Acacia confusa on the soil pH in urban area of Hong Kong.

According to the investigation of Chang-Hung CHOU, it revealed that Acacia confusa can give an effect of allelopathy which will cause inhibition of radical growth of other plants. The inhibition effect came from the extraction of dry leaves of Acacia confusa. Actually, chemicals presented in the leaves of

Acacia confusa like ferulic and m-hydroxyphenylacetic acids were the main components to contribute to the inhibition effect. The test vegetations were

Chinese cabbage, alfalfa and lettuce. Concentration of 5% of extraction shows

10 a very significant of inhibition effect (above 85%). Apart from it, it was found that the total coverage and the diversity of plant species of understory of

Acacia confusa were lower than that of the normal grassland. (CHOU and FU,

1998) Thus, Acacia confusa did hinder the growth of other plants near it. The investigation was carried out in Tai Wan. There was no any such kind of investigation in Hong Kong at the time. And there was no related research has been done on the relationship of Acacia confusa and invertebrates.

Besides, not only the roots of the tree can hinder the growth of understory plant species, the young rod-shape flattened pods and seeds of Acacia confusa were poisonous to faunas like birds and mammals including human. (TaiBIF,

2001)

2.3 Tree survey

In this investigation, it aimed at seeing the performance of Acacia confusa in urban areas. Similar investigations have been done by Croucher Institute for

Environmental Sciences of Hong Kong Baptist University. They have worked on ecological monitoring of recovered landfill in South East New Territories.

(Wong, 2015) They were currently analyzing the soil quality of the recovered

11 landfill. They test the level of heavy metals, nitrogen and phosphorus and also other physical parameters. Transect and quadrats were used for different sampling points. They also monitored the performance of the pioneer plant species, like Acacia confusa, within the area of the restored sanitary landfill.

(Chen, 2015)

It was important to analyze the soil samples in the roadsides as their there were different interferences caused by the urban factors like vehicles. So, in this research, it has filled up the gap that the Acacia confusa in the urban areas.

2.4 Impact of Urbanization on trees

Due to the development in the urban area, there were a lot of impacts caused by urbanization to trees. According to the study of Chi Yung Jim, conflicts do exit between urbanization and trees. First, developments of urban area have increased the area of coverage of concrete surface which was impervious.

Secondly, the underground utilities like TV cables and telecommunication cables have been developed greatly. As a result, it occupies the space for the growth of trees and damage of roots exits during the construction works.

Thirdly, the changes of land use have reduced the green space. The planting

12 space was thus reduced and limited. Improvement and construction works of roads cause damage to the roadsides trees. In addition, constraints led by physical and chemical factors from building have caused the poor performance of trees chronically. Physically, it revealed that urban soil possesses the characteristics of compaction which caused inadequate capacity for air and moisture for growth of plants. Simultaneously, the soil temperature of urban soil was said to be high. According to Phillip J. Craul’s study, higher soil temperature can lead to the increase of reaction rate and biological processes in the soil horizon. Thus, the decomposition rate of organic matter will be increased. It enhances the weathering process of soils. In other words, there will be more available nutrients to be absorbed by roots. (Craul, 1985)

Chemically, the capacity of nutrient-holding was limited for both micro and macro nutrients. Contamination of soil by the fill materials was also the problem faced by urban soil. It was not quite healthy for trees. This study has reviewed the changes of growth of urban area in Hong Kong and trees since the early period of Hong Kong. (Jim, 1998) Similar stud has not been done on

Acacia cofusa. So, this thesis was important to show what urbanization caused problems Acacia confusa are facing in the urban in Hong Kong

In the study of Jian Sheng Liang, it investigated that whether the response of roots of trees to soil compaction and drying can indicate the performance of tree that grow in landfills. It used the seedlings of two tree species, Acacia confusa and Litsea glutinosa to investigate the responses. It aimed to make explanation for the poor performance of some tree species in the restored landfills in Hong Kong. It spotted the differences of the performance of the two species through regulating compaction and the moisture content of the soil.

There were totally four treatments of soil including well-watered and non-compacting, well-watered and compacting, unwatered and non-compacting and unwatered and compacting. It revealed that when facing soil drying, the cell membrane of roots of Acacia confusa was damaged more than that of Litsea glutinosa, while soil compaction had severer effect on

Litsea glutinosa than Acacia confusa. Soil drying also had greater on hydraulic conductivity of roots of Litsea glutinosa than in Acacia confusa. Root water potential and osmotic potential dropped much slower in Litsea glutinosa. (Jian

Sheng Liang et al. 1999) This study was done on the focus of landfill sites. It was important to know soil condition of urban areas.

2.5 Aging problem

For the problem of aging of Acacia confusa, a thesis has worked on spectral analysis and life procession of Acacia confusa by the Horticulture College of

Fujian Agriculture and Forestry University. They made use of the life table of population and the theory of survival analysis to draw a static table, the curve of mortality rate, survival rate, killing rate, survival rate function, mortality rate function, cumulative mortality rate and hazard rate function. In addition, the life process of population has been analyzed. They divided the Acacia confusa in a certain area into different age grades (I, II, III, IV, V, VI, VII and

VIII) according to the diameter of the tree trunk. And each age grade interval was defined by 4 cm. They found that Acacia confusa of age grade IV, VI and

VII have the highest mortality rate and disappearance rate. Acacia confusa age grade VII was said to have highest mortality was due aging and abiotic factors like soil quality, temperature and moisture etc. So, it meant that age grade VII and VIII Acacia confusa (with tree trunk diameter larger than 24-28 cm) can be defined as an old tree with the high risk of death. (YAN, 2005)

According to the Hong Kong Tree Society, the symptoms of aging of Acacia confusa can be tree bark peeling, small amount of leaves and withered branch.

So, all these were the indicators for examining whether an Acacia confusa was aging. (Society, 2013)

Chapter 3: Methodology

3.1 Study area

3.1.1Study sites description

Nine sites were chosen for this study. Acacia confusa in nine different areas within three districts, Sha Tin District, Central and Western District and

Islands District with the ascending order of population was basically compared in the research. For Sha Tin District, it was the most populated district in Hong

Kong, 660200 residences in 2015. For Central and Western District, it was the district with medium population in Hong Kong, 246600 residences in 2015.

For Islands District, it was the district least populated district in Hong Kong,

146 900 residences in 2015. (Census and Statistics Department, 2015) These three districts were chosen for this study to indicate the area with the most urbanization level, medium urbanization level and the least urbanization level.

In Sha Tin, samples were collected at the each of following sites: cycling trial

16 besides Shing Mun River (SMR), Ma On Shan (MOS) and Fo Tan (FT). In

Central and Western District, samples were collected at each of the following sites: Pok Fu lam (PFL), Lung Fu Shan (LFS) and The Peak (P). In Island

District, samples were collected at each of the following sites: Lantau Island

(LT), Lamma Island (LM) and Cheung Chau (CC).

Comparisons were done to contrast the differences between the Acacia in nine urban areas of three districts of their soil properties according to their amount of population. One was the highest, one was the medium and the other one was the lowest. The problems to be addressed were whether the interference of the urban areas will affect the functions of Acacia. Moreover, if they were affected, what the effects will be.

Figure 1. The distribution of the 9 sampling sites in Hong Kong.

3.2 In-situ inspection

3.2.1 Soil sampling

For in-situ inspection, three soil samples were collected in each area. The three samples were collected at around 300 m interval. At each sampling point,

300-400 g of top soil (0-5cm depth) was collected by using scoop. The samples were contained by zip locks in order to keep the soil samples in an undisturbed condition. Soil temperature in different sampling points was measured as well. It was measured by using thermometer. The thermometer 18 was inserted into the soil with a depth of 10cm. The soil temperature at each sampling point was recorded down by log sheet. Totally, there were 27 soil samples.

Apart from it, the frequencies of Acacia confusa in each point were recorded.

3.2.2 Tree analysis

Tree data including the height of trees, the canopy diameter, the health index, the diameter at breast height (DBH) of each sampling point were measured and the presence of invertebrates were measured. The height of the trees and the width of canopy of trees were measured by estimation of eyes. The health index of the trees was estimated according to the peeling condition of the barks of the trees and the amount of leaves and withered branches. For the

DBH of the trees, circumferences of tree trunk at human breast height were measured by using measuring tape first. Later, the circumferences of the tree trunk were converted to diameter by calculation. The presences of invertebrates on the trees were photo taken by using camera. The numbers of species on each tree of each sampling point were recorded down to indicate the species diversity of each area.

3.3 Laboratory analysis

The soil samples were all moved to the laboratory. Physical parameters of the soil, pH, electrical conductivity, soil moisture were analyzed. It aimed to make a comparison of the quality of soil of the three districts mentioned above. Then, analysis can be made to see whether there was the most urbanized district will lead to poorer soil. All the unwanted materials like leaves and stones trapped inside the soil samples were removed. The 27 soil samples were transferred into 27 aluminum foil containers. Corresponding sampling sites were marked on the containers. The 27 soil samples were dried inside the oven at 75oC for seven days.

3.3.1 Soil texture

Each dried sample was ground with mortar and pestle. Around 100g of each ground samples were weighed on the mass balance. The weighed soil samples were sieved by using the 0.5mm sieve for separating the gravel and the remained plant materials. Each of the soil samples was passed through the

0.212mm sieve and 0.055mm sieve respectively. In each soil samples, the weight of the portion that cannot pass through the 0.212mm sieve was measured on mass balance. It was recorded as the sand proportion of the soil

20 samples. The weight of the portion that cannot pass through the 0.055 sieve was measured on mass balance. It was recorded as the proportion of silt of the soil samples. The weight of the proportion of the soil samples that can pass through the 0.055mm sieve was measured on mass balance. It represents the proportion of clay of the soil samples. All the data collected was recorded down on log sheet. The percentage of the proportion of sand, silt and clay of each soil sample was calculated. The soil texture of each of the soil samples was known by the reference of the soil texture triangle. (Bowner, 1983)

The portion of each soil samples that can pass through 0.212mm sieve was collected and transferred into zip lock for later use.

3.3.2 Soil pH & Electrical conductivity

Soil pH of each soil sample was measured by adding distilled water into 5g of

0.212mm dried soil sample. The ratio of water to soil was 5:1. The mixture of water and soil were shaken well for eight to ten minutes. The mixtures were left to stand for a whole night for precipitation before were being tested with pH meter. The data was collected and recorded down on log sheet. Electrical conductivity was then measured followed the soil pH by using EC meter.

3.3.3 Soil moisture

Around 100g of each fresh soil sample was weighed by using mass balance.

The weight of each aluminum foil container was measured as well. The soil samples were transferred into the containers and were all put into the oven at

75oC for at least seven days for drying. After seven days, the weight of the dried soil samples with aluminum foil containers was measured by using mass balance. The moisture content of each soil sample was calculated by the equation:

Moisture content= [weight of soil sample before drying- (weight of dried soil sample- weight of aluminum foil containers)]/ weight of soil sample before drying x 100

The obtained data was recorded down on log sheet.

3.4 Statistical analysis

All the data was entered into Microsoft Excel for data processing. The mean value and standard deviation of the parameters were calculated and obtained.

Later, the obtained data from Microsoft Excel were entered into SPSS 16 statistical package for further statistical analysis. Normality test was carried

22 out each time for testing whether the variables were in normal distribution.

Duncan's Multiple Range Test was used for parameters with normal distribution to calculate the significance statistical difference among all the sampling sites. Kruskal-Wallis H Test was used for parameters with abnormal distribution to calculate the significance statistical difference among all the sampling sites. Pearson and Spearman rank correlation were used to correlate the number of species found on the trees and the frequency of Acacia confusa of different sites. At the same time, correlation was made on the frequency of

Acacia confusa and pH value of the soil samples for different sampling sites, frequency of Acacia confusa and EC value of the soil samples for different sampling sites, the correlation between DBH and health index of Acacia confusa for different sampling sites.

Chapter 4: Results and Discussions

Table 1. The general parameters of the A. confusa in different sampling sites, the height, canopy width and frequency. n=27 Frequency of Site/ parameters Height (m) Canopy (m) A. confusa

SMR 1 5 3 SMR 2 6 9

SMR 3 5 4 63 FT1 5.5 5 FT 2 3 4 FT 3 4.5 2 6 MOS 1 5.5 4 MOS 2 5.5 3.5 MOS 3 5 3 330 PFL 1 5.5 4.5 PFL 2 5 6 PFL 3 5 6 40 LFS 1 4 3 LFS 2 4 6 LFS 3 4.5 2.5 10 P1 4.5 5 P 2 5 6 P 3 5.5 7 17 LT 1 5.5 4.5 LT 2 3.5 4 160 LT 3 6 4 CC 1 6 10 CC2 5 3.5 29 CC3 4.5 10 LM 1 4 3 LM 2 5 4 LM 3 4.5 3.5 120 4.1 Ecological role of Acacia confusa

4.1.1Soil properties

4.1.1.1 Soil pH

(a)

6 a 5

4 pH 3 pH Mean 2

0 SMR Fo Tan MOS PFL LFS P LT CC LM Site

(b)

Figure 2. The comparison of pH value among the different sites (a) and the correlation between the frequency of A. confusa and the pH values of the sites (b).

The columns of data with no difference in letter mean that there was no 25 significant difference. Standard deviations were shown as error bars.

For soil pH value, in Figure 2a, a Kruskal-Wallis H test showed that there was not a statistically significant difference (p > 0.05) in soil pH among the different sites, χ2 (2) = 9.354, p =0.313, with a mean rank soil pH of 16.00 for

SMR, 17.67 for FT, 6.00 for MOS, 17.33 for PFL, 15.00 for LFS, 19.33 for P,

16.00 for LT, 5.67 for CC and 13.00 for LM. It represented that the pH values among all the sites were more or less the same. It might be due to the small size of the samples. Only 3 samples were collected for each site. So, the difference cannot be figured out. The soils of nine sites show an acidic condition which ranges from 4.21 to 5.14 (Figure 2, Table 2). It can be ascribed to the acidification effect of A. confusa as the results obtained by

Tanner, Edwards, Grubb, Kimmins and Berden. (Tanner 1977, Edwards,

Gmbb 1982, Kimmins 1987 and Berden et al. 1987) There was also the chance of acid rain to account for the low pH value in the soils. According to the data provided by the two EPD’s acid rain monitoring stations in Kwun

Tong and Central Western, the average pH value of rain water in Hong Kong from 1993 to 1999 was 4.85. (EPD, 2000) (See Appendix 2) It was near to the mean of the soil pH (4.75) of all the soil samples. It indicates that the effect of

26 soil acidification of A. confusa in urban areas was not justified. The most reasonable explanation was that the soil pH was subjected to the effect to acidification deposition in Hong Kong.

Figure 2b shows the correlation between the frequencies of A. confusa in different sites and the pH values of soils of different sites. (p = 0.357, n =9, r=

-0.350). There was no significant correlation between the two variables (p >

0.05). It meant the changes of pH values of soils in different sites were unrelated to number of A. confusa.

So, from this part, we can draw a conclusion that the acidic conditions in the soils of the samples were not related to the existence of A. confusa.

4.1.1.2 Soil electrical conductivity

(a)

0.8 0.7 0.6 0.5

EC 0.4 a 0.3 EC Mean 0.2 0.1 0 SMR Fo Tan MOS PFL LFS P LT CC LM Site

(b)

Figure 3. The comparison of EC values among the different sites (a) and the correlation between the frequency of A. confusa and the EC values of the sites (b).

The columns of data with no difference in letter mean that there was no significant difference. Standard deviations were shown as error bars.

For EC value, in figure 3a, a Kruskal-Wallis H test showed that there was not a statistically significant difference (p > 0.05) in soil EC among the different sites, χ2 (2) = 12.284, p =0.139, with a mean rank soil EC of 17.17 for SMR,

22.83 for FT, 15.00 for MOS, 7.33 for PFL, 11.33 for LFS, 5.33 for P, 11.00 28 for LT, 18.17 for CC and 17.83 for LM. It represents that the EC values among all the sites were more or less the same. The soils of the nine sites showed a low value of electrical conductivity. It ranged from 0.05 to 0.71 mS (Figure 3,

Table 2).

In Figure 3b, there was not any correlation between frequencies of A. confusa and EC values of soils of different sites, (p = 0.49, n =9, r = -0.266). There was no significant difference between the two variables (p > 0.05). It implied the changes of EC values of soils in different sites were unrelated to number of

A. confusa. It meant that there was no effect on soil EC by A. confusa.

Table 2. Data results for soil pH values and EC values of all sampling points and the standard deviation. (Mean ± SD) n=27.

pH Mean EC Mean Site/ parameters pH (SD) EC (mS) (SD)

SMR 1 4.38 0.3 0.21a 4.74a SMR 2 4.6 0.23 (0.107) (0.447) SMR 3 5.24 0.09 FT1 6.15 0.25 0.38a 5.04a FT 2 4.42 0.71 (0.288) (0.966) FT 3 4.54 0.18 MOS 1 4.22 0.15 0.13a 4.25a MOS 2 4.16 0.11 (0.021) (0.108) MOS 3 4.37 0.14 29

PFL 1 5.28 0.09 0.10a 5.05a PFL 2 4.25 0.08 (0.021) (0.717) PFL 3 5.63 0.12 LFS 1 4.39 0.08 0.13a 4.53a LFS 2 4.55 0.19 (0.057) (0.127) LFS 3 4.64 0.11 P1 4.43 0.12 0.07a 5.07a P 2 5.87 0.05 (0.040) (0.733) P 3 4.91 0.05 LT 1 4.79 0.1 0.11a 5.14a LT 2 6.56 0.11 (0.015) (1.286) LT 3 4.06 0.13 CC 1 4.29 0.71 0.33a 4.21a CC2 4.17 0.1 (0.329) (0.067) CC3 4.18 0.19 LM 1 5.78 0.12 0.24a 4.74a LM 2 4.53 0.13 (0.199) (0.957) LM 3 3.9 0.47

The numbers share the same letter (a) represents that there was no significant difference (p > 0.05) by Kruskal-Wallis H Test.

Values in brackets indicate the standard deviation.

4.1.2 Effects of Acacia confusa on soil texture

120

a 100 ab abcd bcd abc 80 cd d

60 sand

silt

Percentage (%) Percentage 40

clay 20 c

0 SMR FT MOS PFL LFS P LT CC LM Site

Figure 4. The comparison of percentage of sand, silt and clay among all the sampling sites.

Differences in letters (a, b and c) indicated significant difference among all the sites at the level of P < 0·05 (Duncan test). While, no difference in letters represented there was no significant difference among all the sites. Standard deviations were shown as error bars.

From Figure 4, there was significant difference between the sites having the highest percentage and the lowest percentage of sand (at Cheung Chau and

Lung Fu Shan respectively); while there was a less obvious difference among the sites having medium range percentage of sand. f = 3.005, p = 0.025

(Duncan test). For silt content, there was significant difference between the 31

sites having the highest percentage and the lowest percentage of silt; while

there was a less obvious difference among the sites having the middle-range

percentage f= 2.686, p= 0.039 (Duncan test). For clay content, there was no

significant difference among the different site. χ2 (2) = 9.291, p =0.318

(Kruskal Wallis Test), with a mean rank clay content of 21.67 for SMR, 7.33

for FT, 13.00 for MOS, 15.33 for PFL, 16.00 for LFS, 16.33 for P, 10.33 for

LT, 7.33 for CC and 18.67 for LM. Mean and Standard deviation of sand, silt

and clay were summarized in Table3.

Table 3. The soil texture classes of the 9 sampling sites and their relative soil composition percentage. (Mean ± SD), n=27

Site Sampling Sand (%) Mean Silt (%) Mean Clay (%) Mean Soil texture class points (SD) (SD) (SD) SMR 1 76.023 20.356 3.621 Loamy sand 2 78.493 74.65bcd 16.455 20.75abc 5.053 4.60a Loamy sand 3 (4.68) (4.50) (0.85) Sandy loam 69.435 25.434 5.131

FT 1 82.154 15.742 14.91cd 2.105 Loamy sand 82.76abc 2.33a 2 83.459 14.072 (0.83) 2.469 Loamy sand (0.66) (0.02) 3 82.655 14.917 2.429 Loamy sand MOS 1 82.013 16.082 1.905 Loamy sand 85.18ab 11.40c 3.42a 2 82.378 11.748 5.874 Loamy sand (5.18) (4.87) (2.15) 3 91.162 6.362 2.476 Sand PFL 1 82.356 13.713 21.45abc 3.931 3.00a Loamy sand 2 72.852 75.55abcd 24.569 (6.74) 2.578 (0.81) Loamy sand 3 71.439 (5.94) 26.063 2.497 Sandy loam LFS 1 66.056 69.60d 31.758 26.63a 2.186 3.77a Sandy loam

2 69.556 (3.57) 27.023 (5.34) 3.421 (1.78) Sandy loam 3 73.190 21.106 5.704 Sandy loam Peak 1 71.755 73.19cd 18.085 21.70abc 10.160 5.10a Sandy loam 2 80.286 (6.50) 17.514 (6.77) 2.200 (4.39) Loamy sand 3 67.531 29.515 2.954 Sandy loam LT 1 74.113 73.67cd 23.986 23.75ab 1.901 2.58a Loamy sand 2 71.394 (2.09) 26.129 (2.51) 2.477 (0.74) Sandy loam 3 75.501 21.128 3.370 Loamy sand CC 1 94.025 85.86a 5.047 12.19c 0.928 1.95a Sand 2 73.189 (11.13) 23.364 (9.80) 3.446 (1.33) Sandy loam 3 90.373 8.162 1.465 Loamy sand LM 1 87.087 80.55abcd 10.010 15.39cd 2.903 4.06a Loamy sand 2 74.830 (6.17) 18.922 (4.74) 6.248 (1.90) Sandy loam 3 79.733 17.241 3.025 Loamy sand

Values in brackets indicate the standard deviation.

The soil texture of each of the soil samples was determined with the reference

of U.S. Department of Agriculture Classification System. (Appendix 1) The

soil samples were classified as loamy sand, sandy loam and sand. Among all

the sites, the soils were mainly dominated by loamy sand and sandy loam (15

and 10 out of 27 sampling sites respectively). In Sha Tin District, it was loamy

sand dominated. 7 out of 9 sampling points were loamy sand. The fraction of

clay among all the soil samples was relatively poor. It ranges from 0.928 % at

sampling point 1 of Cheung Chau to 10.160 % at sampling point 1 of the Peak.

The range of silt fraction of all the sites was from 5.047 % at sampling point 1

33 of Cheung Chau to 31.758 % at sampling point 1 of Lung Fu Shan. Among all the sites, the proportion of sand was the richest which all exceed 65%. It ranges from 67.531 % at sampling point 3 of the Peak to 94.025 % at sampling point 1 of Cheung Chau.

From the result shown in Table 3, the dominance of loamy sands and sandy loams textural classes revealed that A. confusa was actually contributing to the soil texture. According to Pettinelli and Luce, loamy sands and sandy loams do suitable for cultivation. It can be said that A. confusa can improve the soil texture which was suitable to be topsoil. Of course, the soil texture were dominated by the two type of textural classes can be due to the usage of ideal topsoil (loamy sands and sandy loams) which means the soil textures of the sites were originally ideal at the beginning.

4.1.3 Number of invertebrates’ species

(a)

4.50

4.00

3.50

3.00

2.50

2.00 No. of invertebrates species

Arbitrary unit Arbitrary 1.50

1.00

0.50

0.00 SMR FT MOS PFL LFS P LT CC LM Site

(b)

Figure 5. The comparison among the number of invertebrates’ species found on A. confusa of different sites (a) and the correlation between the frequencies of A. confusa and the number of invertebrates’ species found on it (b).

From Figure 5a, a Kruskal-Wallis H test showed that there was not a statistically significant difference (p > 0.05) in number of invertebrates’ species found on A. confusa among the different sites, χ2 (2) = 11.380, p

=0.181, with a mean rank number of invertebrates species found on A. confusa of 17.67 for SMR, 7.00 for FT, 12.33 for MOS, 19.67 for PFL, 17.17 for LFS,

7.00 for P, 9.83 for LT, 14.83 for CC and 20.50 for LM.

Figure 5b, there was not a statistically significant correlation between the frequencies of A. confusa and the number of species found on it. (p= 0.799, n

=9, r= -0.100) So, there was no correlation between the frequencies of A. confusa and the number of species found on it. It indicated that the change in the frequencies of A. confusa will not cause any change in the number of species found on it. However, it did not reach significance may be due to the small size of samples (3 for each site) which might lead to a misleading result.

(Students, 2008) The mean and standard deviation of number of invertebrates species found on the A. confusa were shown in Table 4.

Table 4. Data result of number of invertebrates’ species of different sampling sites and the standard deviation. (Mean ± SD), n=27

Location No. of invertebrates Mean species (SD)

SMR 1 2 1.67a SMR 2 1 (0.58) SMR 3 2 FT1 1 0.33a FT 2 0 (0.58) FT 3 0 MOS 1 2 1.00a MOS 2 0 (1.00) MOS 3 1 PFL 1 1 2.33a PFL 2 2 (1.53) PFL 3 4 LFS 1 2 2.00a LFS 2 4 (2.00) LFS 3 0 P1 0 0.33a P 2 1 (0.58) P 3 0 LT 1 0 0.67a LT 2 0 (1.15) LT 3 2 CC 1 1 1.33a CC2 1 (0.58) CC3 2 LM 1 2 2.00a LM 2 2 (0.00) LM 3 2

The numbers with the same letter (a) represented that there was no significant difference (p > 0.05) by Kruskal Wallis test.

Values in brackets indicate the standard deviation.

4.2 Urbanization problems of and effect on A. confusa

4.2.1 Soil moisture

(a)

20 Mean of soil moisture

Percentage % 15

0 SMR FT MOS PFL LFS P LT CC LM Site

(b)

Mean soil moisture Percentage (%)

0 Sha Tin District Central and Islands District Western District District

Figure 6. The comparison of soil moisture among all the sampling sites (a) and the trend of mean soil moisture content among the three districts (b).

Standard deviations were shown as error bars.

In Figure 6a, a Kruskal-Wallis H test showed that there was a statistically significant difference (p < 0.05) in soil moisture among the different sites, χ2

(2) = 19.185, p = 0.014, with a mean rank soil moisture of 5.33 for SMR, 5.33 for FT, 17.33 for MOS, 19.00 for PFL, 22.00 for LFS, 5.00 for P, 16.67 for LT,

13.33 for CC and 22.00 for LM. As the data were abnormally distributed, there was no letter used to differentiate them. The mean and standard deviation were shown in Table 5. The ascending order of the soil moisture was P (9.66) <

SMR (10.81) < FT (10.93) < CC (15.77) < MOS (18.72) < PFL (19.72) < LM

(21.40) < LT (23.02) < LFS (24.53).

Refer to Figure 6b and Table 5, it showed the average soil moisture content within the district, Sha Tin District shows the lowest moisture content (13.49

%), Central and Western District has the medium soil moisture content

(17.97%) and Island District has the highest average soil moisture content

(20.06%). It represented that the soil moisture content in the most urbanized district was the lowest. According to the study of Liang, A. confusa will subject to the effect of soil drying. It damaged the root cell membrane of A. confusa. (Jian Sheng Liang et al. 1999) So, from this part, we know that urbanization was damaging the root cells of A. confusa. Moreover, in high-level urbanized areas in Hong Kong, A .confusa was at the risk of damage of root cells.

Table 5. Data result of soil moisture of different sampling sites and the standard deviation. (Mean ± SD), n=27. ST: Sha Tin District, CWD: Central and Western District, ID: Islands District.

Location Soil moisture % Mean Mean of (SD) Districts SMR 1 12.85 10.81 SMR 2 9.73 STD (7.51) SMR 3 9.84 13.49 FT1 11.16 10.93 FT 2 10.89 (7.58)

FT 3 10.74 MOS 1 17.67 18.72 MOS 2 23.75 (7.59) MOS 3 14.75 PFL 1 17.24 19.72 PFL 2 18.90 (8.08) PFL 3 23.02 LFS 1 18.82 CWD 24.53 LFS 2 32.13 17.97 (8.82) LFS 3 22.62 P1 6.43 9.66 P 2 11.55 (8.92) P 3 11.00

LT 1 16.74 23.02 LT 2 11.66 (8.78) LT 3 40.66 CC 1 18.66 15.77 CC2 12.21 (4.42) ID CC3 16.44 20.06 LM 1 19.48 21.40 LM 2 18.98 (3.78) LM 3 25.76

There was a significant difference of soil temperature among the sites. As the data were abnormally distributed, there was no letter used to differentiate them.

Values in brackets indicate the standard deviation.

4.2.2 Soil temperature

(a)

20 18 16 14 12 10 8 Mean of soil temperature

DegreeCelsius 6 4 2 0 SMR FT MOS PFL LFS P LT CC LM Site

(b)

20 18 16 14 12 10 8 Mean of soil temperature

Degree CelsiusDegree 6 4 2 0 SMR FT MOS PFL LFS P LT CC LM Site

Figure 7. The comparison of soil temperature among all the sampling sites (a) and the trend of soil temperature from the most urbanized district to the least urbanized district (b).

Standard deviations were shown as error bars.

In Figure 7a, a Kruskal-Wallis H test showed that there was a statistically significant difference (p < 0.05) in soil temperature among the different sites,

χ2 (2) = 22.39, p = 0.004, with a mean rank soil temperature of 23.83 for SMR,

20.67 for FT, 22.83 for MOS, 15.17 for PFL, 3.83 for LFS, 6.67 for P, 8.67 for

LT, 7.17 for CC and 17.17 for LM. As the data were abnormally distributed and they were significantly different, no letter was used to differentiate them.

Table 6 has summarized the mean and standard deviation of the data.

In Figure 7b, the soil temperatures in SMR, FT and MOS were relatively high

(17.33% to 17.83%). Then it started to drop in PFL, LFS and P (13.33% to

14.17%). It later increased a bit from LT, CC and LM (14.67% to 16.83%). So, the trend first dropped and then increased a little bit. The most urbanized district has the highest average soil temperature. But, it did not show a negative relationship from the medium urbanized district to the least urbanized district. It may due to there were more residential buildings found beside the sampling site in the least urbanized district. To conclude this part, the most urbanized district has a highest average soil temperature, it meant that the A. confusa in this district have the most abundant available nutrients to absorb.

(Craul, 1985) So, urbanization can bring more nutrients to A. confusa.

Table 6. Data result of soil temperature of different sampling sites and the standard deviation. (Mean ± SD), n=27

Location Soil temperature (℃) Mean

(SD) SMR 1 18 17.83 SMR 2 18 (0.29) SMR 3 17.5 FT1 17 17.33 FT 2 18 (0.58) FT 3 17 MOS 1 18 17.67 MOS 2 18 (0.58) MOS 3 17 PFL 1 15.5 16.33 PFL 2 16.5 (0.76) PFL 3 17 LFS 1 14 13.33 LFS 2 12.5 (0.76) LFS 3 13.5 P1 14 14.17 P 2 15 (0.76) P 3 13.5

LT 1 14 14.67 LT 2 16 (1.15) LT 3 14

CC 1 15 14.17 CC2 12.5 (1.44) CC3 15 LM 1 17.5 16.83 LM 2 16.5 (0.58) LM 3 16.5

There was a significant difference of soil temperature among the sites.

However, the data were abnormally distributed, so no letter was used to differentiate them.

Values in brackets indicate the standard deviation.

4.3 Aging problems

4.3.1 Health Index

9 ab 8 abc 7 bcd cd 6 d 5

Mean of health Arbitrary unit Arbitrary 3 index 2 1

0 SMR FT MOS PFL LFS P LT CC LM Site

Figure 8. The comparison of health index of A. confusa among all the sampling sites.

Different letters (a, b, c and d) indicate significant difference between sites at the level of P<0·05 in Duncan range test. Standard deviations were shown as

45 error bars.

In Figure 8, it showed a significant difference among the different sites. f =

5.625, p = 0.001 (Duncan test). There was a significant difference between the largest health index and the smallest health index of A. confusa; while the health index of A. confusa in medium range have less obvious significant difference. Table 7 has shown the mean and standard deviation of the health index of A. confusa of different sampling sites.

Table 7. Data result of health index of A. confusa of different sampling sites and the standard deviation. (Mean ± SD), n=27 Location Health index Mean (SD) SMR 1 8 7.33a SMR 2 7 (0.58) SMR 3 7 FT1 8 7.00ab FT 2 7 (1.00) FT 3 6 MOS 1 7 MOS 2 7 6.67abc MOS 3 6 (0.58) PFL 1 4 4.33d PFL 2 5 (0.58) PFL 3 4 LFS 1 4 4.33d LFS 2 5 (0.58) LFS 3 4 P1 5 4.67d

P 2 3 (1.53) P 3 6 LT 1 5 5.67bcd LT 2 6 (0.58) LT 3 6 CC 1 4 4.67d CC2 6 (1.15) CC3 4 LM 1 5 5.33cd LM 2 6 (0.58) LM 3 5

The numbers with different letter (a, b and c) represented that there was a significant difference (p < 0.05) by Duncan range test.

Values in brackets indicate the standard deviation.

4.3.2 Diameter at breast height (DBH)

60 a ab 50 bcd cd 40 d Mean of DBH

30 Diameter(cm)

0 SMR FT MOS PFL LFS P LT CC LM Site

Figure 9. The comparison of DBH of A. confusa among all the sampling sites. 47

Different letters (a, b, c and d) indicated significant difference between sites at the level of P<0·05 in Duncan range test. Standard deviations were shown as error bars.

Figure 9 showed the differences of health index of A. confusa among the different sampling sites. There was a significant difference between the thickest DBH and the thinnest DBH of A. confusa; while the DBH of A. confusa in medium range have less obvious significant difference. From the result shown in Figure 9, all the A. confusa of all the sampling sites have average DBH above 24 cm, except LFS (averagely 23.04 cm). It indicated that the ages of A. confusa in the sampling sites were mostly over the age grade IV

(DBH 24-28 cm) (YAN, 2005) Table 8 has shown the data of DBH of A. confusa of different sampling sites. So, it meant that A. confusa in the sampling sites are facing the aging problems and are threatened by fungal invasion and have the risk of collapsing.

Table 8. Data result of DBH of A. confusa of different sampling sites and the standard deviation. (Mean ± SD), n=27 Location DBH (cm) Mean (SD)

SMR 1 37.58 38.22bcd SMR 2 39.81 (1.39) SMR 3 37.26 FT1 41.72 27.92cd FT 2 27.39 (13.54) FT 3 14.65 MOS 1 40.76 43.31abc MOS 2 38.22 (6.74) MOS 3 50.96 PFL 1 50.96 60.51a PFL 2 57.32 (11.48) PFL 3 73.25 LFS 1 23.57 23.04d LFS 2 30.57 (7.82) LFS 3 14.97 P1 36.94 46.92ab P 2 50.96 (8.69) P 3 52.87 LT 1 17.20 26.86cd LT 2 21.34 (13.31) LT 3 42.04 CC 1 66.88 55.94a CC2 46.18 (10.40) CC3 54.78 LM 1 36.62 35.77bcd LM 2 40.13 (4.83) LM 3 30.57

The numbers with different letter (a, b and c) represented that there was significant difference (p < 0.05) by Duncan range test.

Values in brackets indicate the standard deviation.

Figure 10. The correlation between the DBH and the health index of A. confusa.

Figure 10 showed there was no statistically significant correlation between the

DBH and the health index of A. confusa. (p= 0.367, n =9, r= -0.343) Due to the small sample size, it might lead to a misleading insignificant correlation.

(Students, 2008) Although the p value was larger than 0.05, the r value shows a moderate negative relationship between the DBH and the health index of A. confusa. It represented that the larger the DBH, the lower the health index of A. confusa will be.

4.4 The ecological role of A. confusa in urban 50 ecosystems and recommendation of native tree species for replacement

According to the finding of the present study, A. confusa in urban area did not lead to acidification of soils and not cause any effect on electrical conductivity of soils. And also, the existence of it did not alter the invertebrates’ richness.

However, it can make the soil to become suitable topsoil for cultivation. And based on the findings in literature review, A. confusa can as act a firebreak to prevent hill fire and windbreak to alleviate the wind power. (Corlett, 1999 and

Cheung, 2011) In addition, it was a nitrogen-fixer which can convert nitrogen in the air into absorbable nitrogenous compound to enrich the soil. It acted as a pioneer plant species to makes the soil becomes more fertile. (Au, 2000, and

Wong et al, 2015) It grew rapidly even on infertile and rough soil. And its fruits were poisonous to faunas and it possessed the allelopathy effect which will inhibit the radical growth of seedling of other species. (Cheung 2011,

CHOU and FU, 1998)

Recommendations of native tree species for replacing the aging A. confusa in urban areas were based on the ecological role of A. confusa in urban ecosystems in Hong Kong. Recommendations of native tree species to replace

51 the aging A. confusa were as followed:

Castanopsis fissa also called Quercus fissa and Evergreen Chinkapin was a fast-growing native tree. It was a kind of evergreen shrub which can adapt to barren soil. It possessed straight tree trunk and can grow up to 20m high. Its flowering period was similar to Acacia confusa, from April to June. Its flowers’ colour was yellow. Its fruiting period starts from October to December. So, it can replace Acacia confusa for its resilient properties. (AFCD, 2006)

Schima superba also called Chinese Gugertree or Schima was an evergreen shrub which can grow up to 20m. It possessed high fertility and resilience. It flowering period begins from April to May. Its beautiful white flowers and aroma attract insects. It can be used as firebreak as it has high water content, which was very similar to Acacia confusa. Apart from it, it was a species suitable for plantation. It had been used as agricultural instruments and furniture. (AFCD, 2006)

Syzygium hancei also called Hance Syzygium was an evergreen shrub. It can grow up to 20m. Its flowering period starts from July to September. White flowers blooms during the flowering period. It can be found easily in secondary forests fung shui woods. Its fruiting period was in November and it possesses spherical fruits. Its fruits can feed the faunas like birds in the forests.

It was a suitable species for plantation. (AFCD, 2006)

So, the above three native species were suggested for replacing the aging

Acacia confusa due to their similar characteristics with Acacia confusa. Thus,

Acacia confusa can be eradicated in long term and the biodiversity of both fauna and flora can be enhanced in Hong Kong.

Chapter 5: Conclusion

5.1 Summary of findings

From the present study, conclusion can be drawn as followed:

Firstly, A. confusa in urban area did not lead to acidification of soils and not cause any effect on electrical conductivity of soils. Secondly, no correlation was found between the frequencies of A. confusa and the of invertebrates’ species found on it. Thirdly, all the soil textural classes were dominated by loamy sands and sandy loam. A. confusa can make the soil to become suitable topsoil for cultivation. Therefore, A. confusa was actually playing the role of enriching the soils.

Fourthly, the moisture content of soil increased with the level of urbanization, the most urbanized district (13.49%), the medium urbanized district (17.97%)

53 and the least urbanized district (20.06%). Urbanization was threatening the root cell membrane damage of A. confusa. In addition, there was a decreasing trend of soil temperature from the most urbanized district to the medium urbanized district and an increasing trend from medium urbanized district to the least urbanized district. Urbanization provides more available nutrients to

A. confusa.

Apart from it, the A. confusa in the sampling sites were generally aging. Most of the DBH of A. confusa recorded were larger than 24 cm. It ranged from

23.04 cm to 60.51 cm. Therefore, there was the risk for these A. confusa to be infected by fungi and collapse.

Moreover, there was a moderate negative correlation between the DBH and the health index of A. confusa. It represented that the larger the DBH, the lower the health index of A. confusa will be.

Finally, Syzygium hancei, Schima superba and Castanopsis fissa were the three common native tree species recommended to replace the aging and aged

A. confusa due to their similar properties with A. confusa.

5.2 Limitations of the study

In the present study, some limitations can be figured out.

To start with, due to the limitation of manpower, only 3 samples were collected for each sampling site (27 samples totally). So, the sample size was not large enough. For small sample size, it can easily lead to a misleading insignificant result which means the results of Anova test and correlation test might not be significant enough. (Students, 2008)

Weather was a source of errors to interfere the results of the data. There were slightly rains during the first and the second field trip. The soil moisture content may be influenced.

Sources of errors also come from the processes of the experiments. When sieving the soil samples, some of the small size particle leaked out from the sieve, which may cause inaccurate results. In addition, when measuring the weight of the sieved soils of the samples, the mass balance might be not well calibrated. Apart from it, the mixtures of sieved soils and distilled water were shaken manually due to the lack of shaker and centrifuge. It might lead to errors to the results.

5.3 Future study recommendations

This study worked on the ecological role of Acacia confusa. More sampling sites can be selected from more different districts to further consolidate the

55 studay. And the study will be more representative.

Chemical properties of the soil samples can be investigated like the total nitrogen and phosphate content instead of only analyzing the physical properties in the future study.

Comparison can be made with other exotic species such as Casuarina equisetifolia, Eucalyptus species, Lophostemon confertus or Melaleuca quinquenervia to further highlight the ecological functions of Acacia confusa.

For the relationship between the species diversity and the existence of Acacia confusa, it is suggested to not only focus on the invertebrates’ species but also the others like birds.

As the time is limited, the present study was carried out to analyzing the variables of soils of one season only. It is recommended to extend the duration of the study to two seasons (summer and winter) for comparison to see if there are any other unknown factors or effects which are seasonal will exerted by

Acacia confusa. It is also suggested to carry out the investigation in continuous years.

Appendix

Appendix 1. The textural triangle displaying the percentages of sand, silt, and clay in the soil texture classes, with the reference to U.S. Department of

Agriculture Classification System.

Appendix 2. The mean pH values of rain water in Hong Kong from 1993 to

1999 from EPD’s acid rain monitoring stations.

Year Station Mean pH 1993 Kwun Tong 4.78 Central/Western 4.72 1994 Kwun Tong 4.88 Central/Western 4.86 1995 Kwun Tong 5.39 Central/Western 4.74 1996 Kwun Tong 4.85 Central/Western 4.84 1997 Kwun Tong 5.13 Central/Western 5.07 1998 Kwun Tong 4.6 Central/Western 4.48 1999 Kwun Tong 4.87 Central/Western 4.75

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(榮譽)學位 課程 Bachelor of Social Sciences (Hons) in Environment and Resources Management

(榮譽)學位課程 Bachelor of Social Sciences (Hons) in Environment and Resources Management