‘ » Vegetation Dynamics and Soil Characteristics of Acacia Plantations in Hong Kong by Au Pui Sze ‘ Thesis submitted to the Graduate School of the Chinese University of Hong Kong • in partial fulfillment of the requirements for the degree of Master of Philosophy December, 2000 Division of Geography Graduate School The Chinese University of Hong Kong The Chinese University of Hong Kong holds the copyright of this thesis. Any person(s) intending to use a part or whole of the materials in the thesis in a proposed publication must seek copyright release from the Dean of the Graduate School. 统系館書圖： tfr— 二j W^IBRARY SYSTE：.!/^"^ ABSTRACT "Vegetation Dynamics and Soil Characteristics of Acacia Plantations in Hong Kong" Thesis submitted by Au, Pui Sze for the degree of Master of Philosophy at the Chinese University of Hong Kong in December, 2000 This study examined the vegetation characteristics and soil properties of six acacia plantations in Hong Kong, with special emphasis on their ecological role in native forest regeneration. These plantations were 5(P5), 6(P6), 13(P13), 14(P14), 25(P25) and 35(P35)-year-old, and were established on borrow areas and fire- disturbed grassy slopes. The sites were chosen for the study of stand growth characteristics and structure, soil ameliorative effects and understorey species regeneration. The acacia species are fast-growing and provide rapid tree cover to the degraded sites. Mean height, stem basal diameter and diameter at breast height increased with stand age for both Acacia auriculiformis and Acacia confusa. The acacia stands closed their canopies in about ten years and divergence in growth pattern increased with stand age afterwards. Tree density varied from 2,150 to 7,625 nos. ha"' and natural die-back was more common in the older plantations than the younger stands. The acacia species cannot regenerate beneath its own shade and the exotic system does not appear to sustain for a long time. Soil organic matter (1.84-9.69%), total Kjeldahl nitrogen (0.08-0.37%) and total mineral nitrogen (20.47-78.37 mg kg"') increased remarkably with age of the i 、 plantations in the top 5 cm soil. Total exchangeable bases followed a similar trend, while changes in exchangeable K, Na, Ca and Mg of the soils with stand age were not conspicuous. The soil pH showed an increased acidity with plantation age. These acidic soils are deficient in available" P. The ameliorative effect took place in both the topsoil and subsoil in fire-disturbed sites, while less effect was detected in the subsurface layers of the borrow areas. A total of 78 native woody species were recorded from the six plots within the area of 100 m" for each site. Most, of the species were bird-dispersed and commonly found in naturally regenerated forest. The Shannon indices ranging from 1.84 to 2.85 did not increase with age of the plantations. The number of understorey species followed the order of P25(45) > P6(39) > P35(34) > PI4(29) > PI 3(19) > P5(7). The proportion of trees was higher in the understorey beneath the « closed canopy in the older plantations. Mid-successional species such as Cinnamomum parthenoxylon, Liquidambar formosana and Schima superba, though scarce, were found in the older plantations. Understorey layer of the borrow areas were weakly developed and deprived of tree species; there were only 7-19 species compared to 29-45 species for the other sites. The effect of antecedent land use, seed source and overstorey structure on species richness and composition were discussed. The ecological role of exotic plantations in the local environment was evaluated with reference to nutrient-supplying capacity of the soil and biodiversity conservation. The strategy of borrow area reforestation and objectives of enrichment planting on grassy slopes were also discussed. % ii 、 摘要 • 本文爲香港六個人工相思林的生長特徵和結構、土壤改良及林下植物區系 組成三方面進行了調查和分析硏究，目的在探討這些外來引種的人工林對於森 林植被恢復的生態效益。這些人工相思林林齡爲5,6,13,14,25及35年，分別栽 種於採泥區或火燒後的草帔上。 . 速生相思林迅速地爲退化地提供植被覆蓋。耳果相思和台灣相思的平均樹 高、平均地徑、平均胸徑都隨著人工林的年齡增加。可是，這些外來弓丨種的樹 種未能在其林冠下自然更新，故此這植林系統將難以長期持續下去。 林地土壤（0-5厘米）的有機質（1.84-9.69%)，全氮（0.08-0.37%)及有 效氮（20.47 -78.37微克/千克）隨林齢有顯著上升的趨勢。代換性驢基總和亦 隨林齢而增加。土壤pH値也隨林齢下降，而這些強酸性土壤大多缺乏有效磷。 在採泥區，人工林對土壤改良的作用只限於表土層，對於亞表土層的改良作用 卻不大。 ， 在六個面積爲100平方米人工林樣方中，共記載了 78種本地木本植物。 其中大部份植物種類爲鳥播，是天然次生林中的常見種�Shannon多樣性指數 在1.84-2.85之間，植物種數在7-45之間，兩者並不一定隨林齡而增加。採泥 區的物種多樣性和樹木種數均較草帔低。文中亦提及退化程度、種軒來源、林 冠結構對林下植物區系組成和結構的影響。 Hi ACKNOWLEDGEMENTS I wish to express my sincere gratitude and appreciation to the following: Dr. Chau Kwai Cheong, my supervisor, for his advice on selecting the research topic, invaluable guidance, endless hours of fruitful discussion, continuous support and encouragement throughout the course of this study; and for his generous gift of time on revising this thesis; Dr. Marafa Lawal Mohammed, for his invaluable advice on site selection and precious comments on data interpretation and my writings; and for his kindly support, encouragement and friendship; The Agriculture, Fisheries and Conservation Department, for the kind approval for the field survey in the country parks. Special thanks must be given to the officers, Mr. Chan Tat Wai and Miss Ngar Yuen Ngor for their assistance in providing information on planting and management record; ， The staff of the Herbarium, The Agriculture, Fisheries and Conservation Department, for their kindness and efficiency in species identification; The United College Campus Work Scheme for the funding and support which made this research possible; Mr. Lee Kwong Chi Kelvin, Mr. Lui Fung Wai, and Mr. Too See Lou, the technicians, for their invaluable technical assistance and helpful discussion; Oscar Wing Wa So, Maxwell Hoi Cheung Luk, Kin Pui Lui, Hoi Yeung Kong and Tin Ho Cho, for their assistance in field work; and for their encouragement and company to work outdoor in harsh weather; The personnel of the Department of Geography, The Chinese University of Hong Kong, for their administrative and technical support; The personnel of the Interlending/Reserve Books Counter of The Chinese University of Hong Kong Libraries, for their kindness, support and efficiency; iv Eva Poon, Regin Leung, Flora Chan, Garfield Yuen and Billy Yu, my fellow colleagues, for their friendship, prayers, encouragement and humour; and for providing a warm working environment and a variety of perspectives on both science and life; Yuen Shan Chan, Tsui Shan Chu, Hoi Yin Tsang, and every brother and/or sister in the Lai Wo Church for their friendship, prayers, love, motivation and encouragement; and their company in my difficult time; My brother, Chun Ting Au, for his love, tolerance, support, and being crazy with me when I need to; and for his help on pre-treatment of soil samples; My sister, Annette Ching Man Au, for her love, tolerance, support; and kind gift of time on reviewing my writings; My parents, who teach me to excel in life and study, for their unfailing love, support, understanding, sacrifice, and tolerance of me spending less time at home. V TABLE OF CONTENTS Page Abstract i Abstract (in Chinese) iii Acknowledgement iv Table of contents vi List of tables x List of figures xi List of appendices xii Chapter one Introduction 1.1 The role of plantations in ecosystem rehabilitation 1 1.2 Reforestation history in Hong Kong 2 1.3 Conceptual framework of the study 5 1.4 Objectives of the study 15 1.5 Scope and significance of the study 16 1.6 Organisation of the thesis 17 Chapter two The Study Area 2.1 Geographical setting of Hong Kong 18 2.1.1 Climate 18 2.1.2 Geology 20 2.1.3 Soils 20 2.1.4 Vegetation 21 2.2 Site selection 23 2.3 Site description 26 2.4 Nature and extent of disturbance prior to planting 30 2.5 Planting techniques and post-planting maintenance 31 vi Chapter Three Stand Characteristics of Acacia Plantations 3.1 Introduction 33 3.2 Methodology 37 3.2.1 Sampling plots design 37 3.2.2 Tree density 37 3.2.3 Tree growth parameters 38 3.2.4 Data processing and statistical analysis 38 3.3 Results and discussion 39 3.3.1 Growth dynamics of acacias 39 3.3.2 Growth characteristics of the reforested species 44 3.3.3 Sustained growth and natural regeneration of acacia plantations. 48 3.3.4 Rehabilitation of degraded lands by exotic species plantations... 52 3.4 Conclusion 54 Chapter Four Cover Soil Characteristics of Acacia Plantations 4.1 Introduction 56 4.2 Methodology 60 4.2.1 Soil sampling 60 4.2.2 Laboratory analysis 61 4.2.3 Texture 61 4.2.4 Soil pH 61 4.2.5 Organic carbon 62 4.2.6 Total Kjeldahl nitrogen 62 4.2.7 Carbon : nitrogen ratio 63 4.2.8 Mineral nitrogen (NH4-N and NO3-N) 63 vii 4.2.9 Available phosphorus 64 4.2.10 Exchangeable cations 64 4.3 Data processing and statistical analysis 64 4.4 Results and discussion 65 4.4.1 Effect of acacias on soil texture and pH 65 4.4.2 Effect of acacias on SOM, TKN and mineral N 69 4.4.3 Effect of acacias on the available P and exchangeable cations..... 77 4.4.4 Soil nutrient status of the plantations 82 4.5 Conclusion 90 Chapter Five Understorey Vegetation of Acacia Plantations 5.1 Introduction 92 5.2 Methodology 96 5.2.1 Understorey plant sampling 96 5.2.2 Species identification and nomenclature 98 5.2.3 Plant growth parameters 98 5.2.4 Data processing and statistical analysis 99 5.3 Results and discussion 100 5.3.1 Floristic composition of the acacia plantations 100 5.3.2 Species richness, diversity and woody abundance of the 101 imderstories 5.3.3 Species composition and structure of the imderstories 106 5.3.4 Mechanisms and pathway of succession in the exotic 115 plantations 5.4 Conclusion 116 viii Chapter Six Conclusion 6.1 Summary of findings 118 6.2 Implications of the study 122 6.2.1 Ecological value of exotic plantations in Hong Kong 122 6.2.2 Restoration strategies for borrow areas 125 6.2.3 Enrichment planting after fire 128 6.3 Limitations of the study 131 6.4 Suggestion for future study.