QUANTIFYING DEGRADED SUBTROPICAL THICKET STRUCTURE AND COMPOSITION: A MULTI-SCALE APPROACH IN THE EASTERN CAPE, SOUTH AFRICA A thesis submitted in fulfilment of the requirements for the degree of MASTER OF SCIENCE of RHODES UNIVERSITY by KYRA ELIZABETH LUNDERSTEDT June 2016 QUANTIFYING DEGRADED SUBTROPICAL THICKET STRUCTURE AND COMPOSITION: A MULTI-SCALE APPROACH IN THE EASTERN CAPE, SOUTH AFRICA MSC THESIS BY KYRA LUNDERSTEDT Supervisors: Professor Charlie Shackleton and Mr. Mike Powell Abstract The loss in canopy cover from over-browsing severely degrades the ecological integrity of spekboom-dominated thicket in the Eastern Cape. A homogenisation of species across the landscape and high heterogeneity at fine scales has been reported with little evidence of recovery. As an interlinked consequence, the loss in important biological structures and composition impacts greatly on soil resources and therefore function. This thesis provides a baseline of degraded thicket abiotic and biotic structure and composition over both spatial and temporal scales within the Greater Addo Elephant National Park (GAENP), including Darlington, Kabouga and Addo Main and within the Baviaanskloof. The study aimed to find correlates of species composition within sites and assess changes in composition with degradation and recovery times. Across the GAENP, landscape degradation was most evident in post-1960 aerial imagery. The duration since sampled sites have been incorporated into the GAENP did not influence species composition, however the period of degradation and severity, did. Across the landscape, communities were strongly associated with each of the four sites and separated predominantly by rainfall, gravel and altitude. A total of 345 plant species were identified across the landscape and despite degradation, each site was characterised into three to five communities each, barring Baviaanskloof which had no significantly different communities. Across the landscape, matrix composition comprised predominantly of Pentzia incana, Drosanthemum hispidum, Galenia pubescens and Cynodon species. Woody cover within patches included smaller Grewia robusta, Rhigozum obovatum and Vachellia karroo and the larger Pappea capensis and Euclea undulata. The succulent shrub Euphorbia caerulescens in Darlington and succulent tree Aloe ferox in Baviaanskloof occurred abundantly as a consequence of degradation. Species richness was significantly correlated to patch size in all sites, but Kabouga had the greatest richness and probability of a suite of species occurring within a patch. Higher woody and succulent cover in Kabouga was therefore associated with higher soil C, root percentage and bulk density. Other sites were higher in Ca, Na, K and P. The method of degradation scoring was not sufficiently accurate and it is recommended that indicator species within the matrix should be used instead of growth forms. The findings of this thesis are conceptualised within a double-cusped catastrophe model and recommendations for restoration are provided. Page | ii Table of contents Abstract..................................................................................................................................................ii Table o f contents................................................................................................................................. iii List o f figures.......................................................................................................................................ix List o f tables........................................................................................................................................xii List o f appendices.............................................................................................................................xiii List of abbreviations and symbols................................................................................................... xiv Acknowledgements........................................................................................................................... xvii CHAPTER 1: GENERAL INTRODUCTION................................................................................1 1.1. Drylands and degradation.................................................................................................... 1 1.2. Concepts and models in understanding degradation processes in semi-arid ecosystems..............................................................................................................................................1 1.2.1. Ecological thresholds, resilience and resistance........................................................ 3 1.2.2. Linear structure-function m odel...................................................................................3 1.2.3. State-and-transition models and alternate stable states............................................. 4 1.2.4. Cusp-catastrophe model.................................................................................................6 1.3. Scale, hierarchy theory and spatial organisation in ecology..........................................8 1.4. Using ecological indicators to monitor degradation and vegetation change............... 9 1.5. Land degradation and implications to ecological integrity...........................................10 1.6. Natural vegetation recovery and restoration....................................................................12 1.6.1. Old lands: recovery and restoration........................................................................... 13 1.7. Desertification in semi-arid South Africa........................................................................ 14 1.8. The case of the Subtropical Thicket Biome.....................................................................15 1.8.1. Degradation and recovery of the Subtropical Thicket B iom e................................15 1.8.2. Global incentives to and restoration of Subtropical Thicket..................................17 1.9. Purpose of the research.......................................................................................................19 CHAPTER 2: STUDY AREAS........................................................................................................22 Page | iii 2.1. The regional landscape: the Subtropical Thicket Biome..............................................22 2.1.1. Defining thicket............................................................................................................24 2.2. Location and history of sites within the Subtropical Thicket Biome.......................... 24 2.2.1. The Greater Addo Elephant National Park................................................................25 2.2.2. The Baviaanskloof Mega-Reserve.............................................................................27 2.3. Climate....................................................................................................................................28 2.4. Geology and soil....................................................................................................................29 2.4.1. Geological types............................................................................................................29 2.4.2. Geology of sites............................................................................................................31 2.4.3. Soils................................................................................................................................ 33 2.5. Topography............................................................................................................................ 34 2.6. Vegetation types and condition.......................................................................................... 35 2.6.1. Thicket types and common species............................................................................ 35 2.6.2. Degradation of thicket types.......................................................................................36 2.7. Fauna..................................................................................................................................... 40 2.8. Conclusion............................................................................................................................. 41 CHAPTER 3: PLANT SPECIES COMPOSITION AND ABIOTIC CORRELATES IN DEGRADED SUBTROPICAL THICKET AT LOCAL AND LANDSCAPE SCALES..... 43 3.1. Introduction........................................................................................................................... 43 3.2. Climatic and geomorphic determinants of vegetation and soil....................................43 3.2.1. The role of drought and herbivory in accelerating degradation.............................. 45 3.3. Abiotic correlates in the Subtropical Thicket Biome..................................................... 45 3.3.1. Degraded ‘pseudo-savannas’ in different thicket types.......................................... 47 3.31.1. Vegetation composition.......................................................................................... 47 3.31.2. Soil composition and litter.....................................................................................48 3.4. Objectives and key questions..............................................................................................50 3.5. Methods..................................................................................................................................50