Environmental and Biogeographic Influences on the Distribution and Composition of the Southern Cape Forests (Veld Type 4)

' ' I ENVIRONMENTAL AND BIOGEOGRAPHIC INFLUENCES ON THE DISTRIBUTION AND COMPOSITION OF THE SOUTHERN CAPE FORESTS (VELD TYPE 4) by Coert Johannes Geldenhuys A thesis presented for the degree of DOCTOR OF PHILOSOPHY in the Department of Botany Faculty of Science University of Cape Town Rondebosch UniversityAugust of 1989Cape Town The copyright of this thesis vests in the author. No quotation from it or information derived from it is to be published without full acknowledgement of the source. The thesis is to be used for private study or non- commercial research purposes only. Published by the University of Cape Town (UCT) in terms of the non-exclusive license granted to UCT by the author. University of Cape Town i CONTENTS Page Abstract i i Acknowledgements vi Preamble 1 Chapter I DISTRIBUTION, SIZE AND OWNERSHIP OF FORESTS IN THE SOUTHERN CAPE 10 Chapter 2 BERGWIND FIRES AS DETERMINANTS OF FOREST PATCH PATTERN IN THE SOUTHERN CAPE LANDSCAPE, SOUTH AFRICA 46 Chapter 3 COMPOSITION OF THE SOUTHERN CAPE FOREST FLORA, WITH ANNOTATED CHECKLIST 74 Chapter 4 RICHNESS, COMPOSITION AND RELATIONSHIPS OF THE FLORAS OF SELECTED FORESTS IN SOUTHERN AFRICA 138 Chapter 5 COMPOSITION AND BIOGEOGRAPHY OF FOREST PATCHES IN THE INLAND MOUNTAINS OF THE SOUTHERN CAPE 209 Chapter 6 DISJUNCTIONS AND DISTRIBUTION LIMITS OF FOREST SPECIES 'IN THE SOUTHERN CAPE 247 Chapter 7 PHYTOGEOGRAPHY OF THE SOUTHERN CAPE FOREST FLORA 276 Chapter 8 ENVIRONMENTAL AND BIOGEOGRAPHIC INFLUENCES ON THE DISTRIBUTION AND COMPOSITION OF THE SOUTHERN CAPE FORESTS: A SYNTHESIS 310 ii ABSTRACT This study aims at explaining the distribution and composition of the southern Cape forests, the largest forest complex in southern Africa. These are the only forests in southern Africa which are actively and scientifically managed for their products and values. Population growth due to forestry, agricultural and economic development and a growing tourism industry exerts increasing pressures on the natural environment of the southern Cape coast and therefore affect the dynamics and conservation of the ,forests. Conservation and sustained utilization of the forests require a sound knowledge of the composition, structure and dynamics of the forests. This study was aimed at an understanding of the biogeography of the forests at the landscape level in order to isolate those variables which contributed to the present distribution and composition of the forests. Determinants of the forest location pattern in the southern Cape were identified as rainfall above 500 mm, which determines the potential limits of the forests, and the bergwind fire pattern, which determines the actual forest distribution. Fires driven by the hot, dry, northwesterly, f6hnlike bergwinds interacted with the terrain physiography since prehistorical times and the forests persisted in ' topographic shadow areas. The largest forests in the area therefore occur on the coastal platform at the foot of the mountains, in the river valleys and on the coastal scarp. Forests in the mountains, with high rainfall, are small and scattered. The results have shown that the bergwind driven fires control the distribution of forests which have important implications for the understanding of forest dynamics and for conservation management of forests in multiple-use management systems. Forest composition at the landscape level was studied by means of plant species 1 i sts. A species 1 i st for the southern Cape forests was annotated with information on the growth form, breeding system, propagule type, forest type, moisture tolerance, abundance and spread in the study area, and the distribution range in southern Africa, of each species. Analyses of the list showed that the iii species/family ratios for the southern Cape forest flora are very low, that woody plants have mostly fleshy propagul es and herbaceous plants mostly dry propagules, and that several species have adaptations to adverse conditions. The species richness and composition, and floristic similarity and relationships were compared between the southern Cape forest flora and the floras of 13 other forests representing particular geographic regions in southern Africa. Forest size explained relatively little of the variation in species richness of the forests. Stepwise multiple regression analyses indicated that the number of dispersal corridors, the proximity to other forests and mean altitude explained most of the variation in number of woody species, whereas the number of landscape types and dispersal corridors explained most of the variation in number of herbaceous species. The high similarity between the southern Cape forest flora and those of the forests along the escarpment from the eastern Cape to northern Transvaal, and the southern attenuation of species suggest that the forests were once continuous. It is suggested that the Sundays river valley east of Port Elizabeth i so 1a ted the southern Cape forests from those to the east a1 ready during the Pliocene or earlier. Patterns in species richness of 23 small, isolated forests in the inland mountains of the southern Cape were not explained by habitat preferences, species-area relationships or long-distance dispersal. However, patterns in species richness and composition show definite trends when they are considered in relation to dispersal corridors along the Gouritz and Gamtoos rivers, the only two rivers which connect the coastal forests with the inland mountains. The patterns relate to the geomorphological history of the river systems. The results indicated that at least four.floras can be recognized and that relative dates can be attached to their domination of the ·area and their contribution to the southern Cape coastal forest flora. The 206 species which reach their distribution limits within the southern Cape forests, or have disjunct distributions within this area and wider distributions outside were found to be mostly concentrated in a few localities within the study area. Biological variables such as growth form and dispersal· type, and habitat iv variables such as forest type and moisture tolerance did not correlate with the drop-out of species. Species which were concentrated in the western limit of the forests, and in other parts of the high forests, relate to the major forest decline with the onset of the Late Pliocene marine regression. Species tolerant of drier conditions are concentrated in various disjunct localities along the coast and this pattern is attributed to the elimination of coastal dune forest and subtropical transitional thicket on the Agulhas Bank during the marine transgression after the Last Glacial Maximum. Finally, the distribution ranges of the 465 species of the southern Cape forest flora were used to establish generalized tracks of the flora in order to test the hypothesis that regional speciation of the floral elements of the tropical, subtropical and temperate regions of southern Africa have enriched the older . Afromontane forest flora. Log-linear analysis was used to determine the dependence between the biological and ecological characteristics of the species and their geographical categories. The results indicated that the ecological characteristics of the species of each geographical group reflected the environmental conditions and disturbance regimes which prevailed in each source area. The results confirmed that the southern Cape forest flora was made up of elements of a narrower cool, humid Afromontane area, elements of a transition zone between the montane zone and the humid, warm subtropical coastal areas with wide moisture tolerances in the southern Cape, elements which had to adapt to more frequent and extreme droughts and drier conditions of the eastern Cape lowlands, and western Cape elements which are associated with either cool, moist montane areas or with more frequently burnt forest margins. The southern Cape forest flora is therefore a product of a long period of forest expansion and contraction which in this study is taken back to the Late Cretaceous-Palaeocene. The increasing aridity since that time contributed to the narrowing and fagmentation of the forest belt. Species developed which were to 1erant of frequent .fires or droughts. Most of the 1arge p1 ant families, and many of the other important families, are shared between the forests and other vegetation types. This sharing suggests that the forests might have been the original gene source for the speciation of many of the families and genera. v The effect of the increasing aridity and disturbance pressure seems to be that the radiation of species occurred in smaller growth forms outside the forests. vi ACKNOWLEDGMENTS I wish to thank the Director-General of the Department of Environment Affairs and the Director of the South African Forestry Research Institute who gave me ample opportunities in an ideal workplace, and provided the funds for this study. They also funded a study visit to the rainforests of Australia, New Zealand and Ch i1 e during which I was ab 1e to gain a Southern Hemisphere perspective of rainforest dynamics. I became aware of the role of disturbance regimes in shaping the physiognomy and dynamics of the different forests and hence influencing the silvicultural systems applicable in the management of those forests. My colleagues at the Saasveld Forestry Research Centre and in the wider South African Forestry Research Institute with their varied scientific interests and approaches helped to broaden my view of my field of study. From them I learned the art of questioning the things

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