Exploring the potential impacts of global change on the woody component of South African savannas Nicola Stevens Town Cape of Univeristy April 2014 A thesis presented to the Faculty of Science, University of Cape Town, for the Degree of Doctor of Philosophy in the Department of Biological Sciences 1 Abstract Land cover change, elevated CO2 and associated climate change are driving changes in ecosystem structure and function across the world. These changes are best described and understood in northern hemisphere temperate systems. Studies, models and reports from these regions are therefore often used as the basis for understanding, predicting and managing change in other biomes, including African savannas. In savannas elevated CO2 has been implicated in driving an increase in woody cover, however attribution to global drivers is often confounded by land-use management which is also implicated as a driver of tree cover increases. Climate change is predicted to directly drive species range shifts in savannas, however several important gaps exist in the literature which prevent the development of a clear predictive framework to describe these changes. Whilst climate is often assumed to be the primary factor underlying savanna tree distribution this has not been demonstrated for savanna tree species. Additionally the determinants of individual savanna species distributions have seldom been investigated. The few existing studies are correlative species distribution models based on adult plant distribution and underlain by the assumption that climate sets plant distribution. Although a demographic approach is very important in understanding tree: grass coexistence in savannas, this approach is seldom used in understanding species distributions. The aim of this study was therefore to assess how global change might influence South African savannas. We place emphasis on the role of elevated CO2 and climate change. I first report on an analysis of historical changes in woody plant abundance at large spatial scales to assess the magnitude of tree cover change in South African savannas. I attempt to untangle the role of local drivers and global drivers in causing tree cover changes from 1940 - 2009. I then report on studies aimed at determining what limits the distribution range of two common savanna trees Acacia nigrescens and Colophospermum mopane, at three critical demographic stages. Here I used a series of field studies, transplant experiments, lab experiments and modelling to determine the critical drivers of the distribution of these plants so as to understand how savanna plants might respond to changing climates. Using a natural experiment of four different land-use systems across the savanna rainfall gradient, I measured the magnitude and trajectory of tree cover change from the aerial photographic records from 1940 to 2009. I used this natural experiment, to untangle the role of local vs. global drivers. Each land use should have different trajectories of tree cover 2 change, unless a global driver is causing the increases. Tree cover increased across all land uses across the rainfall gradient, except in the presence of elephants in the low rainfall areas. The results suggest that elevated CO2 is driving an increase in tree cover across South African savannas To understand savanna tree species response to climate change I investigated the drivers of their distribution limits. I first used a traditional species distribution climate based approach to generate hypotheses as to which regional-scale climatic drivers might determine the distribution of C. mopane. Temperature, rainfall and latitude were the most important predictors of C. mopane range limits, and were therefore considered in subsequent chapters where I examined the drivers of distribution limits at each demographic stage. At the germination and early seedling establishment phases of C. mopane and A. nigrescens, I used a lab experiment and a soil water model parameterized with measurements from the lab study to examine if temperature and water stress interact to limit either germination or seedling establishment of C. mopane. Germination events were not limited but seedling establishment events were limited by water availability. The study suggests that warmer future conditions will accelerate the rate of radicle extension and increase the frequency of seedling establishment events. At the sapling stage I established a transplant experiment where C. mopane and A. nigrescens were planted at four paired, high and low elevation sites across an 850 km latitudinal gradient. This seems to be the first attempt to experimentally explore the climatic limits of African savanna tree distribution by determining tree survival and growth at latitudes and altitudes much broader than the distribution limits of the study species. Neither climate nor presence within a distribution range could explain plant performance. Grass competition significantly affected plant growth rates, but alone could not explain the distribution limit. Species distributions were best predicted when maximum tree growth rates in the presence of grass were considered in relation to local fire return intervals. The probability of sapling escape from the fire trap was the most likely determinant of distribution limits of these two species. At the adult stage I examined the ecophysiology of adult C. mopane using a watering experiment to explore the role of water and temperature in controlling photosynthesis and leaf phenology. Water was a proximate cue in driving leaf loss of C. mopane. Leaf carbon acquisition rates do not decline through leaf aging, but are strongly affected by soil moisture 3 availability and temperature. Although the physiology and phenology of this plant is driven by water and temperature we did not demonstrate, within reasonable climatic limits, any limitations upon this adult stage. Elevated CO2 appears to be driving a large scale regional increase in tree cover across South African savannas with local modification by land use. My studies to identify drivers of the distribution limits of C. mopane and, to a lesser extent, A. nigrescens demonstrated that the determinants of ranges in these tropical savanna species are complex, and multiple factors interact to affect the performance and success of these species at different demographic stages. Critical range limitation is most likely to occur at the sapling stage where a combination of higher grass biomass and frequent fire return intervals are important in determining the escape of these trees from the fire trap. This thesis offers new insights into how savanna species will respond to two critical changes. Tree cover increases and, potentially, associated biome shifts are likely. The lack of any clear direct climate limitation on the study species indicates that range shifts are unlikely to occur in response to altered climates. Consumer controls e.g. by fire are important determinants of the distribution of savanna species. Future research should focus not only on global change impacts on plant growth, but also in how climate change will impact consumer controls. The combination of the findings in this thesis makes a novel contribution to science and savanna ecology by providing a new framework from which to consider global change in savannas. 4 Table of Contents ABSTRACT ........................................................................................................................................................ 2 TABLE OF CONTENTS ....................................................................................................................................... 5 ACKNOWLEDGEMENTS .................................................................................................................................... 8 GENERAL INTRODUCTION .............................................................................................................................. 11 DETERMINANTS OF SAVANNA STRUCTURE AND FUNCTION ............................................................................................. 11 Distribution of the African savannas ......................................................................................................................... 11 Shifting controls on savanna function across environmental gradients .................................................................... 12 Land use and land management in savannas ............................................................................................................ 15 CHANGE IN SAVANNAS ........................................................................................................................................... 17 Regional scale manipulation of fire and herbivory ...................................................................................... 17 Global Drivers of savanna change ............................................................................................................... 19 Climate change .......................................................................................................................................................... 19 Elevated CO2 .............................................................................................................................................................. 20 CONTEXT OF THE THESIS ......................................................................................................................................... 21 Woody encroachment ..............................................................................................................................................