Abstract

Plant-herbivore interactions influence both plant characteristics and feeding patterns of herbivores. In this thesis, I focus on patterns of defence in a wide range of woody species in semi-arid savannas in Botswana, and on responses in woody species to different types of browsing. Methods included both surveys of plant traits relevant to herbivory, and controlled experiments using simulated browsing. On a regional scale gradient, the proportion of spinescent and evergreen species decreased, while fast-growing species increased, with increasing resource availability. On smaller landscape scale gradients, the proportion of spinescent and evergreen species increased, fast-growing species either increased or decreased, while C-based defences generally decreased and nutrients increased, with increasing resource availability. In some species, levels of defence (phenolics) were higher in young trees, compared to mature trees. However, contrary to our predictions, shoots within, compared to above, browsing height on mature trees were less defended (phenolics and tannins). Simulated twig-biting during the dry season (on five species) induced compensatory growth responses in annual shoots during the growth season, and cutting at ground level induced sprouting. Leaf removal late in the growth season (on one species) induced foliage re-growth, although total leaf production during the season did not increase. The application of saliva had a small, but significant, effect on shoot growth following clipping. No evidence was found for a connection between patterns in allocation to defensive traits in shoots of different exposure to browsers, or strength of responses to clipping, and other species characteristics, e.g. leaf fall patterns, growth rate, presence of spines or natural browsing pressure. I suggest further studies focusing on both chemical and structural defences, on different spatial scales and resource gradients. I also conclude that additional studies are needed on potential effects of different traits on different types of herbivores, and stress the advantage of studying a wider range of plant species to obtain more general results.