| Summary: | Thesis (MSc)--Stellenbosch University, 2004. ENGLISH ABSTRACT: There is increasing evidence that the rapid and anomalous changes in climate experienced in the last century have had widespread ecological impacts. Indeed, sub- Antarctic Marion Island has experienced particularly large increases in temperature and declines in rainfall. However, the effects of these changes on the island's extensive fellfield vegetation remain largely unexamined. The aim of this study was to examine the sensitivity of a dominant and keystone fellfield plant species, the cushion-forming Azorella selago Hook. (Apiaceae), to changes in climate. Three complementary approaches (two mensurate, one experimental) were used, and all showed that A. selago is likely to change in response to further changes in climate. First, the unimodal age class distribution of A. selago suggested that the species' establishment is episodic, and therefore reliant on specific (possibly climatic) conditions. Azorella selago growth rate was related to environmental factors, suggesting that both the establishment and growth rate of the species is likely to be sensitive to changes in climate. Second, altitudinal variation in A. selago plant attributes suggested that the species' morphology would be responsive to changes in climate (assuming that a spatial gradient in climate is a suitable analogue for similar changes in climate over time). Plant height, leaf size and trichome density differed most consistently over altitude across the island. The altitudinal range of some epiphyte species, as well as the cover and species richness of epiphytes growing on A. selago, also showed consistent patterns along the altitudinal gradient. These cushion plant and epiphyte attributes appeared to be related to climatic factors, and are therefore predicted to change in response to further shifts in climate. Finally, A. selago showed a rapid vegetative response to short-term experimental reductions in rainfall and increases in temperature and shading. Reduced rainfall ...
|
|---|