Browsing by Author "Randle, Matilda Rose"
Now showing 1 - 1 of 1
Results Per Page
Sort Options
- ItemEffects of warming on the early establishment of an African savanna tree(Stellenbosch : Stellenbosch University, 2019-12) Randle, Matilda Rose; Midgley, Guy F.; Stevens, Nicola; Stellenbosch University. Faculty of Science. Dept. of Botany and Zoology.ENGLISH ABSTRACT: This study investigates the interactive effects of atmospheric warming and the presence of a C4 grass sward on the growth and mortality of establishing savanna tree seedlings (Senegalia nigrescens (Oliv.) P. J. H. Hurter). Atmospheric warming is one of the major drivers of global vegetation change, but has been little studied in tropical African systems. In cool temperature systems of the mid- to high –latitudes, where plant growth is predominantly controlled by climatic factors, warming has beenshown to drive an increase in plant growth and establishment. In systems of the mid – to low – latitudes, such as sub-tropical savannas, the potential role of warming is not well known, and the vegetation structure and functioning of these systems is controlled by the interacting impacts of fire, herbivory and climate. Wildfire and herbivory limit the seedling and sapling demographic stages of savanna trees. This stage is expected to be most vulnerable to warming because seedlings are sensitive to fluctuations in temperature and are highly dependent on water availability, a resource that is competed for strongly by C4 grasses. Furthermore, C4 grasses are unlikely to be as adversely affected by warming as C3 seedlings, due to differences in their photosynthetic pathways. Based on these arguments, this study tests the main hypothesis that there is an adverse interactive effect of warming and grass competition on tree seedling establishment in savannas. In order to test this hypothesis, I carried out a field experiment at Wits Rural Facility in Limpopo Province, South Africa during the 2017/18 growing season. Using passive open-topped, polycarbonate warming chambers, seedlings were warmed on average, by 1-2°C. Soil and plant water content were unaffected by warming but the presence of grass significantly reduced the relative water content of the leaves of establishing seedlings, suggesting competition for water between the different growth forms. Seedling growth rate was unaffected by warming when grown without C4 grasses, but a significant decline was shown by those grown with grasses above a daytime temperature threshold of 30°C. Likewise, seedlings grown with grasses suffered a 65% reduction in survivorship when warmed but those grown in the absence of grass suffered only a 15% reduction in survivorship. The results of this study therefore show that warming and the presence of grasses had an adverse additive effect on seedling survivorship, through which warming enhanced the dehydrating effect of competing grass on establishing seedlings, thus confirming the primary hypothesis. I propose that the cumulative stresses of carbon imbalance due to warming and grass competition for soil water drove this decline in tree seedling growth, resulting in higher seedling mortality with the implication of reduced successful establishment events under warmer conditions. This study makes an important contribution to understanding the impact of warming on African savanna species, in that it suggests a future decline in tree establishment under warmer conditions. To extrapolate these findings, a greater focus on understanding the impacts of warming on a range of savanna plant functional groups across the rainfall gradient, with other global change drivers, is required.