Spatio-temporal analysis of woody vegetation data at various height classes within a semi-arid savanna : insights on environmental drivers, distributions and dynamics.

Abstract

Savanna vegetation is regulated by bottom-up (e.g. soil and rainfall) and top-down (e.g. fire and herbivory) factors. Little is known about how these factors influence biodiversity at regional scales, particularly spatial patterns of woody plant cover, species richness, distribution and composition at various height classes. Vegetation studies are often restricted to hillslope or catchment-level scales due to time and/or logistical constraints, however there is a need to understand patterns of vegetation dynamics at regional scales as well. The main aim of this thesis was to better understand the factors (bottom-up and top-down) that influence vegetation structural dynamics in terms of species richness, distribution, composition and woody cover at regional scales. Using a rapid data collecting technique within n>1800 field sites across the entire Kruger National Park, South Africa, woody species were identified and categorized into one of three height classes; shrub (0.75m – 2.5m), brush (2.5m-5.5m) and tree (>5.5m). From this, several vegetation attributes were collated; such as species composition, richness and woody cover at each height class. A range of covariates related to disturbance (i.e. fire frequency and elephant density), landscape (e.g. aspect, altitude) and climate (temperature, rainfall) were used to identify which covariates were significantly associated with each attribute of interest. A primary finding of this research highlights the importance of bottom-up factors, such as geology and rainfall, for woody vegetation spatial distribution at regional scales. However, depending on the response variable (e.g. species richness, woody cover, species distribution), the various height classes were not influenced equally by each specific environmental factor. Overall, both climatic and non-climatic factors were highly associated with species distribution and richness, but top-down factors such as fire and herbivory were significantly associated with maintaining plant structural variation. This influenced species community composition but not species richness at each height class. Similarities in composition were related to degree of disturbance rather than the disturbance alone at specific height classes. Therefore, while species distribution and woody cover were spatially regulated by specific climatic and non-climatic factors, top-down disturbances were essential in preserving structural heterogeneity. Future vegetation monitoring campaigns in disturbance-driven systems should be cognizant of the importance of vegetation structure.