Drivers of woody plant structural and compositional heterogeneity in a semi-arid African savanna
Woody cover in savannas is essential for maintaining natural processes and ecosystem function. Important drivers of total woody cover in African savannas are rainfall, soil, nutrients, fire and herbivory. However, very little is known about drivers of woody cover at various height classes, in particular, how disturbance factors (such as fire and herbivory) influence woody plant composition. Our analyses aimed to identify which of these drivers are responsible for variation in woody cover at specific structural height classes; shrub (0.75m – 2.5m), brush (2.5m – 5.5m) and tree (>5.5m), and how disturbance influences species composition at these height classes. Data on percent woody cover and species composition at the three structural height classes were collected using Braun-Blanquet monitoring technique over 1700 sites in Kruger National Park, South Africa. The two major geology types (basalt and granite) in the region, mean annual rainfall, fire frequency, and elephant density were considered as potential drivers of woody cover at various height classes, using a Bayesian approach. Thereafter, species community composition was assessed under areas of long-term top-down disturbance (fire frequency and elephant density).
Overall, woody cover decreases with increasing size class, alluding to a shrub-dominated woody plant community. However, drivers of woody cover differed across height classes and geology. Geology type was significant across all height classes, with rainfall negatively associated with shrub cover. Fire frequency was negatively associated with brush cover and positively associated with shrub cover, suggesting that increasing fire frequency suppresses shrubs from entering into the brush class. Elephant density was negatively associated with tree cover and agrees with previous studies conducted on smaller scales. This suggests that while bottom-up factors, such as geology influence the distribution of woody plants, top-down disturbances such as fire and herbivory are important to create and maintain variation in vegetation structure. Additionally, legacy effects of woody plants’ long-term exposure to top-down disturbances have influenced community composition, particularly in areas of high disturbance (areas of frequent fires and higher elephant densities). Therefore, this research shows that specific relationships between woody cover at particular height classes exist and this has an influence on woody plant community assemblages. This research also provides much needed context for future management policies on fire and elephants, as decisions will impact the future structural and compositional trajectory of woody plant heterogeneity.