Density-dependent spatial patterning of woody plants differs between a semi-arid and a mesic savanna in South Africa


Savannas can be defined by the co-dominance of grasses and trees. Interactions between these two life forms are relatively well studied, whereas intra-life form interactions among woody plants only attracted recently increased attention. However, the influence of woody plant density is rarely considered. We analysed intra-life form interactions in a semi-arid and a mesic savanna to test for differences between open and dense woody vegetation in relation to large-scale environmental conditions. We applied spatial point pattern analysis to gain a better understanding of processes, such as competition, facilitation and disturbances, affecting the spatial distribution of trees and shrubs. Our results suggest that competition was most pronounced in dense vegetation, whereas facilitation effects were more common in open vegetation. Further, we conclude that factors shaping the spatial patterns differ with scale. At small inter-plant distances, results indicate limited seed dispersal as the most influential factor explaining the spatial distribution of trees and shrubs. However, with increasing inter-plant distances, environmental heterogeneity in the semi-arid savanna and disturbances in the mesic savanna become more important. We conclude that studying intra-life form interactions in savannas should explicitly consider actual woody plant density, especially if different savanna types are compared.

Journal of Arid Environments
Maximilian H.K. Hesselbarth
Postdoctoral Fellow

Ecological Modelling