Multi-scale patterns and bush encroachment in an arid savanna with a shallow soil layer

Question: Bush encroachment (i.e. an increase in density of woody plants often unpalatable to domestic livestock) is a serious problem in many savannas and threatens the livelihood of many pastoralists. Can we derive a better understanding of the factors causing bush encroachment by investigating the scale dependency of patterns and processes in savannas? Location: An arid savanna in the Khomas Hochland, Namibia. Methods: Patterns of bush, grass, and soil nutrient distribution were surveyed on several scales along a rainfall gradient, with emphasis on intraspecific interactions within the dominant woody species, Acacia reficiens. Results: Savannas can be interpreted as patch-dynamic systems where landscapes are composed of many patches (a few ha in size) in different states of transition between grassy and woody dominance. Conclusions: In arid savannas, this patchiness is driven both by rainfall that is highly variable in space and time and by inter-tree competition. Within the paradigm of patch-dynamic savannas, bush encroachment is part of a cyclical succession between open savanna and woody dominance. The conversion from a patch of open savanna to a bush-encroached area is initiated by the spatial and temporal overlap of several (localized) rainfall events sufficient for Acacia germination and establishment. With time, growth and self-thinning will transform the bush-encroached area into a mature Acacia stand and eventually into open savanna again. Patchiness is sustained due to the local rarity (and patchiness) of rainfall sufficient for germination of woody plants as well as by plant-soil interactions.