Cyanobacterial soil crusts and woody shrub canopies in Kalahari rangelands

Intensive grazing of Kalahari rangelands has led to woody plant encroachment, notably of Acacia mellifera and Grewia flava. The mechanisms causing this process, and the ecological stability of woody plant encroached ecosystems, remain uncertain. Past studies suggest that canopy-soil relations may enhance woody plant competitive dominance. This study aims to investigate one element of this ecological change by examining the spatial distribution of cyanobacterial soil crusts in two vegetation sub-habitats at sites of different disturbance. Crust burial by litter was also assessed to analyse the dynamics of canopy-crust relations. Our results show there is enhanced cyanobacterial crust cover under A. mellifera canopies and that unlike G. flava canopies, the crust cover remains under A. mellifera even at highly disturbed sites. This canopy-crust association suggests A. mellifera encroachment exhibits intrinsic resilience because of the crusts ability to stabilize the soil surface and increase nutrient retention. Crust burial by litter that accumulates under larger woody plants restricts crust development under canopies. Disturbance restricts crust development in plant interspaces and under G. flava. These two mechanisms combine to restrict crust development to an observed 40% threshold, with nonlinear models required to explain spatial patterns of crust dynamics.