Rapid succession of Biological Soil Crusts after experimental disturbance in the Succulent Karoo, South Africa

Abstract

Biological Soil Crusts (BSC) composed of an intimate association of cyanobacteria, algae, lichens and bryophytes colonize the uppermost millimeters of the soil to form a regular component of the soil surface in arid and semi-arid regions throughout the world. We studied the recovery of a BSC that had been disturbed prior to the rainy season in a field experiment in the Succulent Karoo, South Africa. BSC re-established much faster than expected; only eight months (one rainy season) after removal of BSC from the treatment plots, the crust coverage was greater than its pre-disturbance extent and BSC stability (penetration resistance of the crust) was not statistically different to the control plots. However, composition of the recovering crust differed significantly from controls with the major part of the BSC being made up by an initial light-colored cyanobacteria-dominated BSC. In the following two years, coverage of light-colored (early successional type) crust decreased and dark cyanobacteria-dominated BSC (later successional type) increased whilst total BSC coverage was stable over this time span. The first bryophytes were observed 20 months after disturbance. Chlorophyll contents increased continuously and significantly during the experiment. In contrast to published studies, we found that severe, small-scale disturbances could be repaired quite rapidly and important functions, like soil stability and erosion protection, were functional after only eight months or one rainy season. Full recovery from long-term disturbance seems to take longer with climax stages not being reached five years after termination of grazing impact. These results allow important conclusions with regard to rangeland management and imply that timely and moderate grazing can be reconciled with well developed and functional BSC crust development.