Surface instability driven by wetting-drying cycles hinders the colonization by biocrusts in Tabernas Desert

Abstract Catchment asymmetry in the Tabernas Desert suggests a relatively greater instability in the sunnier hillslope in a very early stage of catchment development due to abiotic factors, which would hinder the biocrust colonization. In the absence of erosion, such a difference in stability between opposite hillslopes could be due to differences in wetting-drying cycles. To verify this, as biocrust types assumed as successional stages are associated with different microhabitats, the surface movements of three types of physical crusts (Structural, Depositional and Island) and four biocrust types, representative of different stages of succession (Incipient, Cyanobacteria, Squamarina and Lepraria), were analyzed based on the distances calculated between markers placed on a grid on the soil surface. Two sample groups were analyzed: in situ samples, with four plots per crust type, and ex situ, with four unaltered samples per crust type extracted from the field site to control the effect of slope angle, orientation, trampling by animals, etc. Physical crusts showed greater surface instability compared to biocrusts, and this instability was influenced by the amount and frequency of precipitation. Biocrusts were more stable and elastic to surface movements, often recovering their initial position, and this stability increased throughout succession. Furthermore, the results showed that reducing instability (when sediment deposition ceases) favors colonization. Our results support the hypothesis that, in absence of erosive events, larger surface instability due to wetting-drying cycles hinders biocrust colonization on the relatively sunnier hillslopes with physical crusts; however, it is unknown where (or when) biocrust can develop.