Runoff from biocrust: A vital resource for vegetation performance on Mediterranean steppes

Abstract

AbstractDryland vegetation is limited by water scarcity, and usually appears in the form of sparsely distributed patches within a heterogeneous unvegetated matrix often covered by biological soil crust. Biocrusts usually act as runoff sources, whereas vegetation acts as sinks, reinfiltrating most of the run‐on from upstream biocrusted areas. Alteration of biocrusts by human disturbances or climate change may exert a strong impact on soil erosion, promoting rill formation, increasing flow connectivity between source areas and reducing run‐on inputs to vegetation, strongly affecting ecosystem functioning. The role of biocrust runoff in vegetation productivity, which to date has not been studied in depth, must therefore be understood to predict the consequences of such changes. In this study, we analysed the response of Machrochloa tenacissima to run‐on exclusion for two years and compared it to the performance of plants of the same species receiving runoff from biocrusted source patches. The results showed that plants receiving run‐on had more photosynthetically active biomass, net C uptake rates and water‐use efficiency than plants under run‐on exclusion. Differences between treatments became wider over time, especially after rainfall, when the differences in water availability were greatest. These results lead to the conclusion that run‐on is a crucial water input for dryland vegetation. Any alteration of unvegetated areas usually covered by biocrust may have important effects on vegetation productivity similar to those from changes in precipitation pattern, and would result in a new, eco‐hydrological equilibrium of the whole ecosystem.