Drought at erosion edges selects for a ‘hidden’ keystone species

Abstract

Background: The presence of plants is crucial in securing steep slopes against soil erosion. Inappropriate land use in mountains often leads to vegetation loss and thus soil degradation. Aims: Here we ask if the edges of large erosion gullies select for specialist plant species that reduce or prevent the progression of soil loss. Methods: We quantified species presence and abundance across micro-transects from intact mountain pastures toward the edge of erosion gullies at ca. 1900 m elevation in the Central Caucasus, Georgia. Results: Out of a large species pool, one particular species, Festuca valesiaca, was the dominant species at the very edge of erosion gullies. Increased δ13C values in Festuca valesiaca leaves by 1.1‰ towards the edge confirmed that this species copes best with the dry conditions at the edge. Conclusion: Our findings illustrate the insurance effect of a highly diverse vegetation. The importance of a single species out of this diverse species suite to sustain key ecosystem functions becomes apparent only under extreme environmental conditions; in this case, at edges of erosion gullies.