Precipitation and nitrogen deposition alter biocrust–vascular plant coexistence in a desert ecosystem: Threshold and mechanisms

Abstract

Abstract Dryland ecosystems exist in various vegetation landscapes with contrasting compositions of vascular plants and biocrusts (surface‐soil mats comprised of soil particles, cyanobacteria, lichens, mosses and fungi), and they are very vulnerable to ongoing global change. However, compared to vascular plants, the responses of biocrusts and their coexistence with vascular plants to global change remain elusive. We conducted a 5‐year experiment involving multi‐level precipitation and nitrogen addition in a desert shrubland of northern China to examine these responses. We found that shrubs, perennial herbs and annuals exhibited facilitative, competitive and insignificant effects on biocrusts respectively. Biocrusts coexisted with vascular plants below a threshold of herbaceous productivity of approximately 70 g/m 2 and tended to be replaced by herbs beyond this threshold, resulting from the balance between the shade tolerance of biocrusts and herb‐induced light limitation. Increased precipitation and nitrogen enrichment altered biocrust–vascular plant composition by influencing this balance and tended to shift from a biocrust‐dominated to a vascular plant‐dominated state when the threshold was crossed. Synthesis . Our results suggest that biocrusts can coexist with dwarf shrubs and annuals, but cannot withstand perennial herbs when the herbaceous productivity threshold was exceeded. Our findings provide new insights into the threshold and mechanisms of biocrust–vascular plant coexistence, which may greatly contribute to managing dryland ecosystems for global change.