Null-model analysis and changes in species interactions in biocrusts along a successional gradient in the Tengger Desert, northern China.

Aims: In arid regions, the succession of biological soil crusts (biocrusts) usually accompanies substantial turnover of community composition. How interspecific interactions among biocrust-forming mosses, lichens, algae and cyanobacteria change along the successional gradient is largely unknown. According to the stress gradient hypothesis (SGH), the frequency or intensity of facilitative and competitive interactions within the community will vary along an environmental stress gradient, with an increasing role of competition relative to facilitation under more benign environmental conditions. In the process of biocrust succession, decreasing soil stress also occurs because of slow soil amelioration, which meets the assumption of the SGH. Therefore, we hypothesize that the changes in facilitative and competitive interactions within biocrust communities will follow the prediction of the SGH and that the nature of biocrust interactions will also change. Location: Sand-binding vegetation belts for the Baotou–Lanzhou railway at the southeast fringe of the Tengger Desert in the Shapotou region of the Ningxia Hui Autonomous Region, western China. Methods: We conducted a field evaluation at five adjacent sand-binding vegetation belts built in chronosequence to study the successional gradient. Small-scale patterns of co-occurrence and null models were used to evaluate the community-level interspecific interactions among the biocrust-forming mosses, lichens, algae and cyanobacteria. In addition, an accessory method of species-pair co-occurrence based on null models was also applied to potentially identify the nature of the interspecific interactions within the biocrusts and for the causal investigation of these patterns. Results: A strong spatial segregation of co-occurrence patterns occurred between cyanobacteria and algae and species of mosses and lichens at early succession, and occurred among species of mosses and lichens at late succession, while the weak spatial aggregation between lichen species only occurred at middle succession. Conclusions: Negative interspecific interactions were prevalent within biocrusts throughout succession. The co-occurrence patterns at both the community and species-pair levels are consistent, supporting the unimodal version of the SGH that species interactions shift toward competition under extremely stressed conditions. Given the lack of research on interspecific interactions in biocrusts, our study is important for obtaining a deeper understanding of biocrust succession. [ABSTRACT FROM AUTHOR]