Spatial and temporal scale-dependent feedbacks govern dynamics of biocrusts in drylands

Biota could be ecosystem engineers in generating an intrinsic heterogeneous landscape through scale-dependent feedbacks. Thereby, they can form resource-enriched patchiness or islands of fertility, comprising self-organizing spatial patterns. Research so far has largely focused on the self-organized spatial patterns of plant communities in drylands. It, however, remains unclear whether and how biocrusts having distinct morphology and life history from plant communities could self-organize themselves and form unique spatial patterns. Here, we conducted field observations of biocrusts across successional stages and employed a probabilistic cellular automaton model to investigate the distinct self-organized spatial patterns exhibited by mosaic patches of mosses and lichens with different patch size distributions (PSDs). Our study demonstrates that short-range positive feedbacks initially promote the development of patches, featured with a heavy-tailed PSD, while long-range negative feedbacks subsequently curtail further expansion of big patches, thereby establishing a characteristic patch scale with regular PSDs. Strikingly, only lichens reverted back to the heavy-tailed PSD in the late succession stage, presumably implying self-organized critical fragmentation of lichen patches. Field measurements of biocrust performance at the center and edge of patches of varying sizes along succession stages further support the classic scale-dependent feedback mechanism for Turing pattern formation. Collectively, our results clearly demonstrate the capability of the biocrust communities to self-organize themselves to form distinct spatial patterns governed by the spatial and temporal scale-dependent feedbacks, potentially impacting dryland ecosystem functions and resilience.