Spatial Signatures of Biological Soil Crusts and Community Level Self-Organization in Drylands

Abstract

In dryland landscapes, patches of vascular plants can respond to environmental stress by adjusting their spatial pattern to intercept runoff more effectively, i.e., spatially self-organize, and maintain productivity. However, vegetation patch dynamics in drylands often assumes interspaces of plant patches are composed only of bare soil. Biological soil crusts (BSCs) are complex communities, largely of cyanobacteria, algae, lichens, and bryophytes, living in the soil surface in drylands and often cover more area than vascular plants. BSCs often occur in patches of light cyanobacteria and dark-mixed aggregates and can significantly affect and respond to ecohydrological feedbacks in dryland ecosystems. However, little is known about their spatial patterns and dynamics. In this study, we investigate spatial attributes of BSC patches, their spatial interactions with vascular plants, and factors that drive variation in these attributes. We collected ultra-high-resolution (1-cm) data on spatial patterns of BSCs and vascular plants at 26 sites across three ecoregions of the Southwest of the United States of America. Our analysis shows that light cyanobacterial BSCs vary most in their patch shape complexity along the aridity gradient, while dark-mixed BSCs vary strongly in their abundance. The abundance of dark-mixed BSCs is significantly affected by the soil template, namely soil texture and calcareousness, as well as vascular plants to persist under stress. Furthermore, species associations also change with environmental stress. Light cyanobacteria BSCs, likely a significant source of runoff, may act as a buffer for woody plants against drying, as spatial interactions between these biota become more positive (i.e., spatially aggregated) with greater aridity. While dark-mixed BSCs rely significantly on soil conditions and reduce in abundance as a response to aridity stress, we find evidence that they may have some capacity to spatially adjust under conditions of constant aridity. The interaction of dark-mixed BSCs with light cyanobacteria patches becomes more positive with slope. We conclude that light cyanobacteria BSCs can likely change patch shape in response to water limitation, while dark-mixed BSCs have a reduced capacity to do so - providing further evidence that the abundance of dark-mixed BSCs will decline in the future under drying. BSCs and vascular plants coordinate in space in response to resource availability, suggesting the need to consider self-organization of multiple assemblages to fully understand dryland response to climatic change.