Resilience at the Cost of Productivity: Biological Soil Crusts Mediate Vegetation Spatial Self-Organization in Drylands

Abstract

Water redistribution during rain events in drylands plays a critical role in the persistence and spatial pattern of vascular plants in these patchy ecosystems. Biological soil crusts (BSCs) form a membrane in the soil surface and mediate ecohydrological dynamics. However, little is known about their influence on dryland ecosystem state and spatial pattern under changing climate, which may alter total annual rainfall and intraannual rainfall regime. Building on existing models, we develop a model that considers BSC–vascular plant interactions and realistic ecohydrological dynamics under rainfall pulses. We find that the presence of BSCs often increases ecosystem resilience by promoting runoff to plants under high aridity. However, the benefit of BSCs comes at the cost of plant biomass under relatively wetter conditions; a threshold in BSC effect occurs when water losses from BSCs exceed the benefit by their surface water routing to plants. Increased resilience from BSCs, and their own persistence, can be promoted in finer soils and under rainfall regimes of less frequent events—projected for many drylands. Lastly, we find that BSCs alter feedbacks underlying plant spatial self-organization and hence their formed patterns. In high aridity, BSCs likely ameliorate competition between plants through large scale runoff promotion, reducing plant spatial pattern regularity. Our analysis highlights that BSCs significantly shape drylands' response to climate change and their positive effects on resilience may be stronger and more pervasive in a drier future, but such benefits come at a cost of ecosystem biomass and productivity when aridity is outside a critical range.