Conveners: Jayne Belnap and Ali Nejidat
Assembly rules and patterns in dryland vascular plant communities: biological soil crusts can play an influential role
Jayne Belnap, U.S. Geological Survey, Southwest Biological Science Center, 2290 S. Resource Blvd., Moab, UT 84532, U.S.A
YuanMing Zhang, Key Laboratory of Biogeography and Bioresource in Arid Land, Xinjiang Institute of Ecology and Geography, Chinese Academy of Sciences (CAS), Urumqi 830011, PR China
Factors that influence plant community composition and patterns have long been sought by ecologists, and the concept of assembly rules has influenced much recent thought. The first step in plant establishment is successful seed dispersal/germination and identifying controls on this stage is critical in understanding drivers of vascular plant community structure. Most studies examine how disturbance and plant-plant/plant-animal interactions affect assembly rules. However, we suggest that in drylands, plant-biocrust-disturbance interactions determine plant community composition by controlling what seeds enter the soil bank. We compared plant and seedbanks composition on four surface types (uncrusted, cyanobacteria, lichen, and moss). We also characterized seed weight, size, or plant life form; none of these factors influenced plant community or seedbank composition on the different surfaces. We observed that the highest diversity and richness of plant species and seeds in the seed bank was found on recently disturbed areas that were either uncrusted or covered by early successional cyanobacterial crusts, whose low biomass has little influence on soil characteristics. In contrast, diversity and richness of plants and seeds in the seedbank dropped significantly in late successional lichen- or moss-covered soils, due to the lack of plant seeds with appendages. Lichens and mosses greatly stabilize and reduce permeability of soils, reducing the opportunity for seeds to be buried. However, disturbance readily resets biocrusts to an early successional state, thus providing ample opportunity for seeds to be buried. We propose that the structure, and thus function, of this dryland region is substantially influenced by the tension between the forward forcing of biocrust succession and the backward forcing of disturbance that creates a shifting mosaic of patches with different plant, biocrust, and soil characteristics through time and space and across multiple scales. Therefore, we posit that the interaction between biocrusts and disturbance is an underlying assembly rule for this, and likely most, temperate deserts.