How does elevation and/or substrate affect the composition of biocrusts?

Biological soil crusts (‘biocrusts’) are conglomerations of a variety of organisms including bacteria, lichens and mosses that dominate soil surfaces in arid environments. Biocrusts are important in drylands due to their ability to perform several ecological functions such as soil stabilization and increasing pools of available nitrogen. Higher elevations are typically associated with more precipitation and, consequently, higher vascular plant densities, either outcompeting biocrusts or providing optimal conditions for later successional biocrust communities (lichens and mosses). However, the relationship between the abundance of biocrust and elevation may be obscured by soil type and its effects on nutrient cycling. We tested whether there was a relationship between biocrusts and elevation, and whether differences among biocrust communities were affected by soil type and enzymatic activity (a proxy for nutrient cycling). Biocrust samples were collected from a variety of sites along an elevational gradient between Phoenix (300 m) and Flagstaff (2,100 m) in Arizona while targeting different soil types. We measured % cover of biocrusts and the activity of three enzymes associated with carbon, nitrogen and phosphorus cycles. We found a significant effect of elevation on biocrusts. Lichens and mosses were significantly affected by elevation, while cyanobacteria were not. No significant substrate results were observed, except for the avoidance of granitic soils by mosses and avoidance of basalt by lichens. These results indicate that elevation affects the community composition of biocrusts, but perhaps greater sampling efforts are needed to make more general conclusions about the effects of substrate on biocrust composition