Controls on Distribution Patterns of Biological Soil Crusts at Micro- to Global Scales

Abstract

Biological soil crusts (biocrusts) are heterogeneously distributed in space, and the drivers of their distribution depend on the spatial scale of observation. Globally, there are about 1544 cyanobacteria, algae, bryophyte, and lichen species reported as components of biocrusts. At the global scale, the degree and age of isolation of land masses may dictate distribution of these species and the similarities of the floras of different continents. At intracontinental and smaller scales, climate strongly influences abundance and community composition of biocrusts. Within drylands, biocrusts become more abundant and diverse with increases in precipitation. The seasonality of rainfall is about equally important, with regions receiving the most precipitation in winter exhibiting the highest abundance. At ecoregional and smaller scales, edaphic gradients become particularly influential. The most significant soil properties influencing the cover, richness, and composition of biocrusts in dryland environments are soil texture, pH, and calcareousness. Additionally, gypsiferous soils are often associated with distinct floras and high abundance and diversity of biocrusts, especially lichens. At local to microscales, biocrusts often are better developed in habitats with lower radiation loads such as polar-oriented slopes or shaded habitats. Also at small scales, vascular plant canopies buffer microclimate for biocrusts, but also can exert negative influences such as burial by litter. While our knowledge of biocrust distribution has advanced rapidly, there are considerable geographic and taxonomic gaps in our knowledge and a pronounced lack of truly global studies.