Controls on distribution patterns of biological soil crusts at micro- to global scales

Biocrusts are heterogeneously distributed in space. The drivers of their distribution patterns vary
depending on the spatial scale of observation. Globally, there are about 1337 cyanobacteria,
algae, bryophyte, and lichen species reported as components of biocrusts. At the broadest
biogeographical scales, 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 intra-continental and
smaller scales, climate strongly influences abundance and community composition of biocrusts.
Within drylands, biocrusts become more abundant as precipitation increases. The seasonality of
rainfall is about equally important, with regions receiving most precipitation as winter rain and
snow exhibiting the highest abundance and greatest relative cover of bryophyte and lichens vs.
cyanobacteria. Temperature gradients may dictate the dominant cyanobacterium present in the
community. At eco-regional and smaller scales, edaphic gradients determined by either soil
parent materials or geomorphology or both become particularly influential. Globally, the most
significant soil properties influencing the eco-regional scale cover and richness of biocrusts in
dryland environments are soil texture, pH, and soil CaCO3 content. Sandier soils tend to favour
development of cyanobacterial biocrusts, whereas mosses and lichens tend to be more abundant
on finer textured soils. The alkalinity and CaCO3 content of soils are associated with greater
bioocrust abundance in some regions, and dictates the species composition in the bryophyte and
lichen component. Globally, gypsiferous soils are often associated with distinct floras and high
abdundances of biocrusts, especially lichens. At local to micro-scales, 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
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.