| Abstract |
- This research examines the community composition of biotic soil crusts at nine
sites in central and eastern Oregon, U.S.A. At each site, data were collected in one pair
of livestock-grazed and excluded transects. Variables recorded included: cover of biotic
soil crusts and vascular plant species, soil surface pH, electrical conductivity, soil surface roughness, surface rock, gravel, bare ground, organic litter, and cowpies. Using Non-metric Multidimensional Scaling, we extracted the primary environmental gradients controlling biotic crust community composition among sites. The strongest gradient was related to soil chemistry. Total crust cover, lichen cover, and bryophyte cover were inversely related to soil pH, electrical conductivity, and CIV (Calcareous Index Value; site scores on a scale representing relative soil calcium carbonate content). Total crust cover was also positively associated with temperature, and inversely related to elevation. Biotic crust composition was related to moisture and aspect; cooler, north-facing slopes with greater precipitation and Festuca idahoensis
cover were compositionally distinct from flat sites with greater heat exposure. The weakest pattern in these data appears to represent a disturbance gradient, with bare ground inversely related to exclosure age. Transect scores on this gradient were different between currently grazed and exclosure transects ( p = 0.01, blocked MRPP). On average, bare ground was greater in the grazed transects, while total crust cover,
cover of nitrogen-fixing lichens, biotic soil surface roughness, and species richness were
greater within the exclosures (p 5 0.02 for all, two-tailed paired t-tests). Differences in vascular plant cover, richness, and composition were not detected. CIV was one of the strongest variables related to biotic crust composition. CIV is based on a calcareous index that uses the presence of certain lichen species as indicators of calcareous soils. Using weighted averaging and the abundances of these indicators in
our data, we revised this index, incorporating new indicators and removing poor ones. We conclude that 1) biotic soil crusts are sensitive indicators of disturbance, 2) there are strong compositional differences in shrub steppe crust communities of Oregon, which are related to regional soil and climate differences, and 3) lichens can be used as
sensitive indicators of soil chemistry.
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