Biotic soil crusts of Oregon's shrub steppe: Community composition in relation to soil chemistry, climate, and livestock activity

We examined biotic soil crust cover and composition at nine shrub-steppe si tes in central and eastern Oregon, U,S.A, One pair of livestock-grazed and excluded transects was established at each site. Data were collected on the cover of biotic sail crust and vascular plant species, soil surface pH and electrical conductivity, and other environmental variables Using gradient analysis, Mie found that differences in community composition among sites w ere most strongly related to soil pH, electrical conductivity (EC), and Cal careous Index Value (CIV; a scale representing the relative calcium carbona te content of soils). Other important variables included precipitation, ele vation aspect and temperature. We found total crust cover to be highest at sites with lower pH, EC, and CN. Dominant species differed markedly between the more calcavreous sites with higher pH, and the less calcareous, lower pH sites Livestock exclusion was not an important gradient in the ordinatio n of these darn, being overshadowed by the strong soil chemistry and climat e gradients However; overall community composition of soil crust species wa s different between grazed and long-ungrazed sites (p = 0.02, Blocked Multi -Response Permutation Procedure). Comparison of grazed and long-ungrazed si tes revealed lower cover of biotic crusts, nitrogen-fixing lichens, crust-d ominated soil surface roughness, and lower species (p less than or equal to 0.02 for all, two-tailed pail-ed t-tests). Our results suggested that tota l bunchgrass cover was higher within exclosures, but conclusive evidence wa s lacking (p = 0.1, two-tailed paired t-test). Vascular plant composition, cower; richness, shrub cover electrical conductivity, and pH were Mat diffe rent between the grazed and livestock-excluded transects. Thus, livestock-r elated reductions in cover. and richness of biotic soil crusts were apparen t while significant impacts to vascular plants were riot obvious. We conclu de that 1) biotic soil crusts are sensitive indicators of disturbance and 2 ) there are strong compositional differences in shrub steppe crust communit ies of Oregon, which are correlated with regional soil and climate gradient s.