Biotic and abiotic characteristics interact with fire history to influence biological soil crust cover and composition in the Columbia Basin

Background/Question/Methods

Biological soil crusts (BSCs) are composed of organisms that live at the soil surface in arid and semiarid lands and provide services such as nitrogen fixation and preventing erosion. The relative importance of different factors affecting BSC cover and composition is poorly understood, but can guide decisions to preserve BSC diversity and associated ecological services. Abiotic conditions (topography, soil characteristics) affect available moisture and nutrients. Biotic factors (vegetation cover, litter) affect growing space through shading and shoot density. Disturbances (fire) remove or damage crust organisms or change their microhabitat through the removal of vegetation. These factors also interact, creating complex relationships. We examined the factors that control BSC communities in the sagebrush-steppe in the Columbia Basin. Much of this habitat has been converted to agriculture and pasture, and remaining areas have been invaded by the non-native cheatgrass (Bromus tectorum) and have experienced increased fire frequency. We sampled the BSC and vegetation communities across a range of elevations and disturbance histories. Environmental and soil characteristics were recorded. We used structural equation modeling to understand how explanatory variables interacted and related both directly and indirectly to BSC cover. We related patterns of environmental, vegetation, and disturbance factors to BSC composition.

Results/Conclusions

Fire history affected BSC cover directly and indirectly through its effects on vegetation and litter.  Moss cover was not affected by vegetation type or cover. Edaphic factors such as elevation and soil texture were more strongly related to BSC composition. Small crustose lichens and mosses were associated with more disturbed sites, and foliose and fruticose species with moister microsites. High cover of cheatgrass is strongly related to absence of BSC. Plots showed much higher variation in BSC than vegetation composition, and vegetation was a poor predictor of BSC composition, especially on low elevation, sandy soils. Implications for stewardship to protect extent and diversity of BSC communities in cool deserts include preventing dominance of densely growing invasive species, reducing the frequency and spread of fires, and prioritizing the most affected regions by accounting for interactions among environmental, vegetational, and disturbance characteristics