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Shrubs, bunchgrasses, and biological soil crusts (biocrusts) are believed to contribute to site resistance to plant invasions. 

USGS scientists Lea Condon and David Pyke tested the idea that biotic communities mediate the effects of disturbances such as fire and grazing on site resistance by using structural equation modeling to test relationships between disturbance events, the biotic community, and resistance to cheatgrass invasion. Increased site resistance following fire was associated with higher bunchgrass cover and recovery of bunchgrasses and mosses with time since fire. Fire reduced near-term site resistance to cheatgrass on actively grazed rangelands, and evidence of grazing was more pronounced on burned sites and was positively correlated with cheatgrass cover. Independent of fire, grazing impacts resulted in reduced site resistance to cheatgrass, suggesting that grazing management that enhances plant and biocrust communities will also enhance site resistance.

Condon, L.A., Pyke, D.A., 2018, Fire and grazing influence site resistance to Bromus tectorum through their effects on shrub, bunchgrass and biocrust communities in the Great Basin (USA): Ecosystems, https://doi.org/10.1007/s10021-018-0230-8.

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