Plot-Measured Variables Indicate Landscape-Scale Patterns of Annual Grass Invasion in Northwestern US Rangelands

Introduced annual grasses (IAGs) have invaded tens of millions of hectares of western US rangelands, displacing native vegetation and altering ecosystem functions and services. Our main goal was to assess the influence of introduced annual grasses by 1) exploring the relationships among IAG cover, species richness, diversity, and cover of other plant functional groups; and 2) identifying biotic and abiotic indicators of IAG cover. Using data from 64 sites and 420 plots from rangelands of the northwestern United States, we explored linear correlations between IAG cover and species richness, Shannon index, and plant functional group cover. We also used Least Absolute Shrinkage and Selection Operator (LASSO) regression and nonmetric multidimensional scaling to select environmental variables and plant functional groups that were associated with IAG cover. Our findings revealed a negative correlation among IAG cover and species richness, Shannon index, native perennial grass cover, and other perennial species cover and a positive correlation with other annual species cover. Our results identified several indicators: cover of native perennial grass; other perennial species (forbs and shrubs); litter; bare ground/rock; soil attributes (pH, bulk density, clay, and nitrogen); and mean annual temperatures. Our results showed that perennial vegetation, especially native perennial grasses, have a negative correlation with IAG. Warmer mean annual temperatures, but not higher daily extreme temperatures, and higher bulk density were important indicators for IAG preferred sites, while lower soil nitrogen may denote soil changes due to invasion. Given the fundamental role of disturbance in determining the magnitude and scale of IAG invasion, our results provide important insights for scientists and managers.