Distribution of Russian knapweed in Colorado: climate and environmental factors

Abstract

Russian knapweed (Acroptilon repens (L.) DC.) was introduced to the western United States during the early 1900s. This invasive perennial was a contaminant of alfalfa seed, and was distributed widely across Colorado. Thus, current distributions reflect the climate and soils tolerances of Russian knapweed, and management history, rather than dispersal processes. We surveyed extension and weed agents across Colorado, and were able to locate 528 current or recently eliminated Russian knapweed stands. These patches were superimposed on climate and soils maps to identify 1 km grid cells that were known to contain Russian knapweed. The status of Russian knapweed within a cell was used as the dependent variable in a logistic regression model to define the environmental envelope for this species. At the scale of our analysis, Russian knapweed was most prevalent on fine-textured soils (clay and clay loam), and in warmer, drier regions of Colorado (precipitation 18-73 cm/yr, mean annual temperature 1-12 degrees C). June precipitation was the most important single factor, although nearly all environmental, annual, and monthly climatic factors were significantly related to Russian knapweed occurrence. The multivariate logistic regression model we developed was used to predict the probability of occurrence of Russian knapweed for the entire state of Colorado. Our predictions matched the areas of highest abundance of Russian knapweed from a new field survey, and also indicated areas of high risk that were not identified by the field survey.