Conditions responsible for vegetative differences between the Great Basin and Great Plains are poorly understood. Two hypotheses for the differences are (1) the geophysical barrier of the Rocky Mountains and a slow rate of dispersal of taxa restrict interchange of taxa between the Great Basin and Great Plains, even where gaps in the mountains occur; and (2) differences in climate between the two regions limit floristic intermixing. We examined relationships for both hypotheses using vegetative data from 14 study sites in Idaho, Colorado, Wyoming and Montana. Canopy cover of graminoids, forbs and shrubs was regressed against selected climatic variables and distance of each study site from the Rocky Mountains. Shrub cover diminished, graminoid cover increased and forb cover remained roughly constant on a west-to-east gradient. Graminoid cover increased and forb cover remained roughly constant on a west-to-east gradient. Graminoid cover was positively correlated with summer precipitation, and shrub cover was positively correlated with winter precipitation and negatively correlated with summer precipitation. These vegetative variables were more highly correlated with climatic data than with distance from a broad gap in the Rocky Mountain chain. Current distributions of Great Plains and Great Basin taxa suggest that the Rocky Mountains have not been an effective barrier to plant dispersal in Wyoming. Our data suggest that seasonality of precipitation restricts dispersal of taxa between the Great Basin and Great Plains.
