Influence of Precipitation on Perennial Grass Production in the Semidesert Southwest

Perennial grass production in the semidesert grass—shrub type (with and without a velvet mesquite (Prosopis juliflora var. velutina Woot. Sarg.) overstory) was dependent primarily on current summer rainfall and previous summer rainfall. The influence of previous summer rainfall was an interaction effect–not a direct effect. The best overall relationship involved current August rainfall, previous June through September rainfall, and the interaction product of these two. However, in interaction product along yielded estimates essentially as good as the multiple regression, and explained 64%—91% of the year—to—year variability in grass production. Winter precipitation had no consistent effect on perennial grass production the following summer. The degressing effect of mesquite on perennial grass production was most noticeable at low rainfall levels, but became minor at high rainfall levels.