Influence of Season and Intensity of Defoliation on Bluebunch Wheatgrass Survival and Vigor in Southern British Columbia

Abstract

Bluebunch wheatgrass (Agropyron spicatum Pursh) Scribn. & Smith) response to various clipping regimes which incorporated different times, frequencies, and intensities of defoliation was examined in southern British Columbia. The experiment was repeated for 3 consecutive years at a low (296 m) and a high (1,112 m) elevation site. Plant survival and vigor was evaulated the summer following defoliation. Greatest injury was incurred by treatments involving defoliation to a 5-cm stubble height from mid April to the end of May or from early May to mid June at the low and high elevation sites, respectively. Reduced injury occurred from treatments which left 10 or 15-cm stubble heights or which ceased defoliation earlier in the season. No appreciable damage was incurred by fall clipping to 5 cm or by season-long defoliation to 20 cm. Injury resulting from spring plus fall as compared to spring only defoliation was inconsistent. Greatly reduced injury for many treatments at the low elevation site in one year was attributed to unusually warm spring temperatures and attendant rapid spring growth. Bluebunch wheatgrass (Agropyron spicatum (Pursh) Scribn. and Smith) reaches the northern limits of its distribution on the grasslands of southern British Columbia. Although the species has been studied extensively in the United States, we wished to confirm application of this work, in particular, response to season of defoliation, to southern British Columbia. This information is required for grazing management planning. This study formed part of a larger project on the autecology of bluebunch wheatgrass. Quinton et al. (1982) examined vegetative and reproductive growth of the species in relation to weather. They found that bluebunch wheatgrass in the Kamloops area started growth immediately after snow melt. Growthceased from 7 May to