IMPACTS OF DEFOLIATION ON TILLER PRODUCTION AND SURVIVAL IN NORTHERN WHEATGRASS

Although northern wheatgrass (Agropyron dasystachyum (Hook.) Scribn.) is a dominant or co-dominant species that decreases under grazing in n orthern Mixed Prairie, little is known about its response to herbage r emoval at different times during the growing season. The objective of this research was to determine the effects of defoliation on the tille r production and survival of this native perennial on a clayey range s ite in mixed prairie in south-central Saskatchewan. Vegetation was sub jected to a factorial experiment with an initial defoliation in early- May, June, July, or August and repeated at 2- or 6- week intervals unt il mid-September in the same plots for 3 years. An undefoliated contro l was also included. On average defoliation enhanced tillering (71%) a nd survival relative to the control, and tiller recruitment was greate st during June and September 1989. Generally tiller survival decreased as the date of emergence in the growing season was delayed. Numbers o f tillers emerging was positively correlated with soil water (r = 0.77 ). Some tillers of northern wheatgrass lived 5 years. The 2- and 6-wee k intervals of defoliation had little influence on tiller survival, bu t initiating defoliation near the time of tiller emergence reduced sur vival whereas delaying defoliation until August increased their surviv al. Increased tillering may be an adaptive feature enabling northern w heatgrass to tolerate defoliation by re-establishing lost photosynthet ic area and maintaining or even increasing basal area. Thus, once rele ased from grazing it may rapidly increase phytomass production in a re latively short time. Delaying grazing until August win maximize tiller survival of northern wheatgrass.