Impacts of defoliation on tiller production and survival in northern wheatgrass.

Abstract

Although northern wheatgrass (Agropyron dasystachyum (Hook.) Scribn.) is a dominant or co-dominant species that decreases under grazing in northern Mixed Prairie, little is known about its response to herbage removal at different times during the growing season. The objective of this research was to determine the effects of defoliation on the tiller production and survival of this native perennial on a clayey range site in mixed prairie in south-central Saskatchewan. Vegetation was subjected to a factorial experiment with an initial defoliation in early-May, June, July, or August and repeated at 2- or 6-week intervals until mid-September in the same plots for 3 years. An undefoliated control was also included. On average defoliation enhanced tillering (71%) and survival relative to the control, and tiller recruitment was greatest during June and September 1989. Generally tiller survival decreased as the date of emergence in the growing season was delayed. Numbers of tillers emerging was positively correlated with soil water (r = 0.77). Some tillers of northern wheatgrass lived 5 years. The 2- and 6-week intervals of defoliation had little influence on tiller survival, but initiating defoliation near the time of tiller emergence reduced survival whereas delaying defoliation until August increased their survival. Increased tillering may be an adaptive feature enabling northern wheatgrass to tolerate defoliation by re-establishing lost photosynthetic area and maintaining or even increasing basal area. Thus, once released from grazing it may rapidly increase phytomass production in a relatively short time. Delaying grazing until August will maximize tiller survival of northern wheatgrass.