Invasive Perennial Grasses in Quercus garryana Meadows of Southwestern British Columbia: Prospects for Restoration

Garry oak (Quercus garryana) meadows of the Pacific Northwest are heavily invaded but the dynamics surrounding this ecosystem transformation are poorly understood. Of particular uncertainty is the role of the invasive species in structuring the community, and the potential stability of this invasive-dominated system when disturbed. Clarifying such issues is central to restoration success for Garry oak meadows, one of Canada’s most endangered terrestrial ecosystems. This paper examines the competitive inter-relationships among exotic and native flora, and how these inter-relationships are modified by a series of disturbance treatments (burning, mowing, and selective removal of invasives). Recruitment dynamics are also described. Pre-treatment surveys of vascular plant taxa identified 80 species within the study area located near Duncan, British Columbia, Canada. Thirty-nine of the species were nonnative. Two of the invasives—Kentucky blue grass and orchard grass—were dominant, averaging a combined 80 percent of total cover in all plots. Disturbance treatments, regardless of type or intensity, caused significant decreases in total percent cover of the exotic dominant grasses, and significant increases in ground level light, total species richness, and total percent cover of native flora. A combination of disturbance and native seed additions revealed that native plant recruitment is limited by primarily by dispersal, though the dense invasive sward probably restricts recruitment success also. These results indicate that the invasive sward could be de-stabilized by disturbance treatments, though supplemental recruitment measures are also needed to restore native species dominance to these meadows.