Restoring a Garry oak ecosystem: Native and exotic vegetation recovery following Scotch broom removal

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Cole D. Lysgaard, Nancy Shackelford and Darcy Mathews, School of Environmental Studies, University of Victoria, Victoria, BC, Canada

Background/Question/Methods
This study examined vegetation response to invasive species removal in a Garry oak ecosystem near Victoria, British Columbia. Garry oak ecosystems are characterized by open woodland with a sparse canopy of Garry oak (Quercus garryana). These ecosystems are highly biodiverse and host plants of cultural importance to First Nations, as well as many endemic species. Due to ecosystem degradation, Garry oak ecosystems occupy only a fraction of their former habitat, and approximately 100 species of flora and fauna found within them are considered at-risk. Scotch broom (Cytisus scoparius) is one of the most widespread invasive species in Garry oak ecosystems. This project examined how vegetation communities in Garry oak ecosystems responded to Scotch broom removal. Our aim was to determine whether Scotch broom removal achieved land management goals, namely increased abundance of native species and decreased abundance of invasive species. Twenty 3x3 meter plot pairs were established in a Garry oak ecosystem in 2008. Each June from 2008-2015, Scotch broom was removed from one plot in each of the pairs and percent cover was then recorded for all plant species in all plots. Analyses compared vegetation changes between control and treatment plots, in order to determine effects of Scotch broom removal.

Results/Conclusions
One of the few consistent trends suggested by our results was an increase in cover of sweet vernal grass (Anthoxanthum odoratum) in plots where Scotch broom was removed. Sweet vernal grass is an invasive grass also common in Garry oak ecosystems. Sweet vernal grass cover showed little change in control plots, suggesting that it directly benefitted from Scotch broom removal. Native Camassia spp. (great camas/C. leichtlinii and blue camas/C. quamash) showed rapid increases in cover in treatment plots over the first two years after initial Scotch broom removal, but then declined slowly for the duration of the study. However, Camassia spp. cover showed a net increase in treatment plots that was not seen in control plots. With regard to management goals, these results suggest that Scotch broom removal alone is not effective in promoting native plants. Instead, Scotch broom appears to be replaced by other invasive species, in this case sweet vernal grass. Management efforts will likely be more effective if Scotch broom removal is paired with other methods, such as seeding of native plants and manual or chemical removal of invasive grasses.