Invasive species removal and changing fire regimes in a ləkʷəŋən Garry oak ecosystem

This thesis examines restoration of Garry oak ecosystems in Southwestern British Columbia, Canada. Before the arrival of European settlers, Coast Salish peoples practiced intensive stewardship and cultivation practices that heavily shaped Garry oak ecosystems. These long-standing stewardship practices are responsible for the abundance of culturally important plants found in Garry oak ecosystems today. In addition to their cultural value to Coast Salish peoples, Garry oak ecosystems also support unique biodiversity, including numerous at-risk species. These ecosystems and the values they embody came under threat with the arrival of European settlers, who introduced non-native plants and excluded Coast Salish peoples and their stewardship practices from these ecosystems. Today, Garry oak ecosystems have been reduced to a fraction of their pre-colonial distribution and remaining patches are typically heavily invaded by both native and non-native plants. Their cultural and biological values coupled with ongoing degradation has motivated both Indigenous and non-Indigenous land managers to implement restoration programs in Garry oak ecosystems. To inform future restoration efforts, this thesis examines ecological impacts of a long-term restoration program and a wildfire in a lək̓ʷəŋən Garry oak ecosystem at Mill Hill Regional Park near Langford, British Columbia. In Project 1, vegetation responses to a 13-year invasive species removal program were quantified to determine if native plant populations were successfully bolstered by the removal efforts. In Project 2, impacts of an unintended wildfire on the relative cover of native and non-native plants were examined. This attempted to explore potential ecosystem shifts that may occur as wildfires increase in frequency and severity as predicted by climate models. In Project 1, the greatest change observed after invasive species removal was an increase in other introduced species, while increases in native species were not statistically significant. In Project 2, introduced Anthoxanthum odoratum was facilitated by fire while native Camassia spp. were reduced by it. Taken together, these results demonstrate the complexity of restoring Indigenously managed ecosystems where multiple introduced species have existed for long periods. Invasive species, specifically Anthoxanthum odoratum, showed greater responses to removal efforts and wildfire than native species. Intensive, long-term restoration programs that utilize multiple tools, including low-intensity fire, invasive removal, herbicide, and seeding of native species appear necessary to bolster native species without unintentional facilitation of introduced species. Coast Salish peoples and stewardship practices were integral in maintaining these ecosystems before the arrival of European settlers and should play a key role in their restoration today, though traditional practices will likely need adapted to account for environmental changes caused by colonization. Furthermore, to avoid continuing the cultural damage that began with colonization, it is vital that Coast Salish First Nations lead or be directly involved in restoration of these ecosystems, which continue to hold irreplaceable cultural value.

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