Exotic plant species in the southern boreal forest of Saskatchewan

Exotic species possess abilities to harm the ecosystems they invade. This study assesses the density, frequency and cover of exotic plants in roadside right-of-ways, logged areas and wildfire sites within mixedwood sections of the southern boreal forest of Saskatchewan. A total of 23 exotic species were observed including nine species of Gramineae, seven species of Leguminosae and five species of Compositae. Average density of exotic species in areas recently disturbed by timber harvesting or wildfire was 0.2 stems m−2 with a frequency of 72%. Exotic species adapted for wind dispersal were best represented including common dandelion (Taraxacum officinale), perennial sow thistle (Sonchus arvensis) and annual hawksbeard (Crepis tectorum). Only two exotic species, T. officinale and Canada bluegrass (Poa compressa), were observed in mature forest; both occurred with a frequency of 13% and an average density of 0.002 stems m−2. A total of 22 exotic species was found in the right-of-ways quadrats with an average density of 117 stems m−2 and a frequency of 94%. The most frequently observed exotic species in the roadside right-of-way areas were T. officinale, alsike clover (Trifolium hybridum), S. arvensis, creeping red fescue (Festuca rubra) and smooth brome grass (Bromus inermis). These species are either common agricultural weeds or were part of the original seed mixture used to establish a plant cover in the roadside right-of-ways.Each of the exotic species observed was ranked to determine its current and potential threat to ecosystem attributes. The majority were considered a lesser threat and easy to control, with eight species ranked as a lesser threat and hard to control, and one species, B. inermis, ranked as a serious threat and hard to control.The distribution of exotic species was closely related to the management of roadside right-of-ways, wildfire events and timber harvesting. An increased distribution of exotic species was observed within roadside right-of-ways areas which had been subjected to more intensive management. Although the exotic species observed in this study are maintained and dispersed by management activities, the majority of exotic species are not a threat to this forest ecosystem in the southern boreal forest of Saskatchewan.