Dynamics of weed populations in a northern subarctic community

The introduced flora of Churchill, northern Manitoba, was studied to draw some conclusions about the dynamics of weeds in northern subarctic communities in general. One hundred and six introduced species have been recorded at Churchill. Fifty-five percent of these were "temporary," dying out when original colonizers senesced (e.g., Avena spp., Setaria viridis). Seventeen percent were "established," i.e., able to set viable seed and maintain their populations (e.g., Thlaspi arvense, Crepis tectorum). Twenty-eight percent "persisted" locally by vegetative propagation or repeated seed input (e.g., Linaria vulgaris, Chenopodium album s.l.). The findings were compared with those of an earlier study. The number of species and their local distributions around Churchill have increased in the last 30 years. This may be due to an increase and diversification in the traffic with southern regions or to an amelioration in the climate. Although one in five species in the vicinity of Churchill are introduced, usually fewer than 12 weed species were established at any one site. Abandoned sites exhibited a rapid disappearance of weeds. Depauperate individuals of a few species were found away from inhabited areas, with none occurring in natural communities. Organic material in refuse tips provided a mild environment with a long growing season. Refuse tips may act as refuges for weeds and as potential sources of their seeds. Aliens that were successful at seed production were either perennials, winter annuals, or biennials. These plants flowered early and hence are able to ripen their seeds. Years with long growing seasons may allow seed production in otherwise temporary species. Dynamics of weedy species around Churchill will likely reflect current and (or) future events in other, more isolated northern communities. Key words: subarctic, weeds, introduced plants.