A head start for an invasive species in a strongly seasonal environment? Growth of Elodea canadensis in boreal lakes

Abstract

Many invasive species are expanding northwards into boreal and subarctic habitats, but research on the factors favoring their establishment in these regions remains limited. In three Swedish lakes we investigated the growth of Elodea canadensis Michx, a highly invasive macrophyte that is spreading northwards in Europe and Alaska. We conducted an in situ growth experiment, maintained for ten months, in concert with two field macrophyte surveys, undertaken in summer and spring. We further compared the performance of propagules established during summer with those established under less favorable conditions in late autumn. We found that E. canadensis grew throughout the autumn, followed by winter dieback, with regrowth occurring early in the spring when water temperatures remained under 5 °C. Elodea canadensis plants were frequently found in our spring field survey, soon after ice melt, when almost all other species were still dormant. In the growth experiment, growth of individual shoots was positively associated with key abiotic variables (especially alkalinity) and also with increasing cover of E. canadensis. The tendency of propagule shoots to fragment from the main stems was also positively associated with increasing E. canadensis length and ambient population cover. Although propagules established in November initially did worse the following spring than those established in August, by the start of the following summer both groups were growing equally well, and had converged in morphology. The growth of E. canadensis throughout autumn and its early re-growth in spring, the capacity of propagules established even in late autumn to regrow well the following year, and the apparently self-reinforcing effect of increasing local population size on shoot growth all have the potential to benefit E. canadensis as it spreads north into boreal and subarctic regions.