Potential invasions and extirpations of rare agronomic weeds in Northern New England
Weeds are a major production challenge on vegetable farms. Studies typically focus on abundant weeds that pose the greatest present threat to farm operations. In a changing climate, however, currently rare species warrant consideration. Climate change could lead to local extinctions, or allow for increased abundance and potential new invasions by rare species. To study abundance and distribution of agronomic weeds, we collected soil samples from 77 farms located throughout Maine, New Hampshire, and Vermont. Samples were exhaustively germinated in a greenhouse and seedlings identified to species. Rare weeds in each state were defined as species with 200 or fewer total individuals found in that state. Among these rare weeds, species that may become pernicious in the Northeast were identified as those currently problematic in warmer regions, based on the literature. Similarly, rare species most in danger of extirpation were identified based on the literature.
In Maine, New Hampshire, and Vermont, the ratios of rare weed species to total weed species identified were 67:94, 20:64, and 24:67, respectively. Rare weeds with the potential to become increasingly problematic in the Northeast included Chamaesyce maculata, Conyza Canadensis, Panicum capillare, Panicum dichotomiflorum, Polygonum convolvulus, Polygonum aviculare, and Rumex crispus. Among these, three annual species, P. capillare, P. aviculare, and C. canadensis may pose the greatest risk of increased abundance and new invasions due to their weedy traits including drought tolerance (P. aviculare), persistence in the seedbank (P. aviculare), ability to spread rapidly (P. capillare, C. canadensis), and herbicide resistance (P. capillare, C. canadensis). This study represented the first attempt of which we are aware to specifically identify rare agronomic weeds in our region. Further work integrating naturalistic approaches with climate projections could further help to predict potential invasions and identify conservation targets in a changing climate.