Are Rare Northern Plant Species Retreating from the Southern Edge of Their Ranges in Southern New England?

Abstract - Recent climatic warming is widely expected to cause changes in the geographic distributions of plant and animal species. Potential evidence of such changes includes expansion of populations at the cool range margin and contraction of populations at the warm range margin. We made use of town-level occurrence data from herbaria and state natural heritage programs to assess evidence of potential range contraction in a group of rare (state-listed) northern plant species in southern New England based on differences between historical and current records. For comparison, we examined samples of rare southern species and rare species that were neither northern nor southern (central). The mean annual temperature of towns with current occurrences was significantly lower than in towns with only historical occurrences in the northern and central samples, but not in the southern sample. A potentially confounding variable was human alteration of natural habitats, for which we used human population density as a proxy. Towns with historical occurrences had significantly higher population densities than current towns in the southern and central samples but not in the northern sample, representing a greater overall loss in towns with greater human presence. To separate the potential effects of human habitat alterations and temperature, we analyzed residuals from a regression of town temperature on human population density. When compared between historical and current towns, these residuals were significantly lower in current towns for both the northern and central samples, indicating an effect of climate beyond what could be accounted for by human habitat alterations. For rare southern species, in contrast, the residuals were greater for current than historical towns, indicating that these species were not being disproportionately lost from warmer towns. This is one of a small number of studies that show warm-edge range contractions in a group of plant species, and apparently the only one to consider the potentially confounding effects of human habitat alteration. Our results highlight the value of herbarium specimens in conjunction with recent surveys to document such changes and point to the importance of considering potentially confounding factors in any such analysis.