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Should I stay or should I go? : assessing range stasis versus range shifts of plants in the North Cascades Wilson, Rachel

Abstract

Under the pressure of anthropogenic climate change, species that are negatively impacted must rapidly respond or risk extirpation. The most immediate option for many species will be to track changing distributions of suitable habitat. Comparisons of contemporary data to historical baseline data indicate that climate change has already altered ranges and abundances of numerous species. Though general patterns are slowly emerging, there is still considerable variation in responses among species. Further, a number of species do not appear to be undergoing any change in their distributions or abundances, despite possible fitness costs of stasis. Given this variation, mechanisms underlying whether species shift or do not shift must be elucidated to allow for the creation of a predictive framework that can be extended to other systems. One way to achieve this end is to associate species functional traits with their magnitude of response. To detect elevational range shifts and changes in abundance of plant species, I and a team of surveyors resurveyed historical vegetation plots in North Cascades National Park. Since the original 1983 survey, the area has warmed by approximately 0.8 ⁰ C. I then tested whether variation in range shifts among species could be associated with functional traits. Overall, most species exhibited range stasis. Of the species that initially appeared to exhibit a range shift, more than half were eliminated after accounting for fires and differences in survey effort between years. Species tended to decrease in abundance within their range, though this trend was often not significant. Predictions from trait models were inconsistent, depending on the modeling framework, the metric used for range shifts, and the inclusion of an outlying species. Range stasis was likely driven by dispersal limitation, but may have also resulted from acclimation, slow demographic processes, microclimate buffering of atmospheric temperatures by landscape features, or some combination of these and other factors. The variation in the degree of range shifts could not be explained satisfactorily by functional traits, casting doubt on their use in a general framework to predict future responses.

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