COS 255-2 - Disentangling constraints on plant population establishment following range expansion

Abstract: Species are shifting their distribution limits along elevation gradients in response to changing climate, with upwards shifts occurring on average. But temporal studies typically report substantial lags as well as large variation in the magnitude and direction of range shifts, which might be partly attributable to dispersal limitation or interactions among species that inhibit or promote population establishment. If so, species functional traits related to dispersal or competitive ability might prove useful in explaining variation in range expansion rates.

We ask, firstly, whether lowland plants are already able to establish in high elevation communities under current or warmer climate, indicating dispersal limitation or biotic resistance. To do so, we performed transplant experiments in four mountain regions (Swiss Alps, Norwegian Scandes, Maloti-Drakensberg in South Africa, Changbai mountains in China), where lowland plants are planted several 100 m above their current range limits. We find that most species are able to establish above their limits, suggesting that their current distributions might be dispersal limited. However, performance of most species was substantially suppressed when grown with resident vegetation, suggesting that even without dispersal limitation, competition might present a significant barrier to the establishment of lowland species, at least under current climatic conditions.

Data on plant functional traits are being collected and will be correlated with variation in the demographic performance of lowland species to identify putative traits associated with successful establishment. However, evidence from a separate competition experiment already points to traits being useful predictors of which species are best able to persist with competitors. In this experiment, we grew pairs of 7 low- and 7 high-elevation plants in three sites across an elevation gradient in western Switzerland. We found that many species pairs were predicted to coexist, even when one species in a pair was growing outside of its current range limit. Furthermore, functional traits related to resource capture were correlated with the predicted ability of focal species to persist with competitors, but mainly under warmer climate at the low elevation site. Together, our results suggest that biotic interactions like competition might underlie some of the heterogeneity in species' climate change range shifts, and raises the possibility that some of this variation might be explained using species traits.