Living at the edge: peripheral plant populations and climate change

Abstract

Climate change is expected to place peripheral populations of some species at higher risk of extirpation than others, yet these populations may be significant because they may contain important evolutionary potential. Assessing the conservation significance of peripheral populations is complex, and requires an evaluation of their genetic and ecological distinctness, as well as their likely persistence (extirpation risk) in a changing climate. To address the question of whether rear-edge populations of widespread species are genetically distinct, I used Amplified Fragment Length Polymorphisms (AFLPs) to assess genetic diversity and genetic differentiation of arctic-alpine Anemone parviflora, Dryas integrifolia and Vaccinium uliginosum in Atlantic Canada. The three species showed different genetic patterns at the rear range edge possibly associated with different dispersal capacities and phylogeographic histories. No strong predictors of genetic patterns (disjunction, distance, disturbance) could be confirmed for rear range-edge populations. Some disjunct populations of all three species maintained considerable genetic diversity. To evaluate if rear-edge disjunct populations of A. parviflora, and D. integrifolia at Wilson Brook, NB, are at risk of extirpation from a warming climate, the microclimate of the site was compared to local, regional and northern climates, and short-term responses of population growth to microclimatic variation were evaluated through survey and experimental approaches (Open Top Chambers). The persistence of rear-edge disjunct populations at Wilson Brook may be attributable to a cooler local microclimate. While the temperature manipulation experiment did not increase temperatures in the experimental plots as intended, there were no biologically significant associations of microclimate variables with population growth. Species prioritization based on extinction risk assessments may not be useful for peripheral populations of widespread species, because peripheral populations need to be evaluated at the population rather than species level. I propose a framework that is based on genetic and ecological distinctness as well as extinction risk, and apply it to disjunct range-edge populations of the three species. While none of the disjunct populations were identified as of high conservation significance through this process, my research highlights the value of a population-level approach for these rear-edge disjunct populations.