Conserved thermal performance curves across the geographic range of a gametophytic fern

Species-level responses to environmental change depend on the collective responses of their constituent populations and the degree to which populations are specialized to local conditions. Manipulative experiments in common-garden settings make it possible to test for population variation in species' responses to specific climate variables, including those projected to shift as the climate changes in the future. While this approach is being applied to a variety of plant taxa to evaluate their responses to climate change, these studies are heavily biased towards seed-bearing plant species. Given several unique morphological and physiological traits, fern species may exhibit very different responses from angiosperms and gymnosperms. Here, we tested the hypothesis that previously detected population differentiation in a fern species is due to differentiation in thermal performance curves among populations. We collected explants from six populations spanning the species' geographic range and exposed them to 10 temperature treatments. Explant survival, lifespan and the change in photosynthetic area were analysed as a function of temperature, source population and their interaction. Overall results indicated that explants performed better at the lowest temperature examined, and the threshold for explant performance reflects maximum temperatures likely to be experienced in the field. Surprisingly, explant fitness did not differ among source populations, suggesting that temperature is not the driver behind previously detected patterns of population differentiation. These results highlight the importance of other environmental axes in driving population differentiation across a species range, and suggest that the perennial life history strategy, asexual mating system and limited dispersal potential of Vittaria appalachiana may restrict the rise and differentiation of adaptive genetic variation in thermal performance traits among populations.

Differences in dehydration tolerance among populations of a gametophyte-only fern

PREMISE OF THE STUDY

For many plant species, historical climatic conditions may have left lasting imprints that are detectable in contemporary populations. Additionally, if these historical conditions also prevented gene flow among populations, these populations may be differentiated with respect to one another and their contemporary environmental conditions. For the fern, Vittaria appalachiana, one theory is that historical conditions during the Pleistocene largely shaped both the distribution and lack of sporophyte production. Our goals-based on this theory-were to examine physiological differences among and within populations spanning the species' geographic range, and the contribution of historical climatic conditions to this differentiation.

METHODS

We exposed explants from five populations to four drying treatments and examined differences in physiological response. Additionally, we examined the role of historical and current climatic conditions in driving the observed population differentiation.

KEY RESULTS

Populations differ in their ability to tolerate varying levels of dehydration, displaying a pattern of countergradient selection. Exposure to historical and contemporary climatic conditions, specifically variation in temperature and precipitation regimes, resulted in population divergence observed among contemporary populations.

CONCLUSIONS

Historical conditions have shaped not only the distribution of V. appalachiana, but also its current physiological limitations. Results from this study support the hypothesis that climatic conditions during the Pleistocene are responsible for the distribution of this species, and may be responsible for the observed differences in dehydration tolerance. Additionally, dehydration tolerance may be the driving factor for previously reported patterns of countergradient selection in this species.

Population differentiation and countergradient variation throughout the geographic range in the fern gametophyte Vittaria appalachiana

PREMISE OF THE STUDY

Theory predicts that limited gene flow between populations will promote population differentiation, and experimental studies have found that differentiation is often explained by local adaptation in sexually reproducing angiosperms. However, few experiments have examined the drivers of differentiation among populations in asexual land plants with limited dispersal potential. Here, we evaluated the role of temperature in driving population differentiation in an asexual, obligate gametophyte fern species.

METHODS

We reciprocally transplanted Vittaria appalachiana gametophytes among six populations that spanned the species' geographic range in the Appalachian Mountains and Plateau. Temperature, survival, and senescence rates were measured for 1 year.

KEY RESULTS

Populations had significantly different fitness responses to different sites, consistent with the hypothesis that populations have differentiated across the species' range. There was some evidence for local adaptation in marginal populations and for countergradient selection favoring particularly robust genotypes at the northern range edge. Most populations had relatively high fitness at the site with the most stable temperature conditions and were negatively affected by decreasing minimum temperatures.

CONCLUSIONS

Populations of Vittaria appalachiana exhibit highly variable responses to transplantation across the species' range, and only a small subset of these responses are due to local adaptation. Differences in daily minimum temperature explain some variation in fitness, but other site-specific factors also have significant impacts on transplant fitness. These results indicate that asexual, patchily distributed species with limited dispersal may exhibit population-specific responses to global climate change that have not been elucidated by empirical work focused on sexually reproducing angiosperms.