Analyses of a 30-year-old reciprocal transplant study in northern Alaska with the tussock-forming sedge, Eriophorum vaginatum, showed a pattern of local ecotypic adaptation. We revisited this study to seek evidence for adaptational lag – a condition whereby populations are in a state of disequilibrium with their current environment due to rapid change, which would be manifested in this case by displacement of the climatic optimum northward. To obtain a sensitive measure of this shift in relative performance, we constructed a stage-based matrix model of E. vaginatum tillers using size measurements taken in 2010 and 2011. In order to integrate effects of tiller survival, growth, and reproduction, we estimated tiller population growth rates (λs), separately by garden and ecotype. To analyze the reciprocal transplant as a 2-way factorial design, we performed a Yellow Taxi Analysis – a derivative of Tukey’s Jackknife procedure that allows calculation of pseudovalues (Φi) of λ for each individual tiller. The optimum climate was determined by regressing mean pseudovalues versus the shift in growing degree days (ΔGDD) represented by each ecotype x garden combination. If the optimum had moved northward, the peak of this regression should occur where populations have been transplanted north from their original locations.
Results/Conclusions
Ecotype performance depended on garden (Fecotype*garden=1.84, p=0.0066) in a pattern consistent with local adaptation because in every garden the local ecotype had the highest or second highest mean Φi . When mean Φi values were regressed on the ΔGDD represented by each ecotype-garden transplant, we found that there was in fact an optimum climate for ecotypes, as indicated by a concave response surface. However, this optimum occurred at a ΔGDD = -453 (+/- 5.25), i.e., on average farther north than the home site. This is suggestive evidence that the climate has shifted away from the long-term optimum for each ecotype. Tiller population growth predicted by the regression for home sites (ΔGDD = 0) was 1.04, suggesting that, thus far, the climate shift has not exceeded the thermal niche of E. vaginatum ecotypes. Continued climate change would be predicted to result in tiller population declines, and eventually mortality. Resistance of E. vaginatum populations to rapid climate change could come about via long-distance northward dispersal, by strong natural selection within populations, or a combination of both. Either process requires establishment of new generations of individuals, a process that is inherently slow in tussock tundra in the absence of disturbance.