Tussock tundra, composed largely of the arctic sedge Eriophorum vaginatum L., dominates approximately 336,000 km2 of global surface area. A reciprocal transplant experiment, carried out in 1980-82, demonstrated ecotypic differentiation of Alaskan populations of E. vaginatumacross a latitudinal gradient. While such differentiation may be beneficial during periods of climate stability, it may be detrimental in the face of rapid climate change, such as that occurring in present-day Alaska. Following harvest of the transplant study in summer, 2011, we dissected tussocks and gathered age-specific demographic data on tillers to parameterize a Leslie matrix model and to determine tiller population growth (λ) in order to provide a sensitive measure of performance. We used a derivative of jackknifing called Yellow Taxi Analysis (YTA) to determine the mean pseudovalue of λ (Φ) for each of four populations in three gardens. A 2-way ANOVA was carried out with Φ as the dependent variable and ecotype and garden as main effects. The ‘optimum’ environment was determined by regressing Φ vs. change in growing degree days (GDD) represented by each transplant. If a climate shift had occurred, we would expect the optimum to correspond with a negative (northward) change in GDD, i.e., a northward transplanting effect.
Results/Conclusions
The four populations responded differentially to transplanting along a latitudinal gradient (F=2.8559, p<0.0117). A comparison of home source mean Φ and away source mean Φ indicated that home and away sources did not differ within the same garden (F= 2.8030, p= 0.0608). Mean population growth tended to increase when tussocks from more southern gardens were transplanted into more northern gardens, which is indicated by increasing Φ values with negative (northward) changes in GDD. These patterns are consistent with the hypothesis of adaptational lag, in which climatic warming shifts the position of the optimal environment. Interestingly, mean Φ of home sites was 1.048, indicating that climate shift has not yet exceeded the thermal niche of the ecotypes. Nevertheless, E. vaginatum ecotypes have limited ability to migrate and, as a result of rapid climate shift, southern and northern ecotypes face an increased risk of mortality. If large-scale tiller mortality occurs, tussock mortality and, ultimately, changes in community structure will follow. Adaptational lags are likely to manifest first in northern latitudes, however threats of extinction in locally adapted populations will likely spread to southern latitudes with time.