Increasing temperatures, fluctuating precipitation, and lengthening growing seasons associated with climate change may cause geographical shifts in the climatic niches of many tree species. Stress from these shifting conditions, compounded with interspecific competition, is predicted to force species to migrate in latitude or elevation to avoid extirpation. High-elevation endemics are at particular risk, as they may exist at the highest elevation in a given area without suitable habitat in which to migrate. Research addressing these shifts must specifically quantify the effect of climate on the establishment phase in a tree’s lifecycle, a stage largely ignored in many assessments of future forest conditions, but critical to species persistence. Our research bridges this gap in knowledge by using the Abies concolor-Abies magnifica ecotone in the Sierra Nevada, California, to quantify seedling response to long-term changes in regional climate conditions. Data were collected across the ecotone in Yosemite National Park and the Stanislaus National Forest and paired with a 1981 dataset of collected in YNP. Our primary objectives were to (1) evaluate shifts in forest community composition and ecotone stability potentially associated with climate change and (2) predict seedling survival under future climate scenarios.
Results/Conclusions
Indices of compositional change across the elevation gradient were calculated for 1981 and 2016. Differences in forest composition between sampling years and across elevations were assessed with a permutational multivariate analysis of variance. A. magnifica seedling abundance at lower elevations decreased significantly from historical values (Wilcox Rank-Sum W=60, p=0.005). Elevation explained more of the variation in the overstory than year sampled (elevation F=3.1, p=0.001, year F=8.7, p=0.03), while year explained more variation in seedling abundance (elevation F=3.9, p=0.001, year F=12.1, p=0.001), indicating that the regenerating community has been impacted to a greater extent by changes over time. Changes in community diversity were observed across elevations, with increases in diversity at elevations that were historically dominated by a single species (Sorenson Index A=0.1619, p=0.001). Though A. magnifica seedling density was greater than A. concolor, A. magnifica was more likely to exhibit signs of poor health. This is consistent with the results of the historic study, which found that although A. magnifica reproduces prolifically, younger individuals are less likely to survive to maturity. Our findings indicate that significant changes have occurred in this ecotone in the past 30 years, and that continued compositional shifts can be expected with predicted changes in climate.