Climate change is shifting forest tree species distributions across elevational and latitudinal gradients, and forest composition is expected to change first in ecotones where species meet their climatic bounds and are replaced by other species. Using an extensive ecotone composed of lower-montane white fir (Abies concolor var. lowiana) and upper-montane red fir (Abies magnifica var. magnifica) in the central Sierra Nevada range of California, USA, we (1) assessed how the composition of the ecotone has responded to recent climate change, (2) quantified climate tolerances across species life stages, and (3) tested the potential impacts of future climate conditions on species-specific seedling survival and growth. To do so, we established a field observational study to evaluate contemporary abundances across species life stages (seedlings, saplings, and trees) and compared compositional changes over a 35-year period with a historic dataset. We then assessed climatic drivers of species life stage abundances using contemporary compositional data, and conducted a fully-factorial growth chamber experiment examining climatic controls of seedling growth and survival.
Results/Conclusions
Forest conditions across the A. concolor – A. magnifica ecotone indicate spatial and temporal stability in climate conditions necessary for seed production and tree recruitment of both species within the ecotone. Seedling and mature tree life stages exhibited extensive co-occurrence in elevational distribution, and similar proportional abundances were observed in each species over the 35-year period. Observational and experimental assessments of climatic drivers suggest that temperature and precipitation serve as the governing mechanisms limiting A. concolor versus A. magnifica distributions and are primary stressors at the seedling stage, with A. magnifica exhibiting greater sensitivity to increases in temperature, soil drying, and growing season length under historic and future conditions. Our data suggest an upward shift in climate equivalent to ~500 m in elevation, or the ecotone’s current elevational extent, by the end of the 21st century. Differential drivers of species suggest that projected climate will expand conditions limiting A. concolor abundance and shrink conditions driving A. magnifica abundance across the ecotone. Widespread regeneration failure may be one possible consequence in currently stable ecosystems where species exhibit strong sensitivity to projected climate conditions and demonstrate the importance of quantifying individual species’ tolerances to climate.