Persistent yet vulnerable: resurvey of an Abies ecotone reveals few differences but vulnerability to climate change

Abstract

Climate change is shifting forest tree species distributions across elevational and latitudinal gradients, and these changes are often pronounced at ecotones where species meet their climatic bounds and are replaced by other species. Using an extensive ecotone composed of lower-montane white fir (Abies concolorvar. lowiana) and upper-montane red fir (Abies magnificavar. magnifica) in the central Sierra Nevada range of California, USA, we (1) examined how the demographics of the ecotone have responded to recent climate using a field observational study and a historical dataset, (2) quantified climate drivers across species life stages using contemporary demographic data, and (3) tested the potential impacts of future climate on species-specific seedling survival and growth in a fully factorial growth chamber experiment that varied temperature, growing season length, and water availability. A re-examination of the ecotone midpoint after 35 yr suggested a reduction in A. concolorsapling and tree densities and a rise in A. magnificaproportional dominance between surveys. Seedling abundances across the ecotone indicated that A. magnificatends to dominate the regeneration layer and currently forms an important component of the seedling community at elevations below those where A. magnificasaplings or trees begin to co-dominate stands. Observational and experimental assessments suggest that temperature and precipitation serve as important drivers, differentiating A. concolorvs. A. magnificadistributions, and are primary stressors at the seedling stage. Seedlings of both species were adversely affected by experimental climate treatments, although A. concolorexhibited greater survival and a more conservative growth strategy under extreme climatic stress than A. magnifica. Projections indicate that historical climate conditions will rise by an amount greater than the ecotone’s current elevational extent by the end of the 21st century. Differential drivers of species abundances suggest that the projected climate will expand conditions that promote A. concolorabundance and impede A. magnificaabundance across the ecotone; however, disturbance activity and microclimatic conditions will also influence regeneration and overstory tree dynamics. Our study demonstrates the importance of quantifying species-specific responses to climate and indicates that widespread regeneration failure may be one possible consequence in which species exhibit strong sensitivity to projected climate conditions.

Citation

Nelson, Kellen N.; O’Dean, Emily; Knapp, Eric E.; Parker, Albert J.; Bisbing, Sarah M. 2021. Persistent yet vulnerable: resurvey of an Abies ecotone reveals few differences but vulnerability to climate change . Ecology. 102(12): 1223. https://doi.org/10.1002/ecy.3525.