High-latitude forests are experiencing rapid warming and changes in water availability, disturbance regimes, resource availability, and ecosystem functioning. Compositional shifts due to hotter, drier conditions and increased disturbance frequency associated with climate change are expected to benefit disturbance-adapted, early-successional tree species in boreal forests. However, little is known about how climate change may affect successional trajectories and therefore, compositional shifts in forest communities that span multiple ecotones. Given the broadly distributed tree species that comprise these communities may have highly divergent responses to climate change across their range, it is critical to understand how the long-term response of forest communities to climate change varies within and among tree species and with stand age-dependent succession. Examining three of the most widely distributed, early-successional tree species in North America, Betula papyrifera, Pinus banksiana, and Populus tremuloides, we determined the relative influence of climate, successional stage and species traits on long-term (1978 – 2018) patterns of mortality and recruitment. We used forest inventory measurements and climate data from 4,593 plots located across temperate broadleaf and boreal forests of North America to address how the sensitivity of early-successional tree species to climate change depends on climatic context, successional stage and their interaction.
Results/Conclusions
After accounting for stand age and competitor density, our time-series analyses showed species-specific changes in background mortality rates from 1975– 2018, with P. tremuloides and B. papyrifera experiencing an approximate two-fold increase in their rates of mortality and P. banksiana exhibiting no change. The observed increase in mortality for both early-successional deciduous species was most pronounced at low latitudes and ecotonal boundaries, i.e. temperate broadleaf forests and aspen parkland. Mortality rates increased predominately in young stands (40 – 60 years) for B. papyrifera, whereas P. tremuloides experienced the greatest increase in aspen-dominant rather than mixed stands. We found recruitment rates of all three species did not change significantly over time, but rather were strongly associated with successional stage and mean climate conditions. Our findings suggest that functional trait differences that confer resilience to extreme climatic conditions may be responsible for the contrasting changes to mortality rates between early-successional gymnosperm and angiosperm tree species. Furthermore, for three broadly distributed, early-successional species expected to benefit from rising atmospheric CO2 and climate warming in boreal forests, our results highlight long-term, divergent patterns in mortality and recruitment rates which may influence the rate and extent of compositional shifts in high-latitude forests with climate change.