Implications of Climate Change for Turnover in Forest Composition

Predicting how climate change will influence forests is challenging. Forest communities are expected to lose cold-adapted
trees near their low latitude/elevation range limits, while warm-adapted trees should increase in abundance near their
high latitude/elevation limits ( i.e., ‘thermophilization’). However, slow-growing and long-lived trees, paired with climatic
sensitivities that differ by species could add complexity to these predictions. To address these possibilities, we use
demographic data collected from Mount Rainier National Park to examine: 1) the likelihood of rapid population growth
or decline near range limits; 2) differences in climatic sensitivities between tree species at juvenile and adult stages; and
3) whether forest communities have already changed in response to recent warming. Our results suggest focal species
are unlikely to shift their ranges rapidly with warming because recruit densities are low and time to reproductive maturity
slow near upper range limits, while survival of adult trees is high near lower range limits. Additionally, focal tree
species differ in the magnitude and strength of responses at seed and adult stages to climatic factors likely to be altered
by warming. Consistent with these findings, shifts in forest community composition over the last 35 years were small,
and not consistent with thermophilization predictions. In all, results imply that climate change will only have small, but
unpredictable impacts on forest composition in the near-term. We discuss the possibility of much larger changes in forest
composition with future climate change, if the drastically different climate regimes projected for the region fundamentally
alter demographic processes and disturbance regimes.
Keywords: climate change, community assembly, demography, old-growth, Pacific Northwest