Understanding the effects of climate change on the geographic distribution and abundance of tree species is a pressing issue. Critical questions remain unanswered: how do demographic rates (e.g. growth) and life-history strategies (e.g. shade-tolerance) shift in response to altered temperature and precipitation regimes? What is the lag time in forest response to climate change? How will altered climates affect forest productivity? We argue a major limitation to refining climate change predictions is that rigorous empirical analyses of the effects of climate on tree demography have lagged behind the models. A basic challenge is disentangling climate change effects from local edaphic factors and biotic interactions. We used elevational and latitudinal gradients to measure how current climate variation drives forest dynamics and asked: How is the growth and canopy recruitment of the dominant tree species of the Rocky Mountains responding to current climate variation? We used maximum likelihood analysis to compare the growth of the six dominant tree species of the Rockies—Douglas fir, Engelmann spruce, lodgepole pine, ponderosa pine, quaking aspen, and subalpine fir—along both elevational and latitudinal gradients as surrogates for temperature and precipitation, while controlling for light and soil nutrient availability (measured as covariates for each sapling). We focused on the demography of saplings, long considered the major determinant of patterns of forest succession, measuring actual growth rates of individuals over their entire elevational range at 3 sites located from southern Colorado to northern Wyoming (> 50 saplings sp^-1 site^-1).
Results/Conclusions

Across spatial gradients, a species’ life history (e.g. successional status) interacted markedly with climatic variation: the most shade-tolerant species varied in their response to low light but not high light while shade-intolerant species varied only in their high-light growth (with higher temperatures displacing shade intolerant species northwards faster than shade tolerant species). Species with intermediate shade tolerance varied in their growth amplitude at both high light and low light. In general, growth rates increased for all species at higher temperatures, except at the “bottom” of the gradient where moisture stress was high. Across the latitudinal gradient, mean annual temperature had a greater impact on growth rates than precipitation. Despite the interspecific trade-offs between high- and low-light growth, there was some degree of overlap between species in their relationships with light and climate suggesting the possibility of a community-level response to climate change.