Warming temperatures as a manifestation of climate change are expected to dramatically impact forests, yet we lack vital information on how temperature will drive and interact with future tree growth and demography. Recent observational studies across extensive latitudinal/temperature gradients report weak or non-existent relationships between temperature and decadal-scale growth for many tree species, contradicting clear distributional responses to temperature across elevational gradients and strong short-term temperature effects in ecophysiological studies. To resolve this, we established controlled, in situ experimental gardens across an elevation and temperature gradient with consistent precipitation to assess the direct effects of warming on three montane and one subalpine species from the Rocky Mountains. We transplanted soil from the intermediate elevation to all gardens to minimize edaphic differences and planted seedlings germinated from a single stand to minimize the effects of local adaptation. We assessed whole-seedling performance by shifts in basal area, carbon allocation via height to basal area ratios, and relative growth rates. We compared survivorship rates to those growth parameters to assess the relative contributions of stress and stochasticity, two principle predictors of seedling mortality in forest growth models.
Results/Conclusions
In stark contrast to latitudinal studies, we found warming unambiguously accelerated growth in the montane species while causing a marked decline in the subalpine species. With warming, all species strongly prioritized diameter growth over height, suggesting greater allocation to water transport tissues and sensitivity to indirect effects of warming in water use. After four years, we found no correlation between temperature, growth, or allocation patterns and mortality for the montane species, indicating that survivorship was surprisingly independent of temperature. We observed a strong and mostly positive effect of higher temperatures on multi-year, integrated, whole plant growth in contrast to observational evidence of weak or flat growth rates of wide ranges in temperature that dramatically alter predictions of future forest demography in Colorado.