How trees species cope with novel environmental conditions resulting from climate change may significantly impact western forest composition in the near future. As such, an improved understanding of how trees respond to forecasted change (warming) is vital for devising successful assisted migration strategies and isolating the highest priority species for translocation. Recent evidence for divergence in adult and seedling climate niches in western tree species suggests that climate change is already having important impacts on the ecology of Rocky Mountain forests. To explore these issues, we established three common gardens along a 1,200 meter elevation gradient in Colorado. Each garden was planted with seedlings from a local population with presumed similar genetic composition in soils that were identical at all sites. The species selected – ponderosa pine (Pinus ponderosa), lodgepole pine (Pinus contorta), quaking aspen (Populus tremuloides), and subalpine fir (Abies lasiocarpa) – comprise a large part of the forest biomass in Colorado. Seedlings were planted in 2014, and measured for growth, survivorship and photosynthesis. Our goal was to compare across this elevation and temperature gradient (6 C) how these species differed in ecophysiological and growth responses.
Results/Conclusions
The data collected through the end of 2015 suggest that each tree species responded uniquely to the gradient in temperature imposed by the common gardens. Ponderosa pine displayed a strongly plastic response across the three gardens, with whole plant volume, height, and diameter positively correlated with mean annual temperature. The other three species were more variable in their growth patterns across the sites. We observed significant increases in leaf length (a proxy for leaf area) for quaking aspen but not for the other species. However, in lodgepole pine we observed a consistent shift in the optimal temperature for photosynthesis such that the temperature shift matched the elevation (and temperature) gradient of the gardens. Ecophysiological responses to the temperature gradient imposed by the three sites were inconsistent for the other species. Finally, we did not observe any notable variation in the response for subalpine fir across the gardens suggesting that this species lacks the ability to adopt one of the above mentioned acclimation strategies within the first year. As a result, we suggest that subalpine fir may be a strong contender for assisted migration of warmer adapted genotypes if maintaining subalpine forests in Colorado remains a priority.