Dominant tree species of the Colorado Rockies have divergent physiological and morphological responses to warming

Increasing temperatures worldwide, as a primary manifestation of climate change, may cause substantial alterations in forests, presenting a major challenge to predicting responses in forest composition and function. Yet, recent empirical research on climate and forests has found patterns at odds with theoretical and modeling expectations. Indeed, the need for an improved mechanistic understanding of climate's effects on forests is clear but controlled field studies that address this issue are lacking. Montane and subalpine systems may be particularly sensitive to changes in temperature yet quantifying temperature effects in real-world conditions can be challenging. We used three common gardens arrayed over a 1200 m elevation and 6 °C mean annual temperature gradient in the Front Range of Colorado to evaluate the temperature responses of seedlings of three widespread and dominant tree species in Colorado: lodgepole pine (Pinus contorta var. latifolia), ponderosa pine (Pinus ponderosa) and aspen (Populus tremuloides). Seedlings for the gardens were sourced from populations naturally occurring at the intermediate elevation on this gradient and were planted into identical soils in all three gardens. We focused on in situ photosynthetic performance of seedlings, plasticity in spring phenology, and adjustments in leaf morphology. We found no evidence for clear temperature sensitivities in photosynthetic rates of the two coniferous species, neither across a 6 °C range in growing season temperatures between sites nor across manipulated leaf temperatures of 15-30 °C within sites. Likewise, lodgepole pine exhibited uniform leaf size across the temperature gradient; however, ponderosa pine leaf size did increase significantly at the warmest site. In contrast, aspen displayed pronounced temperature sensitivity in photosynthesis and leaf morphology, with maximum observed values at intermediate temperatures which both declined at the colder or warmer temperatures. Relative to the conifers, aspen also showed reduced phenological responses to warming with ∼12% fewer growing days at the intermediate site, and 6% at the warmest site. These divergent responses suggest that warming temperatures can alter seedling success in a number of different ways, and taken together, are likely to alter forest composition of Colorado in favor of greater dominance by montane conifer species.