Desert riparian ecosystems and the foundation tree species they support, including Populus fremontii are highly valued because they host an exceptional biological diversity and enhanced watershed protection. However, projected climate change potentially threatens the fitness and foundational capacity of this species and consequently the ecosystem structure and function it supports. Thus, the primary questions we are addressing include 1) are P. fremontii populations adapted to local temperature regimes? and 2) if so, will climate change result in populations that will become locally maladapted in the coming decades? In order to address these questions, an experimental common garden was constructed using genotypes, planted in the fall of 2014 of 16 F. populations sourced across this species thermal distribution from central Utah to the US border with Mexico in southwestern Arizona. The experimental garden was placed at the approximate mid-point in P. fremontii’s thermal distribution, with approximately half of the populations coming from warmer locations and the other half sourced from cooler populations. In 2016 temporal changes in radial growth and canopy architecture were measured with a broad suite of leaf functional and structural traits, including specific leaf area, stomatal density, whole-tissue d13C (‰), d15N (‰), leaf, C:N ratios.
We observed a high degree of variability among populations in trunk basal diameter and growth rate, but we did not find a significant correlation of these parameters with elevation. Conversely, we observed negative relationships between provenance elevation and specific leaf area (SLA) (r2=0.34, p=0.017), and stomatal density (p = 0.0022), indicating that high elevation genotypes have a greater investment in leaf biomass with reduced maximum stomatal conductance and carbon gain per leaf tissue investment. Alternatively, a higher SLA, greater area/thinner leaves, coupled with a higher stomatal density suggests a suite of leaf traits associated with rapid utilization of resources. Canopy volume normalized for differences in trunk diameter was positively related to elevation (r2=0.26, p=0.04), suggesting that low elevation trees, from warmer climates, had a denser canopy and consequently, potentially a higher canopy boundary layer resistance. These results indicate that Populus fremontii possess a set of intraspecific traits highly adapted to different climate conditions present along its distribution. By identifying and assessing these different traits and their correlation with local conditions, we will be able to effectively predict how novel environmental conditions derived from climate change will affect this important foundation species across its geographic distribution.