In riparian forests of the southwestern US, tree species Populus fremontii plays a foundational role in community structure and ecosystem processes. However, shifts in temperature induced by climate change are altering its foundational capacity and consequently the ecosystem it supports. We are therefore investigating what are the different physiological mechanisms that drive intraspecific local adaptation in this species. Specifically, the tradeoff between thermal and freezing tolerance that this species exhibits across its elevational and therefore thermal range. Thus, we hypothesize that hot-temperature adapted populations display traits that maximize transpirational cooling increasing leaf thermal tolerance while cold-temperature adapted populations show reduced transpiration rates to minimize the risk of losing hydraulic conductivity from freeze-thaw embolism. To address these questions, a common garden was used to study a set of functional traits on populations sourced across the entire elevational-thermal distribution of Populus. We measured a suite of morpho-physiological leaf and structural traits, including whole-plant leaf area (Al), specific leaf area (SLA), sapwood area (As), characteristic-leaf dimensions (d), stomatal density (SD), leaf area index (LAI), stomatal conductance (gs), net photosynthetic rate (Pn), canopy boundary layer conductance (Gbl), intrinsic water use efficiency (WUEi), sap-flux density (js), water potential (Ψ) and the difference between leaf to air temperature (Tl-Ta).
Results/Conclusions
Low elevation populations exhibited negative differences in their leaves to the air temperatures. We observed negative relationships between provenance elevation and SLA (r2=0.54, p=0.037), SD (r2=0.61, p=0.022), Al:As= (r2=0.74, p<0.001), d (r2=0.61, p=0.021), and Ψ (r2=0.74, p=0.006). Likewise, gs= (p<0.001), iWUE (p=0.003) and Pn (p<0.001) also decreased with elevation of source population. These results suggest low elevation populations display a suite of coordinated functional traits that enhance higher transpiration rates as a strategy to maintain leaf temperature at optimal level for photosynthesis. Thus, while smaller and thinner leaves reduced sensible heating and capacitance from solar radiation, more stomata per area increases the ability to transpire at higher rates to enhance evaporative cooling. Additionally, these hot-adapted populations exhibit lower canopy boundary layer conductance and water potentials through more open canopies and low leaf area to sapwood area ratio respectively. Significant opposite morpho-physiological characteristics were found in high elevation populations. These results indicate a functional trade-off between minimizing the risk of losing hydraulic conductivity from freeze-thaw cavitation and displaying lower stomatal conductance and transpiration rates. Likewise, these results imply water availability will be essential for the hot temperature-adapted populations to endure the projected increase in temperature for the Southwest.