Mechanistic Strategies of Fremont Cottonwood to Thermo-Regulate Leaf Temperature
Davis Blasini1,2*, Dan Koepke2, Kevin Grady3, Kevin Hultine2
1School of Life Sciences, Arizona State University, Tempe, Arizona, USA; dblasini@asu.edu
2Department of Research, Conservation and Collections, Desert Botanical Garden, Phoenix, Arizona, USA; dkopeke@dbg.org, khultine@dbg.org
3School of Forestry, Northern Arizona University, Flagstaff, Arizona, USA; Kevin.Grady@nau.edu
We investigated the different physiological mechanisms that drive intraspecific local adaptation in Populus fremontii and what populations will be more likely affected by climate change and water streamflow modifications in the future. Thus, we used an experimental common garden to study a broad suite of leaf functional and structural traits on populations sourced across the entire thermal distribution of the species. This experimental garden was placed at the mid-point of P. fremontii’s thermal distribution, with an even representation of populations coming from warmer and cooler provenances. We measured whole-plant leaf area (Al), Sapwood area (As), specific leaf area (SLA), characteristic-leaf dimensions (d), stomatal density (SD), leaf area index (LAI), sap-flow density (js), water potential (Ψ) and the difference between leaf to air temperature (T1-T2).
We observed negative relationships between provenance elevation and Al : As= (r2=0.74, p0.001), SLA (June r2=0.54, p=0.037, September r2=0.63, p=0.017), d (r2=0.61, p=0.021), SD (r2=0.61, p=0.022), Ψ (August r2=0.49, p=0.051, September r2=0.74, p=0.006). Simultaneously, low elevation populations exhibited significative negative Tl-Ta. These results suggest lower 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 and faster rates. Additionally, these hot-adapted populations exhibit reduced lower canopy boundary layer conductance and water potentials through more open canopies and low leaf area to sapwood area ratio respectively. These results imply water availability will be essential for the hot temperature-adapted populations to endure the projected increase in temperature for the Southwest.