Leaf thermal tolerance in Populus fremontii: local adaptation and plasticity across its range in the southwestern US

Background/Question/Methods

The southwestern United States is strongly affected by climate change, and will continue experiencing more frequent excessive heat waves, prolonged droughts, intense wildfires, and declining water supply. Riparian ecosystems are particularly at risk under changing climate predictions, but little is known about the thermal tolerance of these riparian plant species. Fremont cottonwood (Populus fremontii), a pioneer and foundation tree species, is already being exposed to increased temperatures and episodic heat waves across its range. Using a common garden located at the low-elevation and warm edge of its range in southern Arizona, we studied the seasonal thermal tolerance, leaf temperature, stomatal conductance, and structural leaf traits of P. fremontii from eight source populations representing a 1200 m elevation gradient across Arizona. We calculated critical temperature (Tcrit) from measurements of chlorophyll a fluorescence at which minimum fluorescence rapidly begins to increase and T50, which is when the fluorescence value reaches 50% of its maximum, thermal safety margins (TSM), and thermal time constants (𝜏). We hypothesized that trees from lower elevation populations would 1) have higher leaf thermal tolerances and 2) would maintain lower leaf temperatures due to higher leaf stomatal conductance values relative to trees from high elevation populations.

Results/Conclusions

From May to August 2021, mean Tcrit increased by 1.21 ± 0.14℃ and mean T50 increased by 1.45 ± 0.14 ℃ across all populations. However, in contrast to our first hypothesis, there was no relationship between population mean leaf thermal tolerance thresholds and source location elevation. TSMs calculated by subtracting maximum recorded leaf temperature from Tcrit and T50 respectively were negatively correlated with elevation during August, indicating that low elevation populations kept their leaves cooler in the summer. We expected larger 𝜏 in low-elevation populations and smaller 𝜏 in high-elevation populations reflecting greater thermal regulatory capacity in low-elevation populations, however there was no relationship between 𝜏 and elevation. Conversely, warm-adapted, low elevation trees maintained lower summer leaf temperatures, likely as a result of higher stomatal conductance values in mid-summer that led to greater leaf evaporative cooling compared to high-elevation populations, supporting our second hypothesis. Results indicate that in absence of greater leaf thermal tolerances, P. fremontii occurring along the warm edge of its range will need to prioritize leaf thermal regulation in response to episodic heat waves predicted for the region, potentially at the risk of hydraulic failure as a consequence of maintaining midday leaf evaporative cooling.