Tradeoffs Between Leaf Thermoregulation and Hydraulic Safety in Warm-Versus Cool-adapted Ecotypes in the Foundation Tree Species Populus fremontii
Kevin Hultine1, Davis E Blasini2, Dan F Koepke3, Madeline E Moran4
1Department of Research, Conservation and Collections, Desert Botanical Garden, Phoenix, AZ, USA; khultine@dbg.org
2School of Life Sciences, Arizona State University, Tempe, AZ, USA; dblasini@asu.edu
3Department of Research, Conservation and Collections, Desert Botanical Garden, Phoenix, AZ, USA; dkoepke@dbg.org
4School of Life Sciences, Arizona State University, Tempe, AZ, USA; memoran@asu.edu
A primary challenge for successful Populus fremontii restoration in warm-desert riparian ecosystems is determining whether genotypes currently adapted to the region will become maladapted under future climate conditions. One way to evaluate genotypes that are best suited to cope with future heat waves is to quantify leaf thermal safety margins: the difference between leaf temperature and the temperature at which leaf photosynthetic capacity is inhibited. Here we define photosynthetic inhibition as the temperature at which electron transport capacity of Photosystem II drops to 50% of its maximum (Tcrit). Mid-summer Tcrit was evaluated in a common garden setting along the Colorado River near Yuma, AZ. The common garden comprised of eight P. fremontii populations sourced across an elevation gradient from 50 m to 1230 m. Mean population Tcrit ranged from 51.1 °C to 52.5 °C. However, contrasts among populations were not correlated with elevation, indicating that low-elevation populations operate with a much narrower thermal safety margin compared to high-elevation populations. Conversely, in a separate common garden study, low elevation populations had a 40% higher mean midday transpiration rate, reflecting a 3.8 °C cooler mean leaf temperature compared to high elevation populations. These data indicate that plants adapted to warm environments are predisposed to tightly regulate leaf temperatures during heat waves via higher evaporative leaf cooling. Combined, these two common garden studies indicate that warm-adapted genotypes operate with a relatively narrow thermal safety margin and as a consequence are adapted to take “hydraulic risks” so that leaf cooling is maximized during heat waves. Identifying genotypes that maintain leaf thermal safety while minimizing hydraulic risk will likely be a necessary feature of future P. fremontii restoration efforts along the lower Colorado River and other warm-desert riparian ecosystems.