Intensive leaf cooling promotes tree survival during a record heatwave

Bradley C. Posch, Susan E. Bush, Dan F. Koepke, Alexandra Schuessler, Leander L.D. Anderegg, Luiza M.T. Aparecido, Benjamin W. Blonder, Jessica S. Guo, Kelly L. Kerr, Madeline E. Moran, Hillary F. Cooper, Christopher E. Doughty, Catherine A. Gehring, Thomas G. Whitham, Gerard J. Allan, and Kevin R. Hultine

PNAS October 14, 2024; 121 (43) e2408583121; https://doi.org/10.1073/pnas.2408583121

Hình: Populus fremontii tree

Significance

High temperatures inhibit leaf function and can cause plant death. Despite ongoing increases in heatwave occurrence, understanding of how plants survive extreme heat exposure is limited. While mechanisms for both leaf heat avoidance and tolerance have been identified, it is unknown how these strategies are affected by water availability during an extended heatwave. We show that the riparian tree Populus fremontii is highly efficient at leaf cooling via transpiration, even when air temperature exceeds 48 °C. However, a minor disruption in soil water availability completely inhibits leaf cooling, causing leaves to exceed critical temperature thresholds. These results provide new insight into the limited capacity forest ecosystems have for cooling their canopies below critical thresholds during extreme heatwaves.

Abstract

Increasing heatwaves are threatening forest ecosystems globally. Leaf thermal regulation and tolerance are important for plant survival during heatwaves, though the interaction between these processes and water availability is unclear. Genotypes of the widely distributed foundation tree species Populus fremontii were studied in a controlled common garden during a record summer heatwave—where air temperature exceeded 48 °C. When water was not limiting, all genotypes cooled leaves 2 to 5 °C below air temperatures. Homeothermic cooling was disrupted for weeks following a 72-h reduction in soil water, resulting in leaf temperatures rising 3 °C above air temperature and 1.3 °C above leaf thresholds for physiological damage, despite the water stress having little effect on leaf water potentials. Tradeoffs between leaf thermal safety and hydraulic safety emerged but, regardless of water use strategy, all genotypes experienced significant leaf mortality following water stress. Genotypes from warmer climates showed greater leaf cooling and less leaf mortality after water stress in comparison with genotypes from cooler climates. These results illustrate how brief soil water limitation disrupts leaf thermal regulation and potentially compromises plant survival during extreme heatwaves, thus providing insight into future scenarios in which ecosystems will be challenged with extreme heat and unreliable soil water access.

See https://www.pnas.org/doi/10.1073/pnas.2408583121

Figure 1:

(A) Number of days per year when maximum air temperature reached or exceeded 45 °C between 1997 and 2023 at the Yuma airport weather station, 75 km from the lowest elevation and hottest site from which trees were sourced for this experiment. The shaded area denotes 95% CI. (B) Daily maximum temperature as measured at the Phoenix International Airport between July 1 and September 30, from 1993–2023. The red line denotes daily maximum temperatures recorded during this period in 2023. The black line denotes daily maximum temperatures calculated for each day as mean from 1993–2022, error bars denote SE (n = 30).