Evidence for efficient non-evaporative leaf-to-air heat dissipation in a pine forest under drought conditions

Drier climates predicted for many regions can result in reduced evaporative cooling leading to leaf heat stress and enhanced mortality. To what extent non-evaporative cooling can contribute to plant resilience to the increasingly stressful conditions is poorly known at present. Using a novel, high accuracy infrared system for continuous measurements of leaf temperature in mature trees under field conditions, we assessed leaf-to-air temperature differences Δ T leaf − air of pine needles during drought. On mid-summer days, Δ T leaf − air remained <1.5 °C, both in trees exposed to summer drought, and in those provided with a supplement irrigation having a 10× higher transpiration rate. The non-evaporative cooling in the drought-exposed trees must be facilitated by low resistance to heat transfer generating large H . Δ T leaf − air was weakly related to variations in the radiation load and mean wind speed in the lower part of the canopy, but highly dependent on canopy structure and within-canopy turbulence that enhanced the sensible heat flux H . Non-evaporative cooling is demonstrated as an effective cooling mechanism in needle-leaf trees, which can be a critical factor in forest resistance to drying climates. The generation of a large H at the leaf scale provides a basis for the development of the previously identified canopy-scale ‘convector effect’.