In-situ energy budget of needle-leaves reveals shift from evaporative to ‘air cooling’ under drought

Abstract

AbstractThe modulation of the leaf energy budget and the balance between its sensible heat (H) and latent heat (LE) fluxes is vital for vegetation functioning and survival, as it is linked to maintaining leaf temperature below the thermal threshold, an increasingly important mechanism under a drying and warming climate, when evaporative cooling is suppressed.Combining measurements and theoretical estimates using a new methodology, we obtained rare and comprehensive energy budgets of leaves on twigs under field conditions in droughted and non-droughted plots of a semi-arid pine forest with low and high evapotranspiration rates, respectively.An examination of all components of the needle-leaf energy budget indicated that under the same radiative load, leaf cooling shifts from nearly equal contributions to H and LE in non-droughted trees to almost exclusively H in droughted ones while maintaining a similar leaf temperature.This LE-to-H shift in leaves of droughted trees highlights the efficiency of the ‘air cooling’ mechanism in maintaining temperature, which can enhance the resilience of trees to drying conditions. Additionally, leaf energy budgets are a fundamental tool to help understand leaf cooling and aerodynamic resistance under field conditions, and to improve modelling of ecosystem activity and its effect on the climate system.