Ecosystem temperature management under water scarcity

Efficient heat dissipation under high radiation load is critical to plant functioning. It includes processes that are clearly observed in dry ecosystems but likely extend to other environments where it will be further enhanced by climate change. We observed that despite near-zero evaporation during the seasonal drought in a semi-arid pine forest, leaf temperatures were within the physiological range at about 35C. At the same time, exposed soil at the same site reached temperatures up to 70C. These leaf temperatures were also similar to that in an irrigated plot where evapotranspiration (ET) was enhanced by x10. A detailed energy budget demonstrates that heat dissipation under drought relies on a large sensible heat flux (H) that must depend, in turn, on reducing aerodynamic resistance to heat transfer. At the canopy scale, a “convector effect” of the high-roughness dry canopies generates a massive H that increases the depth of the planetary boundary layer and induces secondary circulations. Model simulation at larger scales indicated that such a process could modify local and regional climatic conditions. Assessing the global FLUXNET data from different environments further indicates that relying on H as a major heat dissipation process is not limited to dry ecosystems and is not dictated solely by the radiation load.