Energy management in dry canopy starts with efficient leaf-scale heat exchange

Dry forests are expected to heat up considerably more than adjacent shrubland areas due to lower albedo and reduced latent heat flux. Paradoxically, the forest surface at our research site was observed to be cooler than the non-forested neighbouring areas during drought. This reflected the control over canopy temperature through the sensible heat flux, i.e. a 'convector effect'. Our objective was to examine how the efficient non-evaporative energy management, critical to protect the biological functioning of dryland ecosystems, develops at the small, leaf scale. 

We developed a novel system to continuously measure the energy balance and heat dissipation mechanisms on a leaf scale under field conditions. It allows the measurement of emitted leaf and background longwave radiation, and estimating the incoming, absorbed and reflected shortwave radiation using PAR measurements and full spectrum models.  Latent heat exchange and photosynthetic activity were measured with branch chambers. The system was deployed during the long summer drought (>8 months) in drought-exposed and irrigated plots in our semi-arid research Aleppo Pine forest site in southern Israel (mean daytime temperature of >30°C).

Preliminary results showed that in spite of a x10 higher transpiration rate in the irrigated plot compared with the control plots, leaf temperature remains within 1-2°C of air temperature on average in both plots during direct exposure to sunlight at midday. These results suggest an effective leaf to air heat transfer which prevents overheating independent of the latent heat flux. Under the high radiation load, the midday summer value of incoming shortwave radiation was >800 W·m^-2 (mostly absorbed by the low albedo leaves), and background longwave radiation was >500 W·m^-2. In turn, the energy dissipation in the drought-exposed trees was dominated by sensible heat flux of >500 W·m^-2, while the long-wave radiation balance was near neutral (~50 W m^-2), and the residual latent heat flux was <50 W·m^-2. We demonstrated a system that provided new insights to leaf and canopy energy management under drought, which is a basis for the evolution of the convector effect.