Studying the large-scale effect of leaf thermoregulation using an Earth system model

Abstract. Plants have the ability to regulate heat and water losses. This process also known as leaf thermoregulation helps to maintain the leaf temperature within an optimal range. In a number of laboratory and field experiments, the leaf temperature has been found to deviate substantially from the ambient temperature. In the present study, we address the question of whether the negative correlation between the leaf temperature excess and the ambient air temperature, which is characteristic of leaf thermoregulation, constitutes a robust feature at larger scales, across a broad range of atmospheric conditions and canopy characteristics. To this end, we developed a new dual-source canopy layer energy balance scheme (CEBa) and implemented it into JSBACH, the land component of the Max Planck Institute for Meteorology's Earth system model (MPI-ESM). The approach calculates the temperature and humidity in the ambient canopy air space, the temperature of the ground surface, and the temperature of the leaf as well as the energy and moisture fluxes between the different compartments. Here leaf thermoregulation is investigated using different modeling approaches, namely a zero-dimensional instantaneous solution of the energy balance as well as offline FLUXNET site experiments and coupled global simulations. With the help of the simulations at the site-level, we can show that the model is capable of reproducing the effect of leaf thermoregulation even though the simulated signal at the canopy scale is less pronounced than indicated by measurements at the leaf scale. However, on a global scale and over longer-timescales, this negative correlation is only simulated in idealized setups that neglect limitations on the plant available water, and even then, the signal is less pronounced than indicated by the short-term observations of individual leaves. When accounting for moisture limitations, we predominantly find positive correlations between leaf temperature excess and the ambient air temperature.