Estimating leaf damage from hot drought events and predicting the effects on forest productivity from multilayer canopy models

Multilayer canopy models, based on energy balance principles, are appropriate modeling tools for understanding and predicting the effects of hot droughts on forest productivity and leaf damage caused by extreme leaf temperatures. Leaves within canopy vertical layers are exposed to different levels of radiation and momentum flux, resulting in different leaf temperatures and leaf water potentials in the upper, middle and lower canopy. Explicit consideration of these vertical gradients enables more mechanistic predictions without reliance on empiricism and formal model calibration. Here we use two broadleaf forest sites affected by the 2012 American Midwest drought to study the CanVeg2 model’s ability to reproduce the forest canopy responses as measured from eddy covariance towers. This study relies on 3D radiative transfer simulations based on canopy structure information derived from a ground lidar instrument to characterize the radiative forcing on leaves. At both sites the forest productivity was significantly affected by the 2012 drought, as evidenced by the eddy covariance flux tower records. Images from the PhenoCam network show that at one site (Missouri Ozark) there was significant leaf die off, while the other site (Morgan Monroe, Indiana) showed no visual evidence of leaf damage, even though the air temperatures reached were higher at the Morgan Monroe site, why could that be? We will present evidence of the reasons from a modeling perspective, and discuss the conditions under which the canopy microclimate leads to leaf temperatures above critical damage thresholds, as well as where in the canopy leaves reached their highest temperatures for this specific case, and for how long. Using leaf level data on the temperature response of the maximum quantum yield of photosystem II for the species present at both sites, we also present a novel modeling approach to estimate leaf damage levels and its effect on canopy productivity for the rest of the growing season once rains return and the hot drought subsides.