Canopy and physiological controls of GPP during drought and heat wave

Vegetation indices (VIs) derived from satellite reflectance measurements are often used as proxies of canopy activity to evaluate the impacts of drought and heat wave on gross primary production (GPP) through production efficiency models. However, GPP is also regulated by physiological processes that cannot be directly detected using reflectance measurements. This study analyzes the co-limitation of canopy and plant physiology (represented by VIs and climate anomalies, respectively) on GPP during the 2003 European summer drought and heat wave for 15 Euroflux sites. During the entire drought period, spatial pattern of GPP anomalies can be quantified by relative changes in VIs. We also find that GPP sensitivity to relative canopy changes is higher for nonforest ecosystems (1.81 +/- 0.32% GPP/% enhanced vegetation index), while GPP sensitivity to physiological changes is higher for forest ecosystems (-0.18 +/- 0.05 g Cm(-2)d(-1)/hPa). A conceptual model is further built to better illustrate the canopy and physiological controls on GPP during drought periods.