Heat stress: an underestimated impact of climate change on vegetation

Heat stress occurs when plants experience temperature beyond their normal optimum and can disrupt cell functioning and growth. Exposure to extreme heat results in reduced plant productivity and can have lagged effects through accelerated leaf senescence Teskey et al (2014) Rao et al (2023). Few studies exist on evaluation of impact of heat stress at the global scale. It not fully understood, and is thereby only partially represented in land surface models. Detecting the impact of heat stress from space-borne observations is challenging, since it co-occurs with other stressors, such as low water availability. In this paper, we define a method that allows to detect the long term impact of heat stress at the global scale from remote sensing data by disentangling the heat stress effect on vegetation from other covariates, particularly drought. We show that, when maximum surface temperature exceeds a threshold of around 43°C, heat stress is detected on satellite estimates of Leaf Area Index (LAI). Locally, we observe a different critical temperature at which Leaf damage occurs suggesting an acclimation of plants to heat stress. Based on the observed impact of heat stress on leaf foliage, by implementing this relationship in the ORCHIDEE land surface model, we simulated a limited impact of leaf heat damages on net primary productivity (NPP) until the beginning of the 21st century followed by a rapid increase in the last two decades (2000-2020). Simulations for future climate suggest that, if the heat effect on NPP at the global scale is limited (below 2% for RCP 8.5), regionally the impact can be high with up to 25% of reduction of the annual NPP for instance in China and western US. This study emphasizes that we should, in the future, pay more attention to the direct impact of heat stress on leaves and not only to the effect of drought on the negative impact of climate change on vegetation.