Heatwave effects on gross primary production of northern mid-latitude ecosystems

Heatwave events are expected to become more frequent and intense in the mid-latitudes of the Northern Hemisphere in the future. However, our knowledge of the impacts of these extreme temperature events on ecosystem functions is still limited. We investigated the responses of ecosystem gross primary productivity (GPP) to heatwaves for nine sites encompassing a wide variety of ecosystem types with long-term (i.e., >7 years) flux and meteorological observations at northern mid-latitudes. Our results showed that GPP was depressed during heatwaves for most ecosystems except the C4 sites. The evaporative stress index (ESI) was the primary variable responsible for the changes in GPP during heatwaves across the nine ecosystems. Furthermore, forest ecosystems were more resistant and resilient to heatwaves than other ecosystems. Additionally, the asymmetric diurnal pattern of GPP in forest ecosystems was attributed to xylem refilling and high water storage capacity, indicating that GPP was more sensitive to heatwaves in the afternoon. Interestingly, C4 herbaceous ecosystems could promote canopy stomatal conductance (Gc) for avoiding leaf burn and tissue damage during heatwaves. The herbaceous ecosystems had weaker stomatal control on water loss during heatwaves and were more vulnerable to heatwaves owing to the water stress afterwards. This research highlighted the importance of ecosystem type and plant functional type in determining the responses of photosynthesis to heatwaves. Understanding the post-effect after heatwaves is also vital for comprehensively assessing the impacts of heatwaves on ecosystem functions.