When drought, heat and canopy dieback turn a Mediterranean forest into a net annual carbon source for two consecutive years

Increasing temperature and drought conditions can result in leaf dehydration and summer defoliation even in drought-adapted tree species such as the Mediterranean evergreen oak Quercus ilex. This phenomenon was widespread in forests of Southern France in 2022-2023 when record-breaking temperatures of 2022 were associated with two consecutive years of low precipitation, severe drought episodes and recurrent heatwaves.

Using the 20-year data series (2001-2021) of eddy-covariance carbon and water fluxes measured at the ICOS Mediterranean forest site FR-Pue (Puéchabon) prior to this event, we assess the impacts of these two exceptional years on the carbon budget of the forest. While the Puéchabon forest always behaved as a net carbon sink between 2001 and 2021, with an annual net ecosystem exchange (NEE) ranging between -450 and -137 gC m^-2 y^-1, the carbon balance was reversed to a net annual carbon source of +14 and +65 gC m^-2 y^-1 in 2022 and 2023, respectively. This anomaly is caused by a deficit of photosynthetic carbon uptake, as leaf physiology was severely impacted by both water stress and heat stress. Significantly lower photosynthetic rates than in the previous years were, however, not restricted to the most stressful conditions of heat or soil water deficit but manifested under most meteorological conditions even outside the summer period. This observation suggests that neither heat nor drought alone can explain the photosynthesis limitation in 2022 and 2023 but that the two acted in synergy. It also demonstrates that such extreme meteorological events have long lasting effects on tree physiology, mediated by cell physiological damage, leaf hydraulic failure and canopy dieback that limit photosynthetic recovery when favorable temperature and soil moisture conditions return.

Interestingly, these negative effects on photosynthesis were not observed during the following year 2024 when a complete recovery of photosynthetic rates was achieved with the production of new leaves, highlighting a strong resilience of Quercus ilex to drought and heat. Nevertheless, the annual carbon budget in 2024 was also particularly low because of an excess of total ecosystem respiration compared to the long-term mean. The higher respiration rates in 2024 could be caused by the decomposition of dead trees and organs after the extreme years 2022-2023, and by the reallocation of trees carbon reserves to the production of short-lived organs such as new leaves and seeds.

This study is, yet, a rare example of an inversion of a forest carbon balance driven merely by meteorological conditions and it highlights the value of long-term observations to better understand and interpret the consequences of extreme events on ecosystem functioning.