Recent droughts have highlighted an alarming reality: forests, usually considered crucial carbon sinks, can transform into net emitters of carbon dioxide (CO₂). As episodes of water scarcity multiply, this role reversal raises significant concerns for climate change mitigation policies. Although the exact quantification of CO₂ captured by global forests remains complex, it is estimated that they absorb about a quarter of human CO₂ emissions.
The key mechanism lies in the tree leaves. These are equipped with tiny openings, called stomata, which allow CO₂ to enter for photosynthesis. This process converts CO₂ into sugars, fueling the tree's growth and carbon storage. However, the opening of stomata also leads to water loss through transpiration: for every molecule of CO₂ absorbed, approximately 400 water molecules escape.
This transpiration, vital for the plant's metabolism, can amount to hundreds of liters of water per day for a single tree. The transport of this water from roots to leaves occurs through a hydraulic system based on the tension-cohesion principle. This system is fragile, however: excessive tension, due to a lack of soil water and a drier atmosphere, can cause cavitation. This phenomenon creates air bubbles in the hydraulic system, blocking water circulation and leading to irreversible dehydration of the tree's tissues.
Faced with this dilemma between photosynthesis and water conservation, trees close their stomata during droughts. During these periods, forests continue to emit CO₂: firstly, through the consumption of stored sugars for their metabolism; and secondly, when trees die and release the accumulated carbon. This reversal, sometimes amplified by certain forestry practices, can turn a forest into a carbon source.
Drought resistance varies among species, influenced by the vulnerability of their hydraulic system. Trees often operate at the limit of embolism, making them vulnerable to increased droughts linked to climate change. When these droughts exceed the reference conditions, critical impacts such as leaf fall and desiccation can occur, leading to tree death.
The capacity of forests to act as carbon sinks therefore intrinsically depends on water availability. The idea of a fertilizing effect of CO₂ on vegetation is counterbalanced by the intensification of droughts. Furthermore, increased CO₂ concentration can reduce stomatal opening, decreasing transpiration and the recycling of rainwater, which affects the climatic stability of continents.
