Forests may lose their ability to absorb carbon dioxide during severe droughts as trees close their leaf pores to conserve water according to researchers.
Trees absorb CO₂ through tiny openings called stomata while releasing water through the same pores. On average, around 400 water molecules are released for every molecule of CO₂ absorbed during this process.
During drought declining soil moisture and rising atmospheric dryness force trees to close their stomata. This reduces photosynthesis and limits the amount of CO₂ forests can absorb.
Severe water stress can also damage trees’ internal water transport systems. Air bubbles can form inside the vessels that carry water from roots to leaves, blocking the flow and potentially causing irreversible damage and tree death.
As photosynthesis declines forests continue to release CO₂ through respiration. Carbon stored in trees can also return to the atmosphere when drought kills vegetation. As a result, forests can temporarily shift from carbon sinks to carbon sources during extreme drought and heatwaves.
Researchers also noted that different tree species have varying levels of drought tolerance. However all forests remain vulnerable when drought conditions exceed the levels to which trees are adapted.
The study highlights the close connection between the water and carbon cycles. Reduced transpiration can also affect rainfall patterns. In the Amazon for example, researchers estimate that up to half of the rainfall in western Amazonia comes from water transpired by forests in the eastern part of the basin.
With droughts becoming more frequent and intense, understanding how water availability affects forest carbon absorption will be important for predicting the role forests can play in limiting climate change.