This article has been reviewed according to Science X's editorial process and policies. Editors have highlighted the following attributes while ensuring the content's credibility: To survive periods of drought, trees halt photosynthesis, a highly water-intensive process. For every molecule of CO₂ absorbed by a tree leaf, an average of around 400 water molecules are released.
Entire forests may then end up emitting more CO₂ than they capture. This is alarming given that droughts are becoming increasingly frequent and that forests play a key role in climate change mitigation policies. Although quantifying the CO₂ captured by the world's forests remains a difficult task, they are estimated to capture around a quarter of the CO₂ emitted by human activities.
However, when faced with extreme water shortages, forests can cease to act as carbon sinks. To understand why, we need to look at the leaves of the trees. Tree leaves are the main point of entry for CO₂ into the forest ecosystem.
They act as chemical factories, capturing solar energy to convert CO₂ in the air into sugar (i.e., photosynthesis). These sugars meet the plant's energy needs. They also enable trees to grow and, in doing so, to store more CO₂ emitted by human activities.
The surface of the leaves is partly covered with stomata, tiny slits that can open and close. It is through these pores that CO₂ enters the tree. This movement of CO₂ requires no energy from the tree.
It occurs naturally from where CO₂ is more abundant—the atmosphere—to where it is less so, in this case the interior of the leaf. However, the opening of the stomata does not just allow CO₂ to enter. It also results in water loss for the tree, as the cells inside the leaves contain far more water vapor than the atmosphere, even in the most humid regions.
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