During periods of drought, trees pause their photosynthesis, a process that requires significant amounts of water. For every molecule of carbon dioxide (CO₂) absorbed by a tree leaf, about 400 water molecules are released. This can lead to entire forests emitting more CO₂ than they capture, raising concerns as droughts become more frequent and forests play a vital role in climate change mitigation. It is estimated that forests absorb about a quarter of the CO₂ emitted by human activities, though measuring this accurately remains a challenge. Tree leaves are the main entry point for CO₂ into the forest ecosystem, acting like chemical factories that convert CO₂ into sugar through photosynthesis. The surface of leaves contains tiny pores called stomata, which allow CO₂ to enter but also result in water loss through a process called transpiration. On average, for every molecule of CO₂ absorbed, around 400 water molecules are lost. The movement of water from roots to leaves relies on a hydraulic system driven by transpiration, a process described by Henry Horatio Dixon as "tension-cohesion." This mechanism uses the evaporation of water at stomata to create tension that pulls water upwards. However, during extreme droughts, water can enter a "metastable" state and undergo cavitation, forming air bubbles that block water flow and cause hydraulic failure. During droughts, trees face a dilemma: open stomata to allow photosynthesis or close them to conserve water. Stomata open and close dynamically to minimize water loss, and in drought conditions, all vascular plants close their stomata. This closure can lead to forests emitting CO₂ through the consumption of sugars for metabolism and the release of stored carbon from dead trees. Drought tolerance varies among tree species, with those having more vulnerable hydraulic systems, such as birch, closing stomata earlier than more resistant species like the holm oak. Even the most resistant trees operate at the threshold of embolism, making all forests vulnerable to increased droughts caused by climate change. When droughts exceed the conditions to which trees are adapted, physiological impacts become critical, as residual water loss continues through leaves and stems' cuticles, potentially leading to leaf drop, embolism, and tree death. Water and CO₂ dynamics are interlinked, with water regulating the carbon cycle and vice versa. While increased atmospheric CO₂ may enhance photosynthesis, it also reduces stomatal opening, conserving water but reducing transpiration and rainwater recycling. This is critical for climatic stability, as transpired water contributes significantly to rainfall patterns, such as in the Amazon basin, where up to 50% of western rainfall originates from transpiration in the east. Stomatal closure in the east may reduce rainfall in the west, potentially jeopardizing the stability of this key region.