Water that never leaves
It is tempting to picture rain falling on a forest, running into rivers and flowing to the sea. In reality, a large share of the rain that falls on a rainforest goes back into the air. Some of it evaporates from wet leaves and the ground. A great deal more is taken up by roots and released from leaves as vapor. The vapor rises, forms clouds, and may fall again as rain, sometimes hundreds or thousands of kilometers downwind. This reuse of water is called moisture recycling.
How transpiration works
Leaves have microscopic pores, called stomata. When they open to take in carbon dioxide for photosynthesis, water vapor escapes. That loss pulls more water up from the roots through the trunk, a process of water transport covered elsewhere. The whole process is called transpiration.
A large rainforest has a vast leaf surface, and it stays active all year in the wet tropics. Together with evaporation from wet foliage and soil, this release is called evapotranspiration. It is a major supply of water vapor to the lower atmosphere over forest regions.
Deep roots matter too. In places with a short dry season, some trees can reach water far below the surface, allowing them to keep transpiring while shallow-rooted plants would wilt. This may help keep the air humid when rainfall pauses, although the depth and importance of roots differ among forests.
From vapor to rain
Water vapor does not automatically turn into rain. It needs to rise, cool and condense on tiny particles. Forests contribute here as well. They release small organic particles and compounds that can serve as seeds for cloud droplets, and the moist air above them can support convection, where warm humid air rises and builds towering clouds. Scientists are still studying how large these effects are, so they are best described as plausible contributors rather than settled quantities.
Wind carries the humid air onward. In a large forest basin such as the Amazon, air that crosses the forest can pick up moisture along the way, so rainfall in the interior and on the far side may depend partly on the forest it passed over. Estimates of how much rain is locally recycled differ with the region, the season and the method, and they range widely. It is safest to say that recycling is significant in large continental forests and much smaller where moist ocean air supplies most of the rain.
Why scale matters
Moisture recycling works best across large areas. A small forest patch loses most of its transpired water to the wind, which carries it elsewhere. A very large forest, by contrast, keeps much of its own moisture in a long chain: water evaporates, travels downwind, rains, and is taken up and released again. Removing trees in the upwind part of that chain can reduce the water available to the forest downwind.
This is part of why large-scale tropical deforestation is a concern for regional rainfall, not only for the lost trees. Studies and models suggest that extensive clearing could lengthen the dry season and reduce rainfall in some areas, but the exact thresholds are uncertain, and they differ between regions. Claims of a precise “tipping point” should be treated cautiously.
Cooling and the wider water cycle
Transpiration also cools the air, in the same way that sweat cools skin, because evaporation uses heat energy. Forested land often has cooler surfaces on hot days than cleared land does. Pastures and crop fields typically transpire less and heat up more.
These local effects connect to the global water cycle, in which forests are one of several land surfaces that move water between the ground and the atmosphere. In a forest-rich area, healthy streams and groundwater also depend on a steady supply of rain, so changes in recycling can eventually be felt in rivers, farms and cities.
What can change it
- Clearing and burning reduce leaf area and transpiration, and smoke particles can alter cloud formation in ways that are still being investigated.
- Drought stress can cause trees to close their stomata or die, which reduces water release at the time it is most needed.
- Regrowth and restoration can bring some of the water-recycling function back, although young forests may differ from old ones in how much water they use.
The central point is that a rainforest is not just a recipient of the climate. It is also an active participant in producing it, which is why protecting large intact forests is often discussed as a climate and water issue as well as a conservation one.