The water cycle on Earth is driven by solar energy (which evaporates water from oceans and land surfaces) and gravity (which brings water down as rain and flows it toward the sea). In this cycle, trees play the role of an active pump: they absorb soil water through their roots and release it into the atmosphere through leaf transpiration, enormously amplifying the hydrological cycle of land compared to what would happen with bare rock and soil alone. Without trees, Earth's hydrological cycle would be completely different: many inland zones of continents would be far more arid than they are.

The tree as a hydraulic pump: the numbers

A medium-sized adult tree (beech, oak, elm: height 20-25 m, crown 10-15 m in diameter) transpires: in summer (July-August): 150-400 liters of water per day. In spring and autumn: 50-150 liters of water per day. In winter (leaves fallen): almost zero. For comparison: human water consumption is about 2 liters of water per day. A single tree transpires as much as 75-200 people drink. A forest of 100 trees per hectare transpires 15,000-40,000 liters of water per day per hectare = 15-40 mm of water per hectare in a summer day. The entire Amazon rainforest (5.5 million km²) transpires about 20 billion tons of water per day: more than the Amazon River discharges into the ocean. This is the "biotic pump" of tropical forests: water vapor rises, condenses at approximately 5-8 km altitude, and falls again as rain on inland regions of the continent. The physical mechanism of transpiration: water rises from roots to crown through the wood's xylem by negative capillary tension: leaves transpire (evaporate water from stomata), creating negative pressure in the mesophyll that "draws" water from adjacent cells, which in turn pull water from the leaf vein, from the vein, from the branch, from the trunk, down to the roots. A chain of hydraulic tension extending for tens of meters from soil to leaf. The pressures generated by the tensioning of the water column in the xylem reach -20-30 bar (well below atmospheric pressure): a hydraulic system under negative pressure that operates without mechanical pumps.

Trees and local precipitation: rain that creates itself

The influence of trees on local precipitation is more direct than most people imagine. Water vapor transpired by trees contributes directly to cloud formation and local precipitation. The Amazon's "flying rivers": the concept of "atmospheric river" generated by tropical forests was developed by Brazilian physicists Antonio Nobre and Jose Marengo. Water vapor transpired from the Amazon forest (20 billion tons/day) rises in altitude, forms clouds, and is transported by winds toward the west and south, bringing precipitation to agricultural regions of Brazil, Paraguay, and Argentina. This water vapor current has been called a "flying river" (rio voador) and is fundamental for agricultural precipitation in the center-south of the South American continent. Deforestation of the Amazon (already 20-25% of the original forest has been destroyed) is reducing this flying river and increasing drought in agricultural regions most dependent on it. The hydrological scar of deforestation: deforested areas show reductions in local precipitation of 5-30% in the decades following deforestation. In Europe, studies of historical meteorological data show that areas with greater forest cover have precipitation on average more abundant than areas with sparse cover. Trees in cities and urban rain: urban forests increase local (micro-scale) precipitation in a measurable way. A study by Bounoua et al. (2018) showed that urban areas with greater tree cover receive more frequent summer precipitation due to increased evapotranspiration.

The effect of trees on groundwater and streams

The presence or absence of trees profoundly influences the quantity and quality of water in groundwater aquifers and watercourses. Aquifer recharge: forest soil (with its porous structure and humus layer) greatly favors groundwater recharge compared to bare or impermeable soil. Rainwater infiltrates rather than running off the surface. Springs and streams in forested watersheds are more consistent (less seasonal variation) compared to those in deforested watersheds. Water quality: water flowing through the forest litter and humus-rich soil is filtered physically and biologically (the soil microbial community degrades many organic pollutants and retains some heavy metals). Water in forest streams is typically cleaner, with lower sediment loads, nutrients, and pollutants compared to streams in agricultural or urban watersheds. Riparian forest (the tree belt along watercourses): in Italy, traditionally, watercourses were bordered by strips of poplars, willows, alders, and ash trees (riparian forest). The removal of these forests to gain agricultural land or for rigid hydraulic systems has: increased bank erosion, reduced water quality (sediments, nutrients from adjacent agro-ecosystems), eliminated essential habitats for fish, amphibians, waterfowl, and insects. Restoration of riparian forests is one of the most effective rewilding actions to improve the ecological quality of Italian watercourses.

Trees and climate change: the evapotranspiration trap

The relationship between trees and climate change is more complex than simple CO₂ sequestration. Tree evapotranspiration has both cooling effects (already discussed) and potential warming effects under certain conditions. The boreal forest paradox: in boreal coniferous forests (taiga: the forest extending from Siberia to Canada), dark trees absorb much more heat than the white snow that would cover the ground without them. In some taiga zones, this heat absorption (albedo effect) exceeds the cooling produced by evapotranspiration and CO₂ sequestration. The net balance: boreal forest could have a net local warming effect (!) despite CO₂ sequestration. This paradox is called the "albedo-carbon trade-off" and is still subject to scientific debate. Tropical rainforest: no paradox. Tropical forests have much greater cooling effects than boreal forests due to their intense evapotranspiration (in regions where sunlight is always high). CO₂ sequestration, evapotranspiration, and albedo reduction all converge in the direction of cooling. Protecting tropical forests is the most urgent climate mitigation strategy. Temperate forests (Europe, Italy): the balance is generally positive for climate (net cooling), with some uncertainties about more northern alpine forests. Planting trees in temperate zones (especially in degraded or abandoned areas) contributes to climate change mitigation. The risks of massive afforestation in arid zones: in semi-arid zones, massive afforestation can paradoxically reduce local precipitation (trees use all available water by increasing evapotranspiration without increasing local rainfall enough to compensate). China has experience with both successes and failures in its afforestation programs in the Loess Plateau zone: some afforestation efforts have increased local drought. Selecting the right species, the right densities, and the right locations is critical.