Overstory #80 - Forests and Water
Forests and Water
For eons, forests have been slowing water movement and thus precipitating sediments, capturing nutrients, and building the soil. The presence or absence of forest cover may decide the ultimate fate of human society.
Forests have an intimate relationship to water supplies. The delayed release of rainwater from forested soils of the uplands are vital to lowland water supplies. Litter that accumulates on the forest floor absorbs the physical impact of torrential downpours and releases the water gently to the mineral soil beneath. This cushioning action largely prevents the water from suspending large quantities of surface soil particles and thus clogging soil pores beneath. In addition, the decaying litter enriches the water entering the soil and supports organisms that produce porous upper soil layers. These processes are the most obvious ways forests enhance water supplies. The draft on soil water is greatest under forests with their deep-rooted trees and high rates of transpiration. Between storms porous soils again become highly receptive to new water.
Storm water, generally received in torrents, may carry away the litter and surface soil if it cannot promptly percolate into the soil. Such percolation represents the lifeblood of the Tropics--it supplies the forest, and then follows subterranean pathways, reappearing gradually and continuously in springs that feed streams that in turn safeguard and support aquatic life, commerce, irrigation, and urban life downstream. This intimate relation between forests and usable water makes the Tropics habitable. Thus, tropical forests provide soil protection, a high soil infiltration rate, and, where soil is deep, substantial storage (Pereira 1967).
Even after water enters streams, it may continue to be affected by riparian forests. Tree growth on streambanks stabilizes the soil. When floods occur, the forest litter may support aquatic life important as a source of human food. Flooded streamside forests slow water movement, and thus precipitate sediments, capturing nutrients and building up the level of streambanks. At river mouths, estuaries, and along relatively protected seacoasts, mangrove forests retain sediments and provide habitats for important terrestrial, amphibious, and marine fauna.
Many rivers separate nations or run through more than one nation. Therefore, forest benefits to natural water courses are an international concern and expand the self-interest of all nations into a web of interdependence.
Part of the rainfall is intercepted by the forest canopy and evaporates. Interception by rain forest canopies varies widely with the density of the canopy and the intensity and duration of the rainfall. For short, light showers, all water may remain on and evaporate from a dense forest canopy. Measurements over time suggest that, under closed forests, about 15 to 20 percent of the rainwater is held in the canopy (Kline and others 1968, Lawson and others 1981). Whether such water benefits the ecosystem has been debated. Its evaporation cools the vegetation and the air, presumably reducing the draft on soil water for transpiration. Moist vegetation is darker in color than dry vegetation, and therefore absorbs more solar energy, suggesting that at least part of the energy required would not otherwise have been available for the ecosystem's needs (Satterlund 1972).
In the Brazilian Amazon, studies have shown that 62% of the water goes to evapotranspiration, and 90% of this is due to a delicate energy balance (Villa Nova and others 1976). Because the hydrological cycle is so intimately related to the presence of forests, general deforestation can lead to serious consequences.
Forests, Rainfall, and Soil
The relationships between forests and tropical soils are an outgrowth of forest-water relations. Humic acids picked up by rainwater as it passes through the forests accelerate weathering of parent rock and other soil-forming processes. The forest floor minimizes landslides by absorbing the shock of intense rainfall as do the dense and deep tree-root systems. Studies show the superiority of forest over other types of vegetative cover in this function (Lawson and others 1981).
The effectiveness of forests in controlling erosion varies with the climate, slope, soil condition, and the character of the forest. The densest forests, which permit few living plants in the ground layer, may be less protective than more open forests with herbs, grasses, or young trees which hold the litter in place, particularly on slopes. Many trees, and palms in particular, tend to concentrate rainfall towards their main stem. As much as 10% of the rainfall may reach the soil beneath rain forests in this manner (Lawson and others 1981). This stemflow may be rich in particulates washed from the tree bark and thus provide nutrition at the base of the tree, but the concentrated flow downslope from that point can, in extreme cases, cause severe erosion. Where these problems become serious, they can be lessened by silvicultural practices.
Kline, J.R., C.F. Jordan, and G. Drewry. 1968. Tritium movement in soil of a tropical rain forest (Puerto Rico). In: Science. 160:550-557.
Lawson, T.L., R. Lal, and K. Oduro-Afriyie. 1981. Rainfall redistribution and microclimate changes over a cleared watershed. In: R. Lal and E.W. Russell (Eds). Tropical Agricultural Hydrology. John Wiley and Sons. Chichester, UK.
Satterlund, D.R. 1972. Wildland Watershed Management. Ronald Press, New York, NY.
Pereira, H.C. 1967. Afforestation and streamflow in tropical highlands. Commonwealth Forestry Review. 47(4):323-327
Villa Nova, M.A., E. Matsui, and E. Salati. 1976. Estimativa da evapotranspiracao na bacia amazonica. In: Acta Amazonica. 6(2): 215 - 228
Wadsworth, F.H. 1997. Forest Production for Tropical America. International Institute for Tropical Forestry, USDA Forest Service, Rio Piedras, Puerto Rico.
This article is excerpted from the original with the kind permission of the author.
About the Author
Dr. Frank H. Wadsworth has been a silviculturist in Puerto Rico since 1942. He has served as Director of International Institute of Tropical Forestry (USDA Forest Service), Supervisor of the National Forest, and international forestry consultant until retirement at the end of 1999. He is currently editor of ISTF News, the quarterly publication of the International Society of Tropical Foresters.