Issues in Ecology Number 2 Spring 1997 soaked up by soils and gradually meted out to plant roots or into aquifers and surface streams.Thus,the soil itself slows the rush of water off the land in flash bare soil is soil from the full destructive force of raindropsand hod it in place.When landscapes are denuded.rain com pacts the surface and rapidly turns soil to mud (espe cially if it has been loosened by tillage):mud clogs sur face cavities in the soil,reduces infiltration of water.in cre ,and further enhances ed downslope Detache clog arried off b running water (Hille Erosion causes costs not only at the site where soil is lost but also in aquatic systems.natural and human-made,where the material accumulates.Local costs of erosion include losses of production potential diminished infiltration and water availability.and losses Figure 6-Bark of the Pacific vew tree (Taxus breuifolia) of nutrients. which is the source of the new anti-cancer drug,taxol Downstrear cos smay include or lower quality water supplies:si tion that impairs drain Willamette National Forest,Oregon age and maintenance of navigable river channels.har releasing carbon dioxide to the atmosphere (Schneider bors.and irrigation systems:increased frequency and and Boston 1991;Allegre and Schneider 1994). The severity of floods:and decreased potential for hydroelec relative influence of life's stabilizing and destabilizing feed- tric power as reservoirs fill with silt(Pimentel et al.1995). backs remains uncertain:what is clear is that climate Worldwide.the st of reservoir and natural e lost to siltati bility per year pacit system is an importan In addition to protecting soil from erosion,living ecosystems vegetation-with its deep roots and above-ground evapo rating surface-also serves as a giant pump,returning tems also exert direct physical influences that help to water from the ground into the atmosphere.Clearing of moderate regional and local weather.For instance,tran- plant cover disrupts this link in the water cycle and leads spiration(release of water vapor from the leaves)of plants e loss fron the regio nd the in surface temperature. In the Amazon. for example. 50%of the mean annual rainfall is recycled by the forest itself via evapotranspiration-that is.evaporation from wet leaves and soil combined with transpiration (Salati 1987).Amazon deforestation could so dramatically re- duce total pitation that the forest might be tself following complet dest ction (Shukla by forests,which provide shade and surface cooling and also act as insulators,blocking searing winds and trap- ping warmth by acting as a local greenhouse agent. Mitigation of Floods and Droughts An ount of water, about 119.000 Figure 7-Herbal pharmacist in Dai.Yunnan Province. surface-enough to cover the land to an average depth China.An estimated 80 percent of the world's popu- lation relies on natural medicinal products of I meter(Shiklomanov 1993).Much of this water is 7 Issues in Ecology Number 2 Spring 1997 Figure 6-Bark of the Pacific yew tree (Taxus brevifolia), which is the source of the new anti-cancer drug, taxol, Willamette National Forest, Oregon. Photo by Michael Graybill & Jan Hodder/ Biological Photo Service Figure 7-Herbal pharmacist in Dali, Yunnan Province, China. An estimated 80 percent of the world’s popu￾lation relies on natural medicinal products. Photo by Catherine M. Pringle/Biological Photo Service releasing carbon dioxide to the atmosphere (Schneider and Boston 1991; Allegre and Schneider 1994). The relative influence of life’s stabilizing and destabilizing feed￾backs remains uncertain; what is clear is that climate and natural ecosystems are tightly coupled, and the sta￾bility of that coupled system is an important ecosystemservice. Besides their impact on the atmosphere, ecosys￾tems also exert direct physical influences that help to moderate regional and local weather. For instance, tran￾spiration (release of water vapor from the leaves) of plants in the morning causes thunderstorms in the afternoon, limiting both moisture loss from the region and the rise in surface temperature. In the Amazon, for example, 50% of the mean annual rainfall is recycled by the forest itself via evapotranspirationthat is, evaporation from wet leaves and soil combined with transpiration (Salati 1987). Amazon deforestation could so dramatically re￾duce total precipitation that the forest might be unable to reestablish itself following complete destruction (Shukla et al. 1990). Temperature extremes are also moderated by forests, which provide shade and surface cooling and also act as insulators, blocking searing winds and trap￾ping warmth by acting as a local greenhouse agent. Mitigation of Floods and Droughts An enormous amount of water, about 119,000 cubic kilometers, is rained annually onto the Earth’s land surfaceenough to cover the land to an average depth of 1 meter (Shiklomanov 1993). Much of this water is soaked up by soils and gradually meted out to plant roots or into aquifers and surface streams. Thus, the soil itself slows the rush of water off the land in flash floods. Yet bare soil is vulnerable. Plants and plant litter shield the soil from the full, destructive force of raindrops and hold it in place. When landscapes are denuded, rain com￾pacts the surface and rapidly turns soil to mud (espe￾cially if it has been loosened by tillage); mud clogs sur￾face cavities in the soil, reduces infiltration of water, in￾creases runoff, and further enhances clogging. Detached soil particles are splashed downslope and carried off by running water (Hillel 1991). Erosion causes costs not only at the site where soil is lost but also in aquatic systems, natural and human-made, where the material accumulates. Local costs of erosion include losses of production potential, diminished infiltration and water availability, and losses of nutrients. Downstream costs may include disrupted or lower quality water supplies; siltation that impairs drain￾age and maintenance of navigable river channels, har￾bors, and irrigation systems; increased frequency and severity of floods; and decreased potential for hydroelec￾tric power as reservoirs fill with silt (Pimentel et al. 1995). Worldwide, the replacement cost of reservoir capacity lost to siltation is estimated at $6 billion per year. In addition to protecting soil from erosion, living vegetationwith its deep roots and above-ground evapo￾rating surfacealso serves as a giant pump, returning water from the ground into the atmosphere. Clearing of plant cover disrupts this link in the water cycle and leads