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Forests provide remarkable benefits, helping to ensure clean water in sufficient quantities for aquatic habitat, safe drinking water, and recreational opportunities.
Forests are important in the long-term, low-cost protection of water supplies. Concentrations of nitrate from forested watersheds typically average 1 mg/l or less, compared to a drinking water standard of 10 mg/l. Runoff from watersheds dominated by other land uses such as agriculture or development is often 3 to 15 times as much. Maximizing forest resources for water quality improvement depends on the 1) location of forests, and 2) condition of forests. Maintaining forest cover in critical locations such as floodplains, seeps, steep slopes, headwaters, and close to streams can help avoid major deterioration in water quality and increases in treatment difficulty and cost. Forests adjacent to higher sources of nutrients (such as fertilized crops or lawns) can reduce nitrogen before it reaches the reservoir, especially on shallower soils where tree roots reach the groundwater.
The characteristics of a healthy forest which affects its ability to protect water quality includes s tree health, distribution of tree and stand sizes and ages, and number of layers of vegetation (e.g., herbaceous, shrub, subcanopy, midcanopy, upper canopy). Stands with multiple layers of vegetation and a range of ages and sizes of trees can withstand loss of trees most susceptible to damage without losing all of its functions for erosion control and infiltrating water. Forest management near reservoirs takes into account the potential for multiple canopy layers, matching species to site conditions, and opportunities to maintain actively growing forests next to nutrient sources.
Hardwood forests are typically managed using shelter wood techniques removing the commercially valuable stems and leaving the rest of the forest in tack. Soft wood forests typically use clear cuts to properly regenerate those species that need sunlight and no competition.
Case studies of natural forests used to protect water quality:
Baltimore City supported the development of a Comprehensive Forest Conservation Plan for the 17,856 acres of city-owned land around the three reservoirs, Loch Raven, Liberty, and Prettyboy, which supply 1.8 million people in Baltimore City and surrounding areas with drinking water. http://www.dnr.state.md.us/forests/programapps/dwprotection.asp
In 1990 the federal government ordered that all public supplies of surface water be filtered for microbial contaminants. For New York City, the nation's sole million-plus municipality that did not already filter its water, that meant the construction of a $4 billion to $6 billion plant to filter the waters that flow from its upstate watershed. After years of debate, the City and some 30 watershed communities signed the New York City Watershed Agreement, a $1.4 billion deal to protect the working landscape of family farms and woodlots that have protected the Catskills/Delaware watershed for decades. Three-quarters of the City's Catskills/Delaware watershed remains forested. The Catskills landscape is a pastoral setting in comparison to the heavily developed Croton watershed had to be filtered. The plan frees New York City from filtering water from the watershed, provided it can otherwise meet 66 EPA "filtration avoidance criteria" saving billions of dollars by using forest’s natural pollution filtration capacity.
There is a close relationship our water resources and the surrounding land. How land is used — whether it is forested, used for agriculture, or developed with buildings and roads — is a critical factor in determining the quality and quantity of groundwater and surface water. During storms, water adheres to leaves, twigs, and branches, intercepting as much as 25 percent of total rainfall in a dense forest. Rather than flowing into streams and rivers where it would increase flooding, much of this water returns to the atmosphere by evaporation (Figure 1).
In addition to the water intercepted by vegetation, rainfall also collects and forms puddles in depressions in the ground. Forests generally have a greater capacity to store water in this way than agricultural or developed areas because of their irregular ground surface. This water can evaporate or filter into the soil to recharge underground aquifers. These aquifers provide much of our drinking water and also recharge rivers, streams, wetlands, and lakes during dry periods.
Trees are indicators of a community's ecological health. While urban ecology is more complex than just tree cover, trees are good indicators of the health of an urban ecosystem. When trees are large and healthy, the ecological systems-soil, air and water-that support them are also healthy. In turn, healthy trees provide valuable environmental benefits.