The Gift of Urban Trees

(Conservation Currents, Northern Virginia Soil and Water Conservation District)

little girl in leavesIn urban regions around the world, improving the health and vigor of urban forests can yield great economic and environmental benefits for both today and tomorrow. From the street trees in front of a downtown store to the large trees found in many parks and greenways, the urban forest is a critical part of the landscape.

Trees buffer the “wind tunnels” created by buildings and streets, while adding a living element to the concrete, steel, and glass. Trees shade streets, sidewalks, and buildings to reduce the impact of the sun’s rays and the buildup of urban heat. Trees release water back into the air, working as natural air conditioning systems. Trees reduce noise pollution by absorbing unpleasant sounds such as those generated from traffic and construction. Trees in the community filter dirt and chemicals from the air.

In addition to improving air quality, trees are valuable in protecting water quality. Society’s existence depends on fresh, clean water from rivers, streams and groundwater. Unfortunately, unhealthy conditions are common in America’s streams and rivers today due in part to the loss of forest cover.

Stream management has changed in recent years, and restoring stream integrity is now one of the nation’s highest conservation priorities. Formerly, people thought that the way to manage streams was to rework them into more efficient systems to hold and deliver water according to people’s needs and desires. Rivers were dammed and great levees were built to contain floods. Some rivers and streams were cleared and straightened so water could run through faster, and vegetation was often removed to allow farming and development up to the streams’ edges. Where those efforts were accomplished, the landscape was greatly altered, as people built communities, irrigated deserts, and drained wetlands in areas now protected from flooding. Where engineering projects failed, however, the alterations often served to make flooding even worse.

two men fishing from canoeToday’s thinking about stream management focuses on understanding the variable nature of a watershed and its streams and designing management approaches that build upon and take advantage of the stream’s inherent tendencies. Much of what needs to be done to restore the stream’s integrity needs to happen on the land around it.

The riparian area—that area alongside the stream that connects the stream banks to the surrounding uplands—is often the most important management area. If it can be maintained in permanent grass or forest cover, the protection afforded the stream increases significantly. Riparian areas play several roles, but none are more crucial than the trapping of soil and filtering of nutrients as they wash down from the uplands. Caught in the riparian zone, nutrients nourish the vegetation instead of adding to stream pollution. Where trees grow on stream banks, strong root systems help to provide stability and prevent the bank from washing away. Less runoff and erosion allows more recharging of the groundwater supply. Shade keeps water temperatures cool to hold more dissolved oxygen, which is important for maintaining freshwater fish. Leaves fall into the water, where they provide food for aquatic insects and other organisms. Even when a stream bank tree dies and falls into the stream, it forms an important part of the stream structure as shelter and a source of food for stream insects, fish and other animals. Thus, the trees become an important part of maintaining healthy stream communities that help maintain clean water.

Forested riparian buffers are also critical wildlife habitat areas. Because of their nearness to water, these buffers are the most biologically rich areas on any landscape. Maintaining a healthy forest cover, avoiding soil damage by careful machine operations, and controlled use of any chemicals can ensure that we have highly productive forests, clean water, and abundant wildlife.

Good land managers can maintain the quality of streams and rivers and, where those streams and rivers have been damaged in the past, restore watershed function. Trees planted to help maintain stream stability can create a long-term, environmental benefit for everyone who uses water.

The gifts of trees are many. Birds and insects will live and nest in its branches. Its roots will penetrate deeply into the soil, building soil structure and quality as they die and re-grow. The tree’s trunk will thicken with wood that represents tons of carbon dioxide that has been taken out of the air and placed into “carbon storage,” where it is doing its part against the threat of global climate change.

The rain will hit its leaves and trickle gently to the ground to nourish the tree and the other plant life under it, instead of pounding on unprotected soil to create erosion and pollution. The air that moves through its branches will be purified as pollutants are trapped on leaf surfaces, chemicals are absorbed, and carbon dioxide is taken in to provide a basic building block for the tree’s growth.

Give the generations that follow you the gift of trees.

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