Urban Watershed Forestry

What is Urban Watershed Forestry?

Urban watershed forestry is an integration of the fields of urban and community forestry and watershed planning. Urban and community forestry is the management of the urban forest for environmental, community, and economic benefits, while watershed planning promotes sound land use and resource management to improve water resources within a watershed. This integration of urban forestry techniques into urban watershed planning acknowledges the importance of trees and forests in protecting water resources.

The urban watershed forestry approach sets watershed-based goals for managing the urban forest as a whole rather than managing forest resources on a site-by-site or jurisdictional basis. It also encourages watershed managers and urban foresters to systematically assess and manage forest cover at the watershed level with the basic aim of reducing forest loss and increasing forest cover over time. The three major goals include:

Protect undeveloped forests from human encroachment and the impacts of land development by creating and applying various planning techniques, regulatory tools, and incentives. This includes conservation easements that protect forested land from being developed, land use planning that directs development away from forested areas and reduces paved surfaces, ordinances that require developers to physically protect selected forests during construction, and financial incentives that encourage developers to conserve more forest at a development site.

Enhance the health, condition, and function of urban forest fragments. This includes the use of various techniques for increasing and improving structure, hydrologic function, diversity, and wildlife habitat, and improving conditions for tree growth to ensure long-term sustainability of the forest.

Reforest open land through active replanting or natural regeneration to regain some of the functions and benefits of a forest and to increase overall watershed forest cover and increase forest canopy.

How are Forests Important to the Watershed?

Forests play an important role in watershed health by providing certain benefits summarized below.

Watershed Benefits of Forest Cover
Benefit	Description
Reduce stormwater runoff and flooding	Trees intercept rainfall in their canopy, reducing the amount of rain that reaches the ground. A portion of this captured rainwater evaporates from tree surfaces. Trees take up water from the soil through their roots, which increases soil water storage potential and lengthens the amount of time before rainfall becomes runoff. Trees promote infiltration by slowing down runoff and by increasing soil drainage in the root zone. The addition of organic matter (e.g. leaves) also increases storage of water in the soil, further reducing runoff. Forested land produces very little runoff, which can reduce downstream flood flows that erode stream channels, damage property and destroy habitat.
Improve regional air quality	Trees absorb pollutants such as nitrogen dioxide, carbon monoxide, ozone, and particulate matter from the atmosphere. Trees reduce air temperature, which reduces formation of pollutants that are temperature dependent, such as ozone Trees indirectly improve air quality by cooling the air, storing carbon, and reducing energy use, which reduces power plant emissions
Reduce stream channel erosion	Trees growing along a stream bank prevent erosion by stabilizing the soil with root systems and the addition of organic matter
Improve soil and water quality	Trees prevent erosion of sediment by stabilizing the soil, and by substantially dispersing raindrop energy Trees take up stormwater pollutants such as nitrogen from soil and groundwater Forested areas can filter sediment and associated pollutants from runoff Certain tree species break down pollutants commonly found in urban soils, groundwater, and runoff, such as metals, pesticides and solvents
Provide habitat for terrestrial and aquatic wildlife	Forests (and even single trees) provide habitat for wildlife in the form of food supply, interior breeding areas, and migratory corridors Streamside forests provide habitat in the form of leaf litter and large woody debris, for fish and other aquatic species Forest litter such as branches, leaves, fruits, and flowers, form the basis of the food web for stream organisms
Reduce summer air and water temperatures	Riparian forests shade the stream and regulate summer air and water temperatures, which is critical for many aquatic species Trees and forests shade impervious surfaces, reducing temperature of stormwater runoff, which can minimize the thermal shocks normally transmitted to receiving waters during storms.

The important link between forests and watershed health has been documented by several researchers. Booth (2000) found that at least 65% watershed forest cover is needed for the presence of a healthy aquatic insect community in a Puget Sound, Washington study. Other researchers have evaluated the relative impact of streamside forest cover on various indicators of stream health. In a Montgomery County, Maryland study, Goetz and others (2003) found that for streams to have a health rating of Excellent, at least 65% of the length of the stream network in the watershed must be forested (within 100 feet of the stream). At least 45% streamside forest cover was required for streams to have a health rating of Good (Goetz and others, 2003). Other researchers such as Kathy Wolf from the University of Washington are documenting the human dimension of urban forestry with compelling research on the psychological and sociological benefits of trees.

A forested buffer along each side of the stream has been shown to be very important for stream health

More research is needed, but it appears that watershed forest cover, particularly streamside forest cover, may be able to minimize the impacts of land development up to a certain point. For example, Roy et al (2007) found that the presence of streamside forests prevented stream impacts up to the point where > 15% of the watershed was developed as urban land.

A USDA/USFS report entitled “Conservation Buffers: Design Guidelines for Buffers, Corridors and Greenways” was published in Sept., 2008. The report provides over 80 illustrated design guidelines for conservation buffers and represents a synthesis of over 1,400 research publications. Each guideline describes a specific way that a vegetative buffer can be applied to protect soil, improve air and water quality, enhance fish and wildlife habitat, produce economic products, provide recreation opportunities, or beautify the landscape. The online version of the publication is available here.

References

Booth, D. 2000. Forest cover, impervious surface area, and the mitigation of urbanization impacts in King County, WA. Prepared for King County Water and Land Resource Division. Seattle, WA: University of Washington.

Cappiella, K., Schueler, T., and T. Wright. 2005. Urban Watershed Forestry Manual. Part 1: Methods for Increasing Forest Cover in a Watershed. NA-TP-04-05. USDA Forest Service, Northeastern Area State and Private Forestry. Newtown Square, PA.

Goetz, S.; Wright, R.; Smith, A.; Zinecker, E.; Schaub, E. 2003. IKONOS imagery for resource management: Tree cover, impervious surfaces, and riparian buffer analyses in the mid-Atlantic region. Remote Sensing of Environment 88:195-208.

Roy, A.H., Freeman, B.J., and M.C. Freeman. 2007. Riparian influences on stream fish assemblage structure in urbanizing streams. Landscape Ecology 22(3): 385-402.