Urban tree and soil systems provide water quantity and water quality benefits through stormwater infiltration, filtration, interception, evaporation, and uptake of pollutants by trees and associated microbes. Trees are already part of virtually all development and can be integrated even into the densest urban areas. Many cities already have tree requirement ordinances. However, the potential of these trees to provide significant stormwater benefits is largely untapped to date. Integrating urban trees into stormwater management systems provides opportunity to provide significant stormwater benefits using elements (trees and soils) that are already part of most sites and developments.
The soil, trees, and microbes in a bioretention system with trees work together as a system to improve water quality of stormwater that falls on the tree and/or is filtered through the soil volume. Some pollutants are adsorbed or filtered by soil, others are taken up or transformed by plants or microbes, and still others are first held by soil and then taken up by vegetation or degraded by bacteria, “recharging” the soil’s sorption capacity in between rain events.
Trees have a wide range of non-stormwater benefits, such as cleaner air, reduction of heat island effect, carbon sequestration, reduced noise pollution, reduced pavement maintenance needs, cooler cars in shaded parking lots, windbreaks, reduced annual air conditioning costs, and social benefits (e.g. reduced stress, improved outdoor leisure and recreation, reduced crime).
Potential constraints on the use of urban trees for stormwater management include:
Many different types of urban tree stormwater BMP’s exist. Where existing trees exist, tree preservation is highly recommended, as existing trees are typically bigger than newly planted trees, and bigger trees provide significantly more benefits than smaller trees. Incorporating trees into traditional bioretention practices is also highly recommended. Street trees, trees in parking lots, trees in urban plazas, as well as any other trees also provide stormwater benefits.
While trees have tremendous potential to provide stormwater benefits, most urban trees do not provide nearly the magnitude of stormwater benefits they are capable of providing given adequate growing conditions. Large trees provide orders of magnitude greater stormwater benefits than small trees, but the average lifespan for urban trees is only 13 years (Skiera and Moll, 1992), so most urban trees do not survive nearly long enough to reach their mature size and provide the magnitude of stormwater benefits they are capable of at maturity. By far the most important factor to grow healthy trees is to provide an adequate volume of rootable soil, to allow for adequate air, water and drainage (e.g. Coder 2007). Research has shown that trees need 2 cubic feet of rootable soil volume per square foot of tree canopy area to thrive (e.g. Lindsey and Bassuk 1991). To provide 2 cubic feet of rootable soil to allow a tree with a 30 foot canopy to thrive would require 1413 cubic feet of rootable soil, 22 times more than the typical 64 cubic feet an urban tree box has.
Where there is not enough open space to grow large, healthy urban trees, several techniques exist to protect soil volume under pavement from traffic compaction so that this soil can be used both for bioretention and tree root growth. Examples of these techniques include
Same as bioretention, can range greatly in size, and can be sized to meet desired goals.
Pollutants are removed through infiltration and filtration of stormwater. See Calculating credits for tree trenches and tree boxes.