The anticipated construction period for this page is through January, 2014
Contents
Function within stormwater treatment train
Use of trees to manage stormwater runoff encompasses several practices. Tree trenches and tree boxes (collectively called tree BMP(s)), the most commonly implemented tree BMPs, can be incorporated anywhere in the stormwater treatment train but are most often located in upland areas of the treatment train. The strategic distribution of tree BMPs help control runoff close to the source where it is generated. Tree BMPs can mimic certain physical, chemical, and biological processes that occur in the natural environment. Depending upon the design of a facility, different processes can be maximized or minimized depending on the type of pollutant loading expected (Prince George’s County, 2002). As with any filtration and infiltration BMPs, pretreatment is recommended to prevent clogging of the media, particularly when permeable pavement is used in conjunction with the tree BMP.
Tree BMPs are one component of urban forestry. Urban forestry is a broad term that applies to all publicly and privately owned trees within an urban area, including individual trees along streets and in backyards, as well as stands of remnant forest (Nowak et al. 2001). Urban forests are an integral part of community ecosystems, whose numerous elements (such as people, animals, buildings, infrastructure, water, and air) interact to significantly affect the quality of urban life. (Nowak et al 2010 Sustaining America’s Urban Trees and Forests). Trees are already part of virtually all development and can be integrated anywhere in the treatment train, 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.
MPCA permit applicability
One of the goals of this Manual is to facilitate understanding of and compliance with the MPCA Construction General Permit (CGP), which includes design and performance standards for permanent stormwater management systems. Standards for various categories of stormwater management practices must be applied in all projects in which at least one acre of new impervious area is being created.
For regulatory purposes, tree BMPs fall under the “Infiltration / Filtration” category described in Part III.D.1. of the CGP. If used in combination with other practices, credit for combined stormwater treatment can be given. Due to the statewide prevalence of the MPCA permit, design guidance in this section is presented with the assumption that the permit does apply. Also, although it is expected that in many cases tree BMPs will be used in combination with other practices, standards are described for the case in which it is a stand-alone practice.
There are situations, particularly retrofit projects, in which a tree BMP is constructed without being subject to the conditions of the MPCA permit. While compliance with the permit is not required in these cases, the standards it establishes can provide valuable design guidance to the user. It is also important to note that additional and potentially more stringent design requirements may apply for a particular tree BMP, depending on where it is situated both jurisdictionally and within the surrounding landscape.
Retrofit suitability
Tree BMPs are an ideal and potentially important BMP in urban retrofit situations where existing stormwater treatment is absent or limited. Tree BMPs can be utilized in ultra-urban settings.
Special receiving waters suitability
The following table provides guidance regarding the use of tree BMPs in areas upstream of special receiving waters. This table is an abbreviated version of a larger table in which other BMP groups are similarly evaluated. The corresponding information about other BMPs is presented in their respective sections of this Manual.
Design restrictions for special waters - trees
Water quantity treatment
Like bioretention practices, trees used as stormwater BMPs are not typically suitable for providing water quantity control. In limited cases, a tree practice may provide some (albeit limited) storage volume. Tree BMPs can help reduce detention requirements for a site by providing elongated flow paths, longer times of concentration, and volumetric losses from infiltration and evapotranspiration. Experience and modeling analysis have shown that tree BMPs can be used to reduce runoff and maintain the pre-existing time of concentration. This effort can be incorporated into the site hydrologic analysis. Generally, however, it is Highly Recommended that in order to meet site water quantity or peak discharge criteria, another structural control (e.g. detention) be used in conjunction with tree BMPs.
Water quality treatment
Tree BMPs that utilize soil media provide water quality benefits through the same mechanisms as standard bioretention systems. 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.
[Summary of bioretention water quality cleansing mechanisms for common stormwater pollutants]
Several recent literature reviews of lab and field studies concluded that tree BMPs have the potential to be one of the most effective BMPs for pollutant removal. High load reductions are consistently found for suspended solids, metals, polycyclic aromatic hydrocarbons (PAHs), and other organic compounds. Nutrient (dissolved nitrogen and phosphorus) removal has been more variable. Healthy vegetation has been found to be especially crucial for removal of dissolved nitrogen and phosphorus, hence the importance of large trees. Several studies that have compared vegetated media to unvegetated media have found that the presence of vegetation substantially improves TP and TN retention, as vegetated media is much more effective than unvegetated media at removing PO4 from solution and preventing NO3 leaching from media (e.g. Henderson et al 2007, Lucas and Greenway 2007a, 2007b, 2008, May et al 2006). Not only has vegetation been shown to significantly improve nutrient removal, trees also benefit from the nutrients in stormwater (May et al 2006), with greater growth in height and greater root density compared with those irrigated with tap water, turning stormwater nutrients into an asset.
Use of trees to manage stormwater runoff encompasses several practices. Tree trenches and tree boxes (collectively called tree BMP(s)), the most commonly implemented tree BMPs, can be incorporated anywhere in the stormwater treatment train but are most often located in upland areas of the treatment train. The strategic distribution of tree BMPs help control runoff close to the source where it is generated. Tree BMPs can mimic certain physical, chemical, and biological processes that occur in the natural environment. Depending upon the design of a facility, different processes can be maximized or minimized depending on the type of pollutant loading expected (Prince George’s County, 2002). As with any filtration and infiltration BMPs, pretreatment is recommended to prevent clogging of the media, particularly when permeable pavement is used in conjunction with the tree BMP.
Urban forestry is a broad term that applies to all publicly and privately owned trees within an urban area, including individual trees along streets and in backyards, as well as stands of remnant forest (Nowak et al. 2001). Urban forests are an integral part of community ecosystems, whose numerous elements (such as people, animals, buildings, infrastructure, water, and air) interact to significantly affect the quality of urban life. (Nowak et al 2010 Sustaining America’s Urban Trees and Forests). Trees are already part of virtually all development and can be integrated anywhere in the treatment train, 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.
Non-stormwater benefits of trees
In addition to stormwater management, trees provide a host of other benefits including other environmental benefits, energy savings, social and health benefits, wildlife benefits, and economic benefits.
Environmental benefits include
- cleaner air;
- reduction of heat island effect;
- carbon sequestration;
- reduced noise pollution;
- reduced pavement maintenance needs; and
- cooler cars in shaded parking lots.
Strategically placed trees can reduce building and heating energy use. Examples include the following.
- Trees properly placed around buildings as windbreaks can reduce winter heating costs.
- Shade from two large trees on the west side of a house and one on the east side can save on annual air conditioning costs.
Trees have a wide range of social benefits, including
- reduced stress of both body and mind in urban areas (Parsons et al 1998 in USDA Forest Service 2004);
- improved outdoor leisure and recreation experiences (Dwyer et al 1989 in USDA Forest Service 2004) ;
- reduced crime (Kuo and Sullivan 2001 in USDA Forest Service 2004);
- improved recovery from surgery (Ulrich 1984 and Ulrich 1985 in USDA Forest Service 2004);
- improved ability of automobile drivers to cope with driving stresses (Wolf 2000 in USDA Forest Service 2004); and
- improved safety on streets (Wolf 2010)
Monetary benefits as ecosystem services significantly outweighs the cost of utilizing trees in an urban setting. McPherson et al. (2005) observed Minneapolis’s municipal tree resource provides approximately 79 dollars per tree in total net annual benefits to the community. Examples of other economic benefits include the following.*Shoppers in well-landscaped business districts are willing to pay more for parking and up to 12 percent more for goods and services (Wolf 1999 in USDA Forest Service 2004).
- Landscaping, especially with trees, can significantly increase property values (Neely 1988 in USDA Forest Service 2004).
- Desk workers with and without views of nature were surveyed. Those without views of nature, when asked about 11 different ailments, claimed 23 percent more incidence of illness in the prior 6 months (Kaplan and Kaplan 1989 in USDA Forest Service 2004).
- Amenity and comfort ratings were about 80 percent higher for a tree-lined sidewalk compared with those for a nonshaded street (Wolf 1998 in USDA Forest Service 2004).
- Quality of products ratings were 30% higher in districts having trees over those with barren sidewalks (Wolf 1998 in USDA Forest Service 2004).
Trees also benefit wildlife. For example Talamy and Darke 2007 observed that the Oaks genus supports 534 species of Lepidoptera (a large order of insects that includes moths and butterflies) as well as many bird species.
The following table summarizes some additional benefits for select tree species.
Non-stormwater benefits of several trees that can be used in tree trenches/tree boxes. NOTE: this list is not exhaustive and could include dozens of additional species.
Link to this table
| Scientific name1,2,3 | Common name 1,2,3 | Additional benefit (F=human food, W=wildlife habitat/browse, L=lumber)3 | |
|---|---|---|---|
| Acer saccharinum 1,2 | Silver maple | FWL | |
| Amelanchier spp. 2 | Juneberry/serviceberry | FW | |
| Betula populifolia 2 | Gray birch | W | |
| Carya ovata 2 | Shagbark hickory | FWL | |
| Catalpa speciosa 2 | Northern catalpa | WL | |
| Celtis occidentalis 2 | Common hackberry | W | |
| Cercis canadensis 2 | Eastern redbud | W | |
| Cladrastis kentukea 2 | Yellowood, Kentucky yellowood | L | |
| Crataegus crus-galli var. inermis 2 | Thornless cockspur hawthorn | W | |
| Fraxinus americana 2 | White ash | WL | |
| Fraxinus mandschurica | Manchurian ash | WL | |
| Fraxinus nigra 1 | Black ash | WL | |
| Fraxinus pennsylvanica 1,2 | Green ash | WL | |
| Gleditsia tricanthos var. inermis 2 | Thornless common honeylocust | WL | |
| Gymnocladus dioicus 2 | Kentucky coffeetree | WL | |
| Larix decidua 3 | European larch | L | |
| Larix laricina 1 | Tamarack | L | |
| Malus spp. 2 | Crabapple spp. | FW | |
| Ostrya virginiana 2 | American hophornbeam, ironwood | W | |
| Populus grandidentata Michx. | Bigtooth aspen | WL | |
| Populus deltoides 1,2 | Eastern cottonwood | L | |
| Populus tremuloides 1,1 | Quaking aspen | L | |
| Prunus virginiana 2 | Chokecherry | FW | |
| Quercus bicolor 2 | Swamp white oak | WL | |
| Quercus macrocarpa 2 | Bur oak | WL | |
| Quercus rubra 2 | Red oak | WL | |
| Robinia pseudoacacia 2 | Black locust, false acacia, robinia | L | |
| Salix babylonica | Weeping or Babylon willow | L | |
| Taxodium distichum 2 | Common baldcypress | L | |
| Tilia americana 2 | Basswood | WL |
1 Shaw, D. and R. Schmidt. 2003. Plants for Stormwater Design: Species Selection for the Upper Midwest. Minnesota Pollution Control Agency (MPCA)
2 Bassuk, N. et al. 2009. Recommended Urban Trees: Site Assessment and Tree Selection for Stress Tolerance. Urban Horticulture Institute, Dept of Horticulture, Cornell University, Ithaca, NY
3 USDA NRCS Plants Database.
Constraints on the use of trees
Potential constraints on the use of urban trees for stormwater management include:
- above ground space limitations such as utilities, lighting, signs, structures;
- below ground space limitations such as structures, pavement, existing trees, and utilities;
- regulations regarding types and locations of trees planted along public streets and right of ways, such as, for example, minimum sight distances and setbacks from street corners; and
- the need to locate trees outside of snow plow paths and snow storage areas.
While engineers often worry that tree roots will affect underdrains, inspections of underdrains in hundreds of bioretention practices by North Carolina State Engineers did not find roots clogging underdrains (Winston, 2013).
Tree performance can be a considered a constraint. Performance can be adversely affected by a variety of factors, including
- salt (soil slat and salt spray exposure);
- high temperatures, particularly in urban areas due to heat island effects or reflected heat from adjacent buildings;
- vehicle exhaust;
- drought tolerance;
- exposure to frequent inundation;
- atmospheric pollution;
- physical damage from vandalism, mowers, and other maintenance;
- droughty conditions in between rain events, particularly if sandy soils are used; and
- if tree bioretention areas are used for snow storage, compaction from the snow as well as salt from the snow
According to Coder (2007), by far the most important factor to grow healthy trees is to provide an adequate volume of rootable (i.e. not compacted to a level that affects root growth. The top three factors causing the greatest growth limitations for tree growth are soil water availability, soil aeration, and soil drainage. Each of these can be related to site compaction.
Research has shown that trees need 2 cubic feet of rootable soil volume per square foot of tree canopy to thrive (e.g. Lindsey and Bassuk 1991). Most urban trees, confined to a 4’ x 4’ (i.e. 64 c.f. if assumed to be 4’ deep) tree pit hole, have less than 1/10th the rooting volume they need to thrive. To provide 2 c.f. of rootable soil to allow a tree with a 30’ canopy to thrive would require 1413 c.f. of rootable soil! Not surprisingly, studies have found that trees surrounded by pavement in urban downtown centers only live for an average of 13 years (Skiera and Moll, 1992), a very small fraction of their much longer lifespan under natural conditions.