Overview for trees

Photo showing trees BMPs along the Central Corridor, St. paul, MN. Photo courtesy of Capital Region Watershed District.

Tree BMPs on Marquette Avenue, Minneapolis Minnesota. Photo courtesy of the Kestrel Design Group, Inc.

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 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.

Benefits of tree BMPs

Trees have both stormwater management and non-stormwater benefits, as discussed below.