–

U.S. Department of Agriculture 

Urban Forest Systems 

and Green Stormwater 

Infrastructure 

Forest Service FS 1146 FEBRUARY 2020 

Summary 

Trees provide considerable stormwater volume and pollution control 

through rainfall interception and intensity reduction, stormwater 

infiltration and uptake, and nutrient load reduction. This document 

focuses on the effects of trees on urban stormwater runoff, provides 

some helpful urban forest management strategies to maximize 

stormwater benefits, and demonstrates several examples around the 

United States where the stormwater benefits of urban trees are credited 

for reducing stormwater volume and pollutant loading. This document 

serves as a resource manual for natural resource professionals to help 

them communicate with stormwater managers and engineering professionals about the science and benefits of urban trees in stormwater 

management. Resources on accounting for the stormwater functions of 

trees are provided as a starting point for State and local governments 

interested in providing regulatory credit for urban forests in green 

stormwater infrastructure. 

Introduction 

Municipalities are increasingly planning for sustainability and 

improved quality of life for current and future residents as they work 

toward building healthy communities. One method of planning for 

sustainability involves the consideration of social, environmental, 

and economic impacts of proposed development, known as the triple 

bottom line. Trees growing in urban environments provide numerous 

benefits for humanity that improve quality of life and address this triple 

bottom line. 

Summary.............................................................................1 

Introduction......................................................................1 

Overview of the Stormwater Benefits 

of Urban Trees ...............................................................2 

Rainfall Retention....................................................3 

Rainfall Intensity Under Canopy and 

Stormwater Runoff Timing ................................4 

Infiltration of Stormwater Into Soils ...........5 

Transpiration and Stormwater Runoff .......6 

Stormwater Nutrient Uptake 

and Loading................................................................. 7 

Crediting Trees in 

Stormwater Programs...............................................9 

Minnesota Case Study .............................................11 

Vermont Case Study.................................................13 

Chesapeake Bay Case Study...............................16 

Conclusion......................................................................17 

Acknowledgments.....................................................17 

References......................................................................18 

Glossary of Terms .....................................................22 

CONTENTS 
2 | Urban Forest Systems and Green Stormwater Infrastructure 

 

Beyond the stormwater benefits covered in this 

document, more and more scientific evidence shows how 

urban trees and greenspace positively impact physical, 

psychological, emotional, and spiritual well-being in humans (USDA Forest Service 2018). Environmental benefits 

of trees such as improved ambient air quality, carbon 

sequestration, and reduced stormwater runoff can now be 

quantified using public domain software found on the internet, such as the U.S. Department of Agriculture (USDA), 

Forest Service, i-Tree suite of tools. Research has shown 

that trees provide economic benefits by raising property 

value, reducing the amount of time rental property goes 

unrented, and increasing the amount of time customers 

shop at retail establishments (Wolf 2005). 

Strategically planting trees and managing the forest 

within a city can help to mitigate some of the negative 

impacts that come with urban development. A properly 

managed urban forest can help a municipality meet certain environmental regulations and save money through 

avoided costs, particularly related to stormwater runoff. 

To better understand how urban trees improve things like 

human health, economic development, water and air quality, and public safety, visit the Vibrant Cities Lab website. 

This document provides a synthesis of the science 

around how urban trees help mitigate problems associated with stormwater runoff. Several tree crediting tools 

and case studies are provided to help State and local 

governments better account for the stormwater benefits of 

urban forests. A complementary manual for stormwater 

professionals that investigates incorporating forestry into 

stormwater management programs is available through 

the Water Research Foundation. The Urban Watershed 

Forestry Manual, developed by the Center for Watershed 

Protection, provides more detail about methods for 

increasing forest cover in a watershed, conserving and 

planting trees at a development site, and an urban tree 

planting guide. 

Overview of the Stormwater 

Benefits of Urban Trees 

Green stormwater infrastructure (GSI) is defined as stormwater mitigation practices designed to mimic natural 

processes that filter and retain rain where it falls. Typical 

GSI practices include green roofs, urban trees, bioretention, vegetated swales, permeable pavements, and water 

harvesting. GSI includes low impact development designs 

and/or engineered systems that manage stormwater 

runoff at its source in developed landscapes (EPA 2018). 

An urban forest system includes the trees within an urban 

area as well as the ground cover and soil. The parts of this 

system work together as part of a GSI “treatment train” (a 

series of practices designed to mitigate runoff) to provide 

considerable stormwater volume and pollution control 

through rainfall interception and intensity reduction, 

stormwater infiltration and uptake facilitation, and nutrient load reduction. Recent review articles have explored 

how the parts of the system work together to provide these 

benefits (Berland and others 2017, Center for Watershed 

Protection 2017, Kuehler and others 2017). 

The canopy formed by urban trees intercepts rain as 

soon as it starts to fall, with part of that rainfall retained 

on foliage and branches, remaining in the canopy where 

it eventually evaporates back into the atmosphere. When 

the leaf and branch surface area in the upper part of the 

tree canopy is filled and cannot hold additional rainfall, 

excess water drips from these surfaces to those lower in 

the canopy, helping to reduce rainfall intensity and delaying runoff to storm drains or other stormwater control 

measures. This, in effect, allows the stormwater control 

system to work more efficiently and reduces the chances of 

it becoming overwhelmed or of water running over the top 

of drains and other measures. 

Soils provide the bulk of stormwater volume control. 

Macro- and micro-pores—spaces between soil particles— 

allow for temporary water storage from which trees 

acquire water and nutrients. Tree roots condition the soil 

through mechanical, biological, and chemical means, 

increasing its ability to store greater volumes of water. 

Stormwater runoff not retained in the canopy drips off leaf 

surfaces or flows along the branches and trunk (stemflow) 

to the soil at the base of a tree, where it can penetrate deep 

into the soil profile as water moves along the root surfaces. 

Once in the soil, water becomes accessible to tree roots. 

Through the process of transpiration, water is essentially 

pulled from the soil pore space and used by the tree 

between storms. This process allows for greater water 

storage capacity in the soil as water is transpired most 

days during the growing season. 

Soils also filter nutrients and other pollutants from 

stormwater runoff. Trees need many of the nutrients 

found in runoff for growth and survival, especially 

nitrogen and phosphorus which can negatively impact 

water quality when found in excess. The uptake of these 

nutrients from the soil by trees reduces the amount 
Urban Forest Systems and Green Stormwater Infrastructure | 3 

 

leaching into groundwater, helping to retain and improve 

water quality. However, trees also store many of these 

nutrients in their leaves; at the end of the growing season, 

a large amount of these nutrients remain in senesced 

leaves. When the tree sheds these leaves in fall, significant 

amounts of nutrients can find their way to receiving 

waters, especially if leaves fall onto impervious surfaces 

such as streets. 

Quantifying the stormwater benefits of trees is difficult 

because of many factors. These include species differences 

in attributes that affect rainfall storage such as crown 

architecture and leaf structure and surface texture. For 

example, needle-leafed trees generally store more rainfall 

than broadleaf trees, and evergreens intercept more 

rainfall than deciduous trees over the course of a year. 

Natural systems also vary in relation to regional climate 

differences (arid versus tropical) and microclimates, soil 

conditions, tree size and configuration of planting, not to 

mention the average frequency, intensity, and volume of 

local rainfall events. 

In an ideal world, stormwater managers and design 

engineers could calculate the GSI benefits they need for 

planning by entering information into simple formulas 

for stormwater runoff mitigation by urban forest systems. 

Unfortunately, because of all the variables mentioned, 

it is difficult to calculate “the numbers” for stormwater 

benefits. However, good estimates can be made based on 

current research. 

The following sections contain overviews of the various 

benefits that trees provide in mitigating stormwater runoff 

as well as urban forest management strategies that maximize stormwater runoff benefits. Basic “rules of thumb” 

to estimate stormwater benefits are provided where 

appropriate, but it is important to note that since nature is 

infinitely variable, these rules may be superseded by local 

conditions and species variability. For more information 

about the roles that trees play in stormwater management, 

visit www.TreesAndStormwater.org. 

Rainfall Retention 

Tree canopy intercepts rainfall on leaf surfaces, branches, 

and stems. This intercepted rainfall is either retained on 

canopy surfaces and evaporates over time (interception 

loss), flows down branches to stems and eventually to the 

soil (stemflow), or drips off canopy surfaces to the ground 

below (throughfall). Maximizing the amount of rainfall 

retained in the tree canopy (interception loss) is a good 

strategy to help reduce stormwater runoff in urban areas. 

A deciduous tree typically retains approximately 20 

percent of the annual rainfall that falls on its canopy, 

while a conifer retains close to 30 percent (Kuehler and 

others 2017). The amount of intercepted rainfall retained 

in the tree canopy depends on climatic variables such as 

rainfall intensity and duration, ambient air temperature, 

wind speed, relative humidity, and solar intensity. Tree 

crown structure attributes such as leaf architecture, morphology, and water repellency as well as leaf surface area 

and leaf area index (LAI) contribute to interception loss. 

Trees with rigid, rough-surfaced leaves generally retain 

more rainfall than those with flexible, smooth-surfaced 

leaves (Xiao and McPherson 2016). Trees with greater leaf 

area or higher LAI contribute positively to interception 

loss. 

The amount of water remaining on canopy surfaces 

after a rainfall event and after excess water drips off is 

known as “static storage” (Keim and others 2006). This water eventually evaporates back to the atmosphere and does 

not contribute to stormwater runoff. The depth of static 

water storage has been estimated for various species using 

rainfall simulation techniques. Table 1 demonstrates the 

high variability of static storage among species—and even 

among species within the same genus. 

The volume of rainfall retention in tree canopy can be 

estimated from the leaf area of the tree. The average depth 

of static water storage for tree foliage is 0.2 mm/unit leaf 

area (Wang and others 2008). Using local growth equations 

to estimate the leaf area of a tree, one could multiply 

the leaf area by the depth of water storage to estimate 

the maximum volume of rainfall retention by tree for a 

rainfall event (Equation 1) (Hirabayashi 2013). 

Volmax = LA x 0.2 mm x (1 m/1,000 mm) (1) 

where
 

Volmax = maximum volume of rainfall retained by tree foliage
 

(m3

)
 

LA = leaf area (m2

)
 

For example, a tree with 250 m2 of leaf area could be 

expected to retain 0.05 m3

 of rainfall per rainfall event. 

This is equivalent to about 13 gallons of water (1 m3

 of 

water = 264 gallons). This volume may not seem like much, 

but in a city with millions of trees, the impact is multiplied. Therefore, managing the urban forest to maximize 

leaf surface area can help to reduce stormwater volume 

(Box 1). 
 

Table 1. Mean depth of water storage on foliage by tree species 

Species 

Botanical name 

Species 

Common name 

Mean depth 

of water 

storage 

(mm) 

Source 

Rainfall Intensity Under 

Canopy and Stormwater 

Runoff Timing 

Trees help mitigate flooding and 

potential soil erosion by temporarily 

storing rainfall in the canopy formed 

by branches and leaves, thereby reducing the intensity of rainfall below the 

canopy and delaying peak stormwater 

runoff rates. 

Open-grown trees typically found 

in urban landscapes tend to have 

greater crown volume and thus greater 

leaf surface area available for water 

storage than forest-grown trees. As 

tree surfaces in the upper parts of the 

canopy become saturated with rain, 

excess water falls through the canopy. 

Water falling from higher surfaces fills 

lower surfaces in the crown until the 

entire canopy is saturated, a process 

called “dynamic storage” (Keim and 

others 2006). 

Tree canopy essentially acts as a 

stormwater volume control mechanism. Although the canopy can hold no 

additional rainfall once saturated, the 

rain that continues to fall on the crown 

is intercepted and takes time to pass 

from one surface to another, slowing its 

eventual release as stormwater runoff. 

It is worth noting that the excess water 

drips off the tree relatively quickly after 

the rain has stopped, extending the 

rain event for a time under canopy. 

Urban trees also regulate stormwater runoff by moderating rainfall 

intensity underneath the tree canopy. 

Urban trees have been shown to reduce 

rainfall intensity under the canopy 

by 25 to 70 percent (Zabret and others 

2017) depending on species, rainfall 

characteristics, and time of year 

(Figure 1). Stormwater peak flow rate is 

controlled in part by rainfall intensity 

(Kuichling 1889, Bedient and others 

2013); rainfall intensity reductions by 

Acacia longifolia Sydney golden wattle 

Acer macrophyllum Bigleaf maple 0.18 Keim and others (2006) 

Acer saccharinum Silver maple 

Acer truncatum Shantung maple 0.46 Li and others (2016) 

Alnus rubra Red alder 

Catalpa speciosa Northern catalpa 0.13 Holder (2013) 

Eucalyptus cinerea Silver dollar tree 

Eucalyptus dives Broadleaf peppermint 0.07 Aston (1979) 

Eucalyptus maculata Spotted gum 

Eucalyptus mannifera Brittle gum 0.09 Aston (1979) 

Eucalyptus pauciflora Snow gum 

Eucalyptus viminalis Manna gum 0.03 Aston (1979) 

Gleditsia triacanthos Honey locust 

Pinus radiata Monterey pine 0.08 Aston (1979) 

Pinus tabulaeformis Chinese red pine 

Platycladus orientalis Oriental arborvitae 0.38 Li and others (2016) 

Populus deltoides Eastern cottonwood 

Populus tremuloides Quaking aspen 0.15 Holder (2013) 

Pseudotsuga menziesii Douglas fir 

Quercus gambelii Gambel oak 0.15 Holder (2013) 

Quercus variabilis Chinese cork oak 

Thuja plicata Western redcedar 0.26 Keim and others (2006) 

Tsuga heterophylla Western hemlock 

0.08 

0.13 

0.20 

0.11 

0.03 

0.18 

0.18 

0.43 

0.19 

0.26 

0.17 

0.48 

Aston (1979)
 

Holder (2013)
 

Keim and others (2006)
 

Aston (1979)
 

Aston (1979)
 

Aston (1979)
 

Holder (2013)
 

Li and others (2016)
 

Holder (2013)
 

Keim and others (2006)
 

Li and others (2016)
 

Keim and others (2006)
 

Ulmus pumila Siberian elm 0.21 Holder (2013) 

» Where appropriate, increase leaf area by planting smaller, shadetolerant trees under larger dominant trees. 

» Use ground covers (i.e., mulch or vegetation) under tree canopy to 

increase surface area for interception. 

» Encourage the retention and use of conifers and evergreen 

broadleaf trees, where appropriate and desired, to maximize 

interception and evapotranspiration year-round. 

» Plant trees with rigid and/or rough-surfaced leaves and bark. 

» Encourage the use of trees with greater leaf surface area or higher 

leaf area index (LAI). 

» Maximize belowground soil volume to help store stormwater runoff 

and encourage deep root growth. 

» Consider litter accumulation, root growth characteristics, and longterm maintenance in the tree selection process. 

» Ensure proper tree maintenance to maximize health and LAI. 

Urban Forest Management Strategies To Maximize 

Rainfall Retention 

Box 1 

4 | Urban Forest Systems and Green Stormwater Infrastructure 
Figure 1. Growing season throughfall intensity under 

open-grown broadleaf deciduous trees compared to 

rainfall intensity above the canopy. 

Source: Zabret and others 2017. 

tree canopy thus reduce the peak flow of runoff leaving a 

site. Reducing rainfall intensity has also been shown to 

significantly reduce runoff by increasing soil infiltration 

(Nassif and Wilson 1975, Guan and others 2016). Slowing 

runoff flow rate and increasing stormwater storage in soils 

help to reduce incidences of flooding, combined sewer 

overflows, stormwater runoff volumes, and flows that 

erode stream channels and bare soil. 

Tree canopy has been shown to delay stormwater runoff and increase the time it takes runoff to concentrate at 

the outlet of a catchment or drainage area (e.g., a storm 

drain or bioretention practice). Depending on rainfall volume and intensity as well as tree species, this delay can be 

from 10 minutes to over 3 hours (Xiao and others 2000, 

Asadian and Weiler 2009, Gonzalez-Sosa and others 2017). 

Growing trees in a catchment with significant impervious 

surface cover can help delay the runoff hydrograph peak 

(the maximum stormwater runoff volume reported during 

a specified time period, displayed graphically). Trees can 

also reduce the peak flow delivered to the storm drain or 

GSI practice and help prevent that practice from becoming 

overwhelmed, thus allowing it to function more efficiently 

and effectively from a water quality standpoint (Box 2). 

Infiltration of Stormwater Into Soils 

Soils generally have the capacity to store more water than 

tree canopies. Infiltration of stormwater into soil delays 

runoff flow to streams and allows for filtration and adsorption of pollutants. Unfortunately, urban soils tend to be 

disturbed in some way, either from compaction or loss 

of structure, which reduces porosity and inhibits water 

storage. The result is generally diminished infiltration 

capacity and an increase in stormwater runoff. 

» Where appropriate, retain or plant trees 

with a high leaf area index (LAI). 

» Encourage the use of conifers and 

evergreen broadleaf trees in the landscape 

where appropriate. 

» Maximize crown volume by pruning only 

when necessary. 

» Plant trees to encourage crown growth 

over impervious surfaces such as roads, 

sidewalks, and parking lots. 

» When retrofitting a catchment with green 

stormwater infrastructure practices, retain 

as much tree canopy in the catchment as 

possible. 

Urban Forest Management Strategies 

To Reduce Rainfall Intensity 

Box 2 

Trees help increase infiltration of water into the soil. 

Tree roots can condition disturbed soils and loosen 

compacted soils, thus increasing infiltration and percolation of stormwater runoff (Lange and others 2009, Hart 

2017). In a greenhouse study, Bartens and others (2008) 

showed that deciduous trees increased infiltration rates 

of compacted clay loam subsoil by 150 percent compared 

to unplanted controls. In a second study under mature 

urban trees in Iran, Zadeh and Sepaskhah (2016) showed 

that significantly greater volumes of water infiltrated into 

soil under tree canopy compared to soils not under tree 

canopy cover. Depending on soil texture, the cumulative 

infiltration of water under canopy increased by 69 to 354 

percent compared to soil not under the canopy. The rate at 

which water infiltrated into soil under tree canopy cover 

also depended on soil texture. The infiltration rate was 

800 percent greater under the canopy of trees growing 

in clay loam soil compared to that in open clay loam soil; 

however, there was only a 12.5-percent increase in infiltration rates under canopy with loamy sand compared to 

loamy sand in open areas. In both studies tree roots were 

reported to cause this increase in infiltration. 

Stemflow can also help with infiltration of rainfall 

through preferential flow along root surfaces. Unless 

the extent of permeable surface at the base of the tree is 

very limited (as can be the case with some urban street 

or parking lot trees), the stemflow infiltrates into the soil 

macropores along the root surfaces. Quantification of 

the influence of stemflow on infiltration rates or volumes 

continues to be studied (Levia and Germer 2015). 

Urban Forest Systems and Green Stormwater Infrastructure | 5 
Urban Forest Systems and Green Stormwater Infrastructure 

 

Managing urban forests to take advantage of stemflow 

can help mitigate stormwater runoff. Schooling and 

Carlyle-Moses (2015) reported that stemflow accounted 

for 3 percent of rainfall for events greater than 0.4 inches 

(10 mm). In addition to rainfall intensity and wind speed, 

stemflow depends on the smoothness of the bark and 

branch angles. Smooth-barked trees with acute branch 

angles have been shown to produce greater stemflow 

than rough-barked trees or trees with more horizontally 

oriented branches. Staelens and others (2008) also found 

that stemflow volume increased from 6.4 to 9.5 percent of 

total rainfall when leaves were not on the tree (i.e., during 

the dormant season). 

Trees encourage infiltration of rainfall and stormwater 

runoff into the soil by directing water to a single point 

at the base of a tree or by slowing water dripping onto 

permeable surfaces under the canopy. Where appropriate, 

directing stormwater runoff to open green spaces such 

as parks, and planting trees in those green spaces can 

be a useful, efficient, and relatively inexpensive urban 

stormwater runoff mitigation strategy. Strategically 

planting smooth-barked trees with acute branch angles 

near impervious surfaces so that their canopies grow over 

those surfaces could help direct more rainfall to more 

permeable surfaces during the winter months (Box 3). 

Transpiration and Stormwater Runoff 

Trees need water to function and grow. Water stored 

belowground in soil is removed and used by trees and 

eventually returned to the atmosphere through the 

6 | 

» Maximize belowground soil volume and 

quality to enhance infiltration and storage. 

» Where appropriate, use organic mulch 

beneath tree canopy to help improve 

infiltration and retain stormwater runoff. 

» Plant trees in large open areas where 

stormwater is directed. 

» Ensure adequate belowground aeration for 

root growth. 

» Plant trees with acute branch angles near 

impervious surfaces to help direct rainfall 

to permeable surfaces. 

» Ensure adequate permeable soil space 

directly adjacent to tree stems to allow for 

infiltration of stemflow. 

Urban Forest Management Strategies 

To Increase Stormwater Infiltration 

Box 3 

process of transpiration. Trees influence soil water storage 

through this process. As water is removed from the soil 

by trees, soil pore space becomes available to be filled by 

stormwater runoff from subsequent rainfall events. 

Transpiration rates are highly variable by tree species, 

stem size, and leaf area. Average growing season daily 

water use has been reported to be as high as 47 gallons for 

a 23-inch diameter tulip poplar (Liriodendron tulipifera) 

while a 25-inch chestnut oak (Quercus montana) transpired 

6 gallons (Ford and others 2011). In a California study, 15- 

to 22-inch diameter sycamore (Platanus spp.) street trees 

transpired between 27 and 46 gallons of water daily during 

the growing season, but 24-inch pines only transpired 

about 13 gallons (Pataki and others 2011). These differences in the amount of water transpired can be attributed, 

in part, to the tree’s wood architecture or xylem element 

type. Species with deep sapwood and diffuse-porous 

xylem (e.g., yellow poplar, blackgum, birch, dogwood, red 

maple, sycamore) transpire water in greater volumes than 

species with shallow sapwood and ring-porous xylem (e.g., 

oak), species with semi-ring-porous xylem (e.g., hickory), 

or species with tracheid xylem (e.g., conifers). 

Data collected on trees in the mountains of western 

North Carolina to the Gulf Coastal Plain of Georgia show 

that diffuse-porous species can transpire between 0.6 to 

1.5 gallons of water per day per inch of stem diameter 

during the growing season depending on the size of the 

tree, while ring-porous species transpire about 0.3 gallons 

of water per day per inch (Figure 2). Because the trees 

studied were well watered and their roots not impeded by 

urban infrastructure, these rates can be considered an 

upper limit. 

Transpiration rates also depend on many environmental factors. Foliar stomata, the pores in leaves that allow 

for gas exchange with the atmosphere—thus regulating 

water flow in the tree through the release of water 

vapor—open and close depending on light levels, air temperature, humidity, wind, and soil moisture. Using data 

from multiple urban tree transpiration studies and local 

meteorological data, Moore and others (2019) were able to 

estimate that 5,000 m2

 (53,820 ft2) of street tree canopy area 

in Kansas City, KS, could transpire approximately 1,585 

to 1,850 gallons of water from the soil each day during the 

growing season depending on xylem element type and 

thus allow for additional runoff storage between rainfall 

events. They warn, however, that this assumes the soil 

moisture content is not limiting and has enough water for 

the trees to continue transpiring at these rates. 
» Ensure adequate belowground aeration 

for root respiration and increased water 

storage capacity. 

» Select tree species with greater leaf 

surface area. 

» Retain larger trees in the landscape where 

appropriate. 

» Plant larger statured trees where 

appropriate. 

» Plant trees having diffuse-porous xylem 

in large open areas where stormwater is 

directed. 

» Plant ring-porous trees in drier, upland 

sites and in bioretention practices that use 

high infiltration media. 

Urban Forest Management Strategies 

To Maximize Transpiration 

Box 4 

Figure 2. Average growing season daily water use for 

trees growing in western North Carolina and the Gulf 

Coastal Plain of Georgia. Coefficient of determination (R2) 

was determined from the original data.1 

Sources: Ford and others 2011; Ford and others 2008; Ford and Vose 

2007; Hawthorne and Miniat, unpublished data; Oishi and Miniat, 

unpublished data; Vose and others 2016. 

Regional weather patterns may dictate the best trees 

to use in urban systems. For example, in regions with 

a more Mediterranean climate (e.g., California) where 

water for irrigation may be limited, it might be best to 

plant tree species with ring-porous or tracheid xylem 

types that are able to conserve water through reduced 

transpiration. In a region that receives abundant rainfall 

(e.g., the Southeastern United States), planting diffuseporous species could help mitigate stormwater runoff by 

creating increased soil storage capacity through increased 

transpiration. 

Based on this information, it would be advantageous to 

plant trees with diffuse-porous xylem elements in areas 

used to store stormwater runoff, where soils are frequently 

wet, and to plant ring-porous species in drier, upland 

sites or in bioretention practices that use high infiltration 

media. To determine the xylem element type of many 

tree species, search the Wood Finder section of The Wood 

Database (Box 4). 

Stormwater Nutrient Uptake 

and Loading 

Trees require nutrients to grow and remain healthy (Coder 

2013). Urban stormwater runoff contains many of the 19 

or so essential elements used by trees. As stormwater 

infiltrates into the soil profile, filling soil pore space, it 

becomes the soil solution from which tree roots absorb 

nutrients. Most of the chemically charged elements in 

stormwater adsorb to oppositely charged soil particles, 

holding them as exchangeable ions. When the roots 

absorb elements from the soil solution, these exchangeable ions are released through chemical processes into the 

solution, replenishing nutrient levels for plant absorption 

(Brady and Weil 2002). However, excessive water in the soil 

can also cause some of the elements in the soil solution, 

such as nitrate-nitrogen, to be carried or leached by gravity from the root zone to deeper ground water where they 

are unavailable to plants. Eventually these elements can 

make their way to receiving waters and can contribute to 

eutrophication downstream, resulting in overgrowth of 

plant life and the death of fish and other species from lack 

of oxygen. 

Urban stormwater runoff is usually directed to gutters 

and pipes that convey the untreated water to a stream 

and eventually to larger bodies of water or to a treatment 

facility for combined sewage systems. This is done mainly 

to prevent flooding in our cities. However, moving large 

quantities of untreated urban stormwater to downstream 

water sources can decrease water quality, diminish recreational opportunities, negatively impact aquatic life and 

food sources, and increase treatment costs for human use. 

Green stormwater infrastructure practices are designed 

to mimic natural hydrological processes by directing 

stormwater runoff to permeable surfaces that allow soil 

to remove nutrients and other pollutants from runoff 

naturally before it reaches receiving waters. 

1 Ford-Miniat, C. 2018. Personal communication. Research ecologist, USDA Forest Service Southern Research Station, 

chelcy.f.minat@usda.gov. 

Urban Forest Systems and Green Stormwater Infrastructure | 7 
 

 

 

 

 

 

 

 

Nitrogen (N) and phosphorus (P) are two of the most Denman and others 2016). The effects on total N and P, 

essential elements needed by trees. Urban stormwater however, were highly variable. Compared to the amount 

runoff can have substantial concentrations of N and P due of nutrients coming into these bioretention systems, trees 

to natural and human causes. Controlling these elements were found to reduce total dissolved N by 46 to 52 percent 

is critical for municipalities to 

maintain water quality. Research 

studies in urban areas show how 

managing urban forest systems 

can help control N and P from 

stormwater runoff. 

A study in Baltimore, MD, 

showed that intact forested areas 

reduced N leaching by 74 to 81 

percent compared to areas of 

maintained, fertilized turf (Table 

2) (Groffman and others 2009). 

Other studies showed that under 

individual deciduous trees, N 

leaching was 40 to 56 percent 

lower than under turf (Amador 

and others 2007, Nidzgorski 

and Hobbie 2016). In a study 

in Minnesota, Nidzgorski and 

Hobbie (2016) showed that leaching of phosphates was reduced by 

81 percent under deciduous and 

55 percent under coniferous trees 

in municipal parks. Extrapolating 

their data to an urban watershed, 

the authors estimated that 

urban trees reduce P leaching 

to groundwater by 1,175 to 2,648 

pounds per year (18 to 39 pounds 

per square mile). They calculated 

that trees in the watershed saved 

$2 to $5 million per year in 

removal costs compared to 

installing engineered stormwater 

infrastructure. 

Trees in bioretention practices 

have also shown to help reduce 

nutrient loading. Bioretention 

systems with trees reduced 

nitrates by 58 to 97 percent and 

phosphates by 47 to 79 percent 

compared to those without trees 

(Table 3) (Bratieres and others 

2008, Read and others 2008, 

Table 2. Comparison of groundwater nutrient concentrations under turf, deciduous 

trees, and conifers from three field studies 

Nutrient Turf 

(mg / L) 

Deciduous 

trees 

(mg / L) 

Conifers 

(mg / L) Source 

TN 7.32 ± 1.08 3.75 ± 0.55 7.07 ± 0.95 Nidzgorski and Hobbie (2016) 

3.0 

3.1 – 7.3 

5.63 ± 1.00

 1.8 

0.6 – 1.9* 

2.46 ± 0.42 

1.4 

— 

5.95 ± 0.97 

TP 0.159 ± 0.020 0.050 ± 0.004 0.085 ± 0.013 Nidzgorski and Hobbie (2016) 

Amador and others (2007) 

Groffman and others (2009) 

Nidzgorski and Hobbie (2016) 

NOx 

PO 0.131 ± 0.020 0.025 ± 0.003 0.059 ± 0.011 Nidzgorski and Hobbie (2016) 4

3­

TN = total nitrogen | NOx = oxidized nitrogen | TP = total phosphorus | PO4

3- = orthophosphates. *

Forested area. 

Table 3. Water quality data from three bioretention studies comparing effluent 

nutrient concentrations from systems with trees (soil + tree) and without trees (soil 

only) 

Nutrient Soil only 

(mg L-1) 

Soil + Tree 

(mg L-1) 

Reduced 

% Source 

TN 

2.2 

6.68 

1.8 – 2.3 

1.19 

-5% – 18%* 

82% 

Read and others (2008) 

Bratieres and others (2008) 

0.38 

5.23 

7.43 

0.01 – 0.16 

0.38 

1.96 

58 – 97%* 

93% 

74%* 

TP

 0.11 

0.083 

0.06 – 0.10 

0.07 

9 – 45%* 

16% 

Read and others (2008) 

Bratieres and others (2008) 

Read and others (2008) 

Bratieres and others (2008) 

Denman and others (2016) 

NOx 

0.075 .020 – .025 67 – 73%* Read and others (2008) 

PO 0.064 0.034 47% Bratieres and others (2008) 4

3­

0.85 0.18 79%* Denman and others (2016) 

TN = total nitrogen | NOx = oxidized nitrogen | TP = total phosphorus 

PO4

3- = orthophosphates. *

Averaged over entire study period. 

Table 4. Water quality data from two bioretention studies comparing effluent 

nutrient concentrations from systems with trees (soil + tree) and the dose of 

nutrients of the applied stormwater (dose) 

Nutrient Dose 

(mg/L) 

Soil + Tree 

(mg/L) 

Reduced 

% Source 

TN 2.21 1.19 46% Bratieres and others (2008) 

0.79 

2.0 

0.38 

1.96 

52% 

2%* 

TP 0.427 0.07 84% Bratieres and others (2008) 

Bratieres and others (2008) NOx Denman and others (2016) 

0.127 0.034 74% Bratieres and others (2008) PO4

3­

0.6 0.18 70% Denman and others (2016) * 

TN = total nitrogen | NOx = oxidized nitrogen | TP = total phosphorus 

PO4

3- = orthophosphates. *

Averaged over entire study period. 

8 | Urban Forest Systems and Green Stormwater Infrastructure 
9 

 

(Bratieres and others 2008) and P by 70 to 84 percent 

(Table 4) (Bratieres and others 2008, Denman and others 

2016). The authors explained that as trees in these systems 

matured and increased root mass per soil volume, their 

effectiveness improved. These studies suggest that bioretention practices with greater tree root biomass are better 

able to reduce N and P from their stormwater effluent. 

Although trees have been shown to take up substantial 

amounts of nutrients from the soil profile, they can also 

contribute significantly to pollution loading in receiving 

waters by contributing nutrients to impervious surfaces. 

Airborne contaminants, including N and P, deposit on leaf 

surfaces and can be washed off during rainfall events. 

Precipitation dripping from the tree canopy over impervious surfaces has been shown to contribute to increased 

pollutant loading (Halverson and others 1984). Trees can 

move nutrients internally from foliage to other plant tissue for storage before leaves fall off during the autumn; however, about half of 

the N and P content remains in the leaves 

after they fall (Aerts 1996). Studies show 

that approximately 60 percent of the 

annual P yield in urban streams comes 

from autumn leaf fall onto streets (Selbig 

2016). Research also shows a strong 

linear relationship between tree canopy 

cover over streets and mean gutter 

stormwater runoff N and P concentration 

in the autumn (Janke and others 2017). 

From this research, we can expect to see 

an increase in runoff concentration of 

approximately 0.65 mg/L in total organic 

N and 0.35 mg/L in soluble reactive P in 

autumn for every 10-percent increase in 

tree canopy cover over impervious surfaces (Figure 3). 

Litter from urban trees decomposes more rapidly on 

impervious surfaces than in more natural settings due 

mainly to increased ambient temperatures and accelerated fragmentation from tires rolling over it (Hobbie 

and others 2013). Timely and targeted street sweeping, 

especially in areas with high tree canopy cover, has been 

shown to reduce nutrient concentrations in urban streams 

by over 70 percent (Selbig 2016). If tree canopy cover 

over impervious surfaces is desirable in municipalities 

to provide co-benefits and improve quality of life, a 

robust and targeted street sweeping operation is highly 

recommended to help reduce excessive nutrients in urban 

streams and lakes (Box 5). 

Crediting Trees in 

Stormwater Programs 

With the growing body of research on the stormwater 

benefits of urban forest systems, new approaches have 

been developed in recent years to provide regulatory 

credit for trees in stormwater management programs. 

Communities across the Nation are seeking cost-effective 

approaches to meet water quality requirements associated 

with Municipal Separate Storm Sewer System (MS4) permits, Combined Sewer Overflow (CSO) consent decrees, 

and Total Maximum Daily Load (TMDL) pollutant load 

reductions. Urban trees and forests play a central role in 

a community’s green stormwater infrastructure, but they 

are often not accounted for as stormwater management 

practices, in part due to variability or uncertainty in quantifying their function relative to engineered practices. 

Figure 3. Mean nitrogen (N) and phosphorus (P) concentration in stormwater 

runoff from street gutters per street tree canopy fraction in the Minneapolis, 

MN, metropolitan area. 

Source: Janke and others 2017. 

Urban Forest Systems and Green Stormwater Infrastructure | 

» Where appropriate, direct stormwater 

runoff to areas where it can be infiltrated 

into the soil or belowground. 

» Plant trees in large open areas where runoff 

is directed and roots can access it. 

» Ensure adequate belowground aeration for 

root respiration. 

» Identify those areas of the city where 

tree canopy cover overhangs impervious 

surfaces and ensure leaves and debris are 

removed frequently throughout spring and 

autumn. 

Urban Forest Management 

Strategies To Reduce Stormwater 

Nutrient Loading 

Box 5 
 

  

    

 

The Center for Watershed Protection led a thorough 

investigation of crediting approaches for urban trees 

and published a number of valuable resources on 

the subject. Its website, Making Urban Trees Count, 

provides a comprehensive literature review and modeling 

documentation, national spreadsheet tools for calculating 

event-based volume reduction and annual pollutant load 

reduction credits, and sample design specifications for 

urban tree planting as a Best Management Practice (BMP). 

Table 5 gives a summary of the two crediting tools. An 

additional technical guide was developed for stormwater 

engineers entitled “Accounting for Trees in Stormwater 

Models,” which summarizes available tools and outlines 

an array of options for incorporating tree values into 

common stormwater modeling programs (Center for 

Watershed Protection 2018a). 

The following case studies provide practical examples 

of how science-based tree credits have been developed 

and adopted in three different regulatory contexts: 

Minnesota, Vermont, and the Chesapeake Bay watershed. 

They are presented in hopes that other States and localities will learn from and/or adapt these approaches without 

needing to reinvent the wheel. While the tree credits 

are modest relative to other stormwater BMPs, they 

represent an important step towards better accounting 

for the watershed benefits of urban forests. One limitation 

of some of these crediting approaches is that they only 

provide credit for newly planted trees, not for conserving 

existing mature trees that generally provide far greater 

stormwater benefits relative to young trees. Further, 

the credits described below do not account for potential 

pollutant loading (e.g., phosphorus) associated with leaf 

litter falling on impervious surfaces. As the science and 

policy strategies around these issues continue to develop, 

it is anticipated that crediting approaches for trees will be 

strengthened accordingly. 

Table 5. Summary of tree planting credits developed by the Center for Watershed Protection 

Characteristic Pollutant load reduction credit Stormwater performance-based credit 

Use of credit 

» Compliance with nutrient and 

sediment TMDLs 

» Compliance with site-based stormwater management 

requirements (volume-based and pollutant-based) 

Required inputs 

» Climate region 

» Number of trees planted 

» Nearest city (from drop-down list) 

» Tree type 

» Surface over which the tree will be planted 

» Number of trees planted 

» A breakdown of HSG soil type/land cover combinations 

for the entire site 

» The design storm, in inches 

Optional inputs 

(default values 

are provided) 

» Tree type 

» Soil type 

» Surface over which the tree will 

be planted 

» TN, TP, and TSS event mean 

concentrations 

» Tree size (DBH) 

» Tree canopy area 

» TN, TP, and TSS event mean concentrations 

Outputs 

» Annual reduction in TN, TP, and 

TSS loads (lbs/yr) for an individual 

tree and for a tree planting 

scenario 

» Runoff (cubic feet), TN (lbs), TP (lbs), and TSS (lbs) 

reduction for user-defined tree planting scenario for a 

specific storm event (e.g., design storm) 

Key assumptions* 

» TP and TSS load reductions are 

directly proportional to runoff 

reduction 

» The amount of runoff reduction 

achieved by tree planting is not 

uniform across all storm events 

» TN load reductions are 65 percent of runoff reduction 

to account for soluble forms of nitrogen reaching a 

stream or other waterbody through infiltration and 

leaching 

» The annual runoff reduction from the water balance 

model is translated to an event-based reduction using 

a unit runoff reduction value 

TMDL = total maximum daily load | HSG = Hydrologic Soil Group | TN = total nitrogen | TP = total phosphorus | TSS = total suspended 

sediment | DBH = diameter at breast height. *

Refer to the water balance model documentation for more detailed model assumptions. 

10 | Urban Forest Systems and Green Stormwater Infrastructure 
Having supportive State policies in place, as demonstrated in these case studies, is an important condition to 

incentivize the conservation and planting of urban trees 

as a key component of the local stormwater management 

infrastructure. Ultimately, local governments are the 

drivers of community tree management and have a variety 

of policy options to protect and expand the many public 

values provided by trees, as outlined in “Making your 

Community Forest-Friendly: A Worksheet for Review of 

Municipal Codes and Ordinances” (Center for Watershed 

Protection 2018b). Incorporating tree-related targets explicitly in permits and policies related to MS4s, CSOs, and 

TMDLs, as has been done in the District of Columbia and 

other locations, can do much to bolster the role of urban 

forest systems in stormwater management. 

Minnesota Case Study 

Where: Minnesota Stormwater Program 

When: Adopted in 2013 in the online Minnesota 

Stormwater Manual 

What: 

» Volume reduction credit for engineered Tree 

Trench/Box practices based on interception, 

evapotranspiration, and infiltration. 

» Annual pollutant removal credits for total 

suspended solids (TSS) and total phosphorus 

(TP) are calculated based on volume reduction. 

» Requires that users enter soil volume, treatment area, tree size, and other inputs into the 

Minimal Impact Design Standards Calculator. 

Quick facts 

Overview 

Minnesota was the first State to develop a robust, sciencebased approach for crediting engineered tree BMPs within 

State stormwater regulations. With funding allocated in 

2009 from the State legislature, the Minnesota Pollution 

Control Agency convened the Minimal Impact Design 

Standards (MIDS) Working Group to develop new standards that would ultimately be adopted into the Minnesota 

Stormwater Manual (Minnesota Pollution Control Agency 

2013). Sub-committees were formed to develop stormwater 

credits and design specifications for a suite of green infrastructure BMPs, including one focused on trees. The tree 

BMP sub-committee was interested in credits for retaining 

existing trees but ultimately adopted the Tree Trench/Box 

credit, which was easiest to quantify and justify in stormwater standards. One valuable feature of Minnesota’s 

crediting approach is that it encourages well-designed tree 

BMPs with optimal uncompacted soil volume to maximize 

tree growth and function in processing stormwater runoff. 

Key elements of the Minimal Impact Design Standards 

include the following: 

»	 Stormwater volume performance goal for new development and redevelopment projects with greater than 

1 acre of new impervious surface. 

»	 Requires post-construction runoff volume to be retained onsite for 1.1 inches of runoff from impervious 

surfaces. 

»	 Standardized credit calculations and design specifications for a variety of GSI BMPs, including: green 

roofs, bioretention basins, infiltration basins, permeable pavement, infiltration trench/tree box, swales, 

filter strips, and sand filters. 

»	 A model ordinance package that helps developers and 

communities implement the new standards. 

The MIDS approach has received widespread national 

attention for its innovative and robust crediting approaches. The unique manual was designed as an online 

Wiki format so that it could be easily adapted over time 

with new science, technical, and stakeholder input. It has 

been revisited and updated each year. 

The science behind it 

The Tree Trench credit methodology was developed by 

Kestrel Design Group and contract team, with oversight 

from the tree BMP sub-committee and multiple rounds of 

stakeholder input (Kestrel Design Group Team 2013). It is 

based on an extensive literature review of tree interception, evapotranspiration, and infiltration functions. Based 

on mean values found in Breuer and others (2003), the 

interception capacity is assumed to be 0.043 inches for 

a deciduous tree and 0.087 for a coniferous tree, and the 

canopy projection area is based on the diameter of the 

canopy at maturity, dependent on the tree species. The 

MIDS calculator provides default tree size options (small/ 

medium/large) that can be used in place of tree species. 

The team’s report reviews the pros and cons of a 

variety of methods for quantifying evapotranspiration, 

recommending use of the Lindsey-Bassuk (1991) single 

whole tree water use equation. This method relates the 

Urban Forest Systems and Green Stormwater Infrastructure | 11 
12 | Urban Forest Systems and Green Stormwater Infrastructure 

 

total water use of a tree to four measurements: (1) canopy 

diameter, (2) leaf area index, (3) the evaporation rate per 

unit time, and (4) the evaporation ratio. 

Pollutant removal for infiltrated and evapotranspired 

water is assumed to be 100 percent and is calculated by 

multiplying the volume of water reduced by event mean 

concentrations for Total Suspended Solids (TSS) and Total 

Phosphorus (TP) from the International Stormwater 

Database, version 3. 

How the credit works 

Minnesota provides a total runoff volume reduction credit 

for Tree Trench BMPs, by adding together the reductions 

provided by tree canopy interception, soil storage (infiltration), and evapotranspiration. The interception credit is a 

function of tree type and projected leaf area at maturity. 

The storage credit is a direct function of soil volume. The 

evapotranspiration credit is a function of plant available 

water and is indirectly related to soil volume (e.g., available pore space). The total runoff volume achieved for a 

particular storm is calculated as the lower value of the 

total runoff volume directed to the tree trench and the 

total storage provided by that trench through interception, 

infiltration, and evapotranspiration. The total volume 

reduction is also translated into annual pollutant removal 

values for TSS and TP. A Tree Trench BMP without an underdrain is assumed to remove 100 percent of pollutants, 

while a Tree Trench with an underdrain provides lower 

volume reduction and pollutant removal credits 

(Figure 4). 

To calculate the credits, users must enter 

into the MIDS Calculator a suite of inputs based 

on the design of the particular Tree Trench 

BMP such as: 

»	 Site Characteristics 

•	 watershed area/land cover draining to 

the Tree Trench BMP 

• downstream/routing BMP
 

» Soil/Media Characteristics 


•	 soil volume of the tree box 

• hydrologic characteristics of the soil 

Figure 5 shows one of the input screens for the MIDS 

calculator, demonstrating how the volume reduction 

credits are calculated based on the Tree Trench BMP 

characteristics provided. The figure illustrates how, in this 

crediting approach, the volume reduction based on soil 

storage (1201 cubic feet) far exceeds the volume reductions 

for evapotranspiration (72 cubic feet) and interception (5 

cubic feet). Thus, the credit incentivizes providing ample 

soil volume and high quality, uncompacted soil media that 

will promote infiltration and storage in the short term 

and enable trees to grow to their optimal size. A helpful 

summary and example of Tree Trench credits using the 

MIDS calculator is included in the Center for Watershed 

Protection’s Accounting for Trees in Stormwater Models 

(Center for Watershed Protection 2018a). Detailed technical information on the credit equations, input definitions, 

and other guidance can be found in the online Stormwater 

Manual section Calculating Credits for Tree Trenches 

and Tree Boxes. 

In developing the credit calculations, it is assumed the 

tree practice is properly designed, constructed, and maintained in accordance with guidance in the tree section of 

the Minnesota Stormwater Manual. The manual website 

notes that if any of these assumptions is not valid, the BMP 

may not qualify for full credit. 

Some of the model inputs used in the MIDS calculator 

for Tree Trench practices are only applicable to Minnesota 

»	 Tree Characteristics 

•	 number of trees 

•	 most common tree type (deciduous or 

coniferous) 

•	 average tree size at maturity (small/ 

medium/large) 

Figure 4. The Minimal Impact Design Standards (MIDS) total volume 

reduction is translated into annual pollutant removal values for Total 

Suspended Solids (TSS), particulate phosphorus (PP), and dissolved 

phosphorus (DP). A Tree Trench Best Management Practice (BMP) 

without an underdrain is assumed to remove 100 percent of pollutants, 

while a Tree Trench with an underdrain provides lower volume 

reduction and pollutant removal credits based on the type of media 

used. 
Figure 5. One of the Minimal Impact Design Standards (MIDS) Calculator Tree Trench Best Management Practice (BMP) 

input screens showing tree and soil inputs (white boxes) and model outputs (gray). 

and similar climates, so it is not recommended to use the 

calculator itself beyond those geographic zones. However, 

the equations and calculations behind the credit could 

readily be adapted for other climate zones. 

Vermont Case Study 

Quick facts 

Where: State of Vermont 

When: Adopted in 2017 in the Vermont Stormwater 

Management Manual Rule 

What: 

» Volume reduction credit in State stormwater 

permits. 

» Three tree BMPs: Reforestation (active and 

passive), single tree planting. 

» Companion local crediting framework for 

smaller sites not covered by State permit. 

Overview 

The effort to include trees and forests as key components 

of green stormwater infrastructure has been championed 

by the State forestry agency, Vermont Department of 

Forests, Parks and Recreation, for a number of years. 

Starting in 2010, the State’s Agency of Natural Resources 

convened private and public stakeholders in a green 

infrastructure roundtable that resulted in strategic 

plans and initiatives to promote low impact development 

and GSI across State agencies, local governments, and 

professionals. 

As a component of this effort, the State forestry agency 

secured a Federal grant that advanced several strategic 

actions, including hiring a green infrastructure coordinator within the State’s stormwater agency (Department of 

Environmental Conservation) who helped facilitate the 

adoption of new policies and practices. Through the grant, 

a consultant was also hired to complete a comprehensive 

review and set of recommendations on options to 

credit trees within the State’s stormwater management 

framework. 

During this time, the upcoming revision of the State’s 

stormwater management manual provided a key window 

of opportunity to advance the green infrastructure 

recommendations into policy. The initial draft version of 

the manual included stormwater credits for reforestation 

(active and passive) but no credit for single tree plantings. 

In subsequent stakeholder meetings and public comment, 

support for a single tree credit was voiced; the State 

worked with partners to incorporate this into the final 

manual that was officially adopted in 2017. A complementary GSI Toolkit was developed to aid local governments in 

crediting trees and other GSI practices on smaller development sites that are not covered by the State’s permitting 

process (Vermont League of Cities and Towns 2017). 

Urban Forest Systems and Green Stormwater Infrastructure | 13 
 

 

 

The science behind it 

To establish a sound basis for establishing stormwater 

credits for trees, the State forestry agency contracted with 

Stone Environmental, Inc., to review existing research 

and policy examples and draft recommendations. 

Stone Environmental, Inc., developed two white papers 

for the project. The first, describing the stormwater 

management benefits of trees (Moore and others 2014a) 

summarizes scientific knowledge about the tree processes 

that affect stormwater runoff (interception, transpiration, 

infiltration, and pollutant removal) and reviews considerations for maximizing stormwater benefits at the tree or 

site scale (soil restoration, engineered tree systems, tree 

selection, siting, and planting practices). 

The second white paper (Moore and others 2014b) 

reviews examples from 12 States around the country that 

illustrate integrating tree retention or planting practices 

into stormwater programs. It also reviews over a dozen 

examples of green infrastructure crediting/incentives at 

the municipal scale, including examples from Seattle, WA, 

Washington, DC, and Nashville, TN. 

The findings from these reviews helped inform the 

credits that were adopted in Vermont, taking into account 

regulatory concerns and stakeholder input. 

How the credits work–State credits 

The Vermont Department of Environmental Conservation 

Stormwater Program issues permits for post-development 

runoff from impervious surfaces. Permits are required 

for new development and redevelopment projects that 

will include more than 1 acre of impervious surfaces after 

construction. The 2017 Vermont Stormwater Management 

Manual Rule sets forth the treatment standards that 

must be met and the approved methods for calculating 

treatment volume (Tv) credits for the suite of structural 

and nonstructural stormwater treatment practices (i.e., 

BMPs) used onsite (Vermont Agency of Natural Resources 

2017). Using the hydrologic condition method set forth 

in the manual, a suite of practices must be implemented 

to achieve the “hydrologic condition volume,” which is 

calculated as the difference between the pre- and postdevelopment site runoff for the 1-year, 24-hour storm. 

The three types of State tree credits established under 

the reforestation nonstructural practice are summarized 

as follows: 

1.	 Active reforestation involves planting a stand or 


block of trees, or individual trees, at a project site 


with the explicit goal of establishing a mature forest 

canopy or distributed cover that will intercept rainfall, increase evapotranspiration rates, and enhance 

soil infiltration rates. 

Tv credit = 0.1 inches x reforested area 

(i.e., 1 acre of reforested area = Tv credit of 363 cubic 

feet) 

2.	 Passive reforestation consists of protecting a portion 

of a project site from mowing and allowing native 

vegetation to reestablish. 

Tv credit = 0.05 inches x practice area 

3.	 Single tree planting involves planting individual 

trees on a project site. 

Tv credit = 5 cubic feet per tree planted (Box 6) 

Box 6 Requirements for State Credits 

Excerpts from the 2017 Vermont Stormwater 

Management Manual Rule: 

REFORESTATION CREDITS 

» The minimum contiguous area of active or 

passive reforestation shall be 2,500 square 

feet. 

» The minimum width for reforested areas 

shall be 25 feet. 

» The entire reforestation area shall be 

covered with an approved native seed mix 

covered with mulch to help retain moisture 

and provide a beneficial environment for the 

reforestation. 

» Active and passive reforestation areas 

shall not be maintained as landscaped 

areas. Forest leaf litter, duff, and volunteer 

sapling and understory growth shall not be 

removed. 

» The manual lists additional requirements 

regarding tree species selection, soil, slope 

limitations, planting plans, protection from 

development, and other design issues. 

SINGLE TREE CREDIT 

» Trees planted for the single tree credit shall 

be at least 2 inches in diameter at breast 

height (dbh) for deciduous trees, or at least 

6 feet tall for conifers. 

For full details on the State credits, see the 

2017 Vermont Stormwater Management Manual, 

Section 4.2.1 (Vermont Agency of Natural 

Resources 2017). 

14	 | Urban Forest Systems and Green Stormwater Infrastructure 
 

 

 

Box 7 Requirements for Local Credits 

The Green Infrastructure Toolkit lists a number of 

requirements for credit, such as: 

» The tree(s) must be on the development site and 

within 20 feet of new and/or replaced groundlevel impervious surfaces (e.g., driveway, patio, or 

parking lot). 

» Trees must be retained, maintained, and protected 

on the site after construction and for the life of the 

development, or until any approved redevelopment 

occurs. 

» Trees that are removed or die must be replaced 

with like species during the next planting season. 

» See additional criteria regarding soil quality and 

volume and other design requirements. 

RETAINED TREES 

» Retained trees must be a minimum of 6 inches 

dbh. For trees smaller than this size that are 

retained, the newly planted tree credit may be 

applied instead. 

» See additional guidelines for retained trees. 

NEWLY PLANTED TREES 

» New deciduous trees must be at least 1.5 inches 

diameter, measured 6 inches above the ground. 

New evergreen trees must be at least 4 feet tall. 

» See additional tree selection, spacing, planting, and 

maintenance requirements. 

For full details, see Fact Sheet #3 (Vermont League of 

Cities and Towns 2015). 

How the credits work– 

local credits 

Many smaller scale development and redevelopment projects do not meet the greater than 

1-acre impervious surface threshold, and 

thus do not require a State permit or involve 

the standard treatment practice requirements and credits described above. Because 

these smaller projects are governed by local 

ordinances, the Vermont League of Cities and 

Towns worked with State agencies and stakeholders to develop a Green Infrastructure 

Toolkit for local use. The Toolkit features: 

»	 GSI Sizing Tool spreadsheet. 

»	 Set of GSI fact sheets covering credits 

and criteria for 10 stormwater practices, 

including trees. 

»	 Low Impact Development and Green 

Stormwater Infrastructure (GSI) Bylaw 

Template (i.e., model ordinance) that 

can be used or adapted into local policy. 

The crediting approach for retained and 

newly planted trees is based on an impervious area reduction credit, which in effect 

reduces the total volume of runoff that needs 

to be treated through other practices (Box 7). 

Box 8 shows how the credits are calculated. 

BOX 8 Credit Calculation 

How tree credits are calculated using Vermont’s Green Stormwater 

Infrastructure (GSI) Simplified Sizing Tool: 

BMP Tree Type Impervious Area Reduction Credit 

Retained 

Tree 

Evergreen 

Deciduous 

20% canopy area (min. 100 ft2 / tree) 

10% canopy area (min. 50 ft2 / tree) 

Newly 

Planted 

Tree 

Evergreen 

Deciduous 

50 ft2 / tree 

50 ft2 / tree 

TOTAL GROUND LEVEL IMPERVIOUS COVER: ________ sq. ft. 

RETAINED TREES: 

Total evergreen canopy area: _________ sq. ft. 

Evergreen canopy area  0.2 = _________ sq. ft. credit (min. 100) 

Total deciduous canopy area: _________ sq. ft. 

Deciduous canopy area  0.1 = _________ sq. ft. credit (min. 50) 

NEWLY PLANTED TREES: 

Total new evergreen trees meeting requirements: _________ 

# of new evergreen trees  50 = _________ sq. ft. credit (min. 50) 

Total new deciduous trees meeting requirements: _________ 

# of new deciduous trees  50 = _________ sq. ft. credit (min. 50) 

TOTAL CREDIT: __________ sq. ft. 

(Max 25% of proposed impervious cover) 

Source: GSI Simplified Sizing Tool Fact Sheet #3 (Vermont League of Cities 

and Towns 2015) 

Urban Forest Systems and Green Stormwater Infrastructure | 15 
 

 

Chesapeake Bay Case Study
 

Where: Chesapeake Bay Watershed 

(DC, DE, MD, NY, PA, VA, WV) 

When: Adopted in 2016 as approved Total Maximum 

Daily Load BMP credits by Federal and State 

agencies 

What: 

» BMP credits are earned for urban tree canopy 

expansion for dispersed plantings over turf or 

impervious surface and urban forest planting 

for full reforestation. 

» Tree canopy is mapped and credited as a land 

use class in the Chesapeake Bay model, with 

reduced pollutant loading relative to turf or 

impervious cover. 

» States get credit for newly planted trees for 10 

years, after which the tree canopy is tracked 

directly through high-resolution imagery. 

Quick facts 

Overview 

In 2010, the U.S. Environmental Protection Agency (EPA) 

established the Chesapeake Bay Total Maximum Daily 

Load (TMDL)—or “pollution diet”—to reduce the amount 

of nitrogen, phosphorus, and sediment entering the Bay 

through the region’s waterways. The TMDL covers 64,000 

square miles that stretch across parts of six States and the 

District of Columbia. Each of these jurisdictions has committed to reaching ambitious pollutant load reductions by 

2025, as documented in phased watershed implementation 

plans. In order to track and credit progress towards these 

targets, the States and the District of Columbia must 

provide detailed reporting of the number and 

urban tree canopy over turf, and urban tree canopy over 

impervious cover became available in 2016. 

A BMP expert panel was convened in 2015 to provide 

recommendations on how urban tree canopy (including 

urban tree planting) should be credited in the TMDL 

context. All documentation of the literature, modeling 

approaches, and crediting decisions are provided in the 

report the panel developed (Law and Hanson 2016). 

Following review and revision with Federal, State, and 

other stakeholders, a new BMP credit for urban tree 

canopy expansion, as well as a higher credit for urban 

forest planting (i.e., reforestation of developed/turf areas) 

were officially adopted in 2016 for use in the TMDL. 

Having tree BMP credits approved for use in the TMDL 

has helped incentivize the District of Columbia and other 

local jurisdictions to include tree planting targets as part 

of their MS4 permits. 

The science behind it 

The tree canopy BMP expert panel, with support from the 

Center for Watershed Protection, completed a thorough 

literature review on the water quality benefits of urban 

trees and existing tree crediting approaches. Hynicka and 

Divers (2016) constructed a water-balance modeling approach to estimate pollutant loading rates for tree canopy 

over turf grass, tree canopy over impervious cover relative 

to turf, and impervious cover without trees. To account 

for spatial and temporal variation in precipitation, 11 

years (2005 to 2015) of daily weather data were used from 

each of 8 regional locations spanning the Chesapeake 

Bay Watershed. The relative pollutant load reductions are 

summarized in Table 6. 

The expert panel used a variety of tree species, growth, 

and mortality scenarios in i-Tree Forecast to establish an 

average canopy acreage credit per tree planted (144 square 

feet per tree, or approximately 300 trees per acre). 

type of approved BMPs implemented on all 	 Table 6. Tree canopy relative land use loading rate reductions in total 

nitrogen (TN), total phosphorus (TP), and total suspended solids (TSS) agricultural and urban lands. in relation to underlying land use cover While the Chesapeake Bay TMDL and modeling tools have always assigned low pollutant 

loading rates to forest land cover, they did not 

have a way to account for and credit the water 

quality value of urban tree canopy (individual 

and small patches of trees in developed areas Canopy over roads 7.0

not large enough to be classified as forest). 

Thanks to investments by the Chesapeake 

Bay Program partners in high-resolution *

Percent reduction is based on an average Municipal Separate Storm Sewer System 

Land use Total nitrogen 

reduction (%) 

Total 

phosphorus 

reduction (%) 

Total suspended 

solids (TSS) 

reduction (%) 

Canopy over turf 23.8 23.8 5.8 

8.5 11.0 

Forest 85.0 90.7 81.6* 

land cover data, distinct mapping of forest, 	 (MS4) land use loading rate for sediment. Source: Hynicka and Divers 2016. 

16 | Urban Forest Systems and Green Stormwater Infrastructure 
Urban Forest Systems and Green Stormwater Infrastructure | 17 

How the credit works 

Under the Chesapeake Bay modeling and TMDL framework, every acre of land in the watershed has a designated 

land use class and associated pollutant loading rate, based 

on high-resolution land cover mapping, other datasets, 

and best available science. Like many BMPs in the TMDL 

framework, the urban tree canopy BMPs are credited 

based on a land use change or the conversion of a given 

acreage of land from a higher loading land use (e.g., turf 

grass or impervious cover) to a lower loading land use 

(urban tree canopy or forest). For these land use change 

BMPs, States, and local governments track and report 

the total acreage of each BMP implemented on an annual 

basis, and the Chesapeake Bay modeling tools calculate 

the resulting pollutant reductions. 

The urban tree canopy expansion BMP includes tree 

planting projects on developed land that increase the 

tree canopy overlying turf or impervious surfaces but do 

not create forest-like conditions. Trees do not have to be 

planted in a single contiguous area. Trees planted in a riparian forest buffer or as part of a structural BMP, such as 

bioretention practices, are not included; these are tracked 

under separate BMP credits. Each tree planted is given 

credit for creating 144 square feet of urban tree canopy 

(equivalent to 300 trees per acre), which reflects average 

growth at 10 years after planting. The credit is calculated 

within the Chesapeake Bay model based on the percentage 

reduction in nitrogen, phosphorous, and sediment pollutant loads relative to the underlying land use cover. 

The urban forest planting BMP includes projects that 

create forest-like conditions. Trees must be planted in a 

contiguous area specified in a documented planting and 

maintenance plan and conform to the State’s planting 

density and associated standards for forest conditions. 

Urban forest planting BMPs result in a change of land use 

from turf grass to forest land. The credit for this BMP is 

calculated based on the difference between the land use 

loading rate of turf grass and forest land across the acreage of the urban forest planting. 

For both BMP credits, the credit expires after 10 years, 

at which point the canopy coverage is assumed to be 

tracked and directly credited as a land use through new 

high-resolution imagery/land use data. 

Conclusion 

Urban forest systems (trees, soil, and groundcover) help 

manage stormwater runoff by reducing stormwater 

volume, slowing rainfall intensity, delaying runoff, 

improving infiltration into soil, and increasing water storage capacity in soils. Using trees as part of a stormwater 

management “treatment train” can increase the efficiency 

of GSI practices. Larger, mature trees provide greater 

benefits, and healthy trees appreciate in terms of benefits 

over time, so managing the entire urban forest to increase 

leaf surface area is a good strategy to help manage 

stormwater runoff city-wide. Providing credits in State 

and local stormwater programs for retaining mature trees 

and strategically planting new trees is a valuable tool to 

encourage their use as part of a stormwater management 

program. 

Trees increase the quality of life in our cities for 

residents, visitors, and business owners. Using them 

purposefully can help to reduce some of the disservices 

that come with development and improve the long-term 

sustainability of urban ecosystems. 

Acknowledgments 

The Forest Service’s National Urban Forest Technology 

and Science Delivery Team (NTSD) is comprised of urban 

program staff and science delivery experts from across regions and research stations, working collaboratively to deliver quality urban natural resources science, technology, 

and information to improve the long-term sustainability 

of urban ecosystems. This publication is part of the team’s 

effort to deliver urban forestry research and information 

to partners, stakeholders, and customers. NTSD team 

members Eric Kuehler (Forest Service Southern Research 

Station) and Julie Mawhorter (Forest Service Eastern 

Region State and Private Forestry) managed the writing 

of this report. Amanda Perry and Sonja Beavers (Forest 

Service Office of Communication) provided editorial and 

layout reviews, and Zoë Hoyle (retired Forest Service) provided technical editing. Annie Hermansen-Baez (Forest 

Service Southern Research Station) and Lauren Marshall 

(Forest Service State & Private Forestry) provided editing 

advice and supported production of the document. Raghu 

Consbruck provided the graphic design and layout of this 

publication. Cover photo is courtesy of Eric Kuehler. 
18 | Urban Forest Systems and Green Stormwater Infrastructure 

The advisory committee and content reviewers for this 

publication included: 

» Karen Cappiella, Center for Watershed Protection 

» Danielle Fitzko, Vermont Department of Forests, 

Parks & Recreation 

» Robert Goo, Environmental Protection Agency 

» Sarah Hobbie, University of Minnesota 

» Trisha Moore, Kansas State University 

» Randy Neprash, Stantec 

» Aarin Teague, San Antonio River Authority 

» Larry Wiseman, retired American Forest Foundation 

Additional content reviewers included: 

» Joe Burgess, Georgia Forestry Commission 

» Susan Granbery, Georgia Forestry Commission 

» Justin Krobot, San Antonio River Authority 

» Chelcy Ford Miniat, Forest Service Southern Research 

Station 

» Gretchen Riley, Texas A&M Forest Service 

» Andrew Tirpak, University of Tennessee 

The following individuals provided helpful information for the tree crediting case studies: Danielle Fitzko 

(Vermont Department of Forests, Parks and Recreation), 

Amy Macrellis (Stone Environmental Consultants, Inc.), 

Milly Archer (Vermont League of Cities and Towns), Peter 

MacDonagh (The Kestrel Design Group), and Jill Johnson 

(USDA Forest Service). 

References 

Aerts, R. 1996. Nutrient resorption from senescing leaves 


of perennials: Are there general patterns? Journal of 


Ecology. 84 (4): 597-608.
 

Amador, J.A.; Hull, R.J.; Patenaude, E.L.; Bushoven, 


J.T.; Goerres, J.H. 2007. Potential nitrate leaching under 


common landscape plants. Water Air Soil Pollution. 185: 


323-333.
 

Asadian, Y.; Weiler, M. 2009. A new approach in measuring 


rainfall intercepted by urban trees in coastal British 


Columbia. Water Quality Research Journal of Canada. 44: 


16-25.
 

Aston, A.R. 1979. Rainfall interception by eight small 


trees. Journal of Hydrology. 42: 383-396.
 

Bartens, J.; Day, S.D.; Harris, J.R.; Dove, J.E.; Wynn, T.M. 


2008. Can urban tree roots improve infiltration through 


compacted subsoils for stormwater management? Journal 


of Environmental Quality. 37: 2048-2057.
 

Bedient, P.B.; Huber, W.C.; Baxter, E. 2013. Hydrology 


and floodplain analysis. 5th ed. Upper Saddle River, NJ: 


Pearson Publishing Company. 816 p.
 

Berland, A.; Shiflett, S.A.; Shuster, W.D.; Garmestani, 


A.S.; Goddard, H.C.; Herrmann, D.L.; Hopton, M.E. 2017. 


The role of trees in urban stormwater management. 


Landscape and Urban Planning. 162: 167-177. http://dx.doi.
 

org/10.1016/j.landurbplan.2017.02.017. (7 October 2018).
 

Brady, N.C.; Weil, R.R. 2002. The nature and property 


of soils. 13th ed. Upper Saddle River, NJ: Prentice Hall 


Publishing. 960 p.
 

Bratieres, K; Fletcher, T.D.; Deletic, A.; Zinger, Y. 2008. 


Nutrient and sediment removal by stormwater biofilters: a 


large-scale design optimization study. Water Research. 42: 


3930-3940.
 

Breuer, L., Eckhardt, K.; Frede, H.G. 2003. Plant parameter 


values for models in temperate climates. Ecological 


Modelling. 169: 237-293.
 

Center for Watershed Protection. 2017. Review of the 


available literature and data on the runoff and pollutant 


removal capabilities of urban trees. Crediting framework 

Urban Forest Systems and Green Stormwater Infrastructure | 19 

product #1: Making urban trees count: a project to 

demonstrate the role of urban trees in achieving 

regulatory compliance for clean water. Ellicott City, MD: 

Center for Watershed Protection. 44 p. https://owl.cwp. 

org/mdocs-posts/review-of-the-available-literature-anddata-on-the-runoff-and-pollutant-removal-capabilitiesof-urban-trees/. (15 May 2019). 

Center for Watershed Protection. 2018a. Accounting for 

trees in stormwater models. Ellicott City, MD: Center for 

Watershed Protection. 17 p. https://owl.cwp.org/mdocsposts/accounting-for-trees-in-stormwater-models/. (15 

May 2019). 

Center for Watershed Protection. 2018b. Making your 

Community Forest-Friendly: A Worksheet for Review 

of Municipal Codes and Ordinances. Ellicott City, MD: 

Center for Watershed Protection. 37 p. https://owl.cwp. 

org/mdocs-posts/making-your-community-forestfriendly-a-worksheet-for-review-of-municipal-codesand-ordinances/. (15 May 2019). 

Coder, K.D. 2013. Essential elements of tree health. 

Monograph WSFNR13-6. Athens, GA: University of Georgia 

Warnell School of Forestry and Natural Resources. 167. 

Denman, E.C.; May, P.B.; Moore, G.M. 2016. The potential 

role of urban forests in removing nutrients from 

stormwater. Journal of Environmental Quality. 45(1): 

207-214. 

Ford, C.R.; Hubbard, R.M.; Vose, J.M. 2011. Quantifying 

structural and physiological controls on variation in 

canopy transpiration among planted pine and hardwood 

species in the southern Appalachians. Ecohydrology. 4(2): 

183-195. 

Ford, C.R.; Mitchell, R.J.; Teskey, R.O. 2008. Water table 

depth affects productivity, water use, and the response to 

nitrogen addition in a savanna system. Canadian Journal 

of Forest Research. 38(8): 2118-2127. 

Ford, C.R.; Vose, J.M. 2007. Tsuga canadensis (L.) 

Carr. mortality will impact hydrologic processes in 

southern Appalachian forest ecosystems. Ecological 

applications. 17(4): 1156-1167. 

Gonzalez-Sosa, E.; Braud, I.; Becerril, P.R.; Mastachi 

Loza, C.A.; Ramos Salinas, N.M.; Veliz Chavez, C. 2017. A 

methodology to quantify ecohydrological services of street 

trees. Ecohydrology & Hydrobiology. 17: 190-206. http:// 

dx.doi.org/10.1016/j.ecohyd.2017.06.004. (8 October 

2018). 

Groffman, P.M.; Williams, C.O.; Pouyat, R.V.; Band, L.E.; 

Yesilonis, I.D. 2009. Nitrate leaching and nitrous oxide flux 

in urban forests and grasslands. Journal of Environmental 

Quality. 38: 1848-1860. 

Guan, M.; Sillanpaa, N.; Koivusalo, H. 2016. Storm runoff 

response to rainfall pattern, magnitude and urbanization 

in a developing urban catchment. Hydrological Processes. 

30: 543-557. https://onlinelibrary.wiley.com/doi/ 

abs/10.1002/hyp.10624. (8 October 2018). 

Halverson, H.G.; DeWalle, D.R.; Sharpe, W.E. 1984. 

Contribution of precipitation to quality of urban storm 

runoff. Water Resources Bulletin. 20(6): 859-864. 

Hart, T.D. 2017. Root-enhanced infiltration in stormwater 

bioretention facilities in Portland, Oregon. Paper 3468. 

Dissertations and Theses. Portland, OR: Portland State 

University. https://pdxscholar.library.pdx.edu/cgi/ 

viewcontent.cgi?article=4477&context=open_access_ 

etds. (8 October 2018). 

Hirabayashi, S. 2013. i-Tree Eco precipitation interception 

model descriptions. https://www.itreetools.org/eco/ 

resources/iTree_Eco_Precipitation_Interception_ 

Model_Descriptions.pdf. (8 October 2018). 

Hobbie, S.E.; Baker, L.A.; Buyarski, C.; Nidzgorski, D.; 


Finlay, J.C. 2013. Decomposition of tree litter on pavement: 


implications for urban water quality. Urban Ecosystems. 


17(2): 369-385. https://link.springer.com/article/10.1007%
 

2Fs11252-013-0329-9. (8 October 2018).
 

Holder, C.D. 2013. Effects of leaf hydrophobicity and 


water droplet retention on canopy storage capacity. 


Ecohydrology. 6: 483-490. https://onlinelibrary.wiley.
 

com/doi/abs/10.1002/eco.1278. (8 October 2018).
 

Hynicka, J.; Divers, M. 2016. Relative reductions in 


non-point source pollution loads by urban trees. 


In: Recommendations of the expert panel to define 


BMP effectiveness for urban tree canopy expansion. 


Ellicott City, MD: Center for Watershed Protection and 


Chesapeake Stormwater Network: 1-26. https://owl.cwp.
 
20 | Urban Forest Systems and Green Stormwater Infrastructure 

org/mdocs-posts/recommendations-of-the-expertpanel-to-define-bmp-effectiveness-for-urban-treecanopy-expansion/. (8 October 2018). 

Janke, B.D.; Finlay, J.C.; Hobbie, S.E. 2017. Trees and 

streets as drivers of urban stormwater nutrient pollution. 

Environmental Science & Technology. 51: 9569-9579. 

https://pubs.acs.org/doi/10.1021/acs.est.7b02225. 

(8 October 2018). 

Keim, R.F.; Skaugset, A.E.; Weiler, M. 2006. Storage of 

water on vegetation under simulated rainfall of varying 

intensity. Advances in Water Resources. 29: 974-986. 

http://www.rnr.lsu.edu/people/keim/pubs/Keim_etal_ 

RfallSim_AWR06.pdf. (8 October 2018). 

Kestrel Design Group Team. 2013. The roles and effects 

of tree evapotranspiration and canopy interception in 

stormwater management systems and strategies: A review 

of current literature and proposed methodology for 

quantification in the MIDS Calculator application. MPCA 

Stormwater Manual Revisions. Prepared by the Kestrel 

Design Group Team for the Minnesota Pollution Control 

Agency. 21 p. https://stormwater.pca.state.mn.us/index. 

php/File:Trees_Task_13_ET_interception_credits.docx. 

(15 May 2019). 

Kuehler, E.; Hathaway, J.; Tirpak, A. 2017. Quantifying 

the benefits of urban forest systems as a component of 

the green infrastructure stormwater treatment network. 

Ecohydrology. https://onlinelibrary.wiley.com/doi/ 

full/10.1002/eco.1813. (8 October 2018). 

Kuichling, E. 1889. The relation between the rainfall 

and the discharge of sewers in populous districts. 

Transactions of the American Society of Civil Engineers. 

20(1): 1-56. 

Lange, B.; Luescher, P.; Germann, P.F. 2009. Significance 

of tree roots for preferential infiltration in stagnic soils. 

Hydrology and Earth System Sciences. 13: 1809-1821. 

Law, N.; Hanson, J. 2016. Recommendations of the expert 

panel to define BMP effectiveness for urban tree canopy 

expansion. Ellicott City, MD: Center for Watershed 

Protection and Chesapeake Stormwater Network. 237 p. 

https://owl.cwp.org/mdocs-posts/recommendationsof-the-expert-panel-to-define-bmp-effectiveness-forurban-tree-canopy-expansion/. (8 October 2018). 

Levia, D.F.; Germer, S. 2015. A review of stemflow 

generation dynamics and stemflow-environment 

interactions in forests and shrublands. Reviews of 

Geophysics. 53: 673-714. https://agupubs.onlinelibrary. 

wiley.com/doi/epdf/10.1002/2015RG000479. (8 October 

2018). 

Li, X.; Xiao, Q.; Niu, J.; Dymon, S.; van Doorn, N.S.; Yu, X.; 

Xie, B.; Lv, X.; Zhang, K.; Li, J. 2016. Process-based rainfall 

interception by small trees in Northern China: the effect 

of rainfall traits and crown structure characteristics. 

Agricultural and Forest Meteorology. 218-219: 65-73. 

https://doi.org/10.1016/j.agrformet.2015.11.017. (8 

October 2018). 

Lindsey, P.; Bassuk, N. 1991. Specifying soil volumes to 

meet the water needs of mature urban street trees and 

trees in containers. Journal of Arboriculture. 17(6): 141-149. 

Minnesota Pollution Control Agency. 2013. Minnesota 

Stormwater Manual. Published in web-based format 

only. https://stormwater.pca.state.mn.us/index. 

php?title=Main_Page. (15 May 2019). 

Moore, J.; Macrellis, A.; Bailey, K. 2014b. Tree credits 

systems and incentives at the site scale. Montpelier, 

VT: Stone Environmental, Inc. 24 p. https:// 

vtcommunityforestry.org/sites/default/files/pictures/ 

site_scale_tree_credits_2014_02_28_final.pdf. (12 

November 2018). 

Moore, T; Barden, C; Galgamuwa, P.; Nooraei, A. [In 

press]. Predicting urban tree contributions to urban runoff 

budgets with statistical models. [Fact sheet]. Alexandria, 

VA: The Water Research Foundation. 

Nassif, S.H.; Wilson, E.M. 1975. The influence of 

slope and rain intensity on runoff and infiltration. 

Hydrological Sciences Journal. 20(4): 539-553. https://doi. 

org/10.1080/02626667509491586. (8 October 2018). 

Natural Resources Conservation Service. 2010. Time 

of concentration. In: Part 630 Hydrology - National 

Engineering Handbook. Chapter 15. https:// 

directives.sc.egov.usda.gov/OpenNonWebContent. 

aspx?content=27002.wba 
Urban Forest Systems and Green Stormwater Infrastructure | 21 

Nidzgorski, D.A.; Hobbie, S.E. 2016. Urban trees reduce 

nutrient leaching to groundwater. Ecological Applications. 

26(5): 1566-1580. 

Pataki, D.E.; McCarthy, H.R.; Litvak, E.; Pincetl, S. 

2011. Transpiration of urban forests in the Los Angeles 

metropolitan area. Ecological Applications. 21(3): 661-677. 

Read, J.; Wevill, T.; Fletcher, T.; Deletic, A. 2008. 

Variation among plant species in pollutant removal from 

stormwater in biofiltration systems. Water Research. 42: 

893-902. 

Schooling, J.T.; Carlyle-Moses, D.E. 2015. The influence of 

rainfall depth class and deciduous tree traits on stemflow 

production in an urban park. Urban Ecosystems. 18: 

1261-1284. https://link.springer.com/article/10.1007/ 

s11252-015-0441-0. (8 October 2018). 

Selbig, W.R. 2016. Evaluation of leaf removal as a means 

to reduce nutrient concentrations and loads in urban 

stormwater. Science of the Total Environment. 571: 

124-133. 

Staelens, J.; Schrijver, A.D.; Verheyen, K.; Verhoest, N.E.C. 

2008. Rainfall partitioning into throughfall, stemflow, 

and interception within a single beech (Fagus sylvatica L.) 

canopy: influence of foliation, rain event characteristics, 

and meteorology. Hydrological Processes. 22: 33-45. 

USDA Forest Service. 2018. Urban nature for human health 

and well-being: a research summary for communicating 

the health benefits of urban trees and green space. FS1096. Washington, DC. 24 p. 

U.S. Environmental Protection Agency. 2018. What is 

green infrastructure? https://www.epa.gov/greeninfrastructure/what-green-infrastructure. (8 October 

2018). 

Vermont Agency of Natural Resources. 2017. Vermont 

Stormwater Management Manual Rule. Montpelier 

VT: Vermont Agency of Natural Resources. 113 p. 

https://dec.vermont.gov/sites/dec/files/documents/ 

VermontStormwaterManagementManualRule_ 

Ch36_2017_Final_2016-12-20.pdf. (14 November 2018). 

Vermont League of Cities and Towns. 2015. Vermont green 

stormwater infrastructure (GSI) simplified sizing tool for 

small projects fact sheets. Montpelier, VT. https://www. 

vlct.org/sites/default/files/documents/Resource/2015_ 

GSI-Simplified-Sizing-Tool-Fact-Sheets.pdf. (14 November 

2018). 

Vermont League of Cities and Towns. 2017. Green 

stormwater infrastructure toolkit. Montpelier, VT. 

https://www.vlct.org/resource/green-stormwaterinfrastructure-toolkit. (14 November 2018). 

Vose, J.M.; Clark, J.S.; Luce, C.H.; Patel-Weynand, T. , eds. 

Effects of drought on forests and rangelands in the United 

States: a comprehensive science synthesis. Gen. Tech. Rep. 

WO-93b. Washington, DC: U.S. Department of Agriculture 

Forest Service, Washington Office. 231-245. Chapter 10. 

https://doi.org/10.2737/WO-GTR-93b. 

Wang, J.; Endreny, T.A.; Nowak, D.J. 2008. Mechanistic 

simulation of tree effects in an urban water balance 

model. Journal of the American Water Resources 

Association. 44 (1): 75-85. 

Wolf, K.L. 2005. Business district streetscapes, trees, and 

consumer responses. Journal of Forestry. 103 (8): 396-400. 

Xiao Q.; McPherson, G.; Ustin, S.L.; Grismer, M.E.; 

Simpson, J.R. 2000. Winter rainfall interception by 

two mature open-grown trees in Davis, California. 

Hydrological Processes. 14: 763-784. 

Xiao, Q.; McPherson, E.C. 2016. Surface water storage 

capacity of twenty tree species in Davis, California. 

Journal of Environmental Quality. 45: 188-198. 

https://dl.sciencesocieties.org/publications/jeq/ 

abstracts/45/1/188. (8 October 2018). 

Zabret, K.; Rakovec, J.; Mikos, M.; Sraj, M. 2017. Influence 

of raindrop size distribution on throughfall dynamics 

under pine and birch trees at the rainfall event level. 

Atmosphere. 8(12): 240. 15 p. https://www.mdpi. 

com/2073-4433/8/12/240/htm. (8 October 2018). 

Zadeh, M.K.; Sepaskhah, A.R. 2016. Effect of tree roots 

on water infiltration rate into the soil. Iran Agricultural 

Research. 35(1): 13-20. 
22 | Urban Forest Systems and Green Stormwater Infrastructure 

Glossary of Terms 

Bioretention—a green stormwater infrastructure practice 

that uses soil or engineered planting media and plants to 

retain/detain water and filter pollutants from stormwater 

runoff. Raingardens are a subset of bioretention practices. 

Diffuse-porous xylem—water-conducting vessel elements in hardwood tree stems having no clear earlywood 

or latewood arrangement and no discernable difference in 

pore diameter size. 

Dynamic storage—the temporary storage of rainfall on 

tree canopy surfaces eventually released as throughfall or 

stemflow to become stormwater runoff. 

Green stormwater infrastructure (GSI)—stormwater 

mitigation practices designed to mimic natural processes 

that filter and retain rain where it falls. Typical GSI 

practices include green roofs, urban trees, bioretention, 

vegetated swales, permeable pavements, and water 

harvesting. 

Interception loss—the amount of rainfall that is 

intercepted on aboveground surfaces and evaporates back 

to the atmosphere—does not contribute to stormwater 

runoff. 

Leaf area index (LAI)—the total single-side leaf surface 

area per unit of ground surface area. An LAI of 3 indicates 

that a plant has three times as much leaf surface area as 

the ground area under that plant. 

Leaf surface area—the areal sum total of all single sides 

of leaves in a tree. 

Macropores—small holes or pores in the soil greater than 

75 mm from which water drains relatively quickly by gravity, thus providing adequate oxygen for root growth and 

playing a role in stormwater infiltration. 

Micropores—smaller pores in soil (generally 5 to 30 mm) 

that tend to hold water in the soil profile where it is available for plant uptake. 

Preferential flow—the uneven and rapid movement of 

water through soil due to cracks or channels in the soil 

profile caused by the root/soil interface, decayed roots, 

or other biotic and abiotic activities such as geologic 

processes. 

Ring-porous xylem—water-conducting tissue in hardwood tree stems that features earlywood pores that clearly 

form concentric rings. 

Runoff hydrograph peak—the maximum stormwater 

runoff discharge volume reported during a specified time 

period as related in graphical form (hydrograph). The 

runoff hydrograph depicts flow (discharge) versus time. 

Semi-ring-porous xylem—water-conducting tissue in 

hardwood tree stems where pores do not form discernable rows and sizes of pores gradually decrease from 

earlywood to latewood. Static storage—rainfall intercepted 

by tree canopy tissue after a rainfall event that eventually 

evaporates into the atmosphere and does not reach the 

ground surface or become stormwater runoff. 

Stemflow—the movement of water intercepted by tree 

canopy down the stem to the ground. 

Throughfall—rain that passes through the tree canopy 

and drips onto the ground below. 

Tracheid xylem—water-conducting pores in soft-wooded 

trees (i.e., pine). 

Transpiration—the process where plants take in water 

from the soil through their roots, passing it to leaves, 

where it is released as water vapor through pores (stomata) 

to the atmosphere through evaporation. 
Urban Forest Systems and Green Stormwater Infrastructure | 23 

 

 

 

 

 

 

How to cite this publication 

U.S. Department of Agriculture, Forest Service. 2020. Urban forest systems and 

green stormwater infrastructure. FS–1146. Washington, DC. 23 p. 

In accordance with Federal civil rights law and U.S. 

Department of Agriculture (USDA) civil rights regulations and 

policies, the USDA, its Agencies, offices, and employees, 

and institutions participating in or administering USDA 

programs are prohibited from discriminating based on race, 

color, national origin, religion, sex, gender identity (including 

gender expression), sexual orientation, disability, age, marital 

status, family/parental status, income derived from a public 

assistance program, political beliefs, or reprisal or retaliation 

for prior civil rights activity, in any program or activity 

conducted or funded by USDA (not all bases apply to all 

programs). Remedies and complaint filing deadlines vary by 

program or incident. 

Persons with disabilities who require alternative means of 

communication for program information (e.g., Braille, large 

print, audiotape, American Sign Language, etc.) should 

contact the responsible Agency or USDA’s TARGET Center 

at (202) 720-2600 (voice and TTY) or contact USDA through 

the Federal Relay Service at (800) 877-8339. Additionally, 

program information may be made available in languages 

other than English. To file a program discrimination complaint, complete the USDA Program Discrimination Complaint 

Form, AD-3027, found online at http://www.ascr.usda.gov/ 

complaint_filing_cust.html and at any USDA office or write a 

letter addressed to USDA and provide in the letter all of the 

information requested in the form. 

To request a copy of the complaint form, call (866) 632-9992. 

Submit your completed form or letter to USDA by: (1) mail: 

U.S. Department of Agriculture, Office of the Assistant 

Secretary for Civil Rights, 1400 Independence Avenue, SW, 

Washington, D.C. 20250-9410; (2) fax: (202) 690-7442; or (3) 

email: program.intake@usda.gov. 

USDA is an equal opportunity provider, employer, and lender. 

The use of trade or firm names in this publication is for 

reader information and does not imply endorsement by the 

U.S. Department of Agriculture of any product or service. 

A PRODUCT OF THE FOREST SERVICE NATIONAL URBAN FORESTRY TECHNOLOGY AND SCIENCE DELIVERY TEAM