Trees: A Green, Cost Effective Stormwater Management Practice

Vincent Cotrone Extension Urban Forester

Penn State University

Trees & Stormwater • Interception by

Canopy • Increase Infiltration

into Soils • Evapotranspiration • Pollutant Removal &

Phytoremediation • Soil Stabilization • Streamside Buffers

Rainfall Interception by Tree Canopies

• Average interception of rainfall by tree canopy cover ranges from 12-40% (varies by rainfall event).

• Interception: mature deciduous 500 to 2000 gallons per year mature evergreen 4,000 gallons per year.

• In one study, existing canopy in Dayton, Ohio reduced runoff by 7% and could be increased to 12% by planting more trees.

1 inch of rainfall per acre is equal to 27,000 gallons of stormwater

Rainfall Interception by a Tree • Callery Pear -

– 9 yrs old, 28 ft tall, 19 ft crown diameter, 276 sq ft crown projection area, 3,846 sq ft leaf area, 446 sq ft stem area.

• 0.5 inches of rain • Total precipitation in

crown projection area = 86.1 gallons

• Total tree interception = 58.1 gal gallons or 67% of the rain falling within the canopy

USDA Forest Service Research at the Western Center for Urban Forestry, Davis, CA Xiao, Q.; McPherson, E.G.; Ustin, S.L.; Grismer, M.E. 2000. A new approach to modeling tree rainfall interception. Journal of Geographical Research Atmospheres 105: 29173-29188.

Interception By Species 20 Year Old Trees • Crabapple 144 gal/yr 22ft tall x 21ft spread • Kwanzan Cherry 312 gal/yr 17ft tall x 17ft spread • Red Oak 767 gal/yr 40ft tall x 27ft spread • Red Maple 1,014 gal/yr 29ft tall x 24ft spread • Zelkova 1,624 gal/yr 38ft tall x 34ft spread • White Pine 786 gal/yr 32ft tall x 20ft spread • Hackberry 1,394 gal/yr 47ft tall x 37ft spread Source: USDA FS PSW-GTR-202 August 2007 & PSW-GTR-199

Interception by Hackberry

Tree Age in Years

Gal

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of S

torm

wat

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Inte

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ted

per y

ear

Source: Adapted from McPherson et al 2006

NYC Street Tree Interception • New York City’s street

trees intercept rain, reducing stormwater runoff by 890.6 million gallons annually, with an estimated value of $35.6 million.

• Citywide, the average tree intercepts 1432 gallons of stormwater each year, valued at $61 per tree.

Source: NEW YORK CITY, MUNICIPAL FOREST

RESOURCE ANALYSIS April 2007 , Peper, McPherson, Simpson, et.al.

Average Interception by Species London Plane – 2,875 gal/yr Silver Maple – 2,948 gal/yr

Pittsburgh Stratum Project

$334,600 in stormwater reduction savings (42 million gallons or 1,400 gallons per tree) annually.

Interception Avg. 500 – 2000 gal/tree/yr

Increase Urban Tree Canopy Urban Tree Interception rates exceeded 40% for small storm events, but were less than 4% for large storm events (Wang, Nowak, Endreny 2006). Increasing canopy cover over impervious surface had the greatest effect on reducing runoff. UFORE Hydro Study Dead Run, Baltimore, MD

Interception Depends on Rain Event

Xiao, Q.; McPherson, E.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.

Infiltration • Urban/Suburban Soils

become highly compacted (1.8 – 2.0 Bulk Density)

• Loss of Organic Layer • Loss of Root Channels • Infiltration decreased • Virginia Tech Study –

Rooting in to Subsoil below is increased infiltration rates by 153%

Forest Infiltration

• Forests filter and regulate the flow of water.

• The forest floor acts as an

enormous sponge, typically absorbing up to 18 inches of precipitation before gradually releasing it into natural channels and watercourse (sub-surface flows)

Source: Your Water- A Forest Product, Green America

series, 1986

Rainfall Infiltration Natural Areas are Giant Sponges!

When forest understory/ leaf litter was converted to turf in a North Carolina watershed, the mean infiltration rate went from 12.4 in/hr to 4.4 in/hr. (Kays, 1980)

Rooting of Native Trees and Prairie Vegetation

Native perennials & trees have deeper root systems than turf grass.

Hourly Infiltration Rates

Infiltration Rates

Importance of Evapotranspiration

Villanova Urban Stormwater Research

Bioinfiltration Site – 2010: 62% ET capture The remaining portion of

rain goes to infiltration

“the single largest component of the natural hydrologic regime, evapotranspiration (ET).” PA DEP Stormwater BMP manual

Annual Evapotranspiration and Streamflow For Different Land Uses in PA

0

20

40

60

80

100

Forest Meadow Cropland Bare Soil Pavement

Perc

ent

EvapotranspirationStream Flow

Effect of Vegetation on Water Budget

Streamflow (Runoff)

Evapotranspiration

Precipitation = 40 inches This image cannot currently be displayed.

Deciduous Forest Coniferous Forest

23 inches 29 inches

17 inches 11 inches

ET & Vegetation Removal

Stream flow = 16 inches Stream flow = 26 inches

Evap. = 24 inches Evap. = 14 inches

Annual Precipitation for PA = 40 inches

Uncut 100% Cut

Streamflow (Runoff)

Evapotranpiration • Mature Tree – 100 gallons per day

(Akbari, 1992)

• A large oak tree can transpire 40,000 gallons (151,000 liters) per summer or 79 gallons per day. (Thomas 2000)

• A mature maple tree can transpire 65-140 liters per summer day (Cermak et.al., 2000)

• The uptake of soil water by tree roots increases soil water storage potential, effectively lengthening the amount of time before rainfall becomes runoff.

• Factors influence transpiration rates, including leaf shape, size, number of pores (stomata), and waxiness of the leaf surface (Metro, 2002)

Transpiration Studies • Study in Suburban

Minnesota – • Conifers transpired

2 times more than deciduous trees – More Leaf Area – Longer Growing

Season

Peters, E.B.; McFadden, J.P.; Montgomery, R.A. 2010. Biological and environmental controls on tree transpiration in a suburban landscape. Journal of Geophysical Research. 115: G04006.

Evapotranspiration • Bald Cypress – large

wetland species • Evapotranspiration =

880 gallons per day, depending on soil type and saturation (Keating, 2002)

Evapotranspiration

Open Grown Tree Forest Grown Trees

Phytoremediation Pollution Removal

• Plants remove contaminants from soil and water, including metals, pesticides, solvents, oils, hydrocarbons, etc.

• In one study, a single roadside sugar maple removed

60mg of cadmium, 140mg of chromium, 820mg of nickel, and 5200mg of lead

in a single growing season (Coder, 1996)

Traditional Lawnscapes • Turfgrass may be the largest

single crop in the Chesapeake Bay covering up to 3.8 million acres (9.5%)

• Over 100 million tons of fertilizer are applied to residential lawns and gardens annually. (Audubon stat)

• 70 million pounds of synthetic pesticides are used on lawns each year – 10 times the rate/acre used by farmers. (Redesigning the American Lawn by F. Herbert Bormann, Diana Balmori, Gordon T. Geballe, Yale University Press, 1993)

Turf areas are becoming a major source of nutrient loading for waterways

Experiments at Cornell and Virginia Tech using engineered soil mixes and Trees showed an increase of infiltration rate by 27 times compared with unplanted control soils.

Engineered Soils are Gap Graded Soils containing 80% angular 1-1.5” stone and 20% clay loam soil. They were designed to create rooting space along with load bearing for sidewalks or parking lot paving

Pollutant Removal from Structural Soils

Typical Parking Lot Trees

Limited Soil Volumes for Rooting, Compacted Soils, Constant Moisture Stress

Larger Canopy Trees Need Larger Soil Volumes

“ A 30 inch diameter tree provides 70 times the ecological services of a 3” diameter tree” Greg McPherson, et. al. USDA Forest Service Research

Cornell University-Structural Soil

Rooting in to Subsoil below is increasing infiltration rates by 153%

CU Soil Under Porous Paving Installed to a 24” depth

Parking Lot in Ithaca, NY with Porous and Non-Porous Asphalt

Accommodates the 100 year storm for Ithaca ( 6" of rain in 24 hours). Infiltration Rate was > 25”/hr

Bareroot Accolade Elms Planted

Three Years After Installation

Five Years After Installation

Projects

Silva Cell

www.DeepRoot.com

Silva Cell Installation

www.DeepRoot.com

Set Urban Tree Canopy Goals • Perform UTC

Assessment • Set Goals for

Increasing Tree Canopy to 40% or greater.

• Incorporate Tree Planting into Green Infrastructure Plans and MS4 Permits

40% canopy = 0.8”/24hr storm intercepted by the canopy + 1.21”/24hr storm in soil UTC overview - http://nrs.fs.fed.us/urban/utc &

http://www.forestsforwatersheds.org/urban-tree-canopy Examples of UTC reports - http://nrs.fs.fed.us/urban/utc/pubs/

• City of Lancaster’s Green Infrastructure Plan

• Need to capture over 1 Billion Gallons of Stormwater to reduce the Combined Sewer OverFlows

THE GREEN INFRASTRUCTURE BENEFIT CALCULATOR PROJECTS FUTURE BENEFITS FOR CSO AND MS4 AREAS

Manage over 1,200 Acres of Impervious Area Capture over 1 Billion Gallons of Stormwater Runoff over the long term

Preserving Existing Forests During Development

Green Infrastructure Urban Vegetation Benefits

Cooler air temperature Building energy conservation Air quality improvement Water quality improvement UV radiation reduction Greenhouse gas reduction Aesthetics Noise reduction Wildlife habitat Social / physiological benefits Human health

www.itreetools.org

i-Tree

National Tree Benefits Calculator

http://www.treebenefits.com/calculator/ Kingston – 25”diameter Pin Oak annually provides:

$247 in benefits each year Intercepts 3,600 gallons of stormwater Saves 163 KW/hours each year and 53 therms Removes 1,100 lbs of atmospheric carbon Increases Property Value by $95

Summary • Annual Interception

– 500-2000 gal/tree/yr – 15-25% Evergreens – 10-20% Deciduous

• Transpiration – 5-15% – Depends on species & weather

• Plant Large, Broad Spreading Canopy Trees Over Pavement (Conifers or Deciduous)

• Utilize Engineered or Structural Soils to Increase Infiltration, Root Growth, and Tree Health.

• Set Community Wide Canopy Goals – 40%+

Penn State Cooperative Extension

Penn State is committed to affirmative action, equal opportunity, and the

diversity of its workforce.

Vincent Cotrone 570-825-1701 vjc1@psu.edu

www.PATrees.org