Report Contents

2 Project Overview and Major Findings

3 Regional Analysis

4 Local Analysis

6 Using Regional Data for Local Analysis

7 What’s Next for the Atlanta Metro Area?

8 About the Urban Ecosystem Analysis

Urban Ecosystem AnalysisAtlanta Metro AreaCalculating the Value of Nature

August 2001

Sponsored by the Turner Foundation

City of Atlanta, GeorgiaStudy Area

Cobb County

DeKalbCounty

GwinnettCounty

Newton County

Henry CountyFayette County

Clayton County

DouglasCounty

Union City

Atlanta

Forest Park

FultonCounty

Urban Ecosystem Analysis, Atlanta, GA

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Project OverviewAMERICAN FORESTS conducted a Regional EcosystemAnalysis of the Atlanta metro area to determine how the land-scape has changed over time and to calculate the impact of thechanges on community management costs. The study area cov-ers 775,000 acres and is centered around the City of Atlanta,Georgia. The changes in the landscape were determined bymapping land cover using remote sensing techniques. The costsassociated with the changing landscape were then calculatedusing a combination of Geographic Information Systems tech-nology and established scientific and engineering formulas.

The findings from this analysis show that Atlanta’s urban infra-structure has not only expanded rapidly over the last fewdecades, but also produced an urban infrastructure that costsbillions of dollars more to manage than necessary. Simply stat-ed, the development that has taken place has removed far toomany trees and replaced them with too many roads, parkinglots, and buildings. Less vegetation and more impervious sur-faces mean higher costs for clean air and water.

While this report is not intended as an indictment of develop-ment, the findings clearly show that the approach used in thepast is flawed. Land planning needs to be changed if only toreduce the costs of development for tax payers and communi-ty health. Growth and development decisions need to be madelocally, but these decisions should be guided by a complete setof information. Traditionally, development decisions have notconsidered the significant contributions of the green infra-structure—trees, soils, and open space—which provide thecommunity with ecological and environmental benefits.

The study also demonstrates the application of computer tech-nology for measuring the benefits of the “green” infrastructureto the built infrastructure. The technology used in this analysisis available to community leaders and will improve the deci-sion making process.

Community leaders need to build a green infrastructure toreconnect the existing “gray” infrastructure with the underly-ing natural ecology. The benefits of a healthy green infrastruc-ture to the Atlanta metro area mean a cleaner environment,stronger economy, and healthier residents.

Methodology

The analysis assessed the value of ecological features by com-bining advanced image analysis techniques that map the earth’ssurface with time-tested scientific and engineering models thatquantify the effects of the landscape on air, water, and energy.Geographic Information Systems technology is used to con-

Urban Ecosystem AnalysisThe City of Atlanta

nect the maps to the scientific and engineering formulas. Abase map for the Atlanta metro area was created using imagerycollected by Landsat satellites. Landsat satellites have been cir-cling the earth since 1972 and therefore allow us to look backin time. Through the image analysis process accurate maps ofthe earth’s surface are reproduced.

Satellite data from three separate points in time, 1974, 1986, and1996 were used to conduct this analysis. The images selected forthis 22-year time span were chosen to maximize the accuracy ofthe image analysis process. The two most critical criteria used forselecting images were atmospheric clarity (minimum clouds andhaze) and timing. Each satellite image used for analysis targetedthe same date of the selected year and showed trees in a leaf-oncondition. The area covered by the analysis was 775,000 acresand was centered around the city of Atlanta, Georgia.

Major Findings

Trees are an indicator of environmental quality because oftheir ability to moderate the effects of urbanization on air,water, and energy. The economic impact of these changes inland cover are calculated using AMERICAN FORESTS’CITYgreen® software.

The ecology of Atlanta metro area has changed dramati-cally since 1974. Forests have declined and urban devel-opment has expanded.

� Heavy tree cover, where tree canopies covered over 50% ofthe land surface, occupied about half of the Atlanta metro area(368,145 acres) in 1974, but by 1996 heavy tree cover haddeclined to one quarter of the area (204,784 acres. The heavytree cover declined from 47.5% in 1974 to 26.4 in 1996. ). Lossof tree cover in the metro area is largely due to development.

� Areas with less than 20% tree cover expanded from 44% in1974 to 70.8% in 1996. Around Atlanta, areas with little treecanopy are covered with impervious surfaces.

� Average tree cover throughout the 775,000 acre areadeclined from 45% to 29%.

There are economic implications of tree loss for stormwa-ter management and clean air in the Atlanta Metro Area

� Tree loss in the Atlanta metro area from 1974 to 1996 result-ed in a 33% increase in stormwater runoff (from each 2-yearpeak storm event). This translates into an estimated 591 mil-lion cubic feet of water. Costs to build stormwater retentionponds and other engineered systems to intercept this runoffwould cost $1.18 billion ($2/cubic ft. of storage).

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Atlanta Metro Area Satellite ImagesLandsat TM and MSS satellite images show the change in landcover for the Atlanta metro area based on a recent 22-yearperiod. Heavy tree cover (≥50%) is indicated in green andlight tree canopy (<20%) and impervious surfaces associatedwith urban areas are in black. The GIS analysis measures ninecategories of tree cover, and data from the detailed analysis isused in all calculations. The visual images above combine thenine categories into five groupings to accommodate the limi-tations of printing the images at this scale.

Regional Analysis

Landsat MSS 1974 80 Meter Pixel Resolution Landsat TM 1996 30 Meter Pixel Resolution

Low Canopy (<20% Vegetated)

Moderate Canopy (20-49% Vegetated)

High Canopy (≥50% Vegetated)

Key: % Tree Cover

< 20% 20-29% 30-39% 40-49% ≥ 50% Vegetation ChangeAtlanta Metro Area, 1974-1996

Acr

es

� The total stormwater retention capacity of this urban forestcover in 1996 was worth about $2.36 billion, down from 1974'svalue of $3.54 billion. Annually, this translates to $85.9 million.

� The urban forest improves air quality by removing: nitro-gen dioxide (NO2), sulfur dioxide (SO2), carbon monoxide(CO), ozone (O3), and particulate matter 10 microns or less(PM10). Lost tree canopy would have removed about 11 mil-lion pounds of pollutants annually, at a value of approximate-ly $28 million per year.

� The Atlanta metro area’s direct residential summer energysavings, as a result of tree shade, is estimated at $2.8 millionannually. Reduced energy use results in lower pollutantemissions. The reduced energy use as a result of direct resi-dential tree shade reduces the Atlanta area’s emissions about658,000 tons per year.

� Vegetation lost over this 22-year period would have stored11.3 million tons of carbon and sequestered the carbon at arate of 14,000 tons per year (see pp. 4 and 5).

Maintaining and restoring tree cover is a cost effectiveway to improve the environment.

� The natural landscape should be recognized for its econom-ic, as well as ecological, value. Tree cover is a good measureof the ecological health of the landscape.

� Sprawl development has large negative environmental andeconomic consequences.

� Increasing the average tree cover from 29% to 40% in urbanareas would provide sizeable benefits. Stormwater runoff woulddecrease 20% at a value of $1.7 billion. Tree canopy wouldremove 4,000 tons more air pollutants at a value of $26 million.

Graphing ChangeThe change in vegetation depicted in the satellite images(above left) is represented in line graphs (above right). Bothcharts show the change in vegetative cover over a 22-yearperiod for three categories. Natural forest cover is represent-ed by a green line and indicates places with greater than orequal to a 50% tree canopy. Developed areas are representedby a black line and indicate areas where tree canopy is lessthan 20%. The yellow line represents land where the treecover is between 20% and 49%. Open space, residential areas,and park land would all fall into this middle category.

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What is a Local Level Analysis?

The local level analysis is a detailed assessment of a selectednumber of sample study points where high resolution aerialphotographs, a site survey and CITYgreen software are used tocalculate the effects of tree cover on air, water and energy. It isthrough this process that scientific and engineering data is con-nected to the regional assessment. While CITYgreen softwareproduces detailed and accurate information about the functionand value of a sample site, the accuracy of the regional assess-ment is directly related to the quality of the sampling methods.Accuracy is improved by increasing the number of sample sitesand stratifying the sites by land use or land cover. AMERICANFORESTS works closely with local people when selecting sam-ple sites to improve accuracy through stratification. In theAtlanta metro area, data from 41 sample sites were used to cal-culate value. Each sample site was 2-3 acres in size and includ-ed residential, commercial, and open space land use categories.

Using the land cover patterns identified from the Landsatregional image, locations for low-level aerial photographs areselected. Aerial photographs of each sample site provided dataabout trees, grass, and impervious surfaces. Additional informa-tion was collected in the field, including tree species, size, andhealth. Other data is added to the calculations using nationaldata sets for soil types, and rainfall. Land-use data is obtainedfrom local planning agencies. CITYgreen software is used tocalculate ecosystem benefits for each sample site. The results arethen extrapolated to fit the conditions in the Atlanta Region.

Trees as Indicators of a Community's Ecological Health Urban ecology is more complex than tree cover. Nonetheless,trees are good indicators of the health of an urban ecosystem.The greater the canopy coverage, the less impervious surfaceand the more environmental benefits. Trees provide communi-ties with many valuable services that can be measured in termsof dollar benefits. These include: 1) slowing stormwater runoffand reducing peak flow and 2) improving air quality. Thesequantifiable benefits can help community leaders recognize costsavings opportunities from increased tree cover.

Cities spend tremendous amounts of money installing stormwa-ter control systems and repairing damage from flooding.Furthermore, cities that cannot meet EPA attainment levels forair and water quality jeopardize federal funding for capitalimprovements. Trees are an attractive, non-built solution thatreduces stormwater runoff and improves air quality. These ben-efits underscore the importance of maintaining and restoring thenatural infrastructure of our communities.

How to Use CITYgreen To Analyze Local DataAMERICAN FORESTS uses CITYgreen software to conduct adetailed analysis of how the structure of the landscape affects itsfunction. This tool connects research and engineering formulasto place a dollar value on the work trees do.With CITYgreen,it is possible to determine how various canopy covers affectstormwater movement and air quality.

Stormwater RunoffTrees and soil function together to reduce stormwater runoff.Trees reduce stormwater flow by intercepting rainwater onleaves, branches, and trunks. Some of the intercepted waterevaporates back into the atmosphere, and some soaks into theground reducing the total amount of runoff that must be man-aged in urban areas. Trees also slow storm flow, reducing thevolume of water that a containment facility must store. TheTR-55 model, developed by the Natural ResourcesConservation Service, measures stormwater movement in vari-ous storm events (see page 8).

Local governments are looking toward non-built stormwatermanagement strategies, including trees, to reduce the cost of con-structing stormwater control infrastructure. The value of trees forstormwater management is based on avoided cost for storage of

Local Level Analysis

Forest Park Site 11

3.93 acres1% tree cover

Atlanta Site A1

2.69 acres8% tree cover

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Forty-one sample sites were chosen throughout the AtlantaMetro area to represent a range of neighborhoods andcanopy conditions. Five of the forty-one sites are shown indetail here, illustrating canopy coverage from 1-30%.

7.74 acres5% tree cover

Atlanta Site C1

Atlanta Site G1

7.2 acres30% tree cover

Atlanta Site 3D

11.19 acres19% tree cover

treecover

local siteboundary

built structure

impervioussurface

stormwater in retention ponds. Local construction costs forbuilding containment facilities are multiplied by the total volumeof avoided storage to determine dollars saved by trees.

In the Atlanta metro area, the existing tree canopy reduces theneed for retention structures by 591 million cubic feet. Usinga $2.00/cubic foot construction cost, trees in 1996 saved theregion $2.36 billion per construction cycle (maintenance costsare not included). Annually this translates to $85.9 million.

Air QualityTrees provide air quality benefits by removing pollutants suchas NO2, CO, SO2, O3, and PM10. To calculate the dollarvalue for these pollutants (see page 8), economists multiply thenumber of tons of pollutants by an “externality cost” or coststo society that are not reflected in marketplace activity. In theAtlanta metro area, the trees removed 19 million pounds ofpollutants, valued at $47 million in 1996. Tree cover as itexisted in 1974 would have removed 30 million pounds ofpollutants, valued at $75.5 million.

Stored and Sequestered CarbonThis study also analyzed the amount of carbon stored andsequestered per year. Carbon accounts for about half the dryweight of most trees. The carbon-related function of trees is

measured in two ways: storage, or the amount currently storedin tree biomass, and sequestration, the rate of absorption peryear. The region’s trees stored an estimated 8.3 million tons in1996.The trees sequester about 58,000 lbs per year.

Energy UseThe Atlanta metro area experiences a long, hot summer and res-idents spend approximately $400 per home on air conditioningper year. AMERICAN FORESTS’ analysis suggests that the exist-ing tree canopy in the 18 residential sample sites saves an aver-age of $28 per home (based on 1-2 story, single family detachedhomes for which available research numbers are available.)

To estimate citywide energy conservation savings of trees, theaverage savings of $28/home was projected across the city’s esti-mated 99,083 single family homes within the study area. Theestimated annual residential savings is approximately $2.8 million.

Avoided CarbonReducing energy use also reduces the amount of carbon pollu-tion produced by utility companies. CITYgreen calculates theamount of kilowatt hours of electricity conserved as a result ofdirect shading of trees. This number is multiplied by the fuelmix profile of Georgia’s electricity production. Atlanta savesabout 658,000 tons of carbon emission annually as a result ofdirect shading of residences.

Urban Ecosystem Analysis, Atlanta, GA

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Using Regional Data for Local Analysis

The Atlanta Metro area satellite data 1996 (above), theCity of Atlanta clip of satellite data (left).

CITYgreen software can model benefits for different landcover scenarios. By modeling changes in land cover, decisionmakers can see the impacts of various development decisions.

Using the 775,000 acres study site, two scenarios were mod-eled, one used a 40% tree canopy, 10% grass, and 50%impervious surface. This was compared to the 1996 meas-ured canopy from the Landsat data (an estimated 29%

canopy) and a modeled 21% grass, and 50% impervious sur-face land cover.

Increasing the average tree cover from 29% to 40% in urbanareas would provide sizeable benefits. Stormwater runoffwould decrease 20% at a value of $1.7 billion. Tree canopywould remove 4,054 tons more air pollutants at a value of$73.4 million annually.

Table 1. Atlanta Metro Area Benefits

Ecological Benefit 1996 canopy at 29% Modeled at 40 %

Stormwater Management Value (one time) $2.36 billion $4.06 billion

Stormwater Management Value (annual) $85.9 million $197.7 million

Air Pollution Stored (annual) $47 million $73.4 million

Energy Conserved (annual) $2.82 million N/A

Carbon Avoided (annual) $657,000 N/A

Carbon Stored (total) 8 million tons 13.3 million tons

Carbon Sequestered (annual) 58,000 tons 104,000 tons

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RecommendationsThese findings address public policy questions for land-useplanning and growth management. The most fundamentalrecommendation is for the community to use green infra-structure data in the decision making process. Trees, beingthe largest member of the plant world, provide a good meas-ure of how well a community is doing in its effort to become“greener” Trees require good soils, adequate water, and cleanair. Tree cover is also a measure of environmental quality.When urban trees are large and healthy, the ecological systemthat supports them is also healthy.

This report brings together the expertise of ecologists, scien-tists, and engineers with computer mapping technology toevaluate the environment in the Atlanta metro area and charta course of action for future improvement. Local agencies andthe community are encouraged to incorporate this informa-tion into the regional planning process and to gather moredetailed data for local analyses using aerial imagery, site sur-veys, and CITYgreen software.

(1) Expand the capacity and usefulness of this analysis forregional planning and growth management.

� Obtain additional data for this model from city and coun-ty governments.

� Use the data from this analysis as a basis for building aregional model.

� Incorporate a natural resource data layer into the regionalplanning process.

(2) Recruit county and city governments as partners in creating a regional model.

� Local governments should clip their data from the regionalinformation and conduct a more detailed local analysis usingaerial imagery.

� Establish local data collection plots.

� Use local analyses for community planning.

� Utilize CITYgreen software and the AMERICAN FORESTSanalysis technique as a model for community participation.

What’s Next for the Atlanta Metro Area?

(3) Increase and conserve the tree canopy cover in urban areas.

� Develop urban tree canopy goals for the Atlanta metroarea based on AMERICAN FORESTS guidelines for the east-ern US:

40% tree canopy overall 50% tree canopy in suburban residential zones25% tree canopy in urban residential zones15% tree canopy in the central business district

� Implement innovative land-use planning techniques andengineering guidelines for saving existing trees and plantingnew ones.

� Consider the dollar values associated with trees when mak-ing land-use decisions.

� Use trees as a valuable and essential element of the urbanenvironment.

� Use CITYgreen software as a tool to incorporate trees intoland-use planning by collecting data on tree cover and quan-tifying the value of the trees. Use the findings in the localdecision making process.

(4) Increase overall tree cover in the Atlanta metro region to 40%.

� Develop a comprehensive plan for the Atlanta metro area toprotect, expand, and sustainably maintain a “green infrastruc-ture” of urban and rural forests.

� Meet or exceed the goals of major tree-planting initiativesin the Atlanta metro area. These include the Riparian ForestBuffer Initiative of the partners in the Atlanta metro area toplant 2,020 miles of streamside forests by the year 2020.

Urban Ecosystem Analysis, Atlanta, GA

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monoxide are deposited in tree canopies as well as theamount of carbon sequestered. The Urban Forest Effects(UFORE) model is based on data collected in 50 US cities.Dollar values for air pollutants are based on the median valueof the externality costs set by the State Public ServiceCommissions in these states.

Avoided Carbon: CITYgreen avoided carbon module beginswith kWh savings estimated in the energy module. Becausedifferent fuel sources emit different levels of carbon per unit ofelectricity production, the impact of a conserved kWh willvary depending on local fuel sources. To account for this, theamount of saved kWh from the energy module is multipliedby Energy Information Administration (EIA) data for state-level fuel sources used in electricity production.

Acknowledgments for this StudyThis report was sponsored by the Turner Foundation. Wethank our corporate partners: ESRI for GIS software, Emergefor aerial imagery, and ERDAS for remote sensing software.

For More InformationAMERICAN FORESTS, founded in 1875, is the oldest nationalnonprofit citizens conservation organization. Its three centers–Global ReLeaf, Urban Forests, and Forest Policy—mobilizepeople to improve the environment by planting and caring fortrees. Global ReLeaf 2000 is AMERICAN FORESTS’ campaignto plant 20 million trees for the new millennium.

AMERICAN FORESTS’ CITYgreen® software provides indi-viduals, organizations, and agencies with a powerful tool toevaluate development and restoration strategies and impacts onurban ecosystems. AMERICAN FORESTS offers regional train-ing workshops and technical support for CITYgreen® and is acertified ESRI developer and reseller of ArcView products.Prepared analyses such as this report are available by order.

AMERICAN FORESTSP.O. Box 2000 Washington DC 20013 phone:202/955-4500; fax: 202/955-4588email:cgreen@amfor.org, website: www.americanforests.orgClick on “Trees, Cities & Sprawl”

Ecostructure ClassificationAMERICAN FORESTS’ Urban Ecosystem Analysis is based onthe assessment of ecostructures, unique combinations of landuse and land cover patterns. Each ecostructure performs eco-logical functions differently and thus provides different values.For example, a site with a heavy tree canopy provides morestormwater reduction benefits than one with a light tree canopy.

In this study, the regional analysis provided an overview of treecover change in the Atlanta metro region. Using the tree coverpercentage categories to model the area’s ecostructures, samplestudy sites within the Atlanta area were selected to furtherexamine the effects of different tree canopy cover percentageson air quality and stormwater management. Additional localanalysis using aerial photos of representative ecostructures isneeded to refine the local values given in the analysis.

Data Used in this StudyFor regional analysis, Landsat satellite TM (30 meter pixel) andMSS (80 meter pixel) images were used as the source of landcover data. AMERICAN FORESTS used a subpixel classifica-tion technique and divided land cover into nine vegetationcategories. For the local analysis, AMERICAN FORESTS USEDgeo-rectified .tif images (aerial photos) at a 2 foot resolution.

AMERICAN FORESTS developed CITYgreen software to helpcommunities analyze the value of local trees and vegetation aspart of urban infrastructure. CITYgreen is an application ofArcView for Windows, a Geographic Information Systems(GIS) software developed by ESRI.

Analysis FormulasStormwater Runoff: Stormwater runoff calculations incorpo-rate formulas from US Natural Resources ConservationService (NRCS) Technical Release 55 (TR-55). TR-55 is amodel for estimating stormwater runoff in small urban water-sheds and is widely used across the country for stormwaterplanning and urban engineering analyses. Don Woodward,PE, a hydrologic engineer with NRCS, customized the for-mulas to determine the benefits of trees and other urban veg-etation with respect to stormwater management. CITYgreencalculates stormwater runoff volume, peak flow and time ofconcentration and can model the percentage change betweentwo landcover scenarios.

UFORE Model for Air Pollution: CITYgreen uses formu-las from a model developed by David Nowak, PhD, of theUS Forest Service. The model estimates how many poundsof ozone, sulfur dioxide, nitrogen dioxide, PM10 and carbon

About the UrbanEcosystem Analysis