Urban Ecosystem Analysis Atlanta Metro Area Calculating the Value of Nature

Transcription

1 August 2001 Urban Ecosystem Analysis Atlanta Metro Area Calculating the Value of Nature 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 City of Atlanta, Georgia Study Area Cobb Fulton Gwinnett Douglas Atlanta DeKalb Union City Forest Park Clayton Newton Fayette Henry Sponsored by the Turner Foundation

2 Urban Ecosystem Analysis, Atlanta, GA Urban Ecosystem Analysis The City of Atlanta Project Overview AMERICAN FORESTS conducted a Regional Ecosystem Analysis of the Atlanta metro area to determine how the landscape has changed over time and to calculate the impact of the changes on community management costs. The study area covers 775,000 acres and is centered around the City of Atlanta, Georgia. The changes in the landscape were determined by mapping land cover using remote sensing techniques. The costs associated with the changing landscape were then calculated using a combination of Geographic Information Systems technology and established scientific and engineering formulas. The findings from this analysis show that Atlanta s urban infrastructure has not only expanded rapidly over the last few decades, but also produced an urban infrastructure that costs billions of dollars more to manage than necessary. Simply stated, the development that has taken place has removed far too many trees and replaced them with too many roads, parking lots, and buildings. Less vegetation and more impervious surfaces mean higher costs for clean air and water. While this report is not intended as an indictment of development, the findings clearly show that the approach used in the past is flawed. Land planning needs to be changed if only to reduce the costs of development for tax payers and community health. Growth and development decisions need to be made locally, but these decisions should be guided by a complete set of information. Traditionally, development decisions have not considered the significant contributions of the green infrastructure trees, soils, and open space which provide the community with ecological and environmental benefits. The study also demonstrates the application of computer technology for measuring the benefits of the green infrastructure to the built infrastructure. The technology used in this analysis is available to community leaders and will improve the decision making process. Community leaders need to build a green infrastructure to reconnect the existing gray infrastructure with the underlying natural ecology. The benefits of a healthy green infrastructure to the Atlanta metro area mean a cleaner environment, stronger economy, and healthier residents. Methodology The analysis assessed the value of ecological features by combining advanced image analysis techniques that map the earth s surface with time-tested scientific and engineering models that quantify the effects of the landscape on air, water, and energy. Geographic Information Systems technology is used to con- nect the maps to the scientific and engineering formulas. A base map for the Atlanta metro area was created using imagery collected by Landsat satellites. Landsat satellites have been circling the earth since 1972 and therefore allow us to look back in time. Through the image analysis process accurate maps of the earth s surface are reproduced. Satellite data from three separate points in time, 1974, 1986, and 1996 were used to conduct this analysis. The images selected for this 22-year time span were chosen to maximize the accuracy of the image analysis process. The two most critical criteria used for selecting images were atmospheric clarity (minimum clouds and haze) and timing. Each satellite image used for analysis targeted the same date of the selected year and showed trees in a leaf-on condition. The area covered by the analysis was 775,000 acres and was centered around the city of Atlanta, Georgia. Major Findings Trees are an indicator of environmental quality because of their ability to moderate the effects of urbanization on air, water, and energy. The economic impact of these changes in land cover are calculated using AMERICAN FORESTS CITYgreen software. The ecology of Atlanta metro area has changed dramatically since Forests have declined and urban development has expanded. Heavy tree cover, where tree canopies covered over 50% of the land surface, occupied about half of the Atlanta metro area (368,145 acres) in 1974, but by 1996 heavy tree cover had declined to one quarter of the area (204,784 acres. The heavy tree cover declined from 47.5% in 1974 to 26.4 in ). Loss of tree cover in the metro area is largely due to development. Areas with less than 20% tree cover expanded from 44% in 1974 to 70.8% in Around Atlanta, areas with little tree canopy are covered with impervious surfaces. Average tree cover throughout the 775,000 acre area declined from 45% to 29%. There are economic implications of tree loss for stormwater management and clean air in the Atlanta Metro Area Tree loss in the Atlanta metro area from 1974 to 1996 resulted in a 33% increase in stormwater runoff (from each 2-year peak storm event). This translates into an estimated 591 million cubic feet of water. Costs to build stormwater retention ponds and other engineered systems to intercept this runoff would cost $1.18 billion ($2/cubic ft. of storage). 2

3 American Forests Report The total stormwater retention capacity of this urban forest cover in 1996 was worth about $2.36 billion, down from 1974's value of $3.54 billion. Annually, this translates to $85.9 million. The urban forest improves air quality by removing: nitrogen dioxide (NO 2 ), sulfur dioxide (SO 2 ), carbon monoxide (CO), ozone (O 3 ), and particulate matter 10 microns or less (PM10). Lost tree canopy would have removed about 11 million pounds of pollutants annually, at a value of approximately $28 million per year. The Atlanta metro area s direct residential summer energy savings, as a result of tree shade, is estimated at $2.8 million annually. Reduced energy use results in lower pollutant emissions. The reduced energy use as a result of direct residential tree shade reduces the Atlanta area s emissions about 658,000 tons per year. Vegetation lost over this 22-year period would have stored 11.3 million tons of carbon and sequestered the carbon at a rate of 14,000 tons per year (see pp. 4 and 5). Maintaining and restoring tree cover is a cost effective way to improve the environment. The natural landscape should be recognized for its economic, as well as ecological, value. Tree cover is a good measure of the ecological health of the landscape. Sprawl development has large negative environmental and economic consequences. Increasing the average tree cover from 29% to 40% in urban areas would provide sizeable benefits. Stormwater runoff would decrease 20% at a value of $1.7 billion. Tree canopy would remove 4,000 tons more air pollutants at a value of $26 million. Regional Analysis Key: % Tree Cover < 20% 20-29% 30-39% 40-49% 50% Vegetation Change Atlanta Metro Area, Acres Landsat MSS Meter Pixel Resolution Landsat TM Meter Pixel Resolution Low Canopy (<20% Vegetated) Moderate Canopy (20-49% Vegetated) High Canopy ( 50% Vegetated) Atlanta Metro Area Satellite Images Landsat TM and MSS satellite images show the change in land cover for the Atlanta metro area based on a recent 22-year period. Heavy tree cover ( 50%) is indicated in green and light tree canopy (<20%) and impervious surfaces associated with urban areas are in black. The GIS analysis measures nine categories of tree cover, and data from the detailed analysis is used in all calculations. The visual images above combine the nine categories into five groupings to accommodate the limitations of printing the images at this scale. Graphing Change The change in vegetation depicted in the satellite images (above left) is represented in line graphs (above right). Both charts show the change in vegetative cover over a 22-year period for three categories. Natural forest cover is represented by a green line and indicates places with greater than or equal to a 50% tree canopy. Developed areas are represented by a black line and indicate areas where tree canopy is less than 20%. The yellow line represents land where the tree cover is between 20% and 49%. Open space, residential areas, and park land would all fall into this middle category. 3

4 Urban Ecosystem Analysis, Atlanta, GA Local Level Analysis Atlanta Site A1 What is a Local Level Analysis? The local level analysis is a detailed assessment of a selected number of sample study points where high resolution aerial photographs, a site survey and CITYgreen software are used to calculate the effects of tree cover on air, water and energy. It is through this process that scientific and engineering data is connected to the regional assessment. While CITYgreen software produces detailed and accurate information about the function and value of a sample site, the accuracy of the regional assessment is directly related to the quality of the sampling methods. Accuracy is improved by increasing the number of sample sites and stratifying the sites by land use or land cover. AMERICAN FORESTS works closely with local people when selecting sample sites to improve accuracy through stratification. In the Atlanta metro area, data from 41 sample sites were used to calculate value. Each sample site was 2-3 acres in size and included residential, commercial, and open space land use categories. Using the land cover patterns identified from the Landsat regional image, locations for low-level aerial photographs are selected. Aerial photographs of each sample site provided data about trees, grass, and impervious surfaces. Additional information was collected in the field, including tree species, size, and health. Other data is added to the calculations using national data sets for soil types, and rainfall. Land-use data is obtained from local planning agencies. CITYgreen software is used to calculate ecosystem benefits for each sample site. The results are then 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 surface and the more environmental benefits. Trees provide communities with many valuable services that can be measured in terms of dollar benefits. These include: 1) slowing stormwater runoff and reducing peak flow and 2) improving air quality. These quantifiable benefits can help community leaders recognize cost savings opportunities from increased tree cover. Cities spend tremendous amounts of money installing stormwater control systems and repairing damage from flooding. Furthermore, cities that cannot meet EPA attainment levels for air and water quality jeopardize federal funding for capital improvements. Trees are an attractive, non-built solution that reduces stormwater runoff and improves air quality. These benefits underscore the importance of maintaining and restoring the natural infrastructure of our communities acres 8% tree cover Forest Park Site acres 1% tree cover How to Use CITYgreen To Analyze Local Data AMERICAN FORESTS uses CITYgreen software to conduct a detailed analysis of how the structure of the landscape affects its function. This tool connects research and engineering formulas to place a dollar value on the work trees do.with CITYgreen, it is possible to determine how various canopy covers affect stormwater movement and air quality. Stormwater Runoff Trees and soil function together to reduce stormwater runoff. Trees reduce stormwater flow by intercepting rainwater on leaves, branches, and trunks. Some of the intercepted water evaporates back into the atmosphere, and some soaks into the ground reducing the total amount of runoff that must be managed in urban areas. Trees also slow storm flow, reducing the volume of water that a containment facility must store. The TR-55 model, developed by the Natural Resources Conservation Service, measures stormwater movement in various storm events (see page 8). Local governments are looking toward non-built stormwater management strategies, including trees, to reduce the cost of constructing stormwater control infrastructure. The value of trees for stormwater management is based on avoided cost for storage of 4

5 American Forests Report Atlanta Site C1 7.2 acres 30% tree cover Atlanta Site 3D Atlanta Site G acres 19% tree cover 7.74 acres 5% tree cover Forty-one sample sites were chosen throughout the Atlanta Metro area to represent a range of neighborhoods and canopy conditions. Five of the forty-one sites are shown in detail here, illustrating canopy coverage from 1-30%. tree cover local site boundary built structure impervious surface stormwater in retention ponds. Local construction costs for building containment facilities are multiplied by the total volume of avoided storage to determine dollars saved by trees. In the Atlanta metro area, the existing tree canopy reduces the need for retention structures by 591 million cubic feet. Using a $2.00/cubic foot construction cost, trees in 1996 saved the region $2.36 billion per construction cycle (maintenance costs are not included). Annually this translates to $85.9 million. Air Quality Trees provide air quality benefits by removing pollutants such as NO 2, CO, SO 2, O 3, and PM10. To calculate the dollar value for these pollutants (see page 8), economists multiply the number of tons of pollutants by an externality cost or costs to society that are not reflected in marketplace activity. In the Atlanta metro area, the trees removed 19 million pounds of pollutants, valued at $47 million in Tree cover as it existed in 1974 would have removed 30 million pounds of pollutants, valued at $75.5 million. Stored and Sequestered Carbon This study also analyzed the amount of carbon stored and sequestered per year. Carbon accounts for about half the dry weight of most trees. The carbon-related function of trees is measured in two ways: storage, or the amount currently stored in tree biomass, and sequestration, the rate of absorption per year. The region s trees stored an estimated 8.3 million tons in 1996.The trees sequester about 58,000 lbs per year. Energy Use The Atlanta metro area experiences a long, hot summer and residents spend approximately $400 per home on air conditioning per year. AMERICAN FORESTS analysis suggests that the existing tree canopy in the 18 residential sample sites saves an average of $28 per home (based on 1-2 story, single family detached homes for which available research numbers are available.) To estimate citywide energy conservation savings of trees, the average savings of $28/home was projected across the city s estimated 99,083 single family homes within the study area. The estimated annual residential savings is approximately $2.8 million. Avoided Carbon Reducing energy use also reduces the amount of carbon pollution produced by utility companies. CITYgreen calculates the amount of kilowatt hours of electricity conserved as a result of direct shading of trees. This number is multiplied by the fuel mix profile of Georgia s electricity production. Atlanta saves about 658,000 tons of carbon emission annually as a result of direct shading of residences. 5

6 Urban Ecosystem Analysis, Atlanta, GA Using Regional Data for Local Analysis The Atlanta Metro area satellite data 1996 (above), the City of Atlanta clip of satellite data (left). CITYgreen software can model benefits for different land cover scenarios. By modeling changes in land cover, decision makers can see the impacts of various development decisions. Using the 775,000 acres study site, two scenarios were modeled, one used a 40% tree canopy, 10% grass, and 50% impervious surface. This was compared to the 1996 measured canopy from the Landsat data (an estimated 29% canopy) and a modeled 21% grass, and 50% impervious surface land cover. Increasing the average tree cover from 29% to 40% in urban areas would provide sizeable benefits. Stormwater runoff would decrease 20% at a value of $1.7 billion. Tree canopy would 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 6

7 American Forests Report What s Next for the Atlanta Metro Area? Recommendations These findings address public policy questions for land-use planning and growth management. The most fundamental recommendation is for the community to use green infrastructure data in the decision making process. Trees, being the largest member of the plant world, provide a good measure of how well a community is doing in its effort to become greener Trees require good soils, adequate water, and clean air. Tree cover is also a measure of environmental quality. When urban trees are large and healthy, the ecological system that supports them is also healthy. This report brings together the expertise of ecologists, scientists, and engineers with computer mapping technology to evaluate the environment in the Atlanta metro area and chart a course of action for future improvement. Local agencies and the community are encouraged to incorporate this information into the regional planning process and to gather more detailed data for local analyses using aerial imagery, site surveys, and CITYgreen software. (1) Expand the capacity and usefulness of this analysis for regional planning and growth management. Obtain additional data for this model from city and county governments. Use the data from this analysis as a basis for building a regional model. Incorporate a natural resource data layer into the regional planning process. (2) Recruit county and city governments as partners in creating a regional model. Local governments should clip their data from the regional information and conduct a more detailed local analysis using aerial imagery. Establish local data collection plots. (3) Increase and conserve the tree canopy cover in urban areas. Develop urban tree canopy goals for the Atlanta metro area based on AMERICAN FORESTS guidelines for the eastern US: 40% tree canopy overall 50% tree canopy in suburban residential zones 25% tree canopy in urban residential zones 15% tree canopy in the central business district Implement innovative land-use planning techniques and engineering guidelines for saving existing trees and planting new ones. Consider the dollar values associated with trees when making land-use decisions. Use trees as a valuable and essential element of the urban environment. Use CITYgreen software as a tool to incorporate trees into land-use planning by collecting data on tree cover and quantifying the value of the trees. Use the findings in the local decision making process. (4) Increase overall tree cover in the Atlanta metro region to 40%. Develop a comprehensive plan for the Atlanta metro area to protect, expand, and sustainably maintain a green infrastructure of urban and rural forests. Meet or exceed the goals of major tree-planting initiatives in the Atlanta metro area. These include the Riparian Forest Buffer Initiative of the partners in the Atlanta metro area to plant 2,020 miles of streamside forests by the year Use local analyses for community planning. Utilize CITYgreen software and the AMERICAN FORESTS analysis technique as a model for community participation. 7

8 Urban Ecosystem Analysis, Atlanta, GA About the Urban Ecosystem Analysis Ecostructure Classification AMERICAN FORESTS Urban Ecosystem Analysis is based on the assessment of ecostructures, unique combinations of land use and land cover patterns. Each ecostructure performs ecological functions differently and thus provides different values. For example, a site with a heavy tree canopy provides more stormwater reduction benefits than one with a light tree canopy. In this study, the regional analysis provided an overview of tree cover change in the Atlanta metro region. Using the tree cover percentage categories to model the area s ecostructures, sample study sites within the Atlanta area were selected to further examine the effects of different tree canopy cover percentages on air quality and stormwater management. Additional local analysis using aerial photos of representative ecostructures is needed to refine the local values given in the analysis. Data Used in this Study For regional analysis, Landsat satellite TM (30 meter pixel) and MSS (80 meter pixel) images were used as the source of land cover data. AMERICAN FORESTS used a subpixel classification technique and divided land cover into nine vegetation categories. For the local analysis, AMERICAN FORESTS USED geo-rectified.tif images (aerial photos) at a 2 foot resolution. AMERICAN FORESTS developed CITYgreen software to help communities analyze the value of local trees and vegetation as part of urban infrastructure. CITYgreen is an application of ArcView for Windows, a Geographic Information Systems (GIS) software developed by ESRI. Analysis Formulas Stormwater Runoff: Stormwater runoff calculations incorporate formulas from US Natural Resources Conservation Service (NRCS) Technical Release 55 (TR-55). TR-55 is a model for estimating stormwater runoff in small urban watersheds and is widely used across the country for stormwater planning and urban engineering analyses. Don Woodward, PE, a hydrologic engineer with NRCS, customized the formulas to determine the benefits of trees and other urban vegetation with respect to stormwater management. CITYgreen calculates stormwater runoff volume, peak flow and time of concentration and can model the percentage change between two landcover scenarios. UFORE Model for Air Pollution: CITYgreen uses formulas from a model developed by David Nowak, PhD, of the US Forest Service. The model estimates how many pounds of ozone, sulfur dioxide, nitrogen dioxide, PM10 and carbon 8 monoxide are deposited in tree canopies as well as the amount 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 value of the externality costs set by the State Public Service Commissions in these states. Avoided Carbon: CITYgreen avoided carbon module begins with kwh savings estimated in the energy module. Because different fuel sources emit different levels of carbon per unit of electricity production, the impact of a conserved kwh will vary depending on local fuel sources. To account for this, the amount of saved kwh from the energy module is multiplied by Energy Information Administration (EIA) data for statelevel fuel sources used in electricity production. Acknowledgments for this Study This report was sponsored by the Turner Foundation. We thank our corporate partners: ESRI for GIS software, Emerge for aerial imagery, and ERDAS for remote sensing software. For More Information AMERICAN FORESTS, founded in 1875, is the oldest national nonprofit citizens conservation organization. Its three centers Global ReLeaf, Urban Forests, and Forest Policy mobilize people to improve the environment by planting and caring for trees. Global ReLeaf 2000 is AMERICAN FORESTS campaign to plant 20 million trees for the new millennium. AMERICAN FORESTS CITYgreen software provides individuals, organizations, and agencies with a powerful tool to evaluate development and restoration strategies and impacts on urban ecosystems. AMERICAN FORESTS offers regional training workshops and technical support for CITYgreen and is a certified ESRI developer and reseller of ArcView products. Prepared analyses such as this report are available by order. AMERICAN FORESTS P.O. Box 2000 Washington DC phone:202/ ; fax: 202/ cgreen@amfor.org, website: Click on Trees, Cities & Sprawl