Urban Ecosystem Analysis For the Houston Gulf Coast Region Calculating the Value of Nature

Transcription

1 December 2000 Urban Ecosystem Analysis For the Houston Gulf Coast Region Calculating the Value of Nature Report Contents 2-3 Project Overview and Major Findings 4 Regional Level Analysis 5-7 Local Level Analysis 8 Using Regional Data for Local Analysis 9 What s Next for Houston Metro Area 10 About the Urban Ecosystem Analysis Grimes County San Jacinto County Houston, Texas Study Area Waller County Montgomery County Harris County Liberty County Fort Bend County Galveston County Chambers County Brazoria County Sponsored by the USDA Forest Service and the Houston Green Coalition

2 Urban Ecosystem Analysis: Houston, Texas Urban Ecosystem Analysis Houston Project Overview AMERICAN FORESTS conducted an Urban Ecosystem Analysis of the Houston metropolitan area to determine how the landscape has changed over time. The analysis assessed the loss of tree canopy and its associated values using data from satellite images spanning a 27-year period from 1972 to The analysis covered 3.2 million acres of land within a 50 mile radius of Houston (see Data Used in This Study, pg. 10.) Houston is located at the convergence of three distinct ecosystems; prairie, marshland associated with the Gulf of Mexico, and mixed deciduous/pine forest. The Houston area is generally good for growing, with approximately 46 inches of rainfall per year. The analysis used Geographic Information Systems (GIS) technology to measure the structure of the landscape, with emphasis on tree cover. Regional changes in the landscape were analyzed using satellite images. A detailed look at the urban forest s economic value was conducted using low-level digital imagery of 25 study sites and AMERICAN FORESTS CITYgreen software. CITYgreen is a desktop GIS software used by local government agencies and engineering groups for decision support. This report comes at an important time for the City of Houston. Starting in the 1970s, the City was recognized as a non-attainment area for Ozone under the Clean Air Act, potentially leading to significant sanctions. The problem has grown worse and the state is currently developing its State Implementation Plan (SIP) to address this problem. We believe that the Texas Natural Resource Conservation Commission (TNRCC) could use this document to support the inclusion of tree planting and preservation in the State Implementation Plan (SIP) in order to help meet Clean Air Act requirements and as strategies for reducing the urban heat island in Houston. Houston continues to be one of the hottest cities in the nation, with extraordinarily high energy use for air conditioning (an estimated $714 per home annually, according to Reliant Energy). Major Findings From a vegetation perspective, the ecology of the Houston Metro area has changed since Forests have declined and developed areas have expanded In 1972, areas with heavy tree canopy (50% or greater tree cover) covered 31% of the area (1 million acres). Areas with less than 20% tree canopy covered 63% of the land (just over 2 million acres). By 1999, areas with light tree canopy (less than 20%) became even more prevalent, expanding to 71% of the land area, an increase of 13%). By 1999, areas with heavy tree canopy declined by 16%, representing 26% of the land area studied. Tree canopy representing a medium density category (20-49%) had the most significant percentage change, although the total number of acres affected were smaller than either the heavy or light categories. This trend suggests that as new development occurs, tree canopy is not being conserved, and that these areas are probably moving into the light category. The average tree canopy within the Houston study area is approximately 30%. There are economic and ecological implications of tree loss for stormwater management and clean air in the Houston metro area. Lost tree canopy would have removed about 15.3 million pounds of sulfur dioxide (SO 2 ), carbon monoxide (CO), ozone (O 3 ), and particulate matter 10 microns or less (PM10) from the atmosphere annually, at a value of approximately $38 million per year (see pg. 5). Vegetation lost over this 27 year period would have stored 7.5 million tons of carbon and sequestered the carbon at a rate of 138 thousand tons per year (see pp. 5 and 6). Tree loss in the Houston metro area between 1972 and 1999 resulted in an estimated increase of 360 million cubic feet of stormwater flow during a peak storm event (based on an average maximum, two year, 24-hour storm event). Using a local cost estimate of $.66/cubic foot (Harris County Flood Control District) to build stormwater systems, this vegetation loss is equivalent in value to a one-time savings of a $237 million system. The total stormwater retention capacity of this urban forest cover in 1999 is valued at an estimated $1.33 billion, down from 1972 s value of $1.56 billion, based on the avoided cost of having to manage this stormwater (see pp. 5 and 10). Houston s direct residential summer energy savings, as a result of tree shade, is estimated at $26 million annually, an effective strategy to reduce the area s urban heat island (see pg.6). USDA Forest Service research suggests that the indirect cooling from evapotranspiration will produce an additional savings of roughly equal size. 2

3 American Forests Report Reduced energy use results in lower pollutant emissions. The reduced energy use as a result of direct residential tree shade in Houston reduces emissions about 10.8 million tons of carbon per year. Maintaining and restoring tree cover is a cost-effective way to improve urban infrastructure. The natural landscape should be recognized for its economic as well as its ecological value. While urban ecology is more complex than just trees, tree cover is a good indicator of the ecological health of the landscape. When urban trees are large and healthy, the ecological system that supports them is also healthy. Healthy trees require healthy soils, adequate water, and clean air. In turn, when trees are healthy, they provide many valuable services such as improving air quality, reducing atmospheric carbon, slowing stormwater runoff and reducing peak flow. Increasing the average tree cover in the area would provide sizeable benefits. For example a 40% tree cover would provide $3.5 billion in one-time stormwater benefits (a 163% increase from current conditions), $297 million worth of pollutant removal benefits (a 25% increase), and 58 million tons in carbon storage (a 55% increase). Strategically planting trees in urban and suburban areas would improve the effectiveness of tree cover for energy savings. Table 1. Houston Metro Area s Vegetation Change and Associated Benefits* Loss/Gain Acres with more than 50% tree cover 1,004,361 (31%) 844,923 (26%) -16% Acres with 20%-49% tree cover 188,042 (6%) 86,859 (3%) -54% Acres with less than 20% tree cover 2,007,321 (63%) 2,267,942 (71%) 13% Stormwater Management Value $1.56 billion $1.33 billion -$237 million total** -$17 million annually*** Air Pollution Removal Value (annually) $247 million $209 million -$38 million Energy Savings**** (annually) $26 million Avoided Carbon***** 10.8 million tons Stored Carbon 45 million tons 37.5 million tons -7.5 million tons * Numbers may not add to 100% due to rounding. ** Represents a one time savings, and does not include additional savings from annual maintenance. *** Annual benefits are calculated on a stormwater management facility s construction costs, plus the cost of the loan or bond to finance construction (assuming a 6% interest rate for a 30 year lifespan of the facility). **** Residential summer energy savings from trees direct shading of one and two-story detached residences. ***** Avoided carbon emission as a result of reduced air conditioning use. 3

4 Urban Ecosystem Analysis: Houston, Texas Regional Level Analysis Key to satellite images: < 20% 20-29% 30-39% 40-49% 50% Landsat MSS Meter Pixel Resolution Landsat TM Meter Pixel Resolution Houston Metro Area Satellite Images Satellite images show the change in land cover in the Houston metro area over a recent 27-year period. Heavy tree canopy cover ( 50%) is indicated in green while light tree canopy cover (<20%) and impervious surfaces associated with urban areas are in black. The GIS analysis measures nine categories of tree cover. Canopy categories are displayed in five groupings to accommodate the limitations of printing the images at this scale. Graphing Change The changes in vegetation depicted in satellite images (above) are represented by a line graph (right). The chart shows the change in vegetation cover over a 27-year period for three categories. Heavy, natural forest cover is represented by a green line and indicates areas with 50% or greater tree canopy. The black line represents areas where tree canopy is light less than 20%. The yellow line represents land where the tree cover is between 20% and 49%. This analysis also includes a projection of vegetation loss if it were to continue for the next 15 years at the same rate as occurred between 1984 and These projections can be seen in the right hand portion of the graph. Acres Vegetation Change Houston Metro Area, Light Canopy (<20% Vegetated) Moderate Canopy (20-49% Vegetated) Heavy Canopy ( 50% Vegetated) 4

5 American Forests Report Local Level Analysis Using canopy cover classes identified from the satellite image, point samples were selected and low-level aerial imagery was used along with CITYgreen software to calculate the value of the local ecology. The Houston Green Coalition decided to focus on a study area extending in a 50-mile radius of Houston s center. The purpose was to capture the impacts of land-use changes in some of the city s surrounding areas, in addition to the city itself. However, because the satellite imagery available to us does not cover this entire study area (portions of the southwest of the study area extended beyond the imagery s edge), a slightly smaller study area was created (see images on preceding page). Twenty-five sites within this study area were selected to represent a variety of land uses and were analyzed for their ecological value. Sites were selected to represent different land uses (residential, commercial/industrial, and open space). Within each category the sites were also divided into four canopy classes by density (0-19%, 20-29%, 30-39%, 40-49%). A 50% or greater density is considered a natural forest condition. The values of all sites within a class were averaged together for analysis. The resulting average benefits from the sites within each class were multiplied by the total land area of each class. Aerial imagery of study sites provide information about trees, grass, and impervious surfaces. Tree inventory data were collected in the field while other data sources provided data on soil types, rainfall patterns, and land-use configurations. CITYgreen software was used to calculate ecosystem benefits for each sample site. The results were then extrapolated to the entire project area based on the total area for each percentage canopy/landuse category. How CITYgreen is Used To Analyze Local Data AMERICAN FORESTS uses CITYgreen software to conduct a detailed analysis of the structure of the landscape and to calculate the dollar benefits of trees. This analytical technique incorporates research and engineering formulas to place a dollar value on the work trees do. With CITYgreen it is possible to determine how various canopy cover classes affect stormwater movement, air quality, and energy conservation. Stormwater Communities that employ non-structural stormwater management strategies can reduce the cost of constructing stormwater control infrastructure. The value of trees for stormwater management has been calculated based on avoided costs of handling stormwater runoff. Local costs are multiplied by the total volume of avoided storage to determine dollars saved by trees. In the Houston study area, the existing tree canopy reduces the need for stormwater management by 2.4 billion cubic feet. Using a 66 cent- per-cubic-foot stormwater management cost (Harris County Flood Control District), trees currently save the area $1.33 billion in one-time construction costs. 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 into the atmosphere and some soaks into the ground, reducing peak flows and thus reducing the total amount of runoff that must be managed in urban areas. Trees also slow storm flow, reducing the volume of water that must be managed at once. The TR-55 model, developed by the Natural Resources Conservation Service, provides a quantitative measure of stormwater movement called an event model (see page 10). Air Quality Trees provide air quality benefits by removing pollutants such as nitrogen dioxide, carbon monoxide, sulfur dioxide, ozone, and particulate matter less than 10 microns in size. To calculate the value for these pollutants, economists multiply the number of tons of pollutants removed by externality costs, or costs to society not reflected in marketplace activity, as established by state public service commissions. This figure represents costs that society would have paid, in areas such as health care, if trees did not remove these pollutants. In the Houston study area, the existing tree canopy removes 83 million pounds of pollutants, valued at $208 million, annually. Tree cover as it existed in 1972 would have removed 98.5 million pounds of pollutants at a value of $247 million. Specifically, current tree cover removes 35.4 million pounds of ozone (valued at $109 million), 6.55 million pounds of S0 2 (valued at $5 million), 12.2 million pounds of NO 2 (valued at $37 million); 27.3 million pounds of PM10 valued at $56 million), and 1.7 million pounds of CO (valued at $1.7 million). Lost tree cover would have removed approximately 6.55 million pounds of ozone (valued at about $20 million), 1.2 million pounds of SO 2 (valued at about $900,000), 2.2 million pounds of NO 2 (valued at about $6.86 million), about 5 million pounds of PM10 (valued at about $10.3 million), and 387 thousand pounds of CO (valued at about $160,000). Stored & 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 5

6 Urban Ecosystem Analysis: Houston, Texas 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 currently store an estimated 37.5 million tons of carbon and sequester about 138 thousand tons of carbon annually. Energy Use The Houston metro area experiences a long, extremely hot summer and residents spend approximately $714 per home on air conditioning per year (Reliant Energy). AMERICAN FORESTS analysis suggests that the existing tree canopy in the 15 residential sample sites saves an average of $72 per home (Note: value based on 1-2 story, single family detached homes. Also, USDA Forest Service research has thus far only modeled savings to residential-size buildings, values were not calculated for residential homes greater than two stories, commercial or open space sites.) To estimate the city-wide energy conservation savings of trees, the average savings of $72/home was projected across the city s estimated 383,577 single-family detached residences in the city of Houston (American Community Survey, U.S. Census Bureau). Assuming that 94% of these residences use air conditioning (American Community Survey, U.S. Census Bureau), the estimated annual residential savings is approximately $26 million. This savings not only conserves residential energy use, it also cools the urban heat island and improves air quality, since air pollutants increase as temperature rises. Avoided Carbon Reducing energy use also reduces 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 Texas s electricity production. Houston saves about 10.8 million tons of carbon emission annually, as a result of direct tree shading of residences. Modeling 40% Canopy Cover To estimate the impacts of increased tree planting and preservation measures, current study sites were modeled to represent 40% canopy coverage. These results were extrapolated across the entire region to assess the benefits that would be provided if it were all vegetated at this percentage. Results show that 40% canopy coverage across the entire region would result in much higher benefits. One-time stormwater benefits would be about $3.5 billion (a 163% increase from current conditions); pollution removal benefits would equal $297 million (a 25% increase); and 58 million tons of carbon would be stored (a 55% increase). Site # acres 11% tree cover Site # acres 24% tree cover Site # acres 1% tree cover 6

7 Site #24 American Forests Report 2.91 acres 38% tree cover Site # acres 28% tree cover Site #41 Twenty-five sample sites were chosen throughout the city of Houston to represent a range of neighborhoods and canopy conditions. Seven of the twenty-five sites are shown in detail here, illustrating canopy coverage from 1-38% acres 16% tree cover tree cover Site #36 local site boundary built structure 2.19 acres 34% tree cover impervious surface 7

8 Urban Ecosystem Analysis: Houston, Texas Using Regional Data for Local Analysis Houston Metropolitan Area satellite data 1999 (above), Houston clip of satellite data (left). A regional level image contains a great deal of data that can be used by individual local governments. A city or county can obtain a sub-set of the regional data by cutting its boundaries from the regional view. With this information, a local government can determine tree canopy cover. This coarse image can be divided or stratified into various tree cover density classes. These classes form the basis for a more detailed analysis. The Houston City image was clipped from a larger regional image of the ten county Houston Metropolitan area. The regional analysis shows general trends; the more detailed local analysis is used for planning. 8

9 American Forests Report What s Next for the Houston Metro Area? Recommendations The Urban Ecosystem Analysis measures land cover changes over a 27-year period and calculates their impact on fundamental elements of the community such as air, water, and energy. The findings from the analysis show that tree cover, which is used as a key ecological indicator in the Houston study area, has declined significantly over this period (acreage with 50% tree cover or greater declined by 16%). This translates into annual lost benefits to the community of about $55 million. A critical issue for local leaders is to reverse the declining condition of the ecology by establishing public policies that increase the area s tree cover and to assist the community in creating and maintaining a robust green infrastructure. While community leaders have made progress in this direction, the recommendations below are intended to provide a set of additional guidelines for reversing negative trends measured by this analysis and utilizing natural systems as natural capital. 1. Include tree cover data in all public works, transportation, and development decision making. This can be done by incorporating a green data layer into a city s central data base or data engine, usually a Geographic Information System and by recognizing the tree cover as green infrastructure. Establish a system for creating and maintaining a green layer of data in the central data file that is used by all departmental managers in the community for planning and maintenance. Determine the contributions trees make to air, water, and energy needs of the community during the design and engineering phase of construction, repair, or development projects. Use leaf-on aerial photography with 2 foot or better ground resolution for determining the location, size, and value of tree cover. CITYgreen software can calculate the stormwater, air quality, and energy values of trees. The software is an extension of the ArcView, the GIS software already used by the City s government. Include the stormwater and air quality benefits of tree cover in the City s planning models. CITYgreen will allow managers to model various tree cover scenarios and calculate the dollar benefits produced. 2. Encourage the use of increased tree cover as one strategy for meeting air pollution and stormwater management needs. Tree cover can be increased by planting more trees, saving trees during development, and better maintaining trees. 3. Use tree cover and the findings of this study as a guide for land-use planning and growth management. Consider the financial value of natural resources during the public policy decision-making process. Financial benefits of tree cover should be included in land-use planning and growth management plans. 4. Continue to work towards increasing and conserving tree canopy cover in all metropolitan land use areas. Develop specific, measurable urban tree canopy goals for the Houston metro area. AMERICAN FORESTS established the following guidelines for cities like Houston. These goals should be further refined by Houston Green or an equally knowledgeable local group. 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 districts Take specific action to increase medium-density tree canopy (20-49% category from this report). As cities grow, they expand into natural areas and remove tree canopy. The medium density area likely provides a measurement of how well trees are being incorporated into new development. Continue to establish tree planting and protection guidelines for new development 9

10 Urban Ecosystem Analysis: Houston, Texas 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 present in a city. 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 Houston area. Using land use and tree cover percentage categories to model the area s ecostructures, sample study sites were selected to further examine the effects of different tree canopy cover percentages on air quality, energy and stormwater management. 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. The 50-mile radius project area was modified due to the boundary limits of the satellite imagery used in this study, resulting in a slightly smaller area covering 3.2 million acres. AMERICAN FORESTS then used a subpixel classification technique and divided land cover into nine vegetation categories. 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). TRR-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 percent change between two land cover 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 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 state-level fuel sources used in electricity production. 10

11 American Forests Report Acknowledgments We gratefully acknowledge the support of the following: USDA Forest Service Houston Green Coalition: TreeScape Bayou Preservation Association City of Houston s Finance & Administration and Urban Forestry Galveston Houston Association for Smog Prevention Gulf Coast Institute HARC Center for Global Studies Houston-Galveston Area Council Houston Urban Forestry Council Scenic Houston Southwestern Bell Pioneers Texas Forest Service The Park People Trees For Bellaire Trees For Houston UT-Houston Health Science Center For More Information AMERICAN FORESTS, founded in 1875, is the oldest national nonprofit citizen conservation organization. Its three centers Global ReLeaf, Urban Forestry, 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. For further information contact: AMERICAN FORESTS P.O. Box 2000 Washington DC Phone: ; Fax: cgreen@amfor.org Web: Click on Trees, Cities & Sprawl ESRI for GIS software Emerge for aerial photography ERDAS for remote sensing software 11

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