This southwestern jewel, called the Venice of the U.S., shimmers with natural beauty, culture, and economic opportunities. But development and tree loss usually go hand in hand. Here's how one city is gathering the tools to change that scenario.

By Alexis Harte

How an Analysis Inspired Change

San Antonio's residents maintain a proud tradition of civic involvement in protecting their natural resources. In 1921, when a few influential city leaders decided to essentially bury the San Antonio River in concrete after a series of devastating floods killed 50 and cost millions in damages, a diverse citizenry galvanized behind their city's namesake river. Their efforts created the momentum for the city's River Walk, today considered a crown jewel in urban riverfront planning (see "A City Guided by Its River," Summer 2003). When American Forests released the results of its Phase 1 Urban Ecosystem Analysis (UEA) last November, residents were alarmed. San Antonio's population had grown rapidly since 1985, and the study found the city's heavy forest cover (areas with more than 50 percent canopy) had declined by 39 percent. Areas of medium tree cover (containing 20-49 percent canopy) shrunk by 43 percent over the same period. Tree loss was not limited to the urban center; throughout the greater San Antonio area and outlying suburbs, heavy canopy decreased from 26 percent to 20 percent. Residents' alarm quickly became action. Using the UEA's results to fortify their arguments, the Citizens' Tree Coalition, a collection of environmental organizations and neighborhood associations, successfully rallied City Council to pass a long-debated Tree Preservation Ordinance. The ordinance goes further than any past legislation in recognizing the value of San Antonio's urban forest. Some key provisions of the ordinance include protecting all trees in 100-year floodplains, recognizing the value of tree clusters rather than just individual trees, awarding variances to preserve trees in residential setbacks, and preserving any species with a trunk at least 30 inches in diameter. The Phase 1 UEA provided a historical snapshot of declining tree trends and loss of ecological benefits. The new UEA gives city leaders a green data layer with the tools to make decisions on future development.—Alexis Harte

One thousand years before Spanish settlers invoked Saint Anthony from faraway Padua to name their new mission site, the native Payapa people called it simply Yanaguana, or "place of refreshing waters." With dozens of springs and streams still emerging through cracks in one of the nation's largest limestone aquifers, it remains an apt name. Collecting these waters and flowing through downtown, the San Antonio River is the sylvan centerpiece, earning the city the nickname "The Venice of the U.S."

San Antonio's prolific water network drives a highly unique ecological system, defined by the confluence of four distinct ecoregions: Post Oak savanna, Blackland prairie, South Texas plains, and Edwards Plateau. First a sky full of migrating birds, then the Payapa, and later the Spanish missionaries were drawn to the oasis and its remarkable floral and faunal diversity.

Today the influx shows no sign of slowing. With the city boasting a dazzling array of cultural resources, natural beauty, and ample economic opportunities, its population grew an astounding 25 percent over the last 15 years, making it the nation's ninth most populous city. As in the past, the river and the waters that feed it continue to support the region's diverse ecology.

At the same time, Phase 1 of an Urban Ecosystem Analysis (UEA) by American Forests showed that tree canopy throughout the larger San Antonio region diminished over that same 15-year period (see sidebar). When the results were announced last fall, they gave San Antonians a time-lapse, bird's-eye view of their city's forest and confirmed what many had suspected: Unchecked sprawl had taken its toll on canopy cover.

San Antonio Mayor Ed Garza encouraged the city's residents to take a close look at the recent numbers, even if they were troubling. "It's easier for citizens to understand the need for regulatory changes, such as the stronger tree preservation ordinance we passed earlier this year, when we have real numbers and research to back them up," he says. "The American Forests study drew attention to the regional and local changes caused by deforestation in San Antonio since 1985, especially the negative impacts on the urban heat island, stormwater runoff, and air quality."

The UEA demonstrated that the trees lost between 1985 and 2001 would have removed an additional 3.7 million pounds of air pollutants annually, a service valued at about $9 million per year. The city also lost $146 million in stormwater management services. Collectively, residents lost $17.7 million in yearly residential summer energy services from the canopy loss.

A Layer of Green

Building on the UEA Phase 1's results, American Forests is working with city leaders and planners to introduce a "green data layer" as a tool to halt and reverse these trends. Integrated into San Antonio's Geographic Information System (GIS), the layer will provide a digital rendering of regional tree cover. More than simply a static image, the layer will attach to each stand of trees an accurate accounting of their contribution in ecosystem services such as water storage and air cleansing.

As a public policy tool, the green data layer will help locals understand the role trees play in a healthy urban ecosystem and how development will affect their future quality of life.

"The potential applications of this green data are immense," says Mark Peterson, regional urban forester with the Texas Forest Service, which is funding the new project together with the U.S. Forest Service. Peterson says the layer will be instrumental in guiding the region's long-term urban forestry planning.

American Forests has created green data layers for use by local decisionmakers in cities around the U.S. The process is made possible by the recent availability of high-resolution, digital imagery that can detect trees with a 6-foot crown spread. Every land feature—a pond, a road, a roof, or a tree—reflects a distinct bandwidth of light when viewed from the air. From "raw" satellite or aerial images of a landscape, American Forests' remote-sensing specialists can interpret land cover measured in pixels. The smaller the pixel size, the more accurate the analysis; in San Antonio's case, values were derived from aerial photographs taken in 2002 with crystal-clear 1-meter resolution. (By example, it would take roughly 15 individual pixels to represent a single Cadillac on the ground.)

Once the images are classified, users can clearly distinguish land features over an entire region. When combined with American Forests' CITYgreen software, the green data layer provides a ready-to-use foundation upon which local municipalities and conservation groups may undertake their own canopy analyses.

For example, the recent analysis shows that although San Antonio's overall canopy coverage is 27 percent, the Edwards Aquifer recharge zone—with its 43 percent coverage—is responsible for significantly boosting this number. Without considering this zone, overall city coverage would drop to 22 percent. This knowledge allows city planners and conservation leaders to tailor canopy targets to specific areas, rather than generalize over vastly different land uses.

"A green data layer allows communities to look at specific places, analyze the value or dollar benefits of various development options, and in the end make better decisions," says Gary Moll, vice president of American Forests' Urban Forest Center. "We are interested in helping communities ask better questions."

American Forests is working with local partners to develop analyses for areas within the city and the greater metropolitan area. The analyses, designed to initiate a process rather than a product, familiarize a wide group of planners and decisionmakers with the data layer and its many uses.

The test sites were chosen to either broadly represent the region's diverse canopy or to provide baseline information for areas facing potential development.

Keeping Tabs on Growth

Among the areas for which change looms on the horizon is the Southside Initiative, a 57-square mile newly annexed swath of land. From a development perspective, many planners view the land as truly a blank slate, with no past mistakes to confront. "The green data layer will certainly help guide future growth in the Southside area," the Texas Forest Service's Peterson says. "Now we have an opportunity to promote smart growth principles proactively instead of reactively."

A proposed new auto plant in the area has prompted a flurry of growth proposals, but until City Council formally adopts its Long-Range Master Development Plan (LRMDP), a moratorium exists on any new projects. Working with City Council, planners are incorporating into the LRMDP a set of development standards including stream and river buffers, tree planting, incentives for productive agricultural uses, and other conservation principles. Using CITYgreen software with the new green data layer, they can instantly test and refine their plans by modeling air quality and water runoff changes resulting from various canopy scenarios.

For example, canopy cover in the Southside area is 22 percent; those trees annually remove 833,000 pounds of air pollution and manage 402 million gallons of stormwater during an average storm. Even though these services are valued at about $11.3 million, one planned development scenario's commercial and industrial uses would reduce canopy cover to 16 percent.

The loss in tree services could cost the city another $1 million per year and cause higher pollution and stormwater levels. Planting trees on planned vacant areas or parkland could up canopy cover to 30 percent. Air quality and water management services provided by this increase are valued at about $12.5 million annually.

Flooding and Air Quality

By intercepting rainfall and staggering times of peak flow, trees make up a vital and often underrated part of a city's stormwater management system. Using green data and CITYgreen software, planners can calculate the actual volume of rainwater that trees intercept and store and, based on local stormwater management costs, derive a dollar value in saved containment.

In San Antonio, urban growth continues to sprawl on top of the city's aquifers. Recognizing the threat to these precious recharge zones, the city recently mandated that new development over the aquifer must not increase overall flood rates. Based on the recent analysis, American Forests determined that the zone's 43 percent canopy cover provides the city with approximately $51 million in stormwater management services. To prevent flood levels from increasing over this ecologically sensitive area, American Forests has suggested the city maintain a minimum 45 percent tree cover, greater than the citywide goal of 35 percent. With the green data layer, planners can set target canopy goals both for individual sets of plans and collectively over the entire recharge area, making trees a quantifiable part of the development equation.

"Flooding in San Antonio is an age-old issue," says Carol Haywood, a planner with San Antonio's neighborhood and urban design department who is eyeing the green data layer's public education potential. "Most folks think we need more concrete culverts to simply whisk the water away as fast as possible. We will use this [green data layer] to model and demonstrate the ability of trees to perform a similar function without adding new concrete."

The city can also use the green data to educate citizens and improve air quality to meet federal regulations. Since 1999, ozone levels have regularly risen over federal Clean Air Act limits, prompting the Texas Committee on Environmental Quality (TCEQ) to recommend a status of "Non-Attainment Deferred" for Bexar County.

By conferring "deferred" status, TCEQ recognized the county's commitment to confront its air pollution problems through the Early Action Compact, which allows the region to implement local solutions instead of one-size-fits-all national-level programs. By taking an active role in improving their air quality, the counties avoid standard nonattainment penalties such as the withholding of federal transportation funds.

According to Dorothy Birch of the Alamo Area Council of Governments, the green data layer will help local air quality planners design and evaluate air pollution control, satisfying EPA's requirements that the measures are "quantifiable and permanent." Air quality planners can model the effects of increased tree planting on local air pollutant levels in communities like Leon Valley, which are getting squeezed by San Antonio's sprawling development.

For example, a modeling analysis determined that increasing canopy cover by 10 percent in a two-mile radius around the junction of Highway 16 and Interstate 410 would decrease ozone by 25 percent, a service valued at $130,500 annually. According to the Phase 1 study, the total air quality and stormwater benefit would be valued at about $1.1 million per year.

At the neighborhood level, Debbie Reid, city arborist with City of San Antonio Developmental Services (CDS), part of San Antonio's planning department, believes the green data layer will figure prominently in planners' toolkits. Imagine, for example, a developer wants to build a department store and parking lot, requiring the removal of 71 trees, including 37 young Arizona oaks. CDS staff with minimal GIS experience could project the future growth of those trees and derive the actual dollar amount of ecosystem services lost by their removal.

Rather than basing its opposition or support for planned development projects on aesthetic claims, CDS can factor in long-term costs borne by taxpayers in increased water storage and air quality measures. In addition to helping preserve existing trees, the new green data layer will help City Public Services (CPS), the city's publicly owned energy company, identify the most appropriate sites for its Green Shade energy conservation tree-planting program. Says CPS environmental analyst Jenna Terrez, "We can determine which areas have the fewest trees and begin targeting our plantings in those neighborhoods."

Emerging from a period of intense growth, San Antonio is trying to meet the clean air, clean water, and energy needs of its many residents. Accommodating growth without destroying quality AF

Alexis Harte is covering the 2003 National Urban Forest Conference for American Forests. [TOP]