San Antonio: restoring the urban oasis; 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 - Texas

American Forests, Autumn, 2003 by 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 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 a canopy throughout the larger San Antonio region diminished over that same 15-year period (see "How an Analysis Inspired Change, "page 27). 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 $14, 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 "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 static image, the layer will attach to each stand of tree 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 axe 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.