San Antonio: Ripples of Change; Armed with New Information on the Benefits of Their Urban Trees, Officials Plan for a Greener and More Ecologically Based Future. (Perspectives)

Article excerpt

San Antonio successfully straddles the line between historic relic and modern city. Tourists flock by the thousands to visit the Alamo, site of the decisive 1836 battle for Texas independence, or to stroll the banks of the San Antonio River along downtown's commercially successful River Walk. The city is ringed by springs, creeks, and rivers that arise from fault lines along one of the nation's largest limestone aquifers. The lush and varied vegetation that results attracts an array of migratory birds--and nature lovers.

These natural and manmade attractions have brought the city and its surrounding area impressive population growth and urban expansion. Now the greater San Antonio area is trying to meet the clean air, clean water, and energy needs of its many residents. Looking to improve environmental quality, the Alamo Forest Partnership turned to AMERICAN FORESTS for assistance in evaluating the area's tree canopy cover. The Alamo Forest Partnership is a consortium of governmental and environmental organizations spearheaded by the city's publicly owned energy company, City Public Service (CPS).

WHAT'S A COMMUNITY TO DO?

To do that, AMERICAN FORESTS conducted an Urban Ecosystem Analysis (UEA) of the greater San Antonio area. The analysis--which encompassed 788,000 acres of Bexar County, including the city of San Antonio--reevaluated the area's assets to include the environmental benefits provided by trees. The first phase determines landscape changes over time and assesses the impact those changes have had on air and water quality and on residential energy consumption.

Using satellite and aerial imagery along with Geographic Information Systems (GIS) technology, changes in land cover were measured between 1985 and 2001. In autumn, AMERICAN FORESTS will complete the second phase of the study: creating a "green data layer" of information about urban forest ecology that the local community can use when making planning and management decisions.

Municipalities increasingly are turning to the analysis to make the powerful point that trees are an integral and money-saving part of the urban infrastructure. "Although we have acknowledged the benefits of tree cover in reducing air conditioning costs and improving air quality for some time," says Milton B. Lee, CPS's CEO and general manager, "prior to the study, we didn't have any scientific data that measured the extent of our tree canopy or the values it provided."

ECOLOGICAL ISSUES

Bexar County, in south-central Texas, sits at the edge of a wave of urbanization spreading north into the Hill Country and onto the Edwards Aquifer Recharge Zone. The recharge zone, home to endangered bird and aquatic species, is where rain and streamwater enter San Antonio's primary source of drinking water, and it needs protection from pollution.

The greater San Antonio area has a particularly sensitive ecosystem built upon hard limestone bedrock and thin soils. The limestone's faults and sinkholes allow rainwater to drain directly into the aquifer without the benefit of passing through soil, which naturally filters out sediment and urban runoff pollutants. In addition, because its thin soil doesn't hold water well, the region is prone to flash flooding.

These problems have been exacerbated by increased urbanization and a corresponding decrease in tree cover. San Antonio's population has risen 22 percent in 10 years, making it the ninth largest city in the U.S.

In the first phase of the analysis AMERICAN FORESTS determined that since 1985 San Antonio has lost 39 percent of its heavy tree cover (canopy coverage greater than 50 percent). The corresponding decrease in greater San Antonio's tree cover is 22 percent. Urbanized areas (less than 20 percent canopy cover) in greater San Antonio increased from 69 percent in 1985 to 77 percent in 2001.

The trends in moderate tree density (20 to 49 percent) show that as the area is developing, tree canopy is not being conserved. …