San Marcos Greenbelt Alliance •  107 E. Hopkins St. Suite 121A; San Marcos, Texas 78666

The Loop


Green Spaces and the Heat Island Effect

Along with soaring temperatures this summer, there’s been a good bit of discussion about the “heat island” effect, the impact that the built environment and the lack of vegetation have on the ambient temperature. Not surprisingly, this phenomenon is felt most acutely in urban areas, where the increase in temperature over more rural areas can be as much as 8 degrees.

Texas has the dubious distinction of having four cities among the top 10 offenders in the country: #2 Houston, #4 Dallas, #6 San Antonio, and #10 Austin. According to a map featured on KVUE News in Austin on July 27, the heat island effect follows closely along IH-35, with some of the greatest impacts being felt as far south as Buda and Kyle. San Marcos was not included on that map, but it would be logical to assume that a similar increase would be felt here. This would be especially true in locations with sprawling parking lots and a large number of structures, two features that tend to go together.

Mitigating the effects of highways, office buildings, apartment complexes, shopping centers with sprawling parking lots, and all the other structures typical of urban life is a complex process. That said, there are nonetheless some commonsense actions that can make the situation a little more bearable. Chief among these is planting trees—or better yet, not removing mature trees and other vegetation.

According to Melanie Lenart in Trees for Energy Conservation, measuring the precise impact of an urban forest is hard, but “large parks or tracts of urban trees can cool daytime summer air temperatures by about 10°F.” In addition to shading the ground from direct sunlight, something that can reduce ground temperatures by as much as 36 degrees, trees cool the air through transpiration. “When a molecule of water evaporates, it takes with it some heat that could otherwise be used to warm the nearby environment,” writes Lenart. “This effect typically reaches its peak when evaporation levels are highest, usually at midday.”

While most people associate the natural areas in San Marcos with recreation—hiking, birding, mountain biking on the trails—the environmental impact of these spaces is equally important, if not more so. There may be no way to precisely measure that effect, but it’s a given that greenbelts help to mitigate flooding and increase the absorption of rainfall into the aquifer. They also improve air quality and sequester carbon. According to the U.S. Department of Agriculture, “In one year, a mature live tree can absorb more than 48 pounds of carbon dioxide, which is permanently stored in its fibers until the tree or wood experiences a physical event that releases it into the atmosphere, like fire or decomposition.”

If trees are the lungs of the planet, then the work of the San Marcos Greenbelt Alliance is not just to give people a beautiful place to hike and bike. It’s also to improve the quality of the air we breathe and to help make our increasingly hot summers a little more bearable. These maps show just how much green space our natural areas add to the San Marcos landscape.


​​​​​How much is a tree worth? After reading “The Local Power of Trees,” I completed a survey on iTree to determine the environmental value of just six of the hundreds of trees on our three-quarters of an acre property; I selected two large Shumard oaks, two large live oaks, one mid-sized live oak, and a large cedar elm, all adjacent to the house. The results were astounding. If you’re interested in seeing what else iTree can do, click here.







Written by Susan Hanson, editor, The Loop​​.​

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