The Value of Trees
By Lily Auliff
Trees absorb air pollution, control stormwater runoff, and cool our city – we have known these facts for a long time. But new technology has allowed researchers to quantify these functions. The American Forests Urban Ecosystem Analysis for the Houston Gulf Coast region, better known as the Houston Citygreen project, used satellite imagery, aerial photographs, on-the-ground data collection, and unique computer software to calculate the economic value of the tree canopy in the Houston-Gulf Coast region.
Results of the year-long research, conducted by a coalition of regional government, business, and nonprofit organizations in partnership with the national nonprofit American Forests, indicate that current tree cover saves the region $209 million in air pollution removal costs and $26 million in energy costs annually, and $1.33 billion in one-time stormwater management costs.
“We’re going to try to get this information into the hands of policy makers,” explains Mickey Merritt, Staff Forester at the Texas Forest Service, who helped coordinate the project. “I hope we will effect some kind of change in how the urban forest is viewed. Trees should be seen as part of the city’s infrastructure. Like water mains or sewer systems, they add value to a property.”
The Study
The study was completed in two phases. Researchers first used satellite images that were taken in 1972, 1984, and 1999 to calculate the change in land cover over the entire region, focusing on tree canopy. Then, they selected 25 smaller areas as representative samples. Aerial photographs of these plots provided information about trees, vegetation, impervious surfaces, and other features. On the ground analyses backed up these data. “They went out and looked at each tree, determined its species, and noted where it was in relation to any structures,” explains Merritt.
Researchers entered the data on trees, vegetation, and impervious surfaces, along with soil types, rainfall patterns, and land-use configurations into American Forests’ CITYgreen software, which calculated the benefits for each sample site. The benefits from individual small plots were multiplied by the total land in the region represented by those sites.
Study Results
Canopy loss
Not surprisingly, satellite imagery indicates a net tree loss in the Houston-Gulf Coast region over the last three decades. In 1972, more than one million acres in the region had 50 percent or greater tree cover. That figure dropped by 150,000 acres in 1999. Conversely, area with less than 20 percent tree cover increased by 13 percent, or 200,000 acres, in the same time period.
Pollution relief
Tree canopy lost over the past 27 years would have removed about 15.3 million pounds of sulfur dioxide, carbon monoxide, ozone, and particulate matter each year. This service could have saved the region $38 million per year in associated costs, such as health care expenses.
Stormwater management
During peak storm events, the lost vegetation could have reduced stormwater runoff by 360 million cubic feet. Construction of extra management facilities for this water cost about $237 million.
Reduced energy consumption
The average local household spends $714 on cooling costs annually, according to Reliant Energy. Trees save each home approximately $72 annually (based on a one- to two- story single-family, detached house).
Recommendations
The study recommends that tree cover data be considered in decisions concerning public works, transportation, and development. It also encourages using tree planting as a strategy for air pollution reduction and stormwater management. Local project leaders eventually would like to see tree preservation and planting efforts listed in the federally-mandated plan to reduce ground-level ozone in the region.
