During a news conference seven years ago this week, American Forests (americanforests.org) unveiled one my favorite studies - it showed that urban tree deficits were costing tax payers billions of dollars each year in air and water benefits. At that point, San Antonio, Texas, Charlotte, North Carolina and San Diego, California joined in as the first cities in the nation to take action to reverse the tree loss trend by incorporating tree cover data into their infrastructure database - considering trees on the same level as roads, bridges, and treatment plants.
The connection between tree canopy and stormwater may not be immediately clear. Trees reduce the volume of stormwater runoff by capturing some rain on their leaves and branches, which then evaporates back into the atmosphere. Other water infiltrates into the soil rather than running off the land, which must be managed. For example, San Antonio’s urban forest manages 974 million cubic feet of stormwater, valued at $624 million using a $0.64 per cubic foot value based on local engineering, construction, and land costs (Pape-Dawson Engineers).
The findings from American Forests showed that San Antonio had 27 percent tree canopy, seven percent open space, 64 percent impervious/bare urban land, and one percent water. However, if tree cover were increased from 27 to 35 percent citywide, 2.5 million pounds more air pollutants would be removed each year and stormwater runoff would be reduced by 103 million cubic feet during an average 2-year, 24-hour storm event. This service would be worth an additional $200 million in avoided stormwater facility construction.
"Flooding in San Antonio is an age-old issue," said Carol Haywood, a planner with San Antonio's neighborhood and urban design department. "Most folks think we need more concrete culverts to simply whisk the water away as fast as possible. We will use this green (infrastructure) data to model and demonstrate the ability of trees to perform a similar function without adding new concrete."
For its part, the City of Tulsa began a tree planting program in 2007, after that year's devastating ice storm. For comparison, that storm destroyed or damaged as many trees as Up With Trees had planted in its entire 30 year history. Since that time over 5,000 trees have been replanted, with a goal of 20,000 trees by 2010 through ReGreen Tulsa. The City, through its Tree Advisory Oversight Committee, seeks to increase tree canopy by 3% year over year through 2013. Overall, the American Forests recommend 35% canopy coverage in urban areas.
When municipalities are looking to control long-term stormwater infrastructure costs, it should be clear that canopy cover can be a key element. What if we designed for the 2-year 24-hour event with both concrete and canopy? How much less would taxpayers have to spend? And how much more we would enjoy our new public infrastructure.
