Trees: the new sewers

Below are excerpts from an informative story written by Portlander Lisa Ekman, which was published in the Spring 2011 issue of American Forests. You can read the entire story on the American Forests web site.

“When stormwater flows across Portland’s impervious surfaces, it can end up in the Willamette River, which runs through the city’s downtown district.

As the Willamette River winds through downtown Portland, Oregon, it flows past dozens of sewage outfall pipes. On almost any rainy day, you can stand on a riverside walkway and watch as these pipes spew raw sewage into the river.

Over 800 cities across the US, including Portland, have combined sewer systems, which means that they collect and move both sewage and rainwater. During storms, these cities’ treatment plants, and the pipes leading to them, fill up with a combination of sewage and rainwater. That combination often empties into lakes, rivers, and oceans. Some of the measures Portland is taking to solve its problem include investing $1.5 billion in sewage storage, and investing in an older, cheaper, and greener stormwater management solution: trees. The city aims to plant 83,000 trees as part of its five-year, $50 million Grey to Green program. …

83,000 trees

Theoretically, it’s not all that difficult to plant 83,000 trees in five years. A professional can plant more than a thousand trees in one day. The challenge is to find appropriate places for trees in the complex urban grid. Urban foresters contend with overhead and underground utilities, traffic visibility, growing space, and—perhaps the most complicated part of the equation—property owners. Portland has already planted its tree-worthy public spaces, and has the most to gain from tree planting on private property.

To encourage property owners to plant trees, Grey to Green’s canvassers talk to residents about benefits like air quality and increased home value (up to $7,000 for a big, healthy tree). Canvassers also explain the city’s rewards for tree planting: Property owners can claim a ‘Treebate’ for trees they buy, and receive regular discounts on sewer bills. Property owners can get help at every step of the process from the Portland-area nonprofit Friends of Trees, an organization that helps property owners choose, buy, and plant trees. …

How much stormwater can trees manage?

… Portland’s tree-planting program is well underway, but no one is sure how much stormwater the city’s trees can manage. Geoffrey Donovan, a research ecologist for the US Forest Service in Portland, wants to find out. Donovan has studied how urban trees affect crime rates, birth weights, and home values. Later this year he will turn his attention to the science of trees and stormwater.

Other steps of stormwater management include roadside plantings like this one [photo above] by students at the Waldorf School in conjunction with Portland’s nonprofit group Friends of Trees.

‘A significant number of trees could reduce the cost of grey infrastructure,’ Donovan says. ‘The question is how many trees constitute a significant number.’

He explains that trees manage stormwater in three basic ways: Roots take up the water and distribute it to the tree; some water lands on leaves and branches and evaporates there; and roots create gaps in the soil that allow water to seep through.

The new sewer maintenance

… Karps insists that trees are infrastructure, and the city should plant and maintain them accordingly. She is not suggesting that the city take over all tree maintenance, but does think that Portlanders should commit to maintaining green infrastructure at the same level they maintain grey infrastructure. If trees are going to remove stormwater from the system, they need to be kept healthy.

Since no one is sure how significantly trees will solve Portland’s stormwater problem, or even if it is possible to plant all 83,000 planned trees, Karps says she has her own measure of the program’s success: ‘to hit a tree-planting sweet spot—a maximum number of trees along with a maximum number of partners and a maximum number of Portlanders.’”

–Lisa Ekman writes from Portland, Oregon, and teaches writing at Portland State University.