How trees clean our water

March 30, 2012 Irvington, Alameda, Sabin & Grant Park
FOT planting in Irvington, Alameda, Sabin & Grant Park on March 30, 2012 (Holly Talkington)

This is the second excerpt on Growth Rings from a series written by Friends of Trees Crew Leader Neva Knott about the value of city trees. In January, we published the opening story of her blog, “Why Do You Plant Trees?

“Urban forestry is a blend of social and scientific necessity. With 80 percent of the US population living in cities, care of city trees as natural resources takes on a much broader context. As explained by the US Forest Service, ‘urban forests are dynamic ecosystems that provide environmental, social, and economic benefits.’

“Along these lines, Portland’s Climate Action Plan for Urban Forestry and Natural Systems broadens the initiatives of Grey to Green and Friends of Trees. It includes safe-guarding against tree loss during development; treating trees as part of the infrastructure of the city; putting in place codes and policies to maximize tree preservation; expansion of private and public urban forestry programs; removal of regulatory obstacles; reduction of the heat island effect caused by development. This, for sure, is a new way of thinking. Fresh, and aligned with the science of climate change as well as the ideas behind livable cities.

“A city’s tree canopy is important for ecosystems services to function. Urban trees can be used as a cost-saving component of a city’s infrastructure—for storm-water retention, clean water and soil stabilization, clean air and carbon sequestration, cooling and energy efficiency, and by providing habitat and travel corridors for wildlife.

“The canopy of leaves of the 50-foot-tall buckeye in my yard catches rain as it falls; a mature tree can capture up to 700 gallons a year. The paperbark maple planted on Saturday, not yet as leafy and large as the buckeye, holds onto water that falls to the ground and uses it for root growth. A tree’s root system holds soil in place. In turn, some of the captured water is stored in the soil to replenish the ground water supply.  As well, much of run-off water in cities contains chemicals like car oil and other debris—that gunk you see in the street drains during a downpour. When that water moves through the soil, some of the debris is filtered out.  With water held in tree fiber and the soil, and with the soil stabilized and working to filter out toxins, significantly less run-off makes it into the city sewage system, to the nearby Willamette River, and out to the sea.

“A healthy urban forest, one composed of the newest to the oldest trees, slows run-off by about 35 percent; in Portland, this amounts to 500 million gallons of storm water a year. Trees also allow the city to spend less building and maintaining sewage systems. Portland saves $58 million dollars—or 40 percent of traditional sewage repair costs—per year because of its street trees. Deciduous, or leafy, trees aren’t doing all the work; evergreens actually help even more with storm water run-off, because they have needles year-round.”

Read more from Neva Knott on her informative blog, Stand of Trees.