From the American Forests website:
"Goals for the city’s canopy are actually built into multiple management plans, and the improvements the city is seeing are related to the work of a dedicated government, nonprofits and community groups. Meanwhile, the city is using green infrastructure as a cost-effective alternative to gray infrastructure and is protecting its trees through public and private ordinances and a diversification strategy. Portland estimates that its street trees alone have added more than $13 million in property resale value, and its entire urban forest provides more than $38 million in environmental benefits."
American Forests also provides a fact sheet with Portland statistics and their selection criteria.
Trees intercept rain and reduce stormwater runoff, and they're incorporated into the strategies set forth in the 2005 Portland Watershed Management Plan. Together with Friends of Trees, Verde, and many other partners, Environmental Services has supported the planting of over 25,000 new street and yard trees since 2008. (Since Friends of Trees started in 1989 they've planted over 450,000!)