Protecting the Tree Canopy We Take for Granted Requires Action

By Kyna Rubin
As seen in the Irvington Winter Newsletter

My heart plunges. It’s fall 2015. I’m sitting in the audience at the city’s Tree Inventory Summit, looking at a series of maps showing Irvington’s street trees. The Urban Forestry office, under Portland Parks & Recreation, is leading the gathering. City foresters have created these visuals based on data recently collected by an army of volunteer residents trained to identify, measure, and chart our trees. The Irvington survey is part of a citywide tree census that, by fall 2016, will have engaged 1,300 volunteers and staff in surveying nearly 220,000 street trees.

Why worry? Isn’t Irvington one of Portland’s leafiest neighborhoods? Indeed, with 5,601 street trees, it is. But the grids before me are tracking our lavish tree canopy’s vulnerability to pests. One minute we’re seeing a map of fulsome green dots, each marking an existing tree. Next we view what tree loss would look like if Dutch elm disease, bronze birch borer, and emerald ash borer were to strike. Not a pretty sight, but plenty of trees still standing. Actually, bronze birch borer has already killed birches in other parts of Portland, and the city has lost about a third of its elms to Dutch elm disease, including at least one in Irvington, according to Jim Gersbach of Urban Forestry.

The coup de grâce is the final image. That’s showing what would be left of the canopy if the Asian longhorned beetle were also to hit town. The verdant ambiance that makes Irvington so special would be history: More than half our trees could be wiped out. The Asian longhorned beetle targets not only maples, which would be bad enough, as they make up one-third of Irvington street trees. This predator also kills birch, elm, willow, poplar, golden rain, London planetree, horsechestnut, and katsura. We’ve got an awful lot of these trees in Irvington. We’ve also got nearby ports delivering shipments that contain wood packing material from Asian countries including China—the likely source of the beetle’s U.S. debut, in New York, in 1996. 

Some might ask why our trees matter so much. They shade us, lower our energy costs, filter our water, clean our air, increase our property values, and provide wildlife habitat. They promote health and well-being. They are also monetarily valuable. City foresters estimate that every year Irvington’s street trees provide about $1.3 million in environmental services and aesthetic benefits.

The bleak scenario I first learned about in 2015 doesn’t have to play out if we prepare. To do that, we need to understand the range of current and potential weaknesses of our tree canopy. These were brought to light by the tree survey—the city’s first since 1976 and the most comprehensive to date. (To read the full Irvington Street Tree Inventory Report, November 2015, click on Data, Reports, and Maps at https://www.portlandoregon.gov/parks/53181). 

According to that report, here’s the gist of our tree canopy vulnerabilities, listed by what can be done to address them:

  • Expand tree diversity. Planting a wide variety of tree types increases a neighborhood’s resilience to pests, pathogens, and climate change. Irvington does not meet Portland’s 5-10-20 standard for ensuring urban tree diversity. That rule refers to planting no more than 5 percent, 10 percent, and 20 percent, respectively, of any one species, genus, and family. More than 54 percent of Irvington trees fall into only two families—Sapindaceae (e.g., maple, horsechestnut, golden rain) and Rosaceae (e.g., cherry, pear, plum, apple, hawthorn, mountain-ash). Action needed: Increase diversity at the genus and family levels.
  • Fill our available but empty planting spaces. Irvington’s tree stocking level is at 79 percent—not bad compared to the city’s 60 percent. But we’ve got 1,401 empty spots that can accommodate trees. Action needed: Plant trees in empty planting sites, focusing first on placing large trees in large, empty spots, because big trees are key to getting the most canopy from the trees we plant.
  • Reduce underplanting in large sites. In Irvington, 73 percent of all street trees planted in large sites are undersized for their site. Together with Mt. Tabor, we are the city’s “worst offenders” in planting small trees in large sites, according to Urban Forestry’s Jeff Ramsey. This creates missed opportunities for reaping the multiple benefits of large-form trees. Action needed: Replacing all undersized trees in large sites with the right-size tree would add more than 100 acres of tree canopy. Doing this, together with planting trees in all available spaces, would increase Irvington canopy cover by 89 percent.
  • Plant more large-form trees including more conifers. Large-form trees such as oak and Douglas fir make up only 21 percent of Irvington street trees. These trees—growing over 50 feet high—cost the same as small-form trees to maintain, but live four times longer and provide many times the benefits of small ornamentals like the popular dogwood and snowbell. For instance, every year large-form trees remove 60 to 70 times more air pollution than small trees do. Annually a deciduous oak tree produces $505 in environmental and property value benefits, while a cherry tree yields $116. Planting more conifers would also help balance our current mix of deciduous and evergreen trees: broadleaf deciduous trees, which shed their leaves in winter, make up a whopping 95 percent of Irvington street trees. And in the words of tree expert Jim Gersbach, “When the leaves drop, the benefits stop.” Action needed: Plant more large-form trees, including evergreen conifers, in our estimated 470 large planting strips (identified in appendix J of the Irvington Street Tree Inventory Report).
  • Prevent canopy erosion by planting young trees to replace old ones. Our canopy lacks enough young trees to adequately replace aging trees. Action needed: When trees are dead or dying, quickly replace them with new ones.
  • Better maintain young trees. Proper watering and pruning helps increase the likelihood that young trees will reach their full lifespan. Money and time are wasted by planting trees without follow-up maintenance. Action needed: The 27 percent of Irvington trees with a diameter of six inches or less need monitoring: Pruning in the first ten years is critical to preventing breakage from storms and other factors. Pruning needs to be done correctly. The city offers pruning workshops to neighborhoods whose volunteer tree stewards or tree teams/committees request them. 
  • Monitor and replace our “poor condition” dominant trees. The good news (which you can surely use by now!) is that more than 94 percent of Irvington street trees are in good or fair condition. Of our most common trees, deciduous oak, linden, and sweetgum are the healthiest. The bad news is that a large portion (28 percent) of our overrepresented trees—such as cherry, hawthorn, and horsechestnut—are in poor and declining condition. We have 297 trees in poor condition and 28 that are dead. Action needed: Remove and replace these trees, many of which are in the two families that are overrepresented in Irvington, allowing us to improve our tree diversity. Monitor all trees rated as “poor” for safety risks and replacement options.

Taking these actions will require planning, education, and money. Urban Forestry staff can guide and assist neighborhoods that create tree teams to develop a plan to protect canopy, but the city budget for helping out is limited. In Portland, homeowners are financially responsible for maintaining and replacing the street trees on our properties. Buying new trees at low cost isn’t the biggest challenge. Residents can purchase new, city-approved street trees for only $35 by going through the nonprofit Friends of Trees, whose work the city subsidizes. However, homeowners are expected to pay to remove dead and dying trees before planting replacements, and this can be costly. According to the Irvington tree report, 27 percent of Irvington households are low-income.

Money aside, education is crucial. Street trees cannot be removed or replaced without homeowners’ permission. Implementing a tree plan will require educating neighbors about the community-wide benefits of a healthy tree canopy. 

Some neighborhoods have already formed tree committees, developed tree plans, and hit the streets. In Albina, for instance, tree team members knocked on the doors of residents living in distressed properties, securing permission to replace withering trees cost-free, through a grant the team secured after receiving seed money from the city. As a result, says Angie DiSalvo, Portland’s Urban Forestry outreach and science supervisor, dead cherry trees planted in the 1960s will be replaced with longer-lived species. The tree team then applied for a grant from a different organization, which awarded money to continue meeting Albina’s tree plan goals. 

Irvington is graced with some of the best tree canopy in the city, but with our trees comes the need to steward them. In Portland, that stewardship is a partnership between Urban Forestry and neighborhood groups. The 2015 tree inventory provides us with enough data to begin taking informed action. 

Toward that end, the Irvington Community Association has recently created a Tree Committee. For more information, see https://www.irvingtonpdx.com/. If you are interested in joining, please contact me at krubin317@gmail.com

Kyna Rubin is the Friends of Trees Irvington Neighborhood Volunteer Coordinator.

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