Diving Deeper into Portland’s Tree Canopy

Tree Views usually presents community perspectives, not our own. Here we make an exception by responding to the Sightline Institute article Housing Infill and Tree Infill Go Together in Portland, published on Sept. 14, 2018.

By Trees for Life Oregon, April 2021

Something can be true and at the same time misleading. We believe the article’s author, Michael Anderson, paints too rosy a picture of the Rose City’s tree canopy.

Data does support Anderson’s contention that canopy has increased in Portland in the past decade or so. Are there more trees in the Pearl District now than when it was a district of old industrial warehouses? Almost certainly.

Massive infill and high-rises went hand-in-hand with increased canopy because that area had few trees to begin with.

In fact, many plantable spaces in street right-of-ways had become vacant. One reason is that a few decades ago, the city stopped providing free city-planted trees for street tree spaces. As trees died out in many older neighborhoods, residents who had just paid to remove them were often disinclined to pay to plant new ones. This was due in part to the lack of help the city offered to prune, rake leaves, or repair tree-damaged sidewalks. By the 1990s, when the tree-planting organization Friends of Trees was getting underway, the nonprofit had ample opportunity to increase canopy just by filling vacant spots in the right-of-way. Trees for Life Oregon celebrates the private-public partnership that has led to this positive effect on returning trees to public right-of-ways.

If canopy is defined as a tree growing in a space, there are more trees in right-of-ways today than a few decades ago. What the Sightline Institute article doesn’t raise is the age of those new trees and their size at maturity. For example, until rules tightened up in the 1980s, many large-form trees were planted in right-of-ways now considered too small for their root spread. Now, each time a large tree in a three- or four-foot-wide planting strip comes down because of old age or buckling sidewalks, by law it has to be replaced by a tree that will mature at no more than about 35 feet, overhead power lines or not. So canopy numbers can look unchanged or even appear to be expanding, while the structure of the canopy is shrinking.

After all those new buildings go up, is there adequate space left for long-term healthy growth of a large tree--one maturing at more than 50 feet and living more than 75 years?

These are the trees that give us the greatest climate benefits. It’s not hard to find lots where, once developed, builders added street trees that weren’t there to begin with. They now have to do this by law. But look closely at the photos the author presents as proof that the canopy is doing fine and that we have more of it: almost every tree is planted in cutouts. The lifespan of a tree in a four-foot-square cutout surrounded by concrete is not anywhere near that of a tree in a large yard. Trees planted this way are unlikely to reach their full size and longevity, limiting their health and environmental benefits.

Street trees have more legal protection than private yard trees do. Crucially, private trees are what the Sightline Institute author does not address. The bulk of Portland’s large trees are on private residential lots. It is these large- and medium-form trees that are now at risk of being lost to development. How many have already been lost? We have few numbers because the City doesn’t track trees removed on sites 5,000 square feet or less or trees less than 12 inches in diameter.

Trees removed for development are most often replaced with small or, at best, medium trees, because City codes don’t require that space for large trees be preserved. They make it costlier than it once was for developers to cut down large trees on lots 5,000 square feet or more. But developers can and do choose to simply pay fees to remove large trees rather than to design buildings to include space for decent-sized trees. And trees on smaller lots receive no protection.

We have numerous examples of this trend across Portland, even before new infill laws become effective on August 1, 2021. After that, City code will require that triplexes and fourplexes have only half the outdoor area required for a single-family home or duplex. And that outdoor area can be paved. The little dogwoods or Japanese maples plopped next to new buildings may add up to the same number of trees removed, but they will never provide anywhere near the health and environmental benefits of the big trees we are letting slip away.

More and more Portlanders are living in brand-new buildings built over the stumps of big trees like Douglas-firs that might have lived through the rest of this century or beyond.

Not all of Portland’s residential lots will be bought and their trees and houses torn down and replaced in a single year. But the cumulative effect of a tree or grove lost here and then there will take its toll on the most valuable part of the canopy. Not all trees are equal—the large yard trees that have adequate space to reach their full lifespan can do the most to further Portland’s stated climate and environmental equity agenda. We can’t shade the city adequately with street trees alone. Yard trees are an essential part of Portland’s urban forest.

So while overall canopy figures, at least for now, have moved in the right direction thanks to heroic street-tree planting efforts, Trees for Life Oregon believes the data are masking the unfolding deforestation of big trees in yards across the city and the loss of large-form street trees due to overhead wires and narrow planting strips.

We believe that housing infill and tree infill can go together. But let’s do infill without blithely destroying the legacy of large trees we’ve inherited.

Healthy housing across Portland, no matter residents’ income or geography, requires that we consciously help large trees everywhere reach their full potential in order to foster the physical and emotional well-being of all Portlanders.