By Lauren Lewis
When you stumble upon a street where the sidewalk trees are so big they form a full canopy over the road, it just feels good. (I’m picturing 24th Street in the Mission as an example, from Mission to Potrero.) Something about the completeness of the shade, or the feeling of outdoor enclosure, is rare and lovely in an urban setting. Because they are big, long-lasting plants, trees provide structure and large-scale greenness in a city, and it’s very noticeable when they’re lacking. Some interesting recent research has tried to identify the more nebulous benefits that humans derive from trees, and city governments, including San Francisco’s, are starting to prioritize the urban forest.
In 1982 the government of Japan began a campaign encouraging “shirin-yoku”, or forest-bathing, to promote public health. Since then, various studies have shown physiological benefits, like lower cortisol level and lower blood pressure, from time spent in a forest as compared to city. To me this is somewhat unsurprising, though, given the noise and stresses of the city as compared to a natural setting. What’s really interesting is work that has tried to pinpoint trees as the health-promoter, compared to other natural elements. One such study found that high school students whose view out the cafeteria windows was filled with trees showed higher academic performance than those whose views were mostly other landscape elements like lawns. Thoughtful landscape design that mimics wild nature can have benefits that go beyond aesthetics.
Looking at the same idea in reverse, i.e. the public health impact of lost tree cover, yields similarly compelling evidence. When a beetle infestation caused 100 million trees to die across the eastern half of the United States a few years ago, a forest service researcher looked at how human mortality in those areas was affected. He found that infested counties had significantly more deaths per year from cardiovascular and respiratory diseases than uninfested counties, controlling for other factors. Even controlling for income, race, and other factors that impact health, the tree mortality provided a significant explanation for human mortality.
San Francisco’s 13.7% tree canopy is lower than most other major American cities, including Chicago, LA and NYC. Some of this is attributable to the fact that San Francisco’s land was never a treed environment; it was dominantly a shrubby, grassy landscape. Especially in the western part of the city, the fog, wind, and soil properties create a challenging environment for trees. According to Allegra Mautner, tree care manager at Friends of the Urban Forest (our city’s awesome urban forest nonprofit), trees that are planted in the western part of the city have only a 50% survival rate, as compared to the norm of 80%.
But now that we’ve got a dense urban environment on this land, trees are crucial for the all the benefits they provide: quality-of-life, human health, habitat, heat-control, carbon sequestration etc. The city government recognizes this, and in 2015 the Board of Supervisors adopted the Urban Forest Plan to begin building a stronger and larger urban forest. In the 2016 election that effort got a boost when SF voters approved a proposition that moves responsibility for street trees from property owners back to the city. A funding challenge still exists to fully implement the switch, but this change has the promise to bring more consistent and better care to our street trees.