Crosscut recently published a piece by Eric Scigliano exalting the importance of urban tree canopy while decrying density as the “mortal enemy of trees.” The piece was novel in its use of environmental values to discourage one of the single greatest tools we have in promoting sustainability: dense, urban living. And while most of this piece will examine what the author got wrong, there’s plenty he got right.

To begin, tree preservation is a worthy goal. Indeed, urban canopy has a range of benefits for cities, including: cleaning the air, cooling the air, sequestering carbon, enhancing livability, promoting mental and emotional health, and absorbing rainwater. Even NASA thinks you should live on a tree-lined street.

Cascadia Flag
Cascadia Flag

We in the Pacific Northwest pride ourselves on our natural greenery — placing the mighty evergreen front and center on the Cascadian flag. On warm summer days, we flock to the mountains to hike among the coniferous giants that cover our hills. “Seattleites,” remarked a friend returning from DC, “look like they’re either in a band you’ve never heard of or fresh off the REI discount rack — ready for a hike at a moments notice.”

So when someone threatens our trees, they threaten our regional identity and way of life.

But in Scigliano’s call to preserve the landscaping of single-family homes, he misses the forest for the trees.

First, Scigliano presents us with a false choice between growth and protecting our urban canopy. He cites findings from American Forests that show Seattle’s tree canopy grew from 10% in 1996 to 28.5% in 2012. Our population grew 23% over the same period. Correlation by no means implies causation, but Seattle has shown that substantial population growth and tree canopy expansion aren’t mutually exclusive.

Indeed, there appears to be little correlation between population density and urban tree canopy. According to one source, Los Angeles has similar population density (8,282/sq mi vs 7,969/sq mi) and just 18% tree canopy. New York City, meanwhile, has more than three times our population density (2,7857/sq mi) but virtually ties us at 24% tree canopy. To be fair, tree canopy is notoriously hard to measure. It can also change quickly over time, as Seattle has shown. But if the home of Manhattan can match our tree canopy, there’s no reason to fear a few new townhomes in our neighborhoods.

Seattle canopy cover from 2013 report. (City of Seattle)
Seattle canopy cover from 2013 report. (City of Seattle)

In fact, there’s much more to fear from capping growth in the city. People are moving to the region whether Seattle makes room for them or not. But if they don’t move here, they’ll be pushed to the suburbs and exurbs, fueling deforestation as sprawl expands into our meadows and foothills, replacing natural beauty with low density residential subdivision developments and strip malls.

Instead, we should want people to move to the city, where their carbon footprint and energy usage will be dramatically lower. One recent study by UC Berkeley suggests that suburbs effectively cancel out the climate benefits of dense, urban areas. Another by the University of Toronto suggests that as populations grow, regional emissions decline. Dense living is efficient living. Residents share amenities and resources. Multi-unit buildings share walls, preventing heat loss. Walkable and bikable neighborhoods encourage people to leave their cars at home.

Maintaining a development pattern primarily of single-family homes is no way to maintain our urban canopy or our environment. With only one-third of urban canopy growing in single-family lots, suggesting otherwise was always a false start. Instead, we should be looking at smart design guidelines to incorporate trees in new development and public spaces, and improve the health of existing stock (especially those in parks and greenbelts). We should also push for a sustainability agenda that’s broader than tree coverage, like rigorous housing standards, robust and reliable public transportation, and recommendations for inclusive, affordable growth.

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12 COMMENTS

  1. Excellent post Ben. Trees are great! Here are a few ideas for increasing Seattle’s green space and tree canopy while preserving and enhancing our ability to welcome more people to the city:

    • Reduce the width of streets to allow wider tree lawns (or wider “parking strips” as they are called here) and so larger trees.
    • Where streets can’t be reduced in width, replace a parking space here and there with a tree planting area.
    • Start a city-wide program to plant and care for new street trees, which, because they will be in the public right-of-way, will rarely be threatened by development.
    • More woonerfs!
    • Now that the bugs are worked out, more projects like SEA Streets. (http://www.solaripedia.com/13/367/seattle_sea_streets_drain_naturally.html)
    • Care for the public trees we have already.
    • As Mike Eliason and others have suggested, revise zoning to allow courtyard housing, which reduces building footprint and increases accessible private green space.

      • But…but…MORE TREE CANOPY!

        (You can see versions of this idea in parking lots all over, where a few spots are sacrificed to provide shade on the large expanses of blacktop. Or where crossing distances are reduced by extending the sidewalk out and eliminating that last parking spot at an intersection.)

    • This is my favorite solution. Most streets in Seattle are unnecessarily wide, and could really benefit from a planting stripe. Ballard springs to mind immediately – think how nice Leary could be become if a lane of traffic was replaced with a line of trees, or if the new 17th Ave Greenway had trees/gardens instead of the striped concrete sections.

      • Like First Avenue through Pioneer Square…. 🙂 Prime candidates would be any of the many streets in Ballard that were former streetcar routes. I do like the idea of widening sidewalks and planting strips, too, especially in commercial areas. More room for street life!

  2. Great response! I thought many of the same things as I read that Crosscut piece.

    I feel the gist of the article reflects some very cloudy thinking on the part of environmentalists (and I am *not* anti-environmentlist!, trust me). Their thinking goes like this: back in the day the Puget Sound area was native virgin forest, how things ought to be. Then, as the area got settled, humans cut down trees and built ugly cities. The cities just keep getting bigger and uglier, so our job is to try to make the city look as much like the original idyllic forest as possible.

    So when they plant trees in the city, it’s to return as much space as possible back into “nature”. There are several problems with this line of thinking:

    1) Human planted trees in a city are NOT natural, even if they are native. There is no stable state in ecology. Trees go through ecological succession (Disturbance -> Fast-growing Doug Firs -> Shade-loving Western Hemlock & Cedar -> Disturbance). Trees should not be thought of in isolation – they interact with other plants & animals in complex ways that will not occur if planted in your yard in Fremont. People like Eric Scigliano usually have an extremely shallow love and understanding of nature. They think trees are just pretty and green.

    2) They don’t understand how good urban environments are made. Street trees should not be planted as “nature bandaids” (ie, pretty green things to cover up ugly buildings). Rather, they are an integral, technical necessity used to define space, to create a sense of place. When done correctly, trees planted in a row on a street create a magnificent ceiling (not “canopy” — a canopy belongs in the woods, a ceiling belongs in a city), provides shade in the summer, allows sunlight in the winter, and protects the sidewalk from the road. However, the last thing environmentalists want to do is to plant more than 2 non-native trees in a row — how unnatural! Well, yes, it is unnatural — we’re trying to build a city, not reproduce the Sierra Nevada in downtown Ballard.

    So the message is: Yes, trees have an essential role in any good city, especially Seattle. However, let’s try to think a little more clearly about what we’re trying to accomplish.

    • I think most environmentalists would agree with this editorial, and not the piece by Scigliano. It doesn’t take much research, or even much thinking to realize that urbanization is good for the environment. Hell, the cities can be complete hellscapes — void of all trees and flowers and still be great for the overall environment (of course they aren’t, as any visit to Manhattan will show). Cities represent a tiny spot on the map — they are, by their nature a concentration of people and thus human impact. The more you encourage urbanization, the more you reduce human impact overall.

      Your second point is a good one. There are very few places in Seattle that are actual old growth (I believe that Schmidtz park in West Seattle that has some). Just about everything has been mowed over at least once. Some of it (like areas of Ravenna and Discovery Park) have largely been left to regrow like an unmanaged clear cut (thus vaguely resembling second growth). But very few yards are managed that way, and simply planting a rhododendron (which to my understanding is not found natively west of the Olympics) or a Douglas Fir (which is quite common in the woods around here) won’t make the yard native. There is benefit to planting native species, though, and that is that they are unlikely to be invasive. But that isn’t an issue with trees. Meanwhile, trying to mimic the natural environment actually takes a lot of work. If you simply let it grow, it will be taken over by invasive species (first you’ll have a yard full or dandelions, then the blackberries or Scotts Broom will move in). I saw them try and mimic a natural environment at the Knickerbocker Floodplain. They essentially scraped off the first couple feet of soil, then started over and planed native plants. But unless you weed it — unless you go out and remove the invasives, it will look nothing like the forests you can find in Wilderness areas (which look like what Seattle would have looked like a few hundred years ago). Discovery Park, one of the most “natural” parks in Seattle, requires a lot of work to prevent the whole thing from looking like a giant blackberry bramble.

      In general, if you ask anyone in the city, they hate most of our native trees (well “hate” is a strong word). They really don’t like it when you plant most native trees, because most native trees are huge! A Douglas fir, Hemlock or any one of the cedars are enormous, and can take out power lines or tear up the sidewalks or street. It just doesn’t make sense to plant trees like that in most cases. On the other hand, planting vine maple can be very nice and I really like Madrona trees, because we have lost a lot of them (and they don’t seem to reproduce as easily and quickly as trees like Douglas Firs). But that is largely an aesthetic choice. By planting trees like that you may be making a tribute to the native landscape, but you are fooling yourself if you think it is just like the native landscape.

  3. Ben–I wish you wouldn’t misrepresent what I wrote. You
    sliced the quote to eliminate an important qualification: I wrote that “density, absent special protections, is the mortal enemy of trees.” My article closed by suggesting some protections and incentives that would allow higher population density while preserving existing structures and trees.

    Your extraction of statistics is even more selective and misleading. You apparently drew your unsourced assertion that “only one-third of urban canopy grow[s] in single-family lots” from my reference to that estimate in the 2007 Urban Forest Management Plan [it actually said 36 percent]. But as I also noted, that plan’s figures, based on an extremely coarse model, have since been superseded and disavowed by the city. I went on to explain that the best data available comes from the much finer-grained 2009 Urban Tree Canopy Analysis, as affirmed in the 2013 Urban Stewardship Plan. It found that 63 percent of Seattle’s canopy grows on single-family lots and their street frontage, 49 percent on the parcels themselves (a distinction I overlooked in the article but have clarified now). Either way, that’s much more than a third. You ignored it.

    The estimate of 10 percent coverage in 1996, which I cited among many historical estimates and you re-reported as fact, appears to have been misstated in my source, an academic review of canopy measurement; according to a review by the city, American Forests calculated Seattle’s coverage at 18 percent in 1996.

    That 10 percent is as wild an outlier as the “40 percent coverage in 1972” often cited by media and tree advocates, which I debunked. I should have discarded or debunked “10 percent” as well. My mistake: I laid out the range of past estimates and trusted readers to sort them out. Instead, you pounced like a climate-change denialist on two contrary, discredited data points to concoct your case.

    Re. climate: Sure, meaningful carbon sequestration depends on much more than city trees. It’s the biggest but far from the only issue. Rural trees won’t catch urban runoff, clean urban air, support urban wildlife, shade urban heat islands, or directly reduce urban air-conditioner use.

    Other tradeoffs are also more complicated. How do the footprints of the solo drivers who work on the Eastside but are too hip to live there compare to those of their colleagues driving from oversized but nearby suburban lots? But hey, if they shop at REI and drive to the mountains too, everything’s cool.

    All that said, there are some great ideas in the comments here and on Crosscut.
    Planted medians aren’t cheap, but they would help not just on arterials like Leary but on lightly used, absurdly oversized residential streets in newer neighborhoods like Seward Park. It would be much better for canopy, density, and neighborhood vitality to replace small houses with stacked flats on the same or similar footprints than with maxed-out McMansions. I wish I’d mentioned that along with other infill incentives.

    BTW, Seward Park also has some old growth left. And Manhattan scarcely affects New York’s average canopy coverage; it’s only 8 percent of the city’s area.

    • Eric—thanks for taking the time to respond.

      You’re right that finding good data on tree canopy is tricky. The data seems to swing wildly from estimate to estimate. We probably both could have done better digging into the data.

      We also agree that trees in urban environments are important.

      But we fundamentally disagree on a couple points. I’d argue that:

      1.) Urban tree canopy isn’t threatened by density

      2.) To the extent it is, it’s no argument against density, as the same environmental ethic that compels us to care about trees much more strongly compels us to urbanize.

      In the end, we can both advocate for a number of changes highlighted here and on your post to improve the health and coverage of our canopy.

    • In order for the carbon sequestration that urban trees provide to have an impact, you have to stop the displacing much of the growth Seattle metro’s suburban fringe. Those residents are likely to have longer commutes and are more likely to use their cars for many more of their trips. Plus at a certain point, that suburban growth comes up against fairly intact forest ecosystems rather than small scale urban imitations. Sometimes it seems like environmentalists bemoaning upzoning and densification in Seattle city limits don’t expect that growth to happen in the suburbs on much more carbon intensive terms. At worst, it seems they want to brag about Seattle’s tree canopy percentage while ignoring the whole region’s impact on climate change.

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