Since the end of the last recession, Seattle has consistently ranked among the fastest growing major US cities. Is all that growth leaving the Emerald City less emerald?

Not really.

Seattle’s best new data on the change in tree canopy over time does show a 6 percent decline between 2007 and 2015. Here’s the catch, though: most of the confirmed tree loss happened on land reserved for detached houses, the single-family zones that cover over half the city but where population has barely budged for decades.

Meanwhile, the same study found no statistically significant change in tree canopy where the growth actually has been happening: the land zoned for commercial buildings and multifamily housing that absorbed the vast majority of Seattle’s new apartments, offices, and stores.

From 2007 to 2015, all that construction helped make room for 76,000 additional residents and 65,000 new jobs—like adding about half of the neighboring city of Bellevue. It’s a remarkable success story: Seattle increased its stock of homes by 14 percent, confined almost completely to the 18 percent of city land where multifamily housing is allowed—with no measurable impact on trees!

These latest tree findings corroborate a study of Seattle’s commercial centers from 1993 to 2014 that detected tree canopy increases in six of the ten centers studied, and no changes in the remaining four. Another study estimated that Seattle’s citywide tree canopy rose by 2 percent between 2002 and 2007, a period during which the city also grew, albeit not as rapidly as in the current boom.

In 2007, Seattle adopted a goal of 30 percent tree canopy cover by 2037. The city’s most rigorous analysis to date pegged it at 28 percent in 2016. An alternate measurement method showed 33 percent in 2007 and 31 percent in 2015. In 2014, University of Washington researchers applied three different methods that yielded 26 or 30 percent in 2009, and 29 percent in 2012. The varied results highlight the difficulty of measurement, but together these assessments indicate Seattle is already likely close to hitting its 30 percent goal.

All told, Seattle’s tree studies demonstrate that, contrary to what the casual observer might assume, cities can build up—a lot!—and still keep lots of trees around. And that’s great news, because trees bring immense value to city dwellers in numerous ways.

The best available evidence indicates no need for drastic policy measures that could risk thwarting homebuilding to save trees. Even if the data did show some tree loss caused by the construction of apartment buildings, though, Seattle would be ill-advised to reflexively prioritize trees over new homes in a city suffering from a housing shortage that’s been inflating rents and prices ever higher.

Tree preservation rules that would sacrifice new urban homes—that is, housing that can accommodate a lot of people on a small amount of land—becomes even more indefensible when you factor in the resulting shift of development pressure toward places where low-density housing construction obliterates far more trees.

In short, Seattle doesn’t have anything like a tree crisis, and concern over trees is no excuse to stop welcoming more new neighbors to the city. The place growing cities can make room for more trees is the publicly-owned right-of-way, where Seattle’s 500,000 curbside parking spaces alone cover land that could hold perhaps one million trees and boost the city’s tree population by two thirds. Cities throughout Cascadia and beyond can opt to take back underutilized pavement from cars instead of further squeezing much-need housing options. Read on for the longer story.

(Click on the photo below to see a time lapse of the three images.)

We know that urban trees are valuable yet we haven’t been very good at monitoring them

Much has been written about the wide ranging and enormous benefits provided by urban trees: they sequester carbon, absorb stormwater, improve air quality, mitigate the heat island effect, calm car traffic, and improve mental health; plus they’re just plain beautiful. Given the well-recognized high value of urban trees, though, there’s a surprising lack of reliable data to inform comparisons between cities and trends over time.

To help fill that void, researchers at MIT recently developed a new metric derived from Google street view images called the Green View Index (GVI) that quantifies how much tree cover a person at street level experiences. Of 27 international cities Seattle’s GVI came in at 20 percent, ranking in the middle of the pack. For comparison, Vancouver, BC, was 26 percent; Sacramento was 24 percent, Boston was 18 percent; Los Angeles was 15 percent; and Paris was 9 percent.

The most common tree-monitoring yardstick is canopy cover—that is, the portion of the ground that is covered by trees’ branches and leaves when looking down from above. Canopy is usually measured by one of two methods: aerial LiDAR (light detection and ranging) that creates a 3D map of trees; and manual observation of satellite aerial photographs. The two methods tend to deliver differing results.

A comparison of Cascadia’s three major cities—Vancouver, BC, Portland, and Seattle—illustrates the inconsistency. Based on LiDAR, Vancouver’s tree canopy cover was 18 percent in 2013, and Seattle’s was 28 percent in 2016. (Remember, on the Green View Index, Vancouver scored higher than Seattle.) Based on aerial photographs, meanwhile, 2015 coverage for both Portland and Seattle was 31 percent.

Portland’s data indicates canopy rose from 27 percent in 2000 to hit that 31 percent in 2015. In contrast, Vancouver’s LIDAR data showed a decline of 23 to 18 percent from 1995 to 2013—that’s a full one fifth loss of canopy. Based on aerial photos, Seattle measured a smaller decline of 33 to 31 percent from 2007 to 2015, and an earlier study found much lower coverage but with a slight increase over time from 22.5 percent in 2002 to 22.9 percent in 2007.

That these three Cascadian cities—with similar age, climate, environmental ethic, and zoning that reserves about half of their land for detached houses—would report such divergent canopy trends underscores that it’s actually a hard thing to measure reliably, and that they need better methods.

Busting Seattle’s Myth of the Green Seventies

What’s worse, bad data can take on a life of its own. Seattle’s 2007 Urban Forest Management Plan states that “today, about 18 percent of the city is covered by tree canopy as compared with 40 percent just 35 years ago.” Turns out that statement is bunk. Apparently, the 40 percent data point for 1972 was based on a misinterpretation of a 1998 study that was assessing the region, not just Seattle proper. And the 18 percent measured for 2002 was a blatant outlier on the low end, also unreliable.

It’s hard to imagine how a claim that the city lost over half of its tree canopy since the 1970s wouldn’t have failed the laugh test with anyone who had been around during those decades. In the 1970s, tree canopy in much of Seattle was still recovering from historic clear cuts and development, as shown in the accompanying photos, and the city’s tree planting efforts had barely begun. Yet somehow Seattle proceeded to lose something like a million and a half more trees? Furthermore, as the city acknowledges, “natural growth of mature trees and plantings tend to offset canopy loss.” Blaming growth doesn’t hold up either: Seattle lost population during the 1970s and didn’t surpass its late 1960s peak until the mid-1990s.

City authorities have never issued a correction, and so the myth of tree decimation lives on, repeated as recently as last year in Seattle Magazine: “In the 1970s, Seattle was mantled with trees, with about 40 percent canopy cover… but as the number of city inhabitants has increased, we’ve shed at least a quarter of that protective green veil.” Nope.

What the evidence shows: Seattle’s rapid growth has not caused measurable tree loss

The best available evidence busts the myth. The table below presents Seattle’s LiDAR-based canopy cover measurements for 2016, broken out by land use category (“management unit”). Overall, the city’s tree cover is just below the goal of 30 percent by 2037, and most categories are near or exceed their targets. Note that single-family zones, according to this method of measurement, cover 56 percent of the city and provide nearly two thirds of the total canopy cover.

Because Seattle has no comparable LiDAR data from prior years, analysts relied on sampled, manual observation of aerial photography to assess canopy change over time in 2007, 2010, and 2015, as summarized in the chart below. Compared to LiDAR, the aerial photography results are higher across the board. (See this University of Washington study for a review of previous Seattle tree canopy assessments.)

City-wide (magenta bars), the data show a statistically significant decline from 33 to 31 percent between 2007 and 2015, and that change is mirrored in a statistically significant drop from 39 to 36 percent in single family zones (yellow). The only other statistically significant change occurred in institutional areas, falling from 28 to 25 percent, but that land area constitutes only 2 percent of the city.

The 3 percentage point decline in single-family canopy is equivalent to a loss of 8 percent. Assuming for approximation that in 2007 single-family accounted for 63 percent of the city’s canopy cover (as it did in 2016), the 8 percent loss in single-family caused a 5 percent loss city-wide. So the single-family decline accounts for about four fifths of the observed total city-wide loss of 6 percent.

What is happening in Seattle’s single-family zones to cause the tree canopy loss? The city report doesn’t venture to guess. As already noted, population in Seattle’s single family zones has been mostly locked in amber, even during the current high-growth decade. One likely culprit is teardowns replaced by McMansions that take out trees because they cover more lot area, but that’s a small fraction of the city’s total number of houses. Some homeowners cut trees for views or safety reasons, but that also doesn’t seem prevalent enough to be a main cause. Even though the measured decline in Seattle’s single-family zones was statistically significant, is it possibly just bad data? Indeed, the analysts who conducted the study caution that:

“A number of factors could cause differences in canopy estimates including limitations of using a sample‐based approach and that historical imagery from Google Earth is not collected at the same time of day, causing shifts of tree canopy location.”

Where the growth happened and the tree loss didn’t

Nearly all of Seattle’s population and job growth occurred in the multifamily, commercial/mixed-use, and downtown areas where the data show no statistically significant changes in canopy between 2007 and 2015 (see chart above). There is a general trend of slight loss in the measured values, but again, keep in mind the above-quoted caution about error-inducing anomalies in the aerial images.

To restate: from 2007 to 2015 Seattle fit 76,000 new residents, 65,000 new jobs, and 14 percent more housing stock into about one fifth of its land, without causing a measurable decline of tree canopy.

This lack of tree loss corroborates a 2014 study of Seattle’s “urban villages”—designated commercial centers and transit hubs that are zoned for higher capacity to absorb housing and job growth. The study compared 1993 to 2014 and detected tree canopy increases in six of the ten urban villages studied, and no statistically significant changes in the remaining four. During those two decades Seattle’s 32 urban villages captured three quarters of the city’s new housing.

Likewise, Portland attributes its measured canopy gain of 12 percent over 15 years to areas of the city that actually grew, writing that, “residential, commercial, and industrial zones are more likely to undergo development changes and are likely to have more opportunities for planting and growing trees.” It makes sense when you consider that new construction in large, built out cities usually replaces spent buildings or parking lots that have few if any trees to begin with.

Testifying to the success of Seattle’s street tree efforts, city-owned rights-of-way (streets and parking strips, mostly, plus sidewalks that are not on private land) provide an impressive 22 percent of the city’s tree cover (see table above). Rights of way cover a whopping 27 percent of the city, but trees are planted on only a sliver of that area, usually just a narrow strip between sidewalk and curb. The observed loss of canopy in rights of way from 2010 to 2015 (dark blue bars in the chart above) seems particularly suspect: developers, homeowners, and multiple city programs are continually planting new street trees in the rights of way, while the existing trees are growing.

Of course, canopy coverage alone doesn’t tell the whole story of the health or value of a city’s trees.

Conifers, for example, are more effective than deciduous trees at capturing stormwater runoff. Seattle’s 2007 Urban Forest Management Plan notes that “forested parklands have too few conifers, too many deciduous trees, and too many non-native invasive plants when compared with native ecosystems.”

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Street trees may not thrive in hostile, space limited locations. Furthermore, Seattle’s trees are inequitably distributed, with less canopy in areas where “more of the population tends to be people of color and have lower incomes.” Seattle has room to improve in specific areas, but canopy coverage is still the key metric for monitoring trees as the city grows.

The tension between tree preservation rules and housing affordability

Overall, the tree canopy evidence indicates that Seattle has been doing a remarkable job of balancing growth and trees. Credit is due to numerous city tree programs and protection regulations, as well as non-profits and dedicated individuals. Most of these efforts have been focused on street trees, which are currently being inventoried by Department of Transportation analysts who expect the total count to hit 200,000 or so. Street trees are especially beneficial to the experience of pedestrians, as gauged by the MIT Green View Index, noted above.

Despite the evidence, however, the myth is strong: the average person-on-the-street likely assumes that rampant development is stripping Seattle of its trees. Concern over loss of trees is one of the most common reasons people give for opposing homebuilding projects or rule changes to allow more homes.

The risk is that political pressure will beget overzealous tree protection rules that make it harder or more costly to build new homes. And that’s the opposite of what’s needed in a city where a shortage of housing has driven rents skyward; where it’s becoming harder and harder for people with low incomes to find homes they can afford. When tree preservation sacrifices new homes, those at the bottom struggling to pay their rent are the ones who bear the burden.

A good compromise policy would offer an incentive to homebuilders to offset the cost of saving trees. For example, in 2017, Seattle adopted new design review rules that grant an allowance for a larger or taller building in exchange for retention of exceptional trees. In contrast, Seattle’s proposed Residential Small Lot zoning would add tree planting requirements with no balancing incentive. Such rules may increase the tree count but they will also make homebuilding—and therefore homes—more expensive.

Because single-family areas are where most of the city’s canopy is located, and also where it appears to be declining most, Seattle City Council is currently considering new tree preservation rules for single-family zones. The proposed regulations would require homeowners to replace trees they remove to maintain canopy coverage or pay a fee that would then fund planting compensational trees elsewhere. The proposal is fair in that it asks property owners who inefficiently use land to pitch in for trees. But the rules could backfire on Seattle’s affordable housing goals in the case of trees that need to be taken out to make room for a new backyard cottage. Tree removal fees could be the financial straw the breaks the camel’s back, dissuading an owner from building a new home.

The place to add trees is in the publicly-owned right-of-way

In any growing city, conflict between adding homes and preserving trees is inevitable. Seattle’s next assessment may show definitively that canopy is declining. The city’s housing shortage isn’t going away any time soon, and one of the best ways policymakers could help ameliorate that shortage is by relaxing single-family zoning to allow more homes. But that, of course, threatens the two thirds of Seattle’s tree canopy in single-family zones.

From the regional perspective, higher density housing is always a win for trees. Compared to a typical new apartment building in a city center, low-density housing on the metropolitan fringe consumes far more land per home, and that invariably means far more trees disappear—trees that may also have been part of larger functioning ecosystem. Limiting new homes in the city core to preserve trees pushes homebuilding to outlying areas, accelerating the eradication of not only trees, but forests.

But that doesn’t make it any less important to maximize trees in urban areas. How can cities welcome both new neighbors and trees? The most promising solution lies in the publicly owned right-of-way—the land set aside mostly for pavement. The opportunity is enormous: Rights of way cover at least a quarter of a typical city’s land. The small fraction of Seattle’s right of way devoted to trees currently provides over a fifth of the city’s canopy coverage.

As described in this Sightline series, Cascadia’s three major cities are already making progress on greening their public roadways. To achieve their urban forestry goals they can step up these efforts with a focus on adding trees.

Consider: in 2007, Seattle had an estimated 1.4 million trees. The city has roughly 500,000 curb-side parking spaces . Assuming two trees fit on a space, that’s room for 1 million trees. About half of those spaces are in single-family zones.

Similarly, Seattle has about 15,000 street intersections, two thirds of which are in single-family zones. Installing curb bulb-outs one parking space long on every single-family corner (eight spaces per intersection) would make room for 160,000 trees. This reconfiguration would also improve pedestrian safety by reducing the street crossing width. And it would reinforce the revolution that’s brewing in transportation, as bikes, transit, walking, autonomous electric robo-taxis, more-compact neighborhoods, and even electric scooters make private car ownership less important or valuable.

Of course no city will ever convert anything close to all of its on-street parking spaces, or plant 16 trees at every one of its neighborhood intersections, but the upper limit illustrates the huge potential. In principle, Seattle could boost its current tree count by two thirds just by planting in its curb parking spaces. The implication is that the real tradeoff cities face is not between trees and housing for people but between trees and free storage for private cars.

In addition to parking, most North American cities originally laid out their roads with the car in mind, and so they tend to have lots of pavement flab that could be reclaimed for trees, though it’s difficult to estimate that area. Seattle’s Pavement to Parks Program has begun picking off pieces of excess roadway to create small paved parks. Spaces like these throughout the city could instead hold trees. To take the idea further, cities can consider taking back entire roads in select locations to create car-free mini-parks filled with trees.

Cities can welcome new residents and grow greener at the same time

Whenever any growing city proposes loosening rules to allow more homes, opponents invariably raise the issue of tree loss. But that’s a false choice. Seattle’s latest data on tree canopy cover demonstrate that cities can rapidly densify without sacrificing trees. And even if the construction of new homes reaches the intensity where it causes tree loss, it doesn’t justify draconian tree protection rules that could thwart homebuilding and exacerbate the shortage of housing that’s driving Seattle rents more and more out of reach of those on the low end of the economic ladder.

Fortunately, Seattle and other North American cities facing the same affordability challenge have a big escape hatch for accommodating trees: the publicly owned right-of-way—the quarter of urban land typically devoted mostly to pavement, mostly for motor vehicles. By repurposing underutilized land devoted to cars in the right-of-way to land for trees, cities can welcome more people and grow greener at the same time.