Is Seattle's urban forest really in crisis?

A blogger questions the city's claim that the tree canopy has been dramatically reduced over the past 35 years. The city responds.
Crosscut archive image.

Tree canopy in Seattle. (City of Seattle)

A blogger questions the city's claim that the tree canopy has been dramatically reduced over the past 35 years. The city responds.

What is the real status of Seattle's "urban forest?" Do we have fewer trees than we used to, or are the figures quoted by the city of Seattle bogus – a case of comparing arboreal apples and oranges?

Blogger David Sucher questions some numbers I quoted in my recent story about the sad state of the city's tree canopy. On his City Comforts blog, which focuses on urban design, he said some of the numbers were "dubious" and warned that "we should beware governments (and gullible echo-chamber journalists) offering 'facts.'"

The basis of his criticism is that he finds the contention that Seattle's tree canopy has diminished dramatically in the past 35 years hard to believe:

For anyone who has lived in Seattle for the past 40 years and has been watching the physical environment, it's not credible that "Thirty-five years ago, tree canopy covered 40 percent of the city. Today, that's down to 18 percent."

I agree that the number is shocking, but those are the numbers cited by the city in its 2007 urban forest plan [4.2 MB PDF].

Sucher speculates the deforestation numbers are exaggerated because they come from a 1998 American Forests study that covered the greater Seattle metro area and that deforestation outside Seattle has skewed the numbers. He suggests that the numbers for Seattle proper – within the city limits – would not be as bad.

I've been in touch with the city's senior urban forester, Mark Mead, and asked him for clarification on the numbers. Here's what he's told me so far.

First, the city stands by its estimate of the current canopy in the city which is 18 percent – less than half of what is currently recommended for cities like ours. That number refers specifically to Seattle proper, not suburbs and outlying areas. The estimate is not only based on the 1998 study of the Seattle metro area study and a more detailed study in 2000 using LIDAR – which is a high-tech way of measuring topography using light (lasers) instead of radio waves (like radar). He calls the method "highly accurate." That forms the basis for making estimates and canopy models. Those are augmented by other tree data. For more on the city's tree-counting and canopy calculating methodology below, see the footnote below.

The next question is, what was the canopy like in 1972? Unfortunately the city can't retroactively apply its more detailed current methods, so the earlier data is derived from a 1970s satellite survey of the region.

When I pressed Mead, here's what he said:

Today, we know that about 18% of the city is covered by tree canopy. In comparison, past research found that the greater Seattle area had about 40% just 35 years ago. While the improved accuracy and scale of data between these two estimates makes direct comparisons fuzzy, it is fair to say that the number of trees in Seattle has diminished rapidly. Without more accurate information from the past it is a fair estimate to say we have lost over a million trees worth of canopy in the last 30 years.

So the bottom line is that Mead is confident of the current canopy estimate, which measures Seattle proper, and that he believes tree-loss has been substantial since the 1970s.

But Sucher is correct that the 40 percent to 18 percent canopy comparison is, in effect, an apples to oranges comparison. The methodology has changed with time.

So is the trend in the ballpark or is it outrageously overstated? If you accept the city's current, and supposedly more accurate, estimate that there are about 1.4 million trees in Seattle proper and that we've lost – perhaps conservatively – 1 million in the city since the '70s, that amounts to a 40 percent loss of trees. That is less than the over 50 percent cited for the metro area, but still a huge decline.

Sucher's blog post, titled, "Fewer trees in Seattle now? Or more?" seems to wonder if the urban tree population has actually increased in the last 35 years. Mead says no, we've been losing trees rapidly.

Of course, there is almost always a financial incentive to exaggerate a problem. and city-watchers are often cynical with good reason. The city would like more resources to manage the "urban forest," including tools to undertake a more detailed survey of the city's trees. The importance of this is accelerating due to global warming and the need to clean up Puget Sound. A story in The Seattle Times last week reported the results of a study indicating that stormwater runoff is the number one cause of pollution in the Sound. That's a problem that gets much worse when you cut down urban trees. Call it death by a thousand cuts – or a million droplets.

And Seattle city government, developers, and private citizens all share responsibility. Remember the controversy over tearing down the Maple Leaf neighborhood's old hospital? The real concern was less for the building than the grounds, the so-called Waldo Forest and its beautiful old trees. The building was deemed not to be a landmark. The wrecking ball and the chainsaw were then legally let loose: The historic building wasn't up to snuff, therefore the trees were expendable. Seattle's venerable trees don't get the protections or consideration we sometimes accord buildings even though their environmental contribution is far greater.

Such depredations continue, despite the city's own alarm. In response to my story, Duff Badgley wrote to tell me about a major mixed-use development along the Ship Canal that has applied for permits. It would cut down 70 mature trees, he says, and replace them with, among other things, 1,200 parking spaces.

And Tina Cohen, a certified arborist in Seattle, had this to say:

The main problem with tree preservation is Seattle DPD [Department of Planning and Development] allows too large a footprint on rebuilds, and there's no requirement for green space. The current conditions don't allow for retaining existing trees when lots are short platted or monster houses replace modest homes. Last year Richard Conlin proposed an ordinance limiting the size of single family homes, but it hasn't moved forward. Until the building code changes, I am pessimistic about trees in our town.

Sucher might be right to question whether the city is trying to hype the tree holocaust – perhaps there's a tendency to overstate it due to a feeling of urgency or guilt. But unless you're prepared to call city forester Mead a liar, he and the city's staff have concluded that the problem is big, both citywide and regionwide. My sense is that Seattle is trying to walk its environmental talk. It knows to be credible it has to take action to fix a problem that it has enabled, if not created, by decades of forest-unfriendly policies. Turning that around won't happen unless the city stops undermining its green intentions.

It is fair to wonder which city will win out in the end: the tree-loving city or the boomtown that can't build enough fast enough. Now that the city has quantified the problem, maybe it's time to start measuring the results so the future data will be indisputable.

Footnote: Here is Seattle senior urban forester Mark Mead's more detailed description (from e-mail) of the process of measuring the scope of Seattle's tree canopy and how the city came up with goals for future tree plantings:

The forest canopy of city can be measured in a variety of methods. For the purposes of this plan the canopy was measured utilizing a new technology referred to as LIDAR. LIDAR measures the height and location of objects based upon reflection of a laser. The data utilized was collected at a high level of definition, 5 measurements for each 1 meter square. Height elevations from reflection are estimated to be with in 2 feet of actual heights with the locations within the 1 meter square. This data was utilized to create a highly accurate topographic map of Seattle. A by-product of this data is a highly accurate tree canopy map. Using deductive reasoning and data reduction methods, the data was used to isolated all 1 meter grids with secondary measurements 10 feet or higher above the ground that were not identified structures. This process gave a three dimensional model of the canopy. From this data was extracted the two dimensional measure of area covered by canopy, or canopy cover for the City. Existing GIS data for the Management Units was used to develop values for canopy cover in each Management Unit. From the canopy cover the number of trees per Unit was estimated based upon an estimated canopy width for a typical tree with in each Unit. This individual tree area was divided into the total canopy area for the Unit to derive an estimated number of trees per Unit. In two Units, Right of Way and Developed Parks, existing inventories of the trees in these units was substituted for the derived number of trees in the Unit. Based upon currently available national and regional models the number of trees within each units were multiplied by estimated costs to plant an average tree in the Unit, the cost to maintain the tree within the Unit, and the benefits to be derived from each tree within the Unit. ... A panel of City personnel responsible for the management of trees utilized the existing canopy cover percentages to develop goals for the amount of tree canopy that could be potentially achieved in each Unit. The percentage of increase was not linear and was based upon potentials for plantings in regards to space and jurisdictions, the use of the land in the Unit, perceived public/private interest in plantings, and currently applicable land use regulations. The increase in canopy area was translated into numbers of trees to plant to achieve the goals, with corresponding increases in costs and benefits.  

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About the Authors & Contributors

Knute Berger

Knute Berger

Knute “Mossback” Berger is Crosscut's Editor-at-Large.