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.
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