Where have all the trees gone?

I've been following-up on the issue of tree cover in Seattle. The assertion (noted yesterday on this blog) that tree cover in Seattle has diminished from 40% in 1972 to 18% today intrigues me. To me it is intuitively incorrect i.e. it does not fit with my own personal memory nor some basic facts such as the fact that Seattle is about 50% right-of-way and has been for many many decades. So how Seattle was 40% covered by trees in 1972 is hard to grasp.

So I emailed the reporter who wrote the P-I story (Lisa Stiffler) to find the source. She directed me to an earlier story she had written which made the same assertion and again without sourcing it adequately. In fact the story from last September, 2006 uses 1996 as the latest reference date while the story from yesterday pegs the number to "today." Yes the numbersd appear to be virtually the same tjough Seattle has (one would think) has undergone tremendous development in the past 10 years.

Here are the images to which the reporter directed me and which supposedly justify the statement that

Since the early 1970s, Seattle has lost more than half of its tree canopy as more businesses and people have moved into the city and smaller homes have given way to apartments and megahouses.

The problem with the satellite photos (if that is what they are — which seems questionable based on the uniformity of the water color) is that

1. The images show far more than Seattle proper. In fact it shows an area from Tacoma to nearly Everett and from Issaquah to west of Bremertton. Yet the story is explicitly about the city of Seattle.

2. No source is provided for the images except "City of Seattle" and I don't believe that the City has its own satellite nor the expertise to analyze such aerial images with authority.

3. Most importantly it is impossible to tell from these images what is happening on the ground especially when the assertion is so precise: "40% to 18%." The images purport to show "evidence" — "pictures don't lie" — but as presented these two images do nothing.

It's important to determine the truth or falsity of the assertion because public policy is being based upon it.

But hey! "What importance are facts," to paraphrase GW Bush, "when the intentions are good?"

•••

The City of Seattle's own web site repeats the fact and again with no apparent source:

...in 1972 trees covered about 40% of the city, where now tree cover is only about 18%.

•••

Btw, let me make it clear that I neither believe nor disbelieve inthis fact. Yes it doesn't comport with my own perception of how Seattle has changed over the past 40 years. But I don't deny the possibility; I would just like to see some clear documentation.

 

Comments

[Gomez]

Again, the assertion also completely discounts simple growth. Much of that tree cover was on undeveloped land. When you build an office complex, or a shopping center, or a house, a condo building or an apartment building, guess what? Any trees in the area usually disappear to make space for it.

I mean, that's simple logic, but the article's trying to paint it as systematic tree destruction for its own sake.

[David Sucher]

But that is p[art of my point, Gomez, and it makes the numbers even more absurd.
Seattle did not have much undeveloped land in 1972.
Seattle has a land area of about 84 square miles.
Does anyone think that Seattle had 33.6 square miles of "tree cover" in 1972?
And now it is about 15 square miles?
And the difference is because of new development or (even partly) because people are cutting trees to get better views?
It's just so implausible if one follows the senses at all.

[Jesse]

In NYC's comparative environmental assessment it listed seattle's canopy cover as something like 26%. This was a report prepared last autumn by McKinsey and environmental groups.

[M1EK]

I had the same visceral reaction as you, David; it's got to be "metropolitan area" rather than "city of". Suburban sprawl is the real tree-killer.

[JRoth]

No real opinions on this matter either way, but I do want to point out that your one point of comparison - ~50% ROW - is meaningless, since, of course, a street [ROW] lined with mature canopy trees will be 100% tree cover. It's not as if the footprint of a tree is its defining characteristic.

Following on that basic fact is another: much of the loss of tree cover in urban areas is due to the death of mature street trees which are often not replaced, or are replaced with inadequate saplings (a 4" caliper tree is something like twice as likely to survive to maturity as a 3" caliper tree, but also costs more - although not 2X). To some extent this is simply an inevitability once a city reaches a certain age: street trees have a useful life of between 50 and 100 years, and so, starting 50 years after city-wide tree planting campaigns, you start to lose, rather than gain, tree cover. What is not inevitable is that these trees be replaced quickly and appropriately, so that you maintain tree cover.

One final note: redevelopment of urban areas will kill trees as well. I don't mean 60s-style redevelopment, but rather the kind of pop-up gentrification of bungalow neighborhoods that I know is typical in Denver and believe to be common in Seattle. You have a 1920s bungalow with a couple mature trees in the yard; new owner wants more space, and so in comes the heavy equipment. Whether they cut the trees or merely compact the soil around them and pave within the drip line, the effect is the same.

[M1EK]

JRoth,

But equally likely in a healthy urban area is the tearing up of parking lots to put new buildings (often with at least a little bit of treespace). Certainly central Austin has more trees now than it did when I moved here in 1996 - and it's precisely because of urban redevelopment (surface parking lots turned into lofts which have street trees).