The city of Seattle estimates there are almost 1.4 million of them in town, but, as with every census, it depends on how you count. Some of them are old friends – among them the oldest living residents of the city. Some are new immigrants who are thriving in a changing climate. Others are seen as inconveniences, and many are being crowded out.

I'm talking about trees.

On the surface, Seattle is the Emerald City, a place that is green both politically and in terms of the dominant color. We have century-old tree-lined boulevards. Old-growth pockets of fir and cedar survive in some of our parks. There's a tree in Seward Park that was alive when the Pilgrims landed at Plymouth Rock.

Our hottest neighborhood disputes often involve trees. In 2003, there was the case of a federal judge who improved his view by cutting 120 native maples and cherry trees in Colman Park. He was caught and slapped with a $500,000 fine. Public outcry virtually ran him out of town (he moved). Yes, we're tree huggers, but only to a point. If the judge should be ashamed of what he did, then so should we all. In the past 35 years, Seattle has lost 1.7 million trees, more than half the number standing in 1972. Most of that loss has been on private property.

First, a little historical perspective. Seattle was built by tree cutters. Our first big commercial business was a sawmill. The old-growth forest where Seattle now stands was clear-cut to make way for a modern city. We began with 53,000 acres of old growth. We have 200 acres left.

Seattle's founders were not shy about reshaping the natural environment to suit their needs. As author Matthew Klingle points out in his fascinating new book, Emerald City: An Environmental History of Seattle (Yale University Press, $30), we washed away mountains, filled in tidelands, built canals, changed the courses of rivers, lowered Lake Washington and dispatched the forest with great purpose and little conscience. Greed was the main driver.

When the wilderness was tamed, we set about beautifying the town. The cleared land was something of a wreck–steep hillsides were literally slashed and burned. The next phase of improvements was to build parks, lay out boulevards, and plant gardens. The legacy is what you see around you.

Make no mistake. Despite its frowzy greenbelts and old-growth survivors, Seattle is a handcrafted environment. In his invaluable book Trees of Seattle, Arthur Lee Jacobson reminds us that only 33 tree species "were, are or may be" native to Seattle. How many different species live here today? About 3,000.

Unfortunately, our civic garden is in real trouble. This year, the city of Seattle issued a comprehensive plan [4.2 MB PDF] for managing what it calls our "urban forest." But it spells out the problem. Thirty-five years ago, tree canopy covered 40 percent of the city. Today, that's down to 18 percent. That's less than Baltimore. That's less than half of what the American Forests Association recommends for cities like ours.

That hurts, because trees are not only beautiful, they help clean the air of pollutants and help with drainage and storm-water runoff. They combat global warming. They are a source of clean air and clean water. Urban forests are an indicator of environmental and civic health.

Who is responsible for the un-greening of Seattle? We are. "The greatest loss of Seattle's tree canopy has been from private property," says the city. We cut trees because they block views or the sun or because they drop leaves in our yards. Invasive species such as ivy suffocate the reproduction of the old forests. Growth in the name of density is eating up yards, vacant lots, and green spaces. As we upsize our homes, more trees come down. Fifty-six percent of the land in the city is devoted to private single-family homes. The bottom line: As single-family homes go, so go our trees and our urban health.

The city is making efforts to reverse the trend. Mayor Greg Nickels wants to plant nearly 650,000 new trees in the next 30 years. Volunteer groups are working to save parks and wooded land from invasive species. The city is reviewing building regulations to help preserve existing trees.

But greening parks and public streets goes only so far. Ultimately, the success or failure of these efforts comes down to the values of private citizens making private choices about private property. The un-greening or re-greening of Seattle literally starts at home.