SEATTLE — Seattle is the fastest growing big city of the decade, with more construction cranes crowding its skyline than anywhere else in the nation. Neighborhoods buzz with new high-rises and single family homes underway to house newcomers lured by high-paying technology jobs, world-class universities and easy access to outdoor pursuits.

But all that growth appears to be coming at the expense of one of the Emerald City's most iconic features: its urban forest, whose canopy shades roughly one quarter of Seattle's scenic topography of peaks, valleys and waterways.

Despite a 2007 goal to boost the tree canopy to 30% by 2037, not even the city knows precisely how many western red cedars, bigleaf maples, Douglas fir and western hemlocks have been felled on private property in recent years to make way for development, as permits are not yet required for all removals. But as Seattle's elder groves dwindle, a team of tree-loving locals is rallying friends and neighbors to keep track and push back.

"One of the things we have found is when we build community around these trees, it gives [people] a voice. It gives people a way to express themselves. They're a little less powerless," says Jim Davis. He's a retired health care administrator, backpacker, and a leader of The Last 6,000, a civic campaign whose volunteers are attempting to snap and map the city's remaining "majestic" trees – mature trees with a diameter of 30 inches or wider at chest height (imagine the door of a typical home refrigerator). The group's name hails from the results of aerial imaging in 2016 that identified at least 6,338 majestic trees still standing throughout the city.

Research consistently shows that trees help cool residents during warming summers; minimize air pollution; prevent soggy hillsides from becoming landslides during the rainy season; offer habitat for wildlife; contribute to emotional wellness; and absorb stormwater runoff as more ground is paved.

But advocates of Seattle's forest ironically have found themselves at odds with the goals of the Washington State Growth Management Act, which encourages urban density to limit sprawl and preserve the remaining wild spaces that fringe the state's population hubs. The result has been a forest of apartments and townhomes as cities grow ever more dense to meet the voracious appetite for housing, particularly affordable housing to battle another growing conundrum in Seattle: homelessness.

"Even if the data did show some tree loss caused by the construction of apartment buildings … 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 even higher," Dan Bertolet, a senior researcher of housing and urbanism for Sightline Institute, a local think tank focused on sustainability, wrote in 2018. To expand the canopy, he suggests a tree ordinance that rewards homeowners and developers for keeping mature trees and continuing to plant trees in the public right of way.

The Last 6,000 is the latest tree preservation effort to coalesce in Seattle in recent years, including Tree Keepers Alliance and Friends of Urban Forests (keepers of the campaign Don't Clearcut Seattle). All are urging city leaders to do more to protect both existing trees and the space future trees need to flourish – to promote density as well as greenspace.

That's already happened in the neighboring city of Mercer Island, where, similarly, longtime residents grew alarmed by the quickening loss of forest. In 2017, city leaders chose to protect remaining mature trees by restricting home size and how much of the lot it can cover; by requiring a permit to remove trees with a diameter of 10 inches or more; and by regulating when replacement trees can be planted and how long they must be maintained.

Other cities around the country have also taken action. Austin (neck-and-neck with Seattle in terms of record growth over the past decade) and Washington, D.C. both have tree laws that greatly restrict what homeowners can do with existing trees on their property and ban the removal of specially designated "heritage" trees altogether unless dead or a safety risk. The city of Atlanta is also reviewing its tree rules as its population continues to boom: development has felled some 90,000 trees there in the past six years.

Seattle officials are currently reviewing suggested updates to its tree protection code from the city's Urban Forestry Commission. Strategies under consideration include requiring a permit to remove any tree of 6 inches in diameter or wider; improved tracking of tree removal and replacement on both public and private land throughout the city; and requiring that any removed tree be replaced onsite or a removal fee paid into a Tree Replacement and Preservation Fund, according to the Department of Construction and Inspections.

The Last 6,000's first big push over the summer and early fall netted an initial 1,000 trees for its database, drew scores of volunteers and grew its Instagram following. The group is now reviewing and mapping the submissions (which it hopes to share in the future) and encouraging volunteers to continue cataloging the majestic trees still standing in their neighborhoods.