When University of Washington social scientist Kathleen Wolf, Ph.D., needs a break, she rides her bike through the gardens of Ballard’s Hiram M. Chittenden Locks, often ending up at the Washington Park Arboretum. It’s a trip she takes to experience “forest bathing,” or shinrin-yoku, the name Japanese researchers have given to the experience of spending time in naturalistic green spaces at a contemplative pace. Studies in Japan have found health benefits for people who forest bathe, including reduced depression and improved immune system response.

Wolf knows this research well, because her own work is focused on how people experience nature in cities, including the human health and economic benefits of the urban forest—the phrase used to describe the trees and understory plants in a city. Forest bathing aside, research shows our health is boosted by having access to urban trees and other nature. “Neighborhood greening reduces depression, may help kids reduce ADHD symptoms, reduces our stress response, encourages physical activity, reduces signs of mental distress, increases cognitive memory, improves air quality and, overall, promotes significantly higher well-being,” she says.

City trees can also raise property values, reduce crime and muffle urban noise. They help the environment, absorbing water, decreasing flooding and the need for water treatment, and absorbing and storing carbon dioxide, directly fighting climate change. “Up until recently, trees really didn’t have a place at the table with city decision making, but now, because we know about their environmental services and the health benefits, trees are an important component of city decision making,” says Wolf.

Documenting these benefits is more important than ever as Seattle grows. The city’s population has swelled from 608,000 in 2010 to more than 713,000 this year. In the 1970s, Seattle was mantled with trees, with about 40 percent canopy cover—that is, the percentage of the city that is covered by trees, as seen in an aerial view. But as the number of city inhabitants has increased, we’ve shed at least a quarter of that protective green veil. And what’s left is distributed unevenly, and is under threat from disease and development.

This spring, the city released the results of a 2016 study of Seattle’s tree canopy. Several previous assessments, starting in 2001, used a range of methods to measure the tree canopy, making it impossible to directly compare studies. One assessment from 2001 found the canopy was only 18 percent, while the 2016 research found a more robust 28 percent—close to the city’s 2037 goal of 30 percent. It was a respectable number, although well under the “reach” goal of 40 percent, which the city would like to achieve by 2047, based on a recommendation by the national conservation organization American Forests. The 2016 survey was conducted with a technology called light detection and ranging, or LIDAR, in which a sensor-equipped plane collects data to create a three-dimensional model of the Earth’s surface and surface characteristics, such as trees.

But the report doesn’t indicate the condition of these trees. “Many of our urban trees in our natural areas are at the end of their life expectancy,” explains Sandra Pinto de Bader, urban forestry adviser at Seattle’s Office of Sustainability and Environment, in a presentation about the findings. In addition, invasive English ivy throughout the city is weakening some trees, and the impacts of hotter, drier summers, as well as more serious droughts, as we experienced in 2015, are also weakening many trees. “It would just take a couple of nasty storms for us to lose a significant number of trees,” Pinto de Bader says, though she notes that she doesn’t have hard data to back this up.

Knowing something about the state of the city’s tree canopy may help planners as Seattle’s population and density increase. Seattle already has several tree planting and maintenance initiatives, including a program to plant about 2,000 trees every year, and the Green Seattle Partnership (an initiative between the Seattle-based sustainability nonprofit Forterra and Seattle Parks and Recreation), which restores aging, ailing urban forests by planting native understory, pulling invasive species and planting 5,000–7,000 trees annually. A Trees for Neighborhoods program offers residents as many as six trees per household to plant. King County also has a campaign—to plant 1 million trees by 2020.

What’s not known is whether these efforts are keeping up with the pace of loss from disease and development. A “mini assessment” included with the canopy survey provides some insight into how development might impact canopy cover over time. Researchers used historical imagery from Google Earth to look for changes in tree canopy on 80 randomly selected parcels in the city, and found that parcels in the downtown, industrial, single- and multi-family zones lost canopy when developed.

Anyone looking around the city can see how that happens: A small single-family house on a big, treed lot comes down, and the lot is scraped bare, or nearly so. When four units sprout up, there’s scarce room for new trees, particularly ones that need space to grow big. The same can happen with larger developments. And homeowners can remove as many as three trees on their property per year.

Residential areas of the city—with both single- and multi-family housing—are a particular concern: They provide 72 percent of the city’s tree cover, while parks and other natural areas provide only 20 percent. “The city government doesn’t have full control over the majority of our urban trees,” explains Pinto de Bader.

In fact, Seattle has fairly weak protections for trees on private property. An ordinance protects “exceptional trees,” which are generally large, unusual or old, and the city can fine property owners for cutting down those trees. But homeowners often find ways to get around that ordinance, for instance, with a private arborist’s report deeming the tree a hazard. Homeowners can apply for a Heritage Tree designation, which provides a certificate, but no protection.

Large trees, which need plenty of room for canopy and root spread, may be particularly vulnerable. According to the city, we have an estimated 6,338 large trees in Seattle (measured as trees that are at least 30 inches in diameter at breast height) but no idea of how many of those trees are near the end of their life span. Wolf says large trees provide “a much greater proportion of benefit” to cities compared to small trees, in terms of ecosystem services, such as water quality, air quality and carbon sequestration. “Are we going to relegate large trees only to parks and green spaces, or can we incorporate them into development?” she asks. “I’m not sure.”

Seattle residents are also worried about trees lost to larger developments. “We’re proudly the Emerald City, but I don’t know if that’s money or trees,” says Tony Hacker, a psychologist in private practice who lives in Madison Valley. His and other homes on his street face a slope of several dozen trees, including Lombardy poplar, cherry, maple and cedar trees. The slope is the back side of a large commercial property that houses City People’s Garden Store, a home and garden shop. Developers who purchased the site with plans for retail, apartments and a parking garage would like to remove most of that green (as well as the store building). The new view from Hacker’s window would be of a concrete wall.