Ever since I first visited D.C., I was in awe of how full of trees it was. Rock Creek Park, one of the oldest city parks in the United States, is a factor but even walking through residential roads across the District, lush greenery can be found at every corner — from slender maples to towering oaks swelling the sidewalks with their roots.

I decided to find out whether it’s luck or careful planning that is responsible for D.C.’s impressive urban forest. Researchers at the Arbor Day Foundation, the U.S. Forest Service and forestry consultancy PlanIT Geo have mapped tree cover across all urban areas in the country using aerial imagery and artificial intelligence. Their computer model analyzes patterns in color, shape or shadowing to detect tree crowns.

The maps allow us to see what the most forested places in the U.S. have in common.

Zooming in on a few different examples, the first thing you’ll notice is that tree cover depends heavily on the local climate.

example cities from different regions

In the east of the country, a region that was historically forested, trees grow naturally wherever they find an opening.

Traveling west, the natural landscape soon turns into grasslands, and cities begin to have fewer and fewer trees.

In the southwest, where annual rainfall averages just 8 to 14 inches, trees need almost year-round irrigation to survive.

In the east of the country, a region that was historically forested, trees grow naturally wherever they find an opening.

Traveling west, the natural landscape soon turns into grasslands, and cities begin to have fewer and fewer trees.

In the southwest, where annual rainfall averages just 8 to 14 inches, trees need almost year-round irrigation to survive.

In the east of the country, a region that was historically forested, trees grow naturally wherever they find an opening.

Traveling west, the natural landscape soon turns into grasslands, and cities begin to have fewer and fewer trees.

In the southwest, where annual rainfall averages just 8 to 14 inches, trees need almost year-round irrigation to survive.

In the east of the country, a region that was historically forested, trees grow naturally wherever they find an opening.

Traveling west, the natural landscape soon turns into grasslands, and cities begin to have fewer and fewer trees.

In the southwest, where annual rainfall averages just 8 to 14 inches, trees need almost year-round irrigation to survive.

Conservation organizations like American Forests recognize that sustaining trees is much harder in some places than others, so they recommend a tree cover goal of 40 to 60 percent for communities in forested states and a much smaller 15 percent target for desert cities.

Climate is not the only limiting factor for vegetation. Bigger, more densely populated cities need somewhere to put all of their homes, businesses and roads, so they end up with less space to plant trees.

Below, you’ll find a tool to look up your own city. To give some context to each city’s number, you can compare its tree cover to nearby places with a similar climate and population density.

Planting more trees in a crowded city, however, can pose a challenge. Making room for nature means there’s less space for housing and transportation.

“Density has a lot of benefits,” said Rob McDonald, lead scientist at the Nature Conservancy, an advocacy group. “It makes our cities generally lower-carbon as we use less energy, especially for transportation.”

On the other side of that equation are the health benefits urban trees offer — or the risks of not having them. On a hot summer day, afternoon temperatures in treeless neighborhoods tend to be 8 to 11 degrees higher than in the greenest parts of the same city, said Vivek Shandas, professor of urban sustainability at Portland State University.

Unshaded buildings and roads also keep releasing heat long after the sun has gone down. Researchers say that heat waves damage our bodies much more if they’re not broken up by cool nights.

Measuring heat at the neighborhood level is trickier than it may sound. Many studies use satellite readings that capture surface warmth — on asphalt, roofs or treetops. But these statistics don’t always reflect what we actually feel as we move through the city.

Shandas and his colleagues have organized heat mapping campaigns across the U.S. that utilize temperature readings from volunteers driving or biking across a city.

How trees cool down D.C. on a hot summer day

Tree cover

050100%

Afternoon temperature

80859095F

Sources: CAPA Strategies (temperatures), Trees At Work (tree cover). Temperatures were measured on 28 August 2018. The measurements were generalized to the whole of D.C. using an A.I. model.

Studies suggest that across America’s cities, trees prevent several hundred heat deaths per year while also reducing the need for air conditioning.

Then there are the psychological benefits — people report feeling happier when they’re surrounded by nature. McDonald said this extends from spending time in a park to “even just the view outside your window.”

There is some scientific evidence that living near green spaces may help regulate levels of the stress hormone cortisol, but most of these studies have relied on small sample sizes and differing measures of green space exposure.

So which places are leading the way on boosting tree cover? I’ve taken a closer look at three examples that stand out even in the face of development pressures and challenging climates.

Bend, Oregon

Bend, Oregon, originally a small logging town, saw a huge influx of people over the past years, as thousands of remote workers and nature lovers moved in during the pandemic.

The city had to balance the need for housing with preserving its urban canopy, said Corie Harlan, a campaigner at the advocacy group Central Oregon LandWatch who advised the city on updating its tree code.

Under the new rules, enacted last summer, developers have to preserve 20 percent of the largest trees on a site or pay for up to four new trees to be planted for each mature tree they cut down.

Washington, D.C. (and its suburbs)

3d view of DC neighborhood

After crunching the numbers, D.C. does indeed stand out as an unusually green city: together with its sprawling suburbs, it has just under 40 percent tree cover, much more than most other urban areas in the Mid-Atlantic.

Mark Buscaino, executive director of the local nonprofit Casey Trees said the fact that the District started out as a planned capital, “green along the same lines of other great European cities,” helps account for its robust canopy.

Since 2002, the city has enacted several ordinances aimed at protecting its oldest and largest trees. Developers who proceed with illegal tree removals face not just fines but can be issued “stop-work orders” or have their permits revoked.

Frequently, decades- or centuries-old trees that stand in the way of new construction are excavated and relocated instead of being chopped. The District’s urban forestry division also monitors trees throughout the city, identifying ailing trees and replacing them when needed, while also working to plant new trees in low-income areas.

Across the Potomac, the picture is more mixed: According to the data I looked at, some fast-growing neighborhoods in Arlington lost up to 12 percent of their tree cover over the last five years. The county’s own reports show a slight increase over the last decade while acknowledging that the jurisdiction continues to lag behind the District.

Tucson, Arizona

In Tucson, where summer temperatures frequently reach 105 to 110 degrees, trees can make the difference between merely uncomfortable conditions and life-threatening heat.

Until 2020, the city had never taken stock of its existing canopy and didn’t have a dedicated urban forester. “Our past couple of years have been really focused on building up this program from nothing,” said Nicole Gillett, who became the city’s forester.

Today, the city uses a sophisticated mapping tool to track progress on tree cover and zero in on the gaps that remain.

In the face of severe water scarcity, the municipality focuses on planting native, drought-tolerant trees like velvet mesquite. Under a program called “storm to shade,” the city is creating small curbside green spaces designed to capture runoff from occasional rain showers and reduce the need for added watering.

Urban trees will only become more important as climate change makes heat waves more intense and longer-lasting. At the same time, the number of Americans living in cities is expected to increase by 18 to 36 million over the next 25 years.

That growth doesn’t have to come at the expense of our urban forests, McDonald said. He highlighted examples of neighborhoods both dense and green: They often feature multistory apartment buildings, limited parking above ground and tall trees overhanging courtyards and walkways wherever possible.

Shandas said any region not actively growing “verdant urban forests” risks putting the public’s health in danger as temperatures rise. “The coming decade will be the hottest that any of us have experienced,” he said.

“The best time to plant a tree was 20 years ago.”