Two trucks, one hauling a flatbed trailer with a dozen or so balled-and-burlapped trees, pull up next to a small, mostly barren triangle of land in Northwest Washington. Five men in chartreuse shirts descend with shovels, dig five holes in the brown clay and muscle 10-foot-tall overcup oak saplings into them. The workers sweat in the late-morning sun and banter about who’s going to buy lunch.

“It’s the only way to make the day go by,” said Marcus Pinkney, the crew leader.

The oaks are among 30 trees the crew and others were to plant that day at sites throughout the city. “I like it because it’s fast-paced, but sometimes it can be hectic,” said Bridget Cantwell, an urban forester with the District’s Urban Forestry Administration (UFA) who is overseeing the plantings.

Cantwell and her colleagues, along with other city agencies and nonprofits, add more than 10,000 trees annually to the District’s arboreal family. (The number of trees planted in 2017 surpassed 12,000, and the UFA projects a similar total this year.)

The massive long-term effort aims to reverse decades of decline in the city’s urban forest. The District reports success: Its analysis of 2016 data found that 38.7 percent of the city’s land area was forested, up from 35.1 percent in 2006.

But a study published in April by two U.S. Forest Service scientists calls those findings into question. The city lost 850 acres of tree cover — roughly 2 percent of its total surface area — between 2010 and 2015, the researchers reported. That study, which compared the District with states, found that in terms of percentage, the city’s loss of urban forest was among the highest of any jurisdiction over the five-year ­period.

Such a decline would be a startling setback for a $4 million-a-year tree-planting initiative that has been supported by four consecutive mayors — and a sobering reminder of the challenges trees face in urban areas.

A historically verdant tree canopy dating to Alexander Robey Shepherd’s brief governorship in the 1870s earned the nation’s capital its moniker “city of trees,” said Melanie Choukas-Bradley of Chevy Chase, Md., a naturalist and the author of a book by that title. But by the late 20th century, many dying trees were not being replaced, and large areas of the city were losing canopy.

In 1999, the District-based advocacy group American Forests published a satellite map showing a large black swath of treelessness cutting through the city. Mayor Anthony Williams responded by increasing the tree budget from around $3 million to $9 million, stating that trees were critical components of the city’s public spaces. A few years later, philanthropist Betty Brown Casey gave $50 million to endow the advocacy group Casey Trees, which has relentlessly promoted tree planting and care in the District.

By 2010, when the Forest Service study period began, the District and its partners were planting thousands of trees per year. But the city’s trees continued to face challenges. Construction of a Costco-anchored shopping center in Northeast removed acres of young forest, and the June 2012 derecho brought down hundreds of trees. Large trees have also succumbed to Dutch elm disease, development and old age.

“Trees are being lost all the time,” said Earl Eutsler, UFA’s associate director.

PEPCO has led an effort to cut down trees to prevent power outages. But Eutsler said the company’s action has had little impact on the city’s tree canopy.

To measure how effectively his team was combating the losses, Eutsler and his colleagues worked with Forest Service researchers and other outside experts to combine high-resolution aerial imagery from the U.S. Department of Agriculture and height data gathered by lidar, in which light from an airplane-mounted laser is bounced off objects on the ground. Computer analysis then provided a tree-canopy data point for every square meter in the city. The technique is the “gold-standard approach” for measuring urban forests, Eutsler said.

His team then checked a random set of points to make sure the computer’s assessments were accurate. And based on data collected in 2006, 2011 and 2016, they found a slow but steady climb toward a goal set by Mayor Vincent C. Gray in 2011 of 40 percent tree canopy by 2032.

The Forest Service study, which is independent of the District’s assessment, shows the increased emphasis that the traditionally rural-focused agency is placing on urban forests and the benefits they provide to city dwellers, including cleaner air, reduced flooding, lower energy bills, and improved physical and mental health.

Study authors David Nowak and Eric Greenfield used publicly available high-resolution images from Google Earth to randomly distribute 1,000 points throughout the District and each state’s urban land area. They then classified each point as having a tree or not having a tree in images from 2010, and again in 2015.

While most jurisdictions showed some decline in urban forest cover, the District fared worse than every state except Oklahoma.

Nowak and Greenfield estimated that an area of trees equivalent to almost half of Rock Creek Park — or six Malls — had disappeared. In 2015, the city’s tree canopy, according to the researchers, stood at 33.9 percent — more than a percentage point below the assessment when the city’s tree-planting effort began.

Eutsler said the Forest Service’s methods are valuable for generating tree-canopy estimates for large areas. But he’s not buying Nowak and Greenfield’s conclusions about the District. “We looked at more points for the city than [they] did for the entire country,” Eutsler said.

The city’s laser-based methodology is also better able to pick up newly planted trees, said Jessica Sanders, Casey Trees’ director of technical services and research, who independently verified the city’s numbers. Young trees present a narrow profile to an overhead camera, and they start to grow outward only after a few years in the ground.

Nowak said he can spot even small trees in Google Earth imagery. But he acknowledged that the statistical sampling approach gives the study a margin of error around 1.5 percent, meaning that the 2015 number could be closer to the District’s estimate. He added that the city’s method is not perfect, because it can confuse treetops with other kinds of land cover. “I’m not saying they’re wrong,” he said. “But we have a difference of opinion.”

To make matters potentially more confusing — and contentious — there will soon be yet another set of eyes on the city’s trees.

This summer, the District will join the Forest Service’s Urban Forest Inventory and Analysis project — an urban tree census wherein researchers randomly distribute 200 study plots throughout the city and measure trees one by one using tape measures and laser devices. The project will probably provide a third number for tree advocates and researchers to ponder, Eutsler said. “I say to people that this is the most scrutinized forest in the world.”

It’s clearly a good time to be a tree in the District, Choukas-Bradley said. A 2016 law increased fines for removing large trees, giving the city one of the country’s strictest tree ordinances. UFA’s substantial budget also delivers a boost that other jurisdictions lack, said Mark Buscaino, Casey Trees’ executive director.

“Most cities do not have the horsepower that we have here,” he said.

But the last leg of the marathon toward 40 percent canopy is likely to be the hardest, Sanders cautioned. The city has filled nearly all of the 25,000 empty roadside tree boxes Casey Trees found during its first survey. Additional greening will require convincing thousands of religious institutions, business owners and residents to plant trees on their properties.

“The dominant species in the urban forest isn’t trees,” Sanders said. “It’s humans.”