Notes From the Tree Line
How to Grow a Forest in the City
In many ways, citydwellers can’t see the forest for the trees. City trees shade our parks and grow in tree boxes along our sidewalks — but juxtaposed with our urban environment, it’s easy to think of trees as merely decorative.
Ecologically speaking, however, these species make up our urban forest.
“The city might not be the first place that somebody thinks of the forest,” concedes Sophie Earll, a research associate at Casey Trees, a local nonprofit that aims to grow and nurture the tree canopy in Washington, D.C. But urban forests play a critical role in ameliorating some of the harsher trappings of city life, including impervious surfaces and toxins emitted from vehicle exhaust.
Unlike traditional forests, the tree populations that make up an urban forest are often diffuse. The District’s tree canopy, for example, is comprised not only of large wooded parks like Rock Creek Park, but also smaller landscaped parks like Farragut Square and the many trees that line our heavily-trafficked thoroughfares.
“Urban street trees are living in the most hostile place possible,” says Earl Eutsler, associate director for Urban Forestry at the District Department of Transportation’s Urban Forestry Administration. “But it’s also where they do the most direct good in terms of environmental benefits and services to those people walking along streets, riding bikes along streets, even driving — because they’re a direct filter of the particulate matter that is emitted by cars and trucks.”
With so many threats to city trees’ survival, how can an urban forest thrive?
Tracking canopy growth
In the District, strong government support of tree canopy expansion has been a boon to city trees. Mayor Vincent Gray set tree canopy expansion as a bellwether in his administration’s 2012 Sustainable DC plan. The plan set a goal of achieving 40 percent tree canopy cover in the District by 2032.
To achieve that goal, the District Department of Transportation’s Urban Forestry Division (UFD) has partnered with Casey Trees to reach the goal of planting 8,600 trees per year until 2032. Even as the city has transitioned to the Bowser administration, it has remained committed to these sustainability goals. This October alone, Eutsler says, the city saw nearly 1,540 trees planted.
But trees can’t just be planted and forgotten. To determine how effective tree planting programs really are, and measure the overall health of the overall urban forest, continued data collection on canopy growth is essential.
The U.S. Forest Service (USFS) recently funded two studies that used distinct methodologies to measure the District’s tree canopy and produced very different pictures of the success of the city’s efforts. In the first, the District partnered with the U.S. Department of Agriculture, using lidar height data and aerial imagery from the National Agricultural Imagery Program to report a 38.7 percent tree canopy in 2017. In a separate study, USFS used statistical sampling based on Google Earth imagery to report a 33.9 percent canopy in 2015. Lastly, the District and USFS partnered on the Urban Forest Inventory and Analysis study this summer. The study — which is essentially a tree census — has yet to be published. Eutsler says he expects it to produce a third distinct canopy measurement.
For residents trying to make sense of their city’s canopy growth, these differing results can be confusing. Is the District’s tree canopy growing or not?
Eutsler stresses that both of these USFS-funded studies — with their distinct methodologies — provide valuable information that will help the District learn more about the scope and population of its urban forest.
The District-USFS lidar height data study, for example, is “spatially explicit.” Eutsler says it provides a record of each tree in the District, which can be cross-referenced with city records, like requests for tree removal. The USFS statistical sampling study, meanwhile, provides essential data on the populations of different species represented in the District’s urban forest. This sort of population data can help the District understand how to improve species diversity, helping boost overall forest health by preventing the spread of disease.
While previous reporting has focused on the different results of these studies, Eutsler urges a focus on the different methods used.
As the District continues to work toward its tree canopy goal, it will have to conduct further studies to assess tree population health, measure tree growth, and examine tree planting success rates.
It’s hard to be a tree in the city
But no matter which data set you examine, the fact remains that a healthy tree canopy is essential to improving the District’s environmental health.
Looking at Washington’s densely-populated downtown, Earll at Casey Trees points to the benefit provided by parks like Farragut Square and McPherson Square. As the overabundance of pavement limits tree planting downtown, these parks provide essential green space.
These parks, Earll says, “reduce that impervious surface, lower those temperatures, and do some of the work that we need downtown.”
But that work is hard on trees. As German forester Peter Wohlleben notes in The Hidden Life of Trees, trees planted along city streets face a number of challenges: “The roadway restricts growth in one direction, there are pipes under the pedestrian zones, and soil has been compacted during construction.”
All this makes it difficult for a tree to grow. And city trees must also endure scorching temperatures caused by the urban heat island effect; survive extreme weather events, like derecho storms; and intake air polluted with toxins from exhaust fumes. Ensuring the health of these street trees is critical, and it must be complemented by expanding tree planting.
Because D.C. was so vigorously planned, you can find plenty of older trees populating the city’s downtown parks. But much of the room for tree canopy growth can be found in Wards 4, 5, 7, and 8. Eutsler says that those wards have been seeing the most tree planting of late because “there’s just more opportunity, more open spaces, so we’re … going where the need is greatest.”
Building connections between the city and its trees
Given the challenges that city trees face, Casey Trees hosts regular tree inventory events around the District to tally tree populations and assess tree health. Volunteers at these events work with arborists to contribute to the city’s urban forest data as citizen scientists.
“All this data that we collect, we pass to UFD, the Urban Forestry Division, and they use it for their work,” Earll explained.
In October, Casey Trees hosted a tree inventory event at Logan Circle. Small groups of volunteers worked with UFD arborists to canvass the park, recording each tree’s vitals, such as canopy width, trunk diameter, and tree height. Volunteers also identified each tree’s species and assessed its health. The inventory of each tree was then entered into an app that assigns a GPS coordinate to each tree.
Mary Robert Carter, an administrative assistant at a nonprofit, helped collect data at the inventory. Carter went to her first Casey Trees event last year and has kept up with the organization’s volunteer opportunities ever since. She says that tree planting has helped her feel a closer tie to D.C.
“Now, when I walk to work in the morning, I walk past the tree that I planted,” Carter says. “It’s really fun — you feel ownership over that little space in the city.”
Urban forests depend on city residents feeling this kind of buy-in to their surroundings. “You have to connect people to the environment around them,” Earll says. “This urban forest, you know, it’s huge — it’s got millions of trees — and us … as a nonprofit, we don’t have enough people on staff to take care of that. We can’t single-handedly increase the canopy of this city.”
If D.C.’s trees have taught us anything, it’s that it takes a city to grow a forest. With your help planting new trees and caring for and assessing existing trees, our city’s forest can continue to thrive.
