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Urban Forests: What city trees do for us, and what we should do for them

It’s a bit of a no-brainer that the trees gracing our sidewalks, parks and other urban spaces are pleasing to the eye, providing soothing shade in the harsh, barren concrete landscape. In city parks, trees provide a place for citizens to relax and birds and squirrels to reside. What’s not to like about them?

But not many of us realize how much havoc our settlements have wreaked on forests that predate our cities by millions of years, how our unwitting introduction of invasive insects has wiped out billions of our finest native trees and driven some arboreal species to the brink of extinction. And all this occurred without appreciation for the immense eco-system services trees render in carbon storage, energy savings and flood mitigation — services that will only be more urgently required as our planet warms.

As humans continue to urbanize — more than half of us now live in cities — it becomes critically important to restore and conserve urban trees in harmonious coexistence with nature. Many “smart” cities have been doing this for years, of course, and some of the results have been impressive.

Much of this discussion and the overarching history and legacy of city trees is covered by author and historian Jill Jonnes in her recently published Urban Forests: A Natural History of Trees and People in the American Cityscape (Viking). 

“Nature’s largest and longest-lived creations, trees play an extraordinarily important role in our cityscapes, living landmarks that define space, cool the air, soothe our psyches, and connect us to nature and our past,” Jonnes writes on her website. “Today, four-fifths of Americans live in or near cities, surrounded by millions of trees, urban forests containing hundreds of species. Despite the ubiquity and familiarity of those trees, most of us take them for granted and know little of their specific natural history or civic virtues.”

In an e-mail exchange with National Geographic Voices, Jill Jonnes discusses some of the issues she covers in her book:

DB: How much of the American canopy was lost in the process of urbanization and how much has been replaced, and how has the species composition changed in that period?

JJ: In a city like Washington, D.C., (as a fairly typical example) much of the original hardwood forest and its canopy disappeared as new lots and buildings were carved out of what had been wilderness. When Thomas Jefferson was president he denounced the poaching of mature trees intended to cool and beautify the new capitol as “a crime little short of murder.” In the 1870s, when Boss Alexander Shepherd remade D.C.’s streets, sewers, and sidewalks, he instructed “put a tree wherever there is damn room.” By 1900, D.C. had become famous as the “City of Trees,” its streets and parks verdant with familiar urban species like the American elm, maples, lindens, oaks, honey locusts, and a multitude of new species–including the exotic ginkgo, ailanthus, and of course in later decades with thousands of ornamental Japanese cherry trees.

In the decades after World War II, Washington saw its lush canopy (50 percent) begin to diminish — as tens of thousands of trees aged and died, statuesque American elms (the dominant street tree) succumbed to Dutch elm disease, and escalating development and the rise of the automobile meant the continuing loss of trees to wider roads, parking lots, malls and buildings. By 1999, the nonprofit American Forests made front-page news showing with aerial maps that the “City of Trees” canopy had declined to 35 percent. Unlike most American cities, D.C. has managed to re-plant many of its lost trees and prevented further decline. Today, with so much of our cities so paved-over, re-establishing the canopies they once enjoyed is a major challenge. Trees require dirt!

2002 NASA image of Greater Washington, D.C. shows the extent of urban development in what was once a great forest.
2002 NASA image of Greater Washington, D.C., shows the extent of urban development in what was once a great forest.

What were the hardest lessons learned when Dutch elm disease destroyed so many trees in our environment?

The first hard lesson is that global trade (Dutch elm disease came from Asia via Europe) can have devastating environmental consequences. In the past 15 years, the worst tree pest in American history — the Emerald ash borer – snuck in from China and is now expected to spread steadily beyond the 28 states it has infested since 2002 and extirpate all our native ash trees in coming decades. Tree scientists are calling for legislation dubbed Tree-Smart-Trade to fend off further such eco-disasters.

The second hard lesson is do not plant monocultures, as the loss of the American elm was so extreme because far too many of the nation’s streets, avenues, and private yards were dominated by those beautiful trees by World War II. And yet, many cities did not heed that lesson and especially in the Midwest often replaced their lost elms largely with ash trees, now under siege by the Emerald ash borer.

What cities are setting the best examples and what are the most valuable lessons learned about cities re-establishing their forests?

In restoring an urban forest, city arborists must first know the basic facts: How many trees overall, how many street trees, what is the canopy percentage, health, and size of the different species, where are they located? In short, you need a mapped inventory.  With that, a city can learn –as did L.A. back in 2006 — that wealthy council districts like Bel Air have a 47 percent tree canopy, while South Central has a 6 percent tree canopy. And so you can think strategically about where you really need to plant trees.

Most recently, New York City has a been an inspiring (and perhaps surprising) example of a city that used its 2007 tree inventory in conjunction with public domain i-Tree software from the U.S. Forest Service to show then-Mayor Michael Bloomberg that for every $1 spent on a street tree, that oak or honey locust provided more than $5 in  “eco-system” services — everything from energy savings to storm water mitigation to better public health. Bloomberg was so impressed, he quadrupled the Forestry budget and launched MillionTreesNYC. Every step of the way, New York arborists used data and technology to manage this massive greening project. The millionth tree was planted this past spring.

Most valuable lessons? Knowledge is power, even with trees, and the new tree science and technology can be highly compelling ways to move policy.

NASA image of Manhattan shows Central Park and some other tree-friendly areas on Manhattan, New York.
NASA image of Manhattan shows Central Park and some other tree-friendly areas on Manhattan, New York.

What can be done to plant and sustain an urban forest? What are some of the biggest mistakes cities and individuals make planting trees in an urban environment?

To plant and sustain an urban forest, you need to acquire as complete a picture as possible. When Chicago first did this in 1994, foresters were amazed to discover the city had 4.1 million trees, of which 412,000 were street trees. While only a tenth of the overall urban forest was street trees, they provided a quarter of the canopy. The original landscape of prairies, wetland, oak/hickory forests had given way to an industrialized city with an 11 percent canopy. The modern urban forest was dominated by cottonwood (13 percent), green and white ash (12 percent), American elm (7.2 percent) and Japanese cherry (6.5 percent). Fully half of the street trees were Norway maple and honey locusts, making clear that a far wider palette had to be planted.

Biggest mistakes when restoring street trees in the urban forest: Not giving tree roots enough room to produce a healthy, long-lived tree. Or planting short-lived, small trees like red buds or fruit trees– far better to plant a tree that could live 50 to 100 years. Not factoring in the need for regular watering in the early years, and not pruning as the tree grows. Like all urban infrastructure, the green infrastructure that are trees require regular maintenance and — when it comes to pruning — skilled personnel.

Should we be gradually eliminating exotic trees and replacing them with native species? What can individuals do to help the process of reforestation in their own backyards?

Life is very, very tough for city trees, especially street trees, and so while natives are great for fostering local ecosystems, some non-natives – like Japanese zelkova and the Japanese scholar tree — make such hardy, beautiful, and durable urban trees that city arborists find them excellent choices.

Individuals can make a big difference by planting trees on their private property—especially trees that will be long-lived and have significant tree canopies in time. Oaks are especially wonderful because they serve as rich sources of food and habitat, develop big canopies and can share all these good things for as long as a century.

In the book you discuss the story of the loss of the most iconic of urban trees—the American elm. Why did the American elm begin to disappear and how did its gradual restoration come about?

After WW I, when an Asian fungus known as Dutch elm disease began killing European elms by the millions, the U.S. prohibited the import of all live elm nursery stock. But on Aug. 7, 1933, a USDA inspector at the Port of Baltimore spotted hundreds of tiny live beetles infesting Carpathian elm burl logs destined for furniture building. Within the next year, the USDA and local forestry officials identified thousands of infected American elms in the New York region, for these elm log shipments had been coming in for a while. Despite a major effort to contain the disease, DED spread relentlessly across the country, arriving in California in the 1960s, ultimately wiping out about 100 million native elms.

Very gradually, USDA researchers and tree scientists discovered that certain American elm trees seemed able to tolerate and survive Dutch elm disease. Those few rare specimens were then tested and re-tested, and by the beginning of the 21st Century, tree nurseries were again growing and selling American elms. The return of the American elm is great news, as they remain one of the toughest and most beautiful of city trees.

Where can people see American elms in their natural state?

The Jefferson's Elm is one of more famous trees on the National Mall, Washington, D.C. It has been found to be resistant to Dutch elm disease, according to the U.S. National Park Service, which manages the National Mall and its hundreds of trees. Photo courtesy NPS.
The Jefferson’s Elm is one of more famous trees on the National Mall, Washington, D.C. It has been found to be resistant to Dutch elm disease, according to the U.S. National Park Service, which manages the National Mall and its hundreds of trees. Photo courtesy NPS.

The biggest remaining stands of American elms are in cities: Two of the most famous examples: The more than 600 American elms growing along the National Mall in Washington, D.C., their graceful architecture reminding us why these trees were so beloved. In New York City, much of Fifth Avenue along Central Park is lined with beautiful elms. In Central Park itself, there is the exquisite quarter-mile-long Literary Walk, its quadruple row of elegant mature elms towering above expanses of velvet lawns. After it has snowed, seeing these trees etched in white is worth a pilgrimage.

Aerial view of Washington. D.C.'s tree-lined National Mall, courtesy of NASA.
Aerial view of Washington. D.C.’s tree-lined National Mall, courtesy NASA.

The stories of the American chestnut and the American ash do not yet have happy endings, unfortunately. Can you explain what destroyed these trees, and why no viable solution has been found yet?

The American chestnut blight, discovered in 1904 at the Bronx Zoo, was a wind-born fungus. As it wafted its deadly way through all the southeastern states (the chestnut’s range), there was no way to stop it. In 2002, foresters near Detroit inspecting dying American green and white ash found shimmery tiny beetles under the bark. Almost certainly the beetles had stowed away on a wooden pallet from China some years earlier. These munching exotic pests, named the Emerald ash borer (EAB), showed they could kill a tree in a couple of years. The EAB population exploded and is extirpating ash in 28 states. The USDA fully expects EAB to cross the Rockies and eventually wipe out all American ash species (8 billion trees) except those few large mature specimens treated with a pesticide that destroys the beetles. This will mark America’s worst tree loss ever—a historic near-extinction of an entire tree species.

While the American Chestnut Foundation believes it is close to developing a tree that can withstand chestnut blight, they are not there yet. As for the ash tree, if the American elm is any guide, it will be decades before an ash is found that can resist EAB and go back into the nursery trade.

Alarmed tree scientists have proposed legislation that Congress is now working on dubbed Tree-Smart-Trade that would end all use of wooden pallets in shipping and prohibit imports of live nursery stock.

Using new science and technology, scientists are now able to calculate the “benefits” of trees, and can measure the success of individual trees, or entire urban forests, in mitigating air pollution, heat islands, storm water, and myriad economic benefits. Has this data helped to expand the tree population in cities? I wonder also how data can help trees survive in polluted environments. I think I read somewhere some years back that the trees planted in our home town, Washington. D.C. don’t live as long as they should, for example.

When USFS scientist Greg McPherson produced the 2007 i-Tree report for New York City, it showed that its 600,000 street trees saved $35.6 million just in storm water mitigation, not to mention $28 million in energy costs, while each tree annually absorbed about 1.73 pounds of air pollutants each year. As mentioned earlier, this inspired New York to embark on a big tree-planting push. Other cities similarly inspired through i-Tree data have been Philadelphia, Milwaukee, Sacramento, Baltimore, Atlanta, and Denver, to name just a few.

This new i-Tree science that generates basic facts about a city’s urban forest – the actual number of trees, their species, maturity, health, and mapped locations, and then quantify their benefits – means arborists and city officials can be strategic about where and what and how they plant. The status of trees has been transformed from mere ornament to valuable green infrastructure. We know from mapping and data that trees are usually markers of affluence. And, as a growing body of science underscores how essential trees are to human well-being, environmental justice means focusing tree planting in the most barren neighborhoods. With what we now know, any smart city politicians or managers should be striving to create as lush a forest as possible.

I-Tree can answer myriad questions, including how long trees live in specific urban locations. It is no surprise that city trees – especially street trees — do not generally live as long as trees in parks and forests, a far more natural environment. Data might help prolong the life of trees by showing how well certain trees fare in various locations.

Jill Jonnes is the author of Eiffel’s TowerConquering GothamEmpires of Light, and South Bronx Rising. She was named a National Endowment for the Humanities scholar and has received several grants from the Ford Foundation. For more information, please visit: www.jilljonnes.com