Smithsonian National Zoological Park l Friends of the National Zoo



Trees of Life

Living wonders surround you at the Smithsonian’s National Zoo, and not all of them are animals.

Written by Shannon Fischer
Photos by Mehgan Murphy

The most enduring exhibit at the National Zoo are not the elephants, nor the gorillas, nor even the ancient Aldabra tortoises. They are the trees. Some have stood since before the Zoo even existed; some are saplings, planted weeks ago. All are beautiful and wild, and a part of what could be considered the grandest exhibit of all.

flowering ash
In the springtime, trees like this flowering ash (Fraxinus ornus) lend an ethereal beauty to the Zoo’s main walkway with their feathery blooms.

There are perhaps 30,000 trees growing on the Zoo’s 163 acres of land. Of these, 6,500 trees—mostly the ones that grow within and beside the public park area—have been thoroughly inventoried by name, status, and maintenance requirements. There are ash trees, birch trees, beech trees, and elms. There are Japanese zelkovas and Chinese scholars and dozens of variety of oak. They require endless monitoring and care, yet in return, they provide vital shade, feed the animals, and enhance the exhibits. They make the park beautiful and lush—creating an escape from the city and a refuge from the chaos. “This is a place for relief,” says Chuck Fillah, the Zoo’s associate director for exhibits and park management. “You walk through the gate, and to me, it’s like social decompression.”

The thing about trees is that they are so very unassuming and universal that it’s easy to forget that they are actively alive. In many ways, they mirror their animal cousins with startlingly accuracy: They bleed when wounded, and fall prey to viruses and bacterial and fungal infections. They wither when undernourished, but grow an unhealthy abundance of branches when overfed. The arborists who care for them do so with the skill of a surgeon and the devotion of an animal keeper.

The Zoo has two certified arborists on staff. Frank Clements joined the Zoo’s horticulture department 23 years ago, first as a gardener, now as the department’s chief and park manager. Over the years, he’s watched saplings he planted grow into mature trees, and seen mature trees fall prey to age or disease. Clements used to handle the bulk of the arboricultural legwork, but now manages the administrative work and organization regarding not only the trees, but every other plant within the park’s limits. Newly certified arborist Gene Stano has taken up the pruning shears in his stead and begun the task of acquainting himself with all 6,500 trees now under his care.

white oakSometmes, when construction occurs, trees can be left with the short end of the stick. But not at the National Zoo. Before construction begins, the architects, contractors, and head of horticulture meet at the site and take a look at each tree. They mark out which trees can stay and which must be moved or taken down. On those that stay—like this white oak (Quercus alba) standing above the construction site for the new Elephant Trails—the team fences off what’s called the Critical Root Zone (CRZ), or the ground area containing most of the root system. “As much of a tree is under the ground as above,” Clements explains. “They’re engineering miracles.”

No drilling, digging, storage, or equipment use can occur anywhere within the CRZ. The only allowed method of getting down to the tree’s roots, when it’s necessary, is via an air spade, which essentially blows the soil away. “When we build, we take the trees as serious assets,” Fillah says.

For both men, safety is the top priority. In a park inhabited by 2,000 animals in exhibit and visited by more than 2 million people each year, weak limbs or listing trees are potential disasters.

Twice a year (once when the leaves are on, and again after they’ve fallen), an outside company sweeps through the Zoo and evaluates each tree for safety. In-house and contracted arborists follow up on the resulting recommendations—
monitoring those trees that need watching, pruning dying limbs, and, when necessary, removing a failing tree altogether and replacing it with a new, healthier tree.

A Partnership for Life

As much as the Zoo takes care of its arboricultural residents, the trees repay the favor with interest. They absorb carbon dioxide from the city air and filter contaminants from the soil. Their roots control soil erosion from spring thaws and summer storms, and their leaves release cool water into the air, dropping the temperature by at least a few degrees, especially welcome in the heat of a D.C. summer.

Just as valuable are the trees’ other contributions to the Zoo. Many, like the mulberry, tulip poplar, and sugar maple, provide a steady supply of twigs and leaves—called browse—to the Zoo’s herbivores. The horticulture team sends trimmings at least once a week, sometimes more, to the nutrition department to distribute among the animals. It’s an easy convenience for the nutritionists, and an invaluable form of enrichment for elephants, bears, hoofstock, and other animals that enjoy the fresh cuttings.

The trees also promote the Zoo’s conservation and preservation missions by providing a haven for local wildlife. Chipmunks, squirrels, and deer have long recognized the park as a food-rich sanctuary where few predators will disturb their peace. Yet the Neotropical migratory songbirds—the warblers, thrushes, swallows, and others—are some of the most valued unofficial guests. They’re a critical part of the native ecosystems, contributing to insect control, pollination, and seed dispersal. But many of these birds have suffered significant population declines as agriculture and suburban sprawl replace their northern and southern habitats.

white oak

This ancient white oak (Quercus alba) is one of the Zoo’s best examples of strength and endurance. It’s about 150 to 170 years old, Clements estimates, older than the Zoo itself. After thousands of storms and rains and drought, even after a bolt of lightning gouged a deep wound down the length of its trunk, it’s still alive and healthy.

And as long as relatively little trauma occurs to its roots, Clements says, “this tree has every potential to be here for another 100 years.”

Zoo efforts to cultivate native over nonnative trees in the surrounding forest provide a haven for these birds by promoting native insect species: the foundation for the local food web. In fact, oak trees—which make up perhaps a quarter of the National Zoo’s total tree population—are one of the single best trees for insect biodiversity. And, as Greg Gough, an avian ecologist with the Zoo’s Smithsonian Migratory Bird Center, explains: “Since birds eat bugs, oaks are good for birds, too.”

Even a fallen tree continues to serve a purpose. Indeed, at the National Zoo, a downed tree isn’t so much dead as it is about to start a new stage in its career. Almost every animal exhibit includes at least a piece of a tree; wood for scratching, climbing, perching, dam-building, and foodfinding all come from Zoo-grown trees. The endless supply of branches throughout the park means that keepers can create unlimited variation and stimulation for their animal’s exhibits—it’s one of the Zoo’s most celebrated in-house recycling programs. Earlier this spring, when Clements decided that a huge, old red oak standing in the cheetah yard needed to come down for safety reasons, the giant pandas, oryx, and the clouded leopards all got new “furniture” to climb. What isn’t used for the exhibits gets sent out to become compost—nutrition for a new cycle of plant growth. “We always try to be sustainable,” Clements says. “It’s one of the top goals at the Zoo.”

sapling in a protective tubeIn their work to promote native trees in Rock Creek Park, the horticulture team wages a never-ending war against two main opponents: deer and invasive plants. The deer, thanks to a lack of predators and an abundance of food, have become numerous and bold. They meander through the park, snacking on flowers, shrubs, and saplings. It is, Clements grumbles, “a freaking salad bar all the way through the Zoo.” To ward off the nibbling horde, arborists wrap new saplings with protective blue polyester tubes until the trees’ leaves are no longer in danger.

leavesEqually troublesome are the invasive non-native plants that grow abundantly throughout the area. They outpace their native competition and monopolize sun and soil. Some invasives even resemble their native counterpart so closely that the untrained eye can’t readily distinguish the two—like the fast-growing Asian ailanthus tree (Ailanthus altissima) and the native walnut (Juglans nigra). Arborists can easily tell the difference by small details in their bark and the edges of the leaves, so that they can confidently destroy the invaders but not the walnut. The casual observer, however, might try quickly pinching a leaf—the ailanthus will smell like peanut butter.

The walnut leaf is on the right, the ailanthus on the left.

Celebrating in Shades of Green

Ultimately the trees’ greatest gift is in what we learn from them. They tell stories about urban forests and African grasslands. They show a glimpse of life in the Amazon and an East Asian forest. “We have a great opportunity to teach people,” Fillah says. In simply being at the Zoo, a visitor is surrounded by thousands and thousands of trees. Ideally, Fillah explains, people connect to these trees—to nature—at a conscious or subconscious level, and they recognize that this is a place that has a special feeling. The experience communicates the importance of the trees and nature to the welfare of people and the animals. “And hopefully,” he adds, “it leads to them to thinking about the value of nature and conservation.”

Recognizing the importance of the trees and the sensations they invoke, the Zoo is beginning a concerted effort to become an arboretum. It’s a challenging endeavor—in large part because there are few universal rules regulating what specifically does and does not constitute an arboretum. There are, however, several general types. Some exist primarily for research and science purposes, creating what are essentially living laboratories; others simply aim to provide the public with beauty, nature, and an opportunity to learn. Ultimately, this is the kind of arboretum that the National Zoo hopes to become.

fallen oak

Zoo arborists go out of their way to encourage local wildlife to live in the Zoo’s trees. Often, when a tree fails, they leave a length of dead stump like this fallen oak, for salamanders, insects, grubs, and mice to inhabit—creatures critical to the local food web.

Living trees accomplish much the same. In fact, they’re a well-known vacation spot for the hundreds of migrating black-crowned night herons that descend upon the Zoo each spring to roost in the tall trees by the Bird House. It’s the only place in D.C. where these birds nest, and they’ve been doing it for so long—more than 100 years—that they’ve become their own exhibit.

Certainly, the Zoo already satisfies the most fundamental requirement—that of having a living collection of trees and shrubs. What remains is the selection of the collections, and the creation of a mission statement. The presence of the animals and the environments tailored to their needs adds an unusual element, as does the Zoo’s rejection of invasive species. “We have a bit of a unique campus with specific zoogeographic species,” Stano says—one that could provide a defining characteristic in the project’s mission statement.

In essence, the goal is to create educational opportunities through tree labeling and, eventually, classes on horticulture and arboriculture in a zoological setting. Project planning remains at the very earliest stages, but Clements and Stano have begun to tentatively identify candidate trees for the first component. A selected tree will bear a small marker—such as the signs already on some trees throughout the park now—listing its common name, genus, species, origin, and cultivar (cultivated variant—a red “sunset” maple, for example, as distinct from a red maple). At least one representative specimen of every genus (and there are more than 200 different genera on Zoo grounds) will be marked.

But in the end, the arboretum will formalize and spotlight what is already true. These trees are a living exhibit, as beautiful, varied, and valuable as the animals with which they live.


—Shannon Fischer is an editorial intern for Smithsonian Zoogoer magazine, and a graduate student at Boston University’s science journalism program.


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Smithsonian Zoogoer 38(5) 2009. Copyright 2009 Friends of the National Zoo. All rights reserved.