The terms natural and artificial can quickly become somewhat confusing in a place like New York City, where even the so-called natural places, like Central Park and Prospect Park, are built environments.

Constructed on swamp land, Prospect Park was conceived, in the words of James S. T. Stranahan, the President of the Brooklyn Board of Commissioners in 1860, as “the great natural park of the country, presenting the most majestic views of land and ocean, with panoramic changes more varied and beautiful than can be found within the boundaries of any city or the continent.” The trees that Olmsted and Vaux planted to achieve this effect were, for the most part, non-native species, and many of them fared better than their “native” counterparts. Even what is the most famous tree in Brooklyn, Betty Smith’s Ailanthus, was brought from China by Olmsted, who liked it because it looked tropical.

The Ailanthus is now considered a veritable weed, a gritty, street-wise aggressor that can thrive in crowded conditions and contend with a barrage of strange and sometimes dangerous elements, everything from car exhaust and dog urine, to limited areas for root growth, underground heat from subways, and the artificial shade of skyscrapers. Like the Ailanthus (and not unlike its human neighbors), the trees that are best suited to the unique conditions of living in New York City are hearty, adaptable, able to tolerate a lot of abuse, good at making “money” (arborist slang for photosynthesis) and prepared to contend with significant space constraints.

Nina Bassuk, a professor at Cornell University who has been studying urban trees for over 20 years, readily admits that cities were not built for trees. But she also feels strongly that trees need to be where people are, not kept, as she says, “behind glass cages.” Much of Bassuk’s research is focused on understanding what a tree needs from a biological standpoint and what types of trees are best suited to life in the city. Is a planting site extremely wet or windy? If so, she looks at trees that flourish in other rugged environments, such as shorelines or floodplains, conditions, she says, which mimic those of the urban environment. “Looking at the conditions underground is really important, particularly how to find a compatible relationship between sidewalks and roots.”

In studying the various stresses with which urban street trees must contend, Bassuk has found that people have conventional wisdom about what the issues are but often these are not scientifically valid. “Pollution is not a big concern for trees in the Northeast because there’s good air drainage,” she says, “and although some people think vandalism is an issue, it is really a very small issue.” Asking her about my favorite bit of conventional wisdom related to tree damage, namely dog urine, she laughed. “A little bit of urine can be good for a tree since it gives a little bit of nitrogen. It is only when a tree becomes popular, a kind of territory marker, that the urine can cause bark damage.” The effects of urban microclimates are, according to Bassuk, some of the more important, and least understood, stresses that trees face in the city. Her study of trees on Columbus Avenue found that when it was 86 degrees in Central Park, it was 108 degrees on parts of the avenue, a result of re-radiated heat from asphalt, cars, bricks and windows.

Regarding the issue of what is beautiful in a tree, Bassuk, again, takes a scientific approach. Variety, she believes, is really the most important factor in deciding which trees should be planted. She has found that “a reasonable goal for most urban plantings is to place a five percent limit on any one species within the total municipal tree population. Therefore, if a disease or an insect infestation should occur, 95 percent of a tree population would remain intact.” Although Bassuk has compiled a list of over 50 trees suitable to the urban environment, she and other arborists continue to be surprised by the almost miniscule palette of street trees—often made up of less than five species—in New York and other cities.

Christopher Roddick, an arborist at the Brooklyn Botanic Garden, recently completed an inventory of trees in the Brooklyn Heights neighborhood and found that only four species—the Norway maple, the London plane tree, the pin oak and the Bradford pear—were most common. Like Bassuk, he believes there is a wide variety of trees that can prosper in the city, and he talked to me about some of the more canny ways in which urban trees survive. “Have you ever been on a submarine?” he asked. “That’s kind of what trees are like. They are divided into compartments and if one piece is damaged, it seals itself off so the whole tree is not at risk. Trees are easy targets. They can’t run away from predators so they need to figure out other ways to survive, they are either good at propagating or good at compartmentalizing.” And some trees, like the London plane tree, which is a hybrid of the Oriental and American sycamore, have even developed ways to outsmart potential predators. Roddick explained how, in the event of a fungus attack, London Planes are prepared to shed their leaves and protect their buds. “In most trees, the petiole, or the bottom part of the leaf stem, grows right out of the bud. But in this case, the petiole grows around the bud so if the leaf falls from disease, it will sprout another.” I’ve always admired the mottled texture and green brown coloring of the bark. And its appearance, it turns out, is actually a functional trait. The tree sheds portions of its bark to allow photosynthesis to occur on the surface of the trunk and branches.

Another hearty urban dweller that is quite common in New York City is the gingko. Roddick explained that it is a very old species, actually prehistoric, and was once native to North America. But it became extinct and was cultivated in China in nurseries. It was re-introduced to North America in the 18th century and is most readily recognizable by its leaves, which look like small fans or duck feet, and its nubby branches. In the fall, you may see what look like large whitish cherries on the sidewalk below the gingko and notice a smell akin to soiled baby diapers. These are the gingko seeds, which fall to the sidewalks and are trampled by pedestrians. On the streets of Chinatown, women wearing rubber gloves and squatting amongst the stinky “fruit” will gather the seeds in buckets (the gloves are necessary because the seed coat contains an allergen similar to poison oak). The seeds can be roasted and are often served with wine, or sake, since their properties are thought to alleviate hangovers. Another interesting tradition relates to the gingko leaves, which, when used as bookmarks, are thought to ward off booklice and silverfishes. (Note: I tried this with limited success—the leaves are beautiful but sometimes crumble between the pages.)

The painter Piet Mondrian once, allegedly, wrote a letter to the mayor of New York asking that all of the trees be removed from Manhattan. Their naturalness, he felt, was discordant with the angularity, the basic grid pattern, which was part of the aesthetic coherence of the city. One contemporary sculpture, which was in Central Park as part of the 2002 Whitney Biennial, Roxy Paine’s “Bluff,” asked not so much that nature be removed, as replaced, to protect the aesthetic coherence of the city. With the same steel materials used to build the city’s skyscrapers, Paine sculpted a perfect replica of a tree. I was particularly impressed with the delicacy of the sculpture’s branches and the way in which the welded seams looked a little like the real joints between twig and branch. The sculpture was sited just next to Olmsted’s grand Allée Elm, and, when I visited the metal tree, I watched a series of people walk up to it, ask what it was, “not a real tree, huh?” and then slap the trunk a couple of times. People seemed to like having their photographs taken next to the tree, something, I noticed, they were decidedly less interested in doing next to any of the real trees in the area. “Bluff” makes explicit what many of us already know about trees in the city—that they are objects of art as much as of nature.