MANHATTAN’S TREES TEND to get overshadowed—quite literally in some cases—by skyscrapers that line the city’s streets. But they play an essential role in the city’s habitat: providing oxygen, supporting wildlife, and managing the flow and movement of rainwater. Keeping all of those ecological elements in check, though, means the city needs to keep careful tabs on its trees. Starting in May, the NYC Department of Parks and Recreation will begin a five to seven month-long street census to locate and identify the species of every street-side tree.
Once those data are in hand, the city will have to figure out how to visualize them. Brooklyn web developer Jill Hubley is one step ahead: She made this map, pinpointing the 592,130 street trees (52 species!) in New York as they were documented in the last census in 2005. It’s an excellent tool that sheds light on some peculiar patterns in the city’s landscape. And they reveal a lot about how the NYC Department of Parks and Recreation makes its decisions—and how they can be making them better.
Home of the trends that emerge from Hubley’s maps are innocuous: Queens has the most trees, thanks to its spacious, more residential layout, while Manhattan has more trees per block, especially around Central Park. Although the Parks Department has planted a wide variety of trees in order to promote biodiversity, Hubley’s map shows some clustering of certain tree species in different parts of the city. London planes are the most common trees in the city, with heavy concentrations in Brooklyn. Honey locusts, rarely found in the Northeast, are found in great numbers on the streets of Manhattan.
Look closer, though, and the tree map reveals some more concerning patterns. Traditionally, the Parks Department has worked to avoid floral monocultures—homogenous plant populations—by shaking things up and growing nonnatives throughout the city. But if those species aren’t chosen very carefully, they can have a potentially harmful and even dangerous impact on the surrounding area.
For example, Callery pears, a invasive species from China, are all over the region, particularly in Staten Island. Besides bearing responsibility for a repulsive smell (many people think Callery pears give off an aroma reminiscent of rotting fish, chlorine, or semen—so it more-or-less fits in with NYC’s odor profile), the tree is susceptible to losing limbs or keeling over during strong winds, heavy snowfall or ice storms. In other words, it makes no sense to have it present in such large numbers throughout New York City, especially after Hurricane Sandy.
Even when it comes to species you might expect to find in New York—such as oak, maple, and elm—the Parks Department chose to plant Asian cultivars instead of North American ones. “There’s only one oak species on the list that isn’t from Asia—and it’s an English oak, native to Europe,” says Doug Tallamy, a wildlife ecologist from the University of Delaware.
hy is this a problem? According to Tallamy, native trees are better at providing the slew of “ecosystem services” that humans live off of, like the management of watersheds, prevention of floods, and perhaps most importantly, supporting pollinators. Forget about bees and butterflies—80 percent of all flowering plants are pollinated by animals, like birds. Many tree species, like the Callery pear, are just ornamentals that cannot support certain insect populations that birds live off of. Absent birds can lead to ecosystem collapse.
The solutions are actually quite easy to implement. Many of the Asian cultivars can be replaced with a North American counterpart—such as Chinese elms with American elms, or Japanese and Norway maples with sugar maples. Tallamy dissmises any grumblings that native species are too big or too difficult to grow as nonsense. “We have native species that are smaller that we can still grow—we just need to start planting them.”
The city might not be able to maintain all native species—with the emerald ash borer making its way east, the green ash is likely to see its numbers plummet—but many like Tallamy hope to see a much better balance in tree diversity. And digital tools like Hubley’s map make it much easier to observe that imbalance and act on it.
They might not have to wait long. The city’s new survey of street trees begins this summer. They expect to wrap up by the fall and have the data publicly available by fall of next year. Rich Hallett, an ecologist at the US Forest Service’s Northern Research Station, cites plans to integrate the data on a digital platform that can be updated in real time and act as a “living document.”
At the very least, the city might take cues from the new data and decide there are enough Callery pears. Most New Yorkers will agree—they can do without the smell.