“There is, I think, great hope for the continued presence of nature in the metropolis.”
— Elizabeth Barlow, The Forests and Wetlands of New York City, 1971
Surrounded by a grid of residential and commercial streets, playgrounds, and baseball fields, the sandy trails of Brooklyn’s Marine Park wind through thickets of sumac and cedar. Beyond lies Jamaica Bay, hidden but for the salt air that drifts into the woods. A stand of cherry and red oak trees seems at home, here on the edge of the city on the edge of the sea. But this is a new forest, planted as part of an initiative to add one million trees to New York City.
On Earth Day 2007, Mayor Michael Bloomberg announced MillionTreesNYC, a partnership of the nonprofit New York Restoration Project and the New York City Department of Parks and Recreation (NYC Parks). There were many reasons to plant trees in the city. By shading and cooling neighborhoods, purifying air and water, providing wildlife habitat, trees can make urban life better.
Tree-planting campaigns have become popular in cities around the world. But such projects carry risk. Putting trees in the ground feels good and looks good, but planting efforts can fail. Carbon capture claims are often overestimated. And – although this is changing – enhanced green space tends to disproportionately benefit wealthier residents.
In New York City, there was also an immediate challenge: where to equitably plant a million trees in a global metropolis of concrete, pavement, steel, and glass?
Most of the trees were destined for public sidewalks and private yards, aided by thousands of volunteers and property owners. More volunteers and scientists helped NYC Parks plant another 400,000 trees along park edges, rights-of-way, and in scattered fragments of public land, with the intent of establishing, in Parks Commissioner Mitchell Silver’s words, “new, ecologically healthy, multistory forests that benefit all New Yorkers.”
Through this collective effort, the city met its million-tree goal a full two years ahead of schedule. On Friday, November 20, 2015, elected officials, volunteers, and third-graders from a local public school gathered at Joyce Kilmer Park in the Bronx to witness the planting of the millionth tree, a basswood sapling. Bloomberg joined the city’s new mayor Bill de Blasio, actress and New York Restoration Project cofounder Bette Midler, and others for a photo opp, shoveling topsoil around the basswood’s roots. A press release promoting the event called for all New Yorkers “to make a pledge to adopt and care for trees in their communities.”
Since completion of the project, many of the plantings, such as the oaks and cherries at Marine Park, have become established young forests. But not every planting site did so well.
Similar Plantings, Stark Differences
Park staff and their scientist collaborators considered many possible explanations for unsuccessful plantings, including intense heat, pollution, and invasive species, all of which tend to be worse in urban areas. A drought in the first few years didn’t help, nor did accidental mowing by maintenance crews. But after years of trees growing – or not growing – it was clear something deeper was going on.
“When some of the trees were over our heads and other sites had nothing, the visual drama, the differences, became much more stark,” said Timon McPhearson, director of the Urban Systems Lab at The New School, who had helped set up experiments to study the growth of some of the million trees.
McPhearson and other researchers suspected their tree-planting problem might have something to do with the soil. But not until graduate student Gisselle Mejía investigated did they gain a deeper glimpse into what was going on beneath the surface of the urban forest.
Mejía grew up in New York City and has an extensive background in trees. As part of a master’s degree in community forestry at Michigan Technological University, she served in the Peace Corps, working with small-scale woodlot managers in El Salvador.
She returned home to work for NYC Parks, selecting trees to plant in drainage swales and along city streets as part of a “green infrastructure unit.” Her work was separate from, but in parallel with, MillionTreesNYC, both of which were among many forest-focused initiatives in New York.
She followed this with a job at City Harvest, a local food initiative, in her home neighborhood of Washington Heights at the very northern tip of Manhattan Island. “I liked engaging with residents,” she said, “but I wanted to get back into urban forestry.” In 2017, she enrolled in a PhD program in Earth and Environmental Sciences at City University of New York.
Only then did she start thinking about soil.
Mejía joined an interdisciplinary team, including McPhearson and Peter Groffman, a professor at the City University of New York Advanced Research Center, who were evaluating the long-term effectiveness of different tree-planting strategies. Some of the million trees had been planted in experimental plots at Marine Park and eight other parks in Brooklyn, Queens, Staten Island, and the Bronx. Researchers had already studied tree survival, plant growth, and soil composition, but no one had yet focused on soil processes.
Trees take from the earth, but they also give back, via the leaves and needles they shed to the ground and the fungi and bacteria associated with their roots. These microbes help cycle carbon and nitrogen – essential elements for tree growth – in a process that has been extensively studied in rural forests (see the related article on forest soil carbon) but much less so in urban areas.
Perhaps, thought the researchers, this nutrient cycling could explain differences in tree-planting success.
At 10 of the experimental plots, Mejía extracted multiple 1-meter cores of soil – where possible. Some of the sites were so full of construction debris, commonly used for filling in park lands, that she couldn’t get down the full meter. While she and some fellow researchers were sampling at Clearview Park in Queens, a neighbor came out and said that the park used to be a dump; discarded furniture and old appliances were evidence that, to some degree, it still was. The trees, planted on a wedge of sunlit field extending off the sixth green of a golf course, between a residential neighborhood and the Clearview Expressway, were sparse and skinny, and the plot was overgrown with invasive mugwort and briars. Nearby, at Alley Pond Park, and at Canarsie Park in Brooklyn, the trees struggled so much that the sites had to be replanted, but some trees still died.
Mejía’s analysis showed that the sites with poor tree growth had less nitrogen in the soil, less microbial biomass, and fewer roots. They also had histories of disturbance, for example, as dumping grounds for trash, coal ash, and concrete rubble.
A Troubled Legacy
When Elizabeth Barlow published The Forests and Wetlands of New York City in 1971, the parks – indeed much of the city – were in a sorry state. With bankruptcy looming, city parks were neglected. Wooded areas became crime scenes and junkyards for abandoned cars, fields were overgrown with invasive plants, garbage filled the marshes, and sewage steeped in bays and harbors.
Eventually, the city crawled out of crisis. Investment in the parks department and action by neighbors, along with clean air and water legislation, helped revive green spaces. Park staff cleared debris, closed roads, and put up gates to keep out cars. They cut invasive vines and planted (and replanted) native trees. Birds and other wildlife returned, and people reconnected with the forests and wetlands. But the legacy of past abuses lingered.
When Mejía visited Canarsie Park in Brooklyn to collect soil samples, the oaks and cherries were mere stumps – the planted trees died, and park staff later mowed over the area. Cinderblocks stuck out of the grass. The soil at Canarsie is classified as “Bigapple fine sand,” according to the New York State Soil Survey, one of many different soil types formed in “human-altered and human-transported material,” which can have up to 75 percent coarse fragments of brick, concrete, and other debris.
It’s hard to sink roots in such troubled earth. The soil sampled at Canarsie and other failed planting sites showed little capacity to capture, store, and cycle nitrogen.
In contrast, successful planting sites had more nitrogen, especially near the soil surface, suggesting the nitrogen was recently contributed by fallen leaves, tree roots, and other parts of the surrounding ecosystem and not part of the underlying soil layers. Mejía and her team concluded that, rather than being inherently rich in nitrogen, the soil had some characteristic or combination of characteristics, such as texture, moisture, and/or pH, that supported both seedling establishment and nutrient cycling. These were sites where the trees grew tall enough to create a closed canopy, and layers of leaves and other nitrogen-rich organic detritus collected on the ground. They included the Clove Lakes and Conference House parks in Staten Island, which have stayed relatively intact since the 1930s, and Fort Totten in Queens.
“It feels like a forest”
On a sunny day last November, the woods at Fort Totten Park were noisy with rustling leaves, and robins and starlings flew from tree to tree. The planted cherry trees had grown so much their bark was beginning to split into plates, their upper branches merged with those of oak and sweetgum overhead. Some of the young trees had grown faster than others, and shrubs had filled in on their own, overtaking the symmetry of the planting grid.
“Fort Totten is super impressive,” said Groffman, who studies nitrogen dynamics and served as Mejía’s advisor. Groffman and Mejía walked between the trees with their collaborator Elizabeth Cook of Barnard College. Cook was just finishing an analysis of plant-growth data.
Some experimental plots included only two tree species, while others, such as those at Fort Totten, had six. Initial studies found no benefit to soils from tree diversity, but Groffman said that evidence may emerge as the sites mature. He grabbed a fistful of leaves from the ground and held it up to Mejía and Cook. “Who knows what effect all these different species could have?”
Fort Totten has been relatively undisturbed since construction of the fort in the Civil War era. The experimental plot is at the northern end of the park, bordered by a black chain-link fence. On the other side, large maple trees grip the hillside above the confluence of the East River and Long Island Sound. Across the water at Pelham Bay Park in the Bronx, trees planted as part of the MillionTreeNYC goal have grown quickly in the rich, dark soil, and they’re now nearly indistinguishable from the adjacent forest.
Because the trees were put in areas not currently or recently forest, researchers consider the MillionTreesNYC plantings “afforestation,” not “reforestation.” But fragments of the city’s original forest do exist, and they represent what, if given enough time and space, a million new trees could become. To understand this potential, it helps to experience a forest that has been growing for hundreds of years instead of decades. Back on the island of Manhattan, take the A Train all the way uptown, to Inwood.
The Inwood Hill Example
When the last ice sheet melted away from what is now New York City, it left in its wake a layer of rock and mud that over tens of thousands of years became the forest foundation. Trees spread their roots through the earth as they grew tall toward the sun. Lenape families traveled to Man-a-ha-tonh, “place to gather wood for bows and arrows,” and feasted on shellfish from the nearby estuary beneath a towering canopy of oak, chestnut, walnut, tulip, hemlock, ash, and hickory trees.
For Dutch colonists, the forests were an impediment. Trees (and people) were cleared away. Over the centuries, as skyscrapers and highways replaced farms and estates, a fragment of early forest, including old-growth, persisted between the tidal waters of the Hudson and Harlem rivers, not far from where Gisselle Mejía grew up.
“So much of Inwood Hill is woodsy. It was always the park that I went to as a child. It was the closest thing to nature, and Inwood has a big presence in the neighborhood,” she said. A canopy of thick-trunked oak, tulip, and basswood trees still towers over Inwood Hill. The understory is alight with leaves of maple, sassafras, hackberry, and poison ivy.
The intact soils of a forest such as Inwood Hill are not replicable at other city locations, and the future urban forest is likely to include different species of trees and plants, as ecosystems adapt to ever-evolving conditions. Still, Inwood Hill provides a reference for imagining the future forest, how pockets of mature trees can provide spaces for young people, especially, to find refuge and joy, just as Gisselle Mejía did when she was growing up.
Inwood Hill Park also demonstrates the reality that urban forests require ongoing management. NYC Parks staff must continually suppress invasives and prevent erosion. Instead of trying to imitate some idealized forest of the past, said Mejía and Groffman, people need to reconsider expectations of nature in the city – and also have patience, added Cook.
The trees in the experimental MillionTreesNYC plots took about 10 years to create the beginnings of a canopy, said Cook: “By regularly monitoring tree growth in our plots, we are investigating which trees grow fast and survive better in urban environments, in order to help re-establish forests and the benefits they provide, but it will take a long time and a lot of patience and care to develop old forests like we see in Inwood.”
Planting – and Planning – for 30 Percent
The MillionTreesNYC project had an air of excitement. But there is nothing radical about planting trees, said McPhearson. “It’s really just a first step toward making cities more livable. We have to radically think about the ecological nature of cities. To be radical is not simply to make everything green, but to ensure that everyone has access to and benefits from that green.”
With a current grand total of seven million trees, New York City still has fewer than one tree per resident, and despite planners’ efforts, trees are not equally distributed. The city’s canopy cover of 22 percent is below the average for American cities. In The City that Never Sleeps, being below average in anything is unacceptable. Presidents of all five of the city’s boroughs have called for another million trees to be planted.
NYC Parks, with a $112 million allocation from Mayor Eric Adams, has prioritized planting tens of thousands of trees in “shade-deficient” neighborhoods. A new coalition, Forests for All NYC, has an agenda for the city to attain at least 30 percent canopy cover by 2035.
But what about the ground?
Adding fertilizer won’t overcome the deep problems of urban soil, up to 27 percent of which is in the “human-altered and human-transported material” category. Compost helps, although it’s expensive, but so is replanting sites over and over.
Mejía and Groffman argue for finer-scale soil mapping and more research into how to make urban soils more productive. “We need to both map and figure out how to remediate the most challenging areas,” said Groffman.
“We need more site-specific soil analysis to make sure these sites are adequate for planting, so these trees can grow and become the forest we want them to be,” added Mejía. Now a postdoctoral researcher at Dartmouth College, she hopes to return to the MillionTreesNYC sites to learn more about the soil chemistry.
The Soil Remembers
Back in Brooklyn’s Marine Park, home to one of two remaining maritime forests in New York City, trees planted for MillionTreesNYC are growing. The soil that surrounds their roots is getting more attention, thanks in part to the research of Mejía, Groffman, McPhearson, Cook, and their colleagues.
Consider what lies beneath the ground at Marine Park: the deepest layers are bits of gneiss worn down by the weight of ice, washed by glacial meltwater out onto the empty flats. Middle layers, a mix of concrete rubble left over from construction of the Gerritsen Beach public school and sediment dredged from tidal creeks and bays, date to the 1950s. The latest layers, dark and rich, formed organically as trees, shrubs, and salt-tolerant plants returned.
While smaller than the trees at Fort Totten or Pelham Bay, the planted trees at Marine Park are already contributing to wildlife habitat and biodiversity. Some have lichens on their trunks and nests in their branches. MillionTreesNYC has shown that even small areas of planted trees can feel like a forest, that the land holds possibility. But the project has also served as a warning: if you ask for a forest without answering for past mistakes, the soil is always there to remind you.