AMERICAN TREES ARE in trouble. Based on recent estimates, up to one in every six native species in the continental US is in danger of going extinct, due to mounting threats such as invasive species, diseases, climate change, logging, and wildfires. Metropolitan areas, meanwhile, are losing an astounding 36 million trees every year, according to a 2018 study from the US Department of Agriculture’s Forest Service.
This loss of urban trees is a particular problem. They’re a critical part of the green infrastructure of US cities. Without the cooling effect of foliage, a city’s sprawling concrete and asphalt can turn into an urban island of deadly high heat—made even worse by global warming—which then forces buildings to use more energy to stay cool. Trees also lower air pollution and sequester carbon. The Forest Service estimates the annual cost of urban tree loss at $96 million.
But there’s a way to attack this problem on multiple fronts, using undervalued waste—from trees and people—that would otherwise be sent to landfill.
A new analysis from Yale University suggests that the dry waste from urban trees in the US—leaves, cuttings, and so on—could be diverted from landfills or incinerators, where much of it still ends up, and instead be reused to grow new trees, reduce logging, and lower carbon emissions. It’s a potentially huge resource: US cities generate more than 45 million tons of tree waste every year.
“This isn’t a new idea, it just takes money and willpower from city officials to be more sustainable,” says Pete Smith, program manager for urban forestry at the Arbor Day Foundation, a tree-planting nonprofit. Reusing and recycling urban wood is already becoming a growing focal point for cities, he says.
The Yale researchers, led by Yuan Yao at the university’s Center for Industrial Ecology, calculated that converting leaves into compost, wood into chips and lumber, and the remaining tree residues into a charcoal-like substance called biochar, can be environmentally beneficial on several levels. “These products can be substitutes for virgin materials such as fertilizers, and thus reduce associated environmental impacts,” the authors write. Recycling cut-down tree wood for lumber can also store carbon for the long term and reduce logging. Biochar, meanwhile, can be used to improve aeration, water storage, and nutrient retention in soils.
Recycling the country’s urban tree waste would also significantly reduce the amount of greenhouse gasses leaking from landfills, the authors found. They calculated the savings could be equivalent to 28 percent of the total agricultural emissions of the US.
One of the products created through recycling—compost—can then be redirected specifically back to the problem of urban tree loss. For trees, mulch and compost mimic the forest floor and help the soil around their roots retain water and nutrients, which is critical in urban settings where trees can be exposed to hotter, drier, and more stressful growing conditions. For example, although trees planted along city highways can help increase the urban canopy cover, roadway construction can cause soil compaction and topsoil loss, causing lethal stress to saplings planted roadside. A 2020 study showed that roadside trees had a significant survival advantage when the soil around them included 25 percent organic compost made from food and yard waste.
And tree waste isn’t the only unwanted product that can be used to make this compost. Cities could also nourish their trees with a little assistance from their human residents: by recycling their poop.
A growing number of wastewater treatment plants are devising ways to produce safe soil amendments based on recycled biosolids—the feces and other organic matter that pass through water treatment plants and also often end up in landfills or incinerators. DC Water, which provides water and sewer services for the US capital, is one such example, creating a heat- and pressure-treated biosolids product called Bloom. It expects to sell about 60,000 tons of it to farmers, landscapers, and other customers in 2022 alone. The aim is to change the mindset around human waste, says Chris Peot, DC Water’s director of resource recovery. “It’s not a liability; it’s an asset.”
Some cities are already using tree- and human-waste products to improve their urban greenery. In Austin, Texas, treated biosolids are composted with tree and yard trimmings from the city and then cured for several months to create a soil product called Dillo Dirt. It’s a product the city has been making for decades, and among its many applications, it’s being used to aid tree planting and maintenance. Since 2006, the city has managed to increase its tree canopy cover by 20 percent.
Meanwhile, in Washington, DC, Bloom has been used to create a variety of blended products to improve the condition of soils, including one mixed with sand, one blend that’s like potting soil, and a “woody blend” that includes excess bits of aged hardwood. If directly applied to soils, Bloom’s high nitrogen and high phosphorus content would kill plants, Peot explains, but mixing it with tree trimmings, hardwood, or other organic waste helps balance the nutrients. “We’re trying to mimic natural soils,” he says.
April Thompson, Bloom’s director of marketing and sales, says gardeners have reported great results with the soil product on the US Capitol grounds and in community gardens, among other applications. “Hemp, hops, hay. You name it, we’ve probably helped grow it,” she says. That list includes street trees. Under a contract with the city, nonprofit organization Casey Trees has used Bloom as part of its gardening toolkit around the capital. Over the next 10 years, DC aims to buck the trend of urban tree loss and increase its canopy coverage from 38 percent to 40 percent. Recent research suggests the cooling effect this will provide could be life-saving, particularly given the ability of trees to temper the rapid rise in urban surface temperatures.
Of course, recycling trees and human waste won’t be enough, on its own, for increasing the number of trees in cities. But to help stop the decline in urban tree cover, it can be a very useful part of the solution—and it is, hopefully, catching on. DC Water has consulted with plant operators in Virginia, Kentucky, Texas, California, and elsewhere who are installing similar heat and pressure systems to turn their biosolids into plant-nourishing products.
“Contractors are all going to them and saying, ‘Hey, we’ll take this product away from you because it’s a problem,’” says Peot. “And we’re saying, ‘No, it’s not a problem.’”