Cecilia Lee, Senior Illustrator

One of the sneakiest carbon culprits might be sitting right on the curb, according to a recent study by Yale researchers. 

Led by Yuan Yao, assistant professor of industrial ecology and sustainable systems at the School of Environment, the study assessed the environmental footprint of urban tree waste and evaluated the effects of different disposal methods. The study indicates that diverting tree waste from the landfill can significantly reduce carbon emissions.

“Our study highlights the strong need to divert urban tree wastes from landfilling and incineration to valuable utilization,” Yao wrote to the News.

The research team conducted a Life Cycle Assessment that quantified the environmental benefits of reusing, repurposing or recycling organic material. They tested a combination of different pathways to treating waste — ranging from “low utilization” like landfills to “optimal utilization” like biochar production — and determined that even “fair utilization” methods such as incineration could drop the amount of carbon emissions by roughly 115 megatons.

Overall, wood chips and mulch are good options for dealing with fallen trees, but converting them to durable wood products can further reduce greenhouse gas emissions, Yao wrote.

“Our study demonstrates the potential benefits of applying circular economy principles to biomass waste in the urban environment to potentially combat with climate change,” postdoctoral researcher and co-author of the study Kai Lan said. 

The amount of yard waste in landfills has declined since 1990, but the U.S. continues to discard about 10.5 million tons each year. While urban areas are not usually thought of as significant contributors of tree waste, they still leave a sizable footprint. The U.S. urban forest generates over 25 million oven dry metric tons of leaf waste and over 20 million metric tons of tree waste per year. That is more than 20 megatons of carbon mass and minerals which, once left in the landfills, can enter the atmosphere or water streams as they decompose.

Some of the cities that would see the greatest global warming potential reductions from diverting urban tree waste include New York, Atlanta, Boston, Chicago and Philadelphia. The team’s study found that “full utilization” of tree waste through lumber, woodchip or compost use could potentially offset about 127.4 to 251.8 megatons of overall carbon dioxide each year. It would also remove anywhere from 93.9 to 192.7 kilotons of nitrogen in bodies of water annually.

Even so, the most effective waste treatment methods may vary by state. Differences in geography mean that certain regions would benefit from approaches tailored to their local flora. Northeastern regions with dense, deciduous forests would decrease their carbon outputs most by using fallen leaves as compost or mulch. On the other hand, states like California would see the greatest global warming potential reductions by repurposing lumber.

For many cities though, putting these findings into practice is expected to be a long-term project. The research paper recognizes that illegal dumping, a lack of municipal facilities and infrastructure stand in the way of maximizing urban yard waste’s potential.

Efficient use of tree waste is a joint effort between individuals, government and communities. The trees on private property vastly outnumber those on public land, explained Colleen Murphy-Dunning, lecturer in urban and community forestry at the School of the Environment and director of the Urban Resources Initiative. Applying this circular economy concept to urban forests will require raising awareness about tree waste disposal methods to not just the public work department, but private landowners as well.

Some of the furniture industry is responding to this trend. In an effort to decrease waste, a handful of companies like Room & Board and Brick + Board have cropped up over the past decade, which repurpose fallen trees into high-end fixtures.

Local businesses like CityBench have meanwhile collected trees throughout New Haven and given fallen timber a second life in dining tables, chairs and shelves. CityBench co-owner Zeb Esselstyn was inspired to start the company after reading that “35-40% of hardwood lumber that’s used each year could be replaced if you just tapped into the urban forest.” Since 2009, he and his brother have made thousands of furniture pieces from salvaged urban hardwood.

The city of New Haven has approximately 29,000 street trees.

HANWEN ZHANG