City trees are leaking greenhouse gases—but U of T researchers say there's a fix

It is well known that urban trees help mitigate climate change impacts by shading and evaporative cooling, and also are important sinks for atmospheric CO₂. But what about the other major greenhouse gases, namely methane (CH₄) and nitrous oxide (N₂O) that together account for about one-third of global warming?

Daniels forestry PhD student Mohd. Rezaul (“Rony”) Karim and Professor Sean Thomas made an educated guess about urban trees that grow in soils subject to all manner of insults and injuries, ranging from compaction to dog pee: namely, that these soil conditions would lead to production of non-CO₂ greenhouse gases both from the soil and, when channeled though the water-conducting xylem vessels, these gases would then be emitted from tree leaves.

Equipped with new-generation ultra-sensitive gas analyzers and a measuring system developed by U of T forestry affiliate startup company CredoSense LLC, Karim and Thomas found that, sure enough, representative urban street trees in Toronto do show greenhouse gas emissions both from the soil surface of planting pits, and from leaves. The emission rates are much lower, than, say, landfills—but the total leaf area of urban trees is hundreds of times larger, so the total effect is significant.

Karim and Thomas also examined street trees where the soil was treated with biochar, a charcoal-like material made from wood waste materials, and currently being used in a city-wide trial coordinated by the Thomas Lab (with funding from the Natural Sciences and Engineering Research Council’s Alliance Mission grants program). Remarkably, trees with surface treatments of biochar showed not only reduced emissions of methane and nitrous oxide, but in the case of methane both soils and tree leaves were taking up methane from the atmosphere.

The takeaway from this research is both cautionary and hopeful: urban trees remain vital climate allies, but their role is more complex than previously thought. With better soil management, Toronto and other cities may be able to unlock even greater climate benefits from the forests already growing along our streets.

Karim and Thomas’s paper, “Urban Tree Channeling of Soil Methane and Nitrous Oxide and Its Mitigation Using Biochar,” is published today (27 April 2026) in Environmental Science & Technology.

Rony Karim with master’s student Christine Bogle making measurements on College Street.