Planting trees in cities can bring a host of benefits. They pull pollutants and carbon dioxide out of the air, cool neighborhoods, reduce stormwater runoff, and make streets more inviting places to walk, bike, and live. These are especially poignant in a place like Cleveland, where over half of the tree canopy has been lost over the last 75 years.

Since 2019, Cuyahoga County’s Healthy Urban Tree Canopy (HUTC) grant program has been working to reverse canopy loss. To date, the program has awarded over $4.75 million to local groups, who have planted more than 11,000 trees across the county.

About 1,100 of those trees are participating in a collaborative research study by Holden Forests & Gardens, the County Board of Health, and the County Planning Commission. They’re asking the question: What if we added soil from healthy forests to each tree planted? Could we successfully restore native soil fungal communities — and would that benefit the trees?

Claudia Bashian-Victoroff, research specialist in the Burke lab and a Ph.D. student at Cleveland State University, is leading the study. The trees were planted by community partners who received a HUTC grant, including nonprofits like Western Reserve Land Conservancy and Heights Tree People, city Community Development Corporations, and local governments. Each group agreed to let HF&G researchers inoculate the trees with forest soil, take measurements, and monitor their growth year after year. Early results are showing promising signs for our native fungi.

Restoring healthy soil

In healthy, intact natural areas, mature trees form partnerships with beneficial soil fungi that help them absorb nutrients. But in cities, these fungi may be degraded or lost over time.

The Healthy Urban Tree Canopy Soil Inoculation Project is testing a simple idea: Take small samples of soil from an intact natural area at the Holden Arboretum, and add it to the base of newly planted trees to reintroduce the native fungi. If it works, it could support both tree health and conservation of the fungi.

Before launching this countywide effort, the team ran a preliminary study at Acacia Reservation in Lyndhurst, Ohio — a collaboration between HF&G, Case Western Reserve University, and Cleveland Metroparks. Andy Lance, then a Ph.D. student at Case Western Reserve University and now conservation manager at HF&G, led the study.

Lance’s work showed that this approach works: Fungal communities not only established, they also improved the ability of trees to acquire nutrients from the soil.  It was the proof of concept the team needed before scaling up and into more urbanized landscapes.

Reforesting Cleveland

The first large-scale soil inoculations added to HUTC plantings began in 2022. Bashian-Victoroff prepared the packets of inoculum, each consisting of 50 grams (less than a cup) of air-dried soil harvested from one of Holden Arboretum’s best natural areas, Bole Woods. The amount needed per tree is so small that even harvesting soil for a thousand trees doesn’t harm the forest.

Half of the trees received inoculation at planting, and researchers have monitored them annually since. Additional trees were added in 2023 and 2024, bringing the total to 1,100 trees across 15 different planting sites, each with its own unique setting. 

One group planted street trees throughout Cleveland’s Midtown neighborhood. Another focused on a single pocket park. A third added trees to Lake View Cemetery, where mature trees are already present. The diversity of sites — including differences in size, urban density, traffic, and pollution — adds another layer of potential insight to the research.

Fungi are moving in

The first round of soil testing was completed in 2024. Researchers were eager to know: Did the inoculation work? 

They’ve found that the fungal communities on inoculated trees vary more from one another than the control trees do—an increase in what ecologists call beta diversity.

There’s not yet a clear pattern in which specific fungi show up under inoculated vs. control trees, which was unexpected. But site-to-site differences are emerging.

“This is where some heavy statistics are going to come in,” says Bashian-Victoroff. “We’re looking at thousands of different fungal species on the roots of a thousand different trees.”

As for the trees themselves, there’s been no difference yet in growth or survival—though that’s not surprising, given how slowly trees grow. 

Bashian-Victoroff will be back out measuring trees again this August.

Unlocking a rich dataset

With so many trees, so many sites, and so many fungal species, the dataset offers a rare opportunity to explore deep ecological questions. Which fungal species are best at colonizing and persisting? Which ones thrive under tough urban conditions like lead or cadmium contamination, road salt, or industrial pollutants?

The data could also help answer broader questions: Which tree species do best where? How does pollution impact their survival? How does urbanization affect fungal communities—something we still know surprisingly little about?

“There are so many connections to be made between those different threads of data—tree growth and survival, soil nutrient availability, soil fungal community, even soil enzymatic activity—it’s an incredible amount of information to work through,” Bashian-Victoroff says.

Restoring Trees, Restoring Soil

The study isn’t just about helping trees—it’s about helping fungi, too. Reestablishing soil fungi that are otherwise restricted to conservation areas can boost their persistence in the region. That’s why it’s so important that this effort uses soil from nearby healthy forests, rather than commercially available inoculum that might not be adapted to our environment and may even outcompete native fungi.

And as interest in soil fungi grows, Bashian-Victoroff is glad to see the spotlight widen. “Sometimes I feel like I’m the town crier, ‘What about the fungi!’ So it’s been cool to see our community partners curious about whether the inoculation is helping their trees—and wanting to know about the soil fungi.”