Efforts are underway to plant trees in Cuyahoga County’s urban areas by the thousands. In these cities, canopy cover is well below the county average, which brings well-documented ill effects to its residents. Trees shade and cool, they take up CO2 and pollutants from the air, and they provide important habitat for our native birds, insects, and other animals. With support from Cuyahoga County’s Healthy Urban Tree Canopy (HUTC) Grant, organizations and local governments began planting en masse in 2019.

But urban environments bring extra challenges for new trees. City soils tend to be more compacted, worse at water retention, contaminated by road salt, pesticides, and other pollutants, and missing essential nutrients and healthy soil organisms that you’d find in undisturbed natural areas. With efforts ramping up to get trees in the ground, it begs the question: How can we ensure these trees will survive?

Soil scientists at Holden Forests & Gardens may have the answer: mycorrhizal fungi. These soil organisms form partnerships with the roots of trees and other plants, providing them with important nutrients in exchange for sugars the plant makes through photosynthesis. Researchers have been learning more and more about the importance of these fungi for plant growth and survival, as well as their potential for use in ecological restoration, but they haven’t yet been adopted into mainstream restoration practice.

The Holden team has partnered with the county and select community groups to run a soil inoculation experiment on trees planted through the HUTC program. Soil inoculation is when a small amount (in this case, about a handful) of soil collected from a healthy natural ecosystem is added to the soil during planting. This re-introduces soil microbes and mycorrhizal fungi to the soil, along with their potential benefits to the trees.

Using soil from Holden Arboretum’s Bole Woods, the researchers are inoculating trees being planted at several HUTC sites. For each tree in the study, they measure the height and diameter after planting, and will continue to monitor growth and survival of inoculated and non-inoculated trees year over year.

Throughout the fall and winter of 2021 and 2022, the Holden researchers, in collaboration with Bartlett Tree Experts and the Holden Arboretum Community Forestry team, inoculated, measured, and sometimes planted around 450 trees across dozens of species. They added almost 300 more this spring, and this summer, they collected soil samples from trees planted in 2022 to see how well the inoculum established. Not only will data from this experiment reveal if and how soil inoculum can benefit urban tree plantings, it will also provide evidence for which tree species fare best in urban environments.

Be on the lookout for trees recently planted in your neighborhood. And if you see a see a Holden scientist measuring trees, feel free to say hello. We’d love to chat about soil fungi with you!

By Lydia Jahn and Anna Funk