Oaks are a keystone species, supporting more native caterpillars and other wildlife than any other tree in North America, the gardening public learned when Douglas Tallamy published his influential “Bringing Nature Home” in 2007.

We have more than a dozen oak trees on our house lot of a little more than half an acre. We also have black cherries, red maples, a shagbark hickory, a pine and many other native perennials and self-seeding annuals. We are doing our part to sustain wildlife.

For that reason, I didn’t run out to buy Tallamy’s “The Nature of Oaks” when it was published in 2021. But in the last month, I’ve enjoyed reading it as a way to keep me in a gardening frame of mind during winter. Reading the book was time well-spent.

One of the main benefits of oaks is the environmental value of the oak leaves left on the ground in the fall. Gardeners may have noticed, and maybe complained, that oak leaves are thicker than the leaves of maples, cherries, poplars, birches and willows. If you don’t rake these other leaves, many will decompose on their own by early spring.

Not so oak leaves, which are not only thicker but also contain lots of tannin and other chemicals, which taste unpleasant to many wildlife species. When wildflowers begin poking up and blossoming, most oak leaves will still be whole and either blowing around or bunched up against stone walls or other wind-protected areas.

“In many places,” writes Tallamy, an entomologist and conservationist, “oak-dominated forests are the last refuge for many spring ephemerals like trilliums and trout lilies.”

Yellow tout lilies happily bloom in early spring in oak leaf litter. Yet another reason not to rake in fall. Photo by Tom Atwell

Sure enough, when I scrolled back to the wildflower photos I took last April and May, the photos of bloodroot, trout lily and others all show intact oak leaves around them. And I probably moved many of the leaves when I took the pictures so the blossoms would show up better.

The relationship of oaks and those wildflowers has existed for millennia. But another benefit the trees offer is fairly new. Amynthas, sometimes called crazy or jumping worms for how quickly they can climb out of the soil,
don’t like oak leaves. In non-oak forests, these invasive Asian worms strip leaf litter, leaving bare, easily eroded soil, and they eat almost all the organic matter in the soil for good measure.

“Apparently oak litter can be too tough for Asian worms, and they rarely penetrate areas with abundant oak leaves on the ground,” Tallamy writes.

Oak leaves also fight drought and slow pollution from rapid runoff after heavy rainstorms. That runoff carries topsoil and pollutants to streams, rivers and eventually, the ocean.

Oaks lessen those problems in several ways. First, the heavy canopy of oak leaves slows the rainfall, which allows more of it to soak into the ground instead of running off. Also, the infiltration through leaf litter and soil “scrubs the water clean of its nitrogen, phosphorous and heavy metal pollutants” before sending it more slowly and less destructively toward the ocean, Tallamy writes.

In addition, he said, each tree’s canopy can intercept and allow evaporation of up to 3,000 gallons of rainwater each year. Tallamy also argues that oaks are one of the best trees for carbon sequestration.

“Tree species that are ideal for carbon sequestration are large, long-lived, dense trees – like oaks – that will safely keep carbon out of the atmosphere for hundreds of years,” he wrote. The trees’ root systems, which extend to three times the width of the tree canopy, do much of that sequestration.

I’ve touched just a little of what Tallamy writes about in “The Nature of Oaks” – and probably not the parts that he would think most important. While it is his book, it’s my column. And to that point, I was struck by a Maine fact he related that is only tangentially related to gardening.

Early in the gardening season – April in Delaware, where Tallamy lives, later in Maine – Thaxter’s Sallow is among the first caterpillars to emerge from oak twigs. It is difficult to see, because it’s gray, like the color of oak bark. The moth is named for Roland Thaxter, a lepidopterist from Maine and the son of Celia Thaxter, a famous writer and gardener, who lived for many years on several islands off York County.

Tom Atwell is a freelance writer gardening in Cape Elizabeth. He can be contacted at: tomatwell@me.com