Nature on Trail: Flying Squirrels, Fungi and Forests

By Sylvia Feder

Dusk in a Northwest forest. You are almost back at the car after another long and satisfying hike. Suddenly you hear a soft rustle in the leaves alongside the trail. In the fading light, you strain to see, but the shadows keep their secrets. You continue on your way, leaving the woods to its creatures.

That quiet rustle may have belonged to a unique denizen of our lowland woods. Soft fur, big eyes, fluffy tail—it looks like a toy-shop caricature of our more familiar red and gray squirrels. Its nocturnal nature means that most of us hike and perhaps even live with these squirrels nearby, yet we never see them.

Introducing the northern flying squirrel, Glaucomys sabrinus, whose genus name means, appropriately, “gray mouse.” This diminutive squirrel is most at home in old-growth coniferous and mature mixed forests. Despite the name, flying squirrels don’t really fly; instead, they glide, thanks to skin stretched from wrist to ankle and a laterally flattened tail that acts as a rudder. Glides as long as 40 meters (130 feet) have been recorded, though most flights are shorter.

With their loose-fitting skin, flying squirrels are more agile in the trees than on the ground, where they are at high risk of predation from owls as well as foxes, martens, and other mammals. Nevertheless, they do venture down from the trees to forage.

Flying squirrels eat a variety of foods ranging from nuts and berries to insects and birds’ eggs. Other important components of the diet are fungi that grow just underneath the leaf or needle litter on the forest floor. These are no ordinary mushrooms; they are highly specialized components of the ecosystem that serve as an important link between flying squirrels and their forest home.

Mycorrhizae, the fungi eaten by flying squirrels, live in a symbiotic relationship with virtually all Northwest trees. They colonize tiny rootlets of large trees and then extend far beyond the roots’ reach in a dense web that infiltrates an enormous volume of soil. From this soil, they draw water and nutrients, which they ultimately transport back to the tree. In exchange, the tree provides the fungi with sugars and other carbohydrates. In a further, even more amazing twist, some fungi contain nitrogen-fixing bacteria, providing nitrogen in a usable form to both the tree and the fungus.

Fossil data suggests that the mutually beneficial relationship between trees and fungi has existed for millions of years. Growing trees under experimental conditions without  mycorrhizae results in stunted, unhealthy plants. But how do seedlings and young trees become colonized with mycorrhizae in the first place?

Until about twenty years ago, this was a mystery. Then a landmark paper hypothesized that flying squirrels (as well as other rodents such as voles) helped distribute mycorrhizal spores. Further studies supported this hypothesis; as underground fungi “fruit” into what we know as mushrooms, they exude a distinctive odor that attracts flying squirrels. The squirrels ingest the fungi, and the fungi’s spores pass through unharmed, to be deposited in a different part of the forest in the squirrel’s droppings. Mycorrhizal spores recovered from spotted owl droppings suggest that the spores can be carried even farther when squirrels fall prey to owls.

From the diminutive flying squirrel, to the network of fungi beneath our feet, to our iconic old-growth trees—as hikers, we are in a unique position to appreciate these essential strands in the web of life in our Northwest forests.

This article was originally published in the January+February 2011 issue of Washington Trails magazine.