A Forest in an Acorn

Dusk was settling in as I coasted down one of the many hills on the Rock Lake Trails near Cable. With no recent snow, and strong, bitter winds, tree debris was strewn all over the ski trail, and had gathered in the tracks. Feeling like Super Mario jumping over obstacles, I lifted my skis—Right! Left! Left! Right!—over a cluster of oak leaves, a knobby stick, a white pine cone, and a single oak leaf. As my speed gathered on the hill, I could barely keep up. And then: scrrraaape…the sickening noise of an acorn cap vibrating on my skis’ fish scales was the sound effect for losing one of the lives on my fresh coat of glide wax.

Acorn caps; pointy-tipped red oak leaves; and knobby oak twigs were by far the most common debris on the trail. The dry, sandy, well-drained soil on this glacial pitted outwash plain makes great habitat for oaks. As far as I can tell, this year was a mast year in the Cable area. As mast trees, oaks save up their energy for three to five years before producing thousands of acorns all at once. All the trees in an area will mast in the same year, and with thousands of acorns available at once, the hordes of acorn-eating seed predators cannot possibly eat them all. Some, at least, will survive to grow a new tree.

Judging by all the tracks along the trail, there are plenty of critters already on-hand to eat the acorns. Four-footed red squirrel tracks crisscrossed the trail and connected every tree. I suspect that the bounty of acorns stored in their bellies and their food caches will give them a very successful breeding season starting in March. 

In the midst of the mess of squirrel tracks, there were the loping, five-toed tracks of a fisher taking advantage of the easy travel in the groomed trail. Along with hares and porcupines, red squirrels are one of the fisher’s favorite prey. As the acorns feed the squirrels, the squirrels will soon feed the fishers. The predator’s tracks are already common here. Will this mast year allow them to increase even more?

The old snow pack held a perfect record of days of activity in the woods. Mouse tracks caught my eye next. Their tiny, four-footed patterns scurried among the shrubs, connected by a thin tail-drag. Mice are food for almost everything in the forest, including fishers. And deer ticks. As the cases of Lyme disease in Wisconsin have increased – 300% since 2000 – both mice and ticks are getting the blame. Mice are the preferred hosts for deer ticks (also known as black-legged ticks), and are very effective at transmitting the bacterium that causes Lyme disease.

Mice can also raise several litters of up to 9 young per year. They mostly breed in spring and late summer, but increased food availability or mild weather can extend the season from March through October. These local mice may have already had an extra litter or two in response to the acorn mast.

The repercussions of mast years on mice are wide-ranging. Scientists have noticed that an increase in mouse hosts for deer ticks results in an increase in Lyme disease cases two years after the mast. First the mice increase, then the ticks increase. Then the plentiful mice eat themselves out of house and home. Their population crashes without the mast, and the ticks go looking for a new host – you, or me. By this pattern, this summer should be fine, but prepare to tuck you pants into your socks during summer 2016!

A few minutes after the fisher tracks veered off into the woods, I noticed that I was following the dainty tracks of a fox up a hill. Oak leaves in the tracks caught my skis at awkward moments, and I suspected that the fox was here in the oak grove because that’s where the mice are.

Red foxes are often held up as the champions in our fight against Lyme. They are quite good at hunting mice, and will cache mouse-snacks for later consumption when numbers are high. As a result, they are much better at controlling mouse populations than their coyote cousins. But coyotes don’t tolerate foxes, and coyotes have greatly expanded their range in the absence of wolves. Our historical predator management policies may have doomed us to disease. I’m happy to note that my favorite haunts in the area all boast more fox and wolf tracks than coyote sign.

Near the end of the loop trail, on the way back to the car, I came across a wide area of messy snow. Deer had been pawing through the shallow drifts to get at acorns in the leaf litter. Their pointy hooves marred the ski tracks, too. Deer also benefit from mast, and they have often been implicated in the spread of Lyme disease. But they are host to adult ticks, not the nymphs that are so efficient at making us sick. Studies show that deer numbers don’t correlate with tick abundance. They may wreck the ski trails more than a mouse, but the mouse is more of a threat to my health.

Ralph Waldo Emerson was very poetic when he philosophized that “The creation of a thousand forests is in one acorn,”  but he didn’t yet know wide-ranging effects that the thousands acorns in just one of those forests could bring. 

Oaks and their acorns are part of a cascade of effects that impact mice, foxes, ticks, humans, and more. Photo by Emily Stone. 

Mice leave an out-sized footprint on the forest as their populations rise and fall. Photo by Emily Stone.