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Fierce gusts of wind rocked the treetops above me, sending shivers down their trunks. Among the boles of swaying aspens, maples, and firs, I skied with one ear to the sky. Twigs clattered, snow plopped, and cold wood fibers moaned up and down the musical scale. One tree almost buzzed with a high-pitched popping, while another groaned from deep within. I listened carefully to determine the location of the loudest trees, and to gauge their ability to strike me if they came crashing to the ground.
 Although the bluster threatened above me, it didn’t quite reach all the way down to the swooping hills and rippled snowpack. I felt protected in the understory of this forest. My eyes were scanning for animal tracks in the days-old snow, but I only saw signs of other skiers. Deep drifts and bitter temperatures have driven many creatures down into the next level of protection: the subnivean layer.
 Fluffy snow captures summer warmth still radiating up from the ground. A thin zone opens up under the snow, right at the surface of the ground. Here, the temperature stays at a fairly stable 32 degrees Fahrenheit. Compared to a negative 45 degree wind chill at the surface, that feels pretty balmy!
 American martens are one of the many creatures that exploit the subnivean microclimate. These small weasels tunnel through the snow to find food, stay warm, and escape predators. Phil Manlick, a Master’s student at UW Madison, has been studying the “Habitat-mediated foraging and predation risk in reintroduced American Martens.” Recently he gave a dinner lecture on the topic at the Rookery Pub and Fine Dining near Cable, WI. One of his hypotheses is that martens need lots of structural diversity – lots of fallen logs – in their habitat to help them gain access to the subnivean layer.
 Once under the snow, martens can forage along the log-lined runways where red-backed voles, mice, shrews, and squirrels also travel. When satiated with a tasty meal, martens have been known to curl up in the den of their prey for a nice warm nap. The snow is an excellent blanket for this lean mammal, who stores little fat and burns lots of fuel to stay warm. Snow also provides cover from other predators. The diets of foxes, fishers, and bobcats overlap with martens’ diets, and those larger carnivores will kill martens to eliminate competition.
American martens were extirpated from Wisconsin in the 1920s, due to unregulated fur harvest and widespread habitat loss. In 1986, wildlife managers developed a marten recovery plan with the goal of reestablishing two self-sustaining populations of martens in the Chequamegon-Nicolet National Forest. But even with cooperation between the Wisconsin DNR, the US Forest Service, and the Great Lakes Indian Fish and Wildlife Commission (GLIFWC), and multiple reintroductions, marten populations in the Chequamegon National Forest are not rebounding as well as wildlife managers and researchers hoped. Phil is trying to figure out why.
 Although he’s still mostly in the data collection phase, preliminary results indicate that one issue limiting the American martens’ recovery in the Chequamegon National Forest might be poor juvenile survival. Adults are doing fine, he says, but the young martens, kicked out of their parents’ home range after just a few months, may not be finding enough high quality food to make it to adulthood.
 Phil’s research techniques involve gathering hair samples from martens as they explore PVC tubes baited with beaver meat and equipped with a brush. By comparing the ratios of stable isotopes of nitrogen and carbon in the martens’ bodies to the ratios of those isotopes in their various prey species, Phil can get an idea of what martens are eating. Other researchers also identify animal remains found in marten scat to identify specific prey.
 The surprising results are that in northern Wisconsin, martens’ diets consist of 40% shrews and 30% deer. In contrast, healthier populations of martens out west and in Minnesota eat many more red-backed voles and squirrels. Why might eating shrews and deer be a problem? Shrews, like the martens themselves, don’t store fat for the winter. With an extraordinarily fast metabolism, shrews must feed voraciously night and day. This makes them the skim milk of the small mammals.
 Deer might seem like an improbable food source for a two-pound marten, but evidently, martens are feeding opportunistically on winter-killed and wolf-killed carcasses. The trouble is that deer carcasses are a risky food source. Fishers, wolves, coyotes, and other scavengers are attracted to those same banquets, and make things dangerous for the martens. That competition with other species may be another important factor limiting the martens’ recovery.
 Phil suspects that habitat quality has an impact on both food availability and competition. During his summer habitat surveys, Phil found that martens need “nasty forests that you don’t want to trudge through.” By which he means forests with lots of blow downs, snags, large trees (greater than 39 cm diameter), and coarse woody debris. If further data analysis supports that hunch, it might influence how foresters manage the landscape in the future.
 Thinking back to those trees groaning and swaying in the tempest above me, it might not be a bad thing if a few of them fell down...after I’ve skied past, of course.
 “But these are the woods you love, where the secret name of every death is life again…”  - Mary Oliver, from Skunk Cabbage

For over 45 years, the Cable Natural History Museum has served to connect you to the Northwoods. Come visit us in Cable, WI, at 13470 County Highway M. The current exhibit, “Deer Camp: A Natural and Cultural History of White-tailed Deer,” opened in May 2013 and will remain open until April 2014.  

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