Trails in the Snow: The Effects of Reforestation on Mammal Distribution in the Baraboo Hills
The first snowfalls of winter have transformed the landscape into a pristine, silent, unscathed world. The sounds of birds and animals that are so casual in other seasons are now magnified by silence. The bark of a squirrel is crisp and suspended in time, like the lingering imprint her paws leave on the snow. With the absence of so much life from the forests and meadows, winter evokes a heightened sense of detail. It does not bring the dramatic migrations of spring and fall, or the vibrant colors and noises of summer. Winter is the invigorating cold, the silent struggles for survival, the stories written by paws and hooves on the snow; small details that could easily not be noticed.
Winter also provides a glimpse into the lives of mammals that are very elusive. Following the tracks of animals in the snow can be as insightful as actually watching them rambling, grazing or hunting. I began tracking with more intent this winter, with an enlivened interest in mammals and an eye for details in the snow. My main curiosity was where the animals had been. What habitats did they most frequent? How did they cross streams? What caught their attention during their nocturnal expeditions? The list was almost as long as the winding trails I followed.
From an environmental standpoint, this is a great time to be living in the Baraboo Hills of southern Wisconsin. Rugged and inaccessible terrain combined with years of conservation efforts have created the largest block of upland forest in the lower half of the state. The Nature Conservancy has been involved with the Baraboo Hills since the 1960s. It has joined local environmentalists in the goal of acquiring and managing properties. The preservation and restoration of the forest ecosystem has been main
ly aimed at conserving songbirds, which require unbroken tracts of forest for nesting. However, large carnivores are benefiting as well, as increasing sightings of timber wolves, fishers and black bear inform us.
The intent of my research was to discover whether or not mammals in the Baraboo Hills are benefiting from the reforestation that is currently taking place. Although all of the mammals I tracked for this project have Least Concern conservation status, according to the IUCN Red List, their presence is a big part of the ecosystem. While some fields in the Baraboo Range are being farmed and logging is still occurring in some places, the general trend is that old fields are either being planted back to forest or are growing back on their own. I set out to determine if mammals will need to adapt to the expansion of the mature forest habitat, or if they already favor this habitat more than they favor fields.
One of the Nature Conservancy preserves, Pine Hollow, is the perfect location for studying reforestation. With its three different stages of forest expansion, it is a good representation of forest regeneration throughout the Baraboo Hills. There is mature woodland that connects to an extensive forested block of more than 10,000 acres, and an old field in which red oak and white pines have been planted. The planting happened in two phases. The first planting occurred in a five-acre corner of the field about 15 years ago. The second planting was completed four years ago and covers the remaining 38 acres of the field.
Both plantings were done by tractor and alternated rows of red oak and white pine. The pines were introduced only to provide shade and shelter for the more slowly growing red oak. Because the upcoming forest is intended to be primarily an oak forest, the white pines have been removed from the 15-year-old planting and will eventually be removed from the 4-year-old planting as well. The younger planting retains the look of a field, but the 15-year-old planting has become shrubby and natural-looking. Within each habitat, the trees were very equal in size. I chose one average-sized tree (two in the 4-year-old planting) from each habitat to measure. I measured the shadows of the taller trees and determined their height through calculations based on the comparison of a 24-inch rope to its 40-inch shadow.
Walking through the property is almost like walking through time: the young trees in the field, the older planting and finally (the goal of conservationists), a mature forest. The environmental effort behind the three distinct habitats I described convinced me to use Pine Hollow as a site to study the reaction of mammals to reforestation in one location.
After reading about the habitat preferences of mammals in Wisconsin, I hypothesized that in Pine Hollow the most valuable habitat would be the mature forest, followed by the 15-year-old planting, and then the 4-year-old planting. For this study, I am defining the term valuable as favored by the greatest number and most species of mammals. In my reading and background research, all of the mammals, except prairie deer mouse and meadow vole, were recorded as living in a variety of habitats, but they were all at least partially dependent on the forest habitat. I hypothesized that some mammals would remain in the mature forest habitat all of the time. These mammals would be the squirrels, because of their tendency to live and forage near large trees, and the long-tailed weasel, because of its sensitivity to mature forest habitat fragmentation (Gehring 2004).
Rabbits, opossums, least weasels, short-tailed weasels, raccoons and skunks are more inclined to stay near dense cover, but they may live in a variety of habitats (Forrest 1988). Similarly, deer prefer brushy habitats (Whitaker 1996) and usually avoid large open fields (Jackson 1961). I hypothesized that all of these mammals would be most common in the 15-year-old planting, then in the forest, and would only rarely venture into the 4-year-old planting.
Coyotes also favor second-growth habitat and scarcely leave dense cover (Jackson 1961), but they are very dependent on mice and other small rodents that inhabit fields. So I hypothesized that they would be found mostly in the 15-year-old planting and the mature forest, but they would frequent the 4-year-old planting (for hunting) more often than other mammals, excluding the small rodents.
I hypothesized that the mice, voles and shrews, being so adaptable, would be fairly evenly distributed over the three habitats.
To conduct my research, I decided to record the number and species of mammals that left tracks in the three different habitat zones. I went tracking as often as conditions permitted (fresh snowfall) throughout my study so that my data would be as conclusive as possible. This proved to be six sessions between 11 December 2009 and 14 February 2010.
Using a map of the Pine Hollow Preserve, I outlined my three areas of study (mature forest, 15-year-old planting, 4-year-old planting). As I mentioned earlier, there was quite a difference in the size of each habitat, so I chose one square equal to 10,000 square yards (100 x 100 yards) within each of the three habitats I would survey for tracks.
Within one to three days after a snowfall, I walked evenly across each square and counted tracks, being extremely careful not to count the same mammal twice. If I was in doubt as to which species had left the tracks, I followed the trail until I had gathered enough evidence to identify it before I counted it. I found the Field Guide to Tracking Animals in Snow by Louise Forrest to be invaluable while I was tracking.
The weather, from the time the last snow had fallen to the time I was tracking, was extremely important because it affected the liveliness of the mammals and also the state in which I found their tracks. Therefore, I recorded weather conditions, snow conditions and temperature from the snowfall to the time of tracking. While tracking, I took note of the number of mammal tracks in a field notebook. The following is an excerpt from my records.
Results and Discussion
I found 13 different species of mammals in Pine Hollow, which I divided into three size-based categories.
- Large mammals: white-tailed deer, coyote
- Medium-sized mammals: eastern cottontail rabbit, striped skunk, opossum, raccoon, long-tailed weasel, least weasel, gray squirrel, red squirrel
- Small rodents: mice, voles, shrews
I did not delineate between the different species of shrew, mouse and vole. It was impossible to tell their tracks apart, and I had no other method of identification. There have been several species of small rodents found in the Baraboo Hills in habitats similar to Pine Hollow: northern short-tailed shrew, masked shrew, meadow mouse, and white-footed mouse (Mossman and Lange 2006). The scientific names for all of the species above are listed in Appendix A.
The distribution of mammal species throughout the three study areas supported part of my hypothesis. The tracks of all 13 species were found in the mature forest; nine of the 13 were found in the 15-year-old planting; and six of those nine were found in the 4-year-old planting.
In order to understand why certain mammals were found in the different study areas, I found it helpful to refer to the characteristics and habitat needs of individual mammal species. For example, many of the mice, voles and shrews stayed under the snow most of the time because of the extreme cold and deep snow.
Despite a few minor differences, the voles and shrews did not strongly favor any habitat more than the others. Mice favored the mature forest much more than the plantings. According to the data that I collected, mice were nearly twice as plentiful in the forest. My data also suggests that mice venture out of the snow more than the other rodents and therefore are more susceptible to predation. Living in the forest shelters them from some hawks and owls, which have a very keen eye for the mice that live in the 4-year-old planting. I came across five different wing prints in the snow; they all were in the vicinity of mice tracks in the 4-year-old planting, and three of the five were successful.
My observation of red and gray squirrel tracks showed that both species stayed in the mature forest at all times, thereby confirming my hypothesis. I tracked two different long-tailed weasels in the mature forest habitat. They stayed very far away from the open field and the 15-year-old habitat at all times, which supported my hypothesis and implied that they are very sensitive to mature forest fragmentation.
I found the tracks of an entire family of raccoons who had been wintering in a hollow tree. They never left the mature forest either, which is contrary to my assumption that raccoons would favor the 15-year-old planting over the mature forest habitat.
During a January thaw, an opossum and a striped skunk woke up from hibernation and wandered around the mature forest and the 15-year-old planting, spending equal amounts of time in both habitats. Because these mammals went far into the mature forest to reach the square where I was surveying, I had to reject the part of my hypothesis that assumed that these two species would prefer the forest edge.
The cottontail rabbits were the only medium-sized mammals that ventured out into the 4-year-old planting. In keeping with my hypothesis, the habitat that hosted most of their trails was the 15-year-old planting, followed by the mature forest.
My results concerning white-tailed deer are inconclusive. Early on in my study, I followed the tracks of a pack of coyotes chasing five deer out of the mature forest. Throughout the rest of my study, the deer remained very wary, judging by the low numbers of their tracks. The data that I collected implies that the deer spent more time in the forest than the other two habitats. This was surprising, as I had been expecting to see more deer in the transitional habitat. However, the small amount of deer tracks in all three areas cannot be relied upon for finding any major trends.
The 4-year-old planting habitat was favored by coyotes while they were hunting, but unlike the forest or the 15-year-old planting, where there is thick vegetation, it did not provide shelter for them. It has been shown that coyotes are resourceful and gravitate to where there is a steady food supply (Whitaker 1996). Therefore, the coyotes are not so much dependent on the field as they are dependent on the resident mice, voles and shrews in the form of available prey. Because of the brambles that provide shelter for the small rodents and create obstacles for the coyotes in the 15-year-old habitat, it is presumably easier for the coyotes to hunt in the 4-year-old planting.
The mature forest is able to support the most mammal species of any of the three habitats. Of the 13 species of mammals whose tracks were found there, four of the mammals were found in no other habitat (long-tailed weasel, red squirrel, gray squirrel, raccoon). Based on the number of tracks, two of the mammals that were found in the other habitats favored the mature forest (white-tailed deer, mouse).
The 15-year-old planting is not as valuable as the mature forest, but two mammals favor this habitat above the others (cottontail rabbit, least weasel).
Mice, voles and shrews are the only mammals that are able to acquire both food and shelter from the 4-year-old planting. Because these mammals do not live solely in this habitat and because they are all found fairly evenly throughout the other two habitats, the
The general trend is that mammals depend more upon the mature forest habitat than any other habitat. However, there are eight species in my study that also use the 15-year-old planting or forest edge habitat (shrew, mouse, vole, cottontail rabbit, opossum, striped skunk, coyote, white-tailed deer). Therefore, as long as the second growth or edge habitats are not fully eradicated, all of the mammals that I tracked would respond positively to the expansion of mature forest habitat in the Baraboo Hills.
If I were to do this study again, I would add three more areas of mature forest and transition forest in order to gather more data on each mammal. It would be very interesting to see if the distribution of mammals changes at other times of the year. I would also start the study in the summer and live-trap the shrews, mice and voles for identification.
Completing this study led to the discovery of many more research topics. One that stands out in my mind is a study of the large carnivores that are repopulating the Baraboo Hills and their effect on the ecosystem.
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Gehring, Thomas M., and Robert K. Swihart. "Home Range and Movements of the Long-tailed Weasel in a Landscape Fragmented by Agriculture." Journal of Mammology 85.1 (2004) 79-86.
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