ARBORETUM NEWS (NATURALISTS’ NOTES)
Let It Snow!
SATURDAY, DECEMBER 5, 1998As I write these notes, snow is falling outside the windows—finally! Precipitation started around 1:30 p.m., with sleet. It shifted to snow between 2:30 and 3 p.m. The first flakes were huge, about one inch in diameter; however the flakes gradually became smaller, and covered the ground in a white blanket. It was quite a shift from yesterday, which featured a glorious, sunny afternoon, and a high temperature of 62 degrees Fahrenheit.
A number of plants were unprepared for winter’s arrival. These include some of the lilacs (Syringa vulgaris) in the garden, which had sent out a few, tentative leaves; many magnolias, budding out; and most boldly, the Vernal Witchhazel (Hamamelis vernalis), which was in glorious bloom. These plants are from milder climes than ours, and do not have extended dormancy periods to prevent early flowering or leaf-out.
Species native to Wisconsin (or ecotypes from Wisconsin of widely distributed species) typically must be exposed to extended cold temperatures for dormancy to be broken and flowers and leaves to emerge. This prevents plants from flowering during a warm spell in January, when the flowers would soon be killed by a harsh blast of winter. The length of the cold period is called the chilling requirement. The magnolias and Witch hazel either have a very short chilling requirement, or do not enter true dormancy at all.
There is an animal species found in Wisconsin that is also ill-adapted for our typically harsh winters. This is the opossum (Didelphis virginiana); North America’s only marsupial. The opossum has gradually moved into the northern United States in the past 300 – 500 years, reaching the Great Lakes area in the past 200 years.
Opossum populations in Wisconsin fluctuate, varying in part with the harshness of winter. Every winter, opossums get frostbite on their ears and tails, which are naked. In very cold winters, opossums may freeze or starve to death, leading to a population drop.
Now that the ground is snow covered, it will be easier to follow the life of an opossum. Tracks may be seen heading to or from a hollow tree, a brush pile, or under a house. The opossum has distinctive tracks—both front and hind feet have five toes, and the inside hind toes are opposable, like our thumbs. The front tracks may be almost star shaped; this print is often obscured by the print of the hind foot.
The hind print often looks like a tiny human hand print, whose thumb comes out nearer the wrist than the palm. The opossum is typically nocturnal, but I have seen one wading through the snow in Longenecker Garden in the afternoon. It hid under some shrubs when it noticed me. Perhaps the opossum was on its way to eat persimmons behind the McKay center. Or, it was going to scrounge through some fallen leaves in the forest for worms and grubs.
The opossum is not a fussy eater, consuming fruit, nuts, worms, eggs, and carrion. The opossum needs to eat a lot as winter approaches to build up an insulating and nourishing layer of fat. The easily frostbitten tail is useful towards this end – it serves as a fat storage organ! The opossum’s tail also can stabilize a climbing opossum, wrap around a branch from which the opossum is suspended, and carry leaves and other items.
Look for opossums and their tracks in the woods and gardens of the Arboretum. They are not considered as beautiful as many other mammals, but they are quite fascinating. Happy Tracking.
—Judy Kingsbury, Arboretum Naturalist