By Boak Wiesner
A touch of winter blew into Minnesota this past week with the northern edge of our state getting school-closing enough amounts of snow at the same moment as my friends and I were relaxing in my front yard on probably the last balmy evening of the year. And “blew in” is an apt way to describe it. As air cools, it contracts and thus becomes more dense so gravity pulls it down. When that cold air hits the Earth, it spreads out like syrup on pancakes. What we call a cold front is the leading edge of this blob of coldness. Since there’s more air in a given space, the pressure goes up. As this denser air seeks a way to let itself go, as it were, it heads off towards an area of low pressure. And if you’re in the way, you experience a lot of wind. Minnesota is smack dab in the pathway for classic mid-latitude cyclonic systems as they “migrate” west to east due to the Coriolis Effect.
One starts with cold air moving in, pushing up warmer air, thus creating low pressure, which begins to spin, counter-clockwise, of course, here in the Northern Hemisphere, like hands on a backwards clock, with the warm front always leading the cold front. Since cold fronts move faster that warm ones, the cold air “catches up” to the warmer, moister air pushing it aloft. This is called occlusion. And when it happens, there’s a lot of rapid, heavy precipitation. Ergo, Roseau got socked. The low tracked well north up in Manitoba, but its effects were felt down here as it drew warm air from the south up into our area. Hence Wednesday’s lovely pleasant evening.
The storm brought the juncos back to our area. Flocks of them are all over. White flashes on the edges of their tails are the best field marks. Fox sparrows can be heard rustling loudly through the dry leaves. Once in a while, their melodious songs break forth. White-throated sparrows are also passing through. This year’s yound still needs practice with their singing so the familiar “Oh sweet Canada, Canada, Canada” sounds pretty scratchy.
To be in the Arboretum these days is an experience in pure color. Just off the road by the Berens Cabin, the yellow light seemingly suffuses even the very air through which it’s passing. Most of the yellow we see is the reflection from a pigment called xanthophyll, which, being yellow, absorbs ultraviolet light, and so protects the electron-releasing chlorophylls in the leaf. Leaves are like peanut-butter-and-jelly sandwiches in waxed paper. Photosynthesis occurs mostly in the top layer in the middle which looks like a lot of Velveeta cheese boxes standing side-by-side.
Sugar maples, elms, basswoods, and ashes all glow yellow in the fall. Oaks show more browns as the color of tannins becomes visible. Tannins are fairly toxic which means that the decomposition of oak leaves so they make a good choice to cover gardens with in the fall. Acorns, too, are toxic to many domesticated animals but they’re a favorite food of black bears, who prefer them over most other nuts or crops. Research is being done to try to dissuade bears from chowing down on the oily sunflower seeds in the northwest part of the state and using acorns seems to be the way to go.
Analysis of pollen from cores taken in bogs and ponds around here indicate that our area has undergone major changes in its dominant ecosystems since the Ice Ages evaporated. Frequent fires kept the eastern forest at bay allowing red and white oak acorns to sprout and grow. Once the fires were suppressed, the sugar maple-basswood climax forest we see today advanced to the west. As the climate warms, the Big Woods we’re familiar with will dry out and may return to the previous oak savannah still found in parts of our state. Seeing stands of big oak trees is a treat for Arboretum visitors.
The big change of weather is part of the coming of fall. In the crabapple grove, I watched robins and their taxonomic cousins the bluebirds stoking up for the migration south on the ripe fruit. The Arboretum has been involved in bluebird restoration for many years and around 150 new bluebirds join the ranks each year from its grounds. Thinking of changes this week, I reflect that when I was young, I never saw any bluebirds but these days, with the concerted efforts of many people who put up houses and keep away the tree swallows, bluebird numbers have soared.
Boak Wiesner is a Minnesota Naturalist volunteer.