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In a winter lineup of leafless Wisconsin trees, white birch, Betula papyrifera, would be the most readily recognized and identified because of its smooth, white bark.
Although commonly found in northern Wisconsin, white birch is primarily a Canadian species whose range extends north to the tree line and east to west from coast to coast. Within its range, white birch thrives in a variety of habitats from well-drained sandy soils to moist areas along streams and bogs. Often it is found in the company of pines, oaks, and aspens, but pure stands can be found.
White birch is a pioneer species, one of the first to colonize a new or disturbed area. Colonizers are fast-growing, short-lived, sun-loving trees that lose their zest for survival and perish as other trees invade, eventually dominate the area, and close in the canopy. A few lone, but hardy, white birches may survive in a maturing forest and grow to old age. A 140-year-old white birch would be an ancient relic compared to a 140-year-old bur oak that would still be in its youthful prime.
Fire is important in regenerating white birch because fire opens up an area and removes competing trees and shrubs. Birch seeds won't germinate in the shade. Once they find the sun, they will sprout and grow in the nutrient-rich ashes fires leave behind. Thus fire can ultimately contribute to pure stands of the stately white trees. White birch also will sprout from burned or cutover stumps, growing into the elegant multiple-trunked clumps we so admire.
In winter, white birches already give evidence of preparing for spring. Small, slender lateral buds are spaced alternately along zigzagging branches. The branch ends are tipped with clusters of three inch-long male catkins that formed in September, long before the serrated heart-shaped leaves turned pale yellow and dropped.
Small green pistillate or female catkins (composed of bracts and numerous minute flowers) will form in early spring and by April into May, the male catkins will lengthen to about three inches, open and release pollen. After the wind-carried pollen lands in the receptive flowers and fertilizes them, the catkins seal up so seeds can develop. In about four months, beginning in late August and progressing through October, those catkins disintegrate, releasing thousands of individual seeds, each less than a millimeter long. They are released as stacks of samaras or winged fruits.
About that white bark – each tree grows, the thin, tough bark curls back on itself and peels into papery-thin layers that gives rise to white birch's alternate name, paper birch. Below recently-peeled bark is a colorful palette of muted pinks and oranges which whiten with age. Although it is tempting to peel the bark and watch it separate from the trunk, please resist the temptation. This harms the tree and increases the possibility that insects or fungi can invade, attack and weaken it. Each tree sheds its bark when ready and has replacement bark underneath. When bark is removed prematurely, the trunk turns black.
In addition to papery curls, fine, horizontal black lines of up to 1 1/2 inches long, randomly mark the white bark. These lines are lenticels, openings in the bark to allow for gas exchange.
When I gaze upon white birch, several questions intrigue me. Why is the bark white? Is the color an adaptation for survival? If so, how? Also, how does this fragile-looking tree with the thin bark survive our long, cold winters? These unanswerable questions are worth contemplating as you gaze upon a stand of white birch, silhouetted against a clear blue sky on a crisp winter day.
Anita Carpenter writes from Oshkosh. Winter is her favorite season.