WINTER BUDS - Winter World: The Ingenuity of Animal Survival - Bernd Heinrich

Winter World: The Ingenuity of Animal Survival - Bernd Heinrich (2003)

WINTER BUDS

By the end of January, those of us who live in the Northeast have been looking out on starkly bare trees for three months. “Only four more months,” we think, before the buds break and trees are again resplendent in green. The wait is all the more difficult when you realize that the buds are there all winter, biding their time. Indeed, they were fully formed on the trees the previous summer, long before the shows of brilliant fall foliage.

A bud can consist of bare clusters of miniature leaves harboring a new shoot (as in hickory and butternut); an embryonic flower or inflorescence (as in alder, hazelnut, and birch); or both incipient leaves and flowers encased together under protective scales (apple, cherry, shadbush). The large buds of mountain ash and poplars have a sticky, resinous covering that helps protect them from hungry animals. All leaf and flower buds are packed with nutrition and are prized winter food for many northern herbivores. Ruffed grouse live all winter on aspen and birch buds, and I’ve watched purple finches devouring sugar maple flower buds. I’ve seen red squirrels decapitate almost all the young balsam fir trees in some patches to eat the large terminal shoot buds. In the tops of mature fir trees they snip off hundreds of terminal twigs to eat the twenty or more flower cone buds on each one like corn from a cob, as shown in the figure on page 44. Then they discard the twig, which drops onto the snow below. Moose break off branches of poplar saplings and of red and striped maple to feed on the terminal buds and twigs. In some patches of my woods in Maine, I can hardly find a single sapling that is untouched by moose, deer, or snowshoe hares.

Red maple shoot with deer browse and an empty sawfly cocoon.

A twig of mountain ash that has been browsed off by hares three years in succession.

Buds vary greatly in size, and it is primarily the trees of northern regions that have large buds. Southern transplants to the north, such as black locust (Robinia pseudoacacia) and honey locust (Gleditsia trichanthos), have only miniscule buds.

For large-budded northern trees, the prepackaging of leaves and flowers into buds must have some advantages that outweigh the considerable cost of maintaining buds for so long before they are activated. I suspect the main benefit of having the leaves and flowers preformed all winter is for a quick start-up in the spring—the buds are ready to break out quickly on cue, thereby allowing the tree to make the most of a short growing season. In New England, trees have only three short months in which to produce leaves—their photosynthetic machinery—and then use them to make anenergy profit. When the long-awaited spring finally arrives in the north, it does so suddenly. Over a week or two, the ice and snow melt, and the bare ground begins to absorb heat. In only three or four days in mid-May, a bare beech forest is crowned with a canopy of pea-green leaves soaking up sunlight.

Pin cherry, with flowers and leaves from same buds.

Butternut, with separate leaf and flower buds.

There is, however, a major caveat in the trees’ race to grab light: the new leaves are vulnerable to frost damage. Buds, as long as they are dormant, can, like hibernating insects, survive winter’s lowest subzero temperatures. Once they awaken and begin to draw water into their tissues, however, they are at risk. They in turn can put the tree at risk, because leaves can collect wet snow that can break the tree branches. It’s a dilemma. The buds need to open as early as possible, but not too early. Trees, being long-lived, can perhaps afford to lose flowers to frost in any one year, because energy saved one year by not fruiting can instead be invested in growth and can lead to the production of even more fruits the next year. Losing leaves to frost, however, is more serious; if the tree misses out on the sunlight sweepstakes, or loses limbs to snow-loading, growth and reproduction will suffer. Releafing is sometimes possible but is energetically costly. However, trees are seldom fooled by a false start, such as a midwinter thaw. How do they know when to start their metabolic engines and break bud?

Buds follow local schedules that are dictated by an interplay of cues involving day length, seasonal duration of cold exposure, and warmth. Warmth alone is not enough. For example, sugar maples from the north, if transplanted to Georgia, won’t break bud there because they need a long period of cold beforehand, a kind of reminder that winter has occurred. The strategy is a bit like that of northern cecropia moth pupae, which do not stir unless first chilled for a sufficiently long time.

Where I spend most of my time, in western Maine and in central Vermont, new leaves of all the deciduous tree species usually emerge relatively synchronously in mid-May, over the short span of about two weeks. First to appear are quaking aspen and birch leaves; last are oaks and ash. Beech, maples, and the others are in between. Flower buds of the native forest trees, however, open in a progression and over a six-month span, starting in March or April with poplars, alders, red maple, and beaked hazel; moving on to basswood in June and American chestnut in late July; and ending in October with witch hazel. (Significantly, the latest-blooming species do not have their flowers prepacked in buds in winter.) In trees such as apple, cherry, and shadbush, with flowers and leaves packed in the same bud, the flowers generally bloom in one quick burst; the leaves follow almost immediately.

Willow showing swelling of flower buds, but not of leaf buds, in response to warmth.

Having separate buds for leaves and flowers—such as in willow, poplar, and alder—allows a tree to open its flower buds a month before the leaf buds or up to five months after them, such as in witch hazel. Wind-pollinated trees may produce flowers a month or more before leaves, which tend to block wind flow. In contrast, a beepollinated basswood may flower a month or more after the leaf buds have opened, when bee populations peak in late summer. Witch hazel takes advantage of the pollination services of winter moths of the genus Eupsilia, which are active in fall and winter (see Chapter 14).

Bud opening is a wonder but can easily be taken for granted. I like to be reminded of the spring miracle, especially in the depth of winter, when the vibrantly alive trees look so dead. Every year in January, February, and March I go into the woods, pick some twigs of trees and shrubs, then bring them home and stick them in a jar of water. Indoors, some buds can be coaxed (or “forced,” according to botani-cal usage) to open at least three months ahead of their normal outdoor schedule. Twigs of trembling aspen, willow, beaked hazel, speckled alder, and red maple, picked as early as January and brought inside, will flower and then shed their pollen. (The flowers of these trees and shrubs are also the first to open in the woods, in early March or April, when there is still snow on the ground.) In contrast, most leaf buds, as well as the flower buds of late-blooming trees such as basswood, don’t respond to the warmth of my office until March.

Nannyberry.

Hobblebush.

Elderberry.

Beech.

When snowstorms rage outside and temperature may dip below 0°F (-16°C), the jar of twigs reminds me of the winter trees’ vibrant life. The buds on the trees outside are like runners who have prepared for a big race for more than six months and who now wait for the signal to start.

In late April or early May, a warm temperature pulse will be the “go” signal, starting the twigs and leaves on their race to the sunlight and summer. In 2002, that pulse, two days of 90°F (34°C) on April 19 and 20, was unusually early. It forced the poplars, cherries, serviceberries, and sugar maples to start unfurling their leaves, to be followed two days later by the flowering of the serviceberries and sugar maples. Two snowstoms and frosts then followed within ten days. A week later sticky snowflakes the size of miniature snowballs were plummeting down and sticking onto the leaves and flowers. It looked like the trees had made a false start—as though they had jumped the gun. If so, they would be penalized themselves. This time they seemed okay, but I think they were cutting it close. The costs involved only a few broken limbs, a few lost leaves. As in the bees’ early exits from their hive to forage, the gamble paid off.