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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