Summer World_ A Season of Bounty - Bernd Heinrich [7]
Fig. 4. Willow twig on 23 October, before the leaves were dropped, showing the “pussy willow” flower buds (along with two tiny leaf buds on the base of each twig, and a portion of the same twig drawn again the following April).
Prepackaging the leaves and flowers into buds the summer before they open normally has advantages that outweigh these costs. The main advantage is probably that it helps the tree to flush out quickly, and thereby to maximize the short growing season of about three months. In those three months the trees must not only produce their photosynthetic machinery, the leaves, but also use them long enough to repay their production costs to make an energy profit. Many animals take advantage of the early bud production, but trees are seldom fooled by a false start—which could occur because of a midwinter thaw, making them lose all their investment. As long as the buds maintain dormancy, they remain safe from freezing. Dormancy and cold-hardiness go together, through an evolved mechanism: the cold-hardiness is achieved in large part by withdrawing water from the tissues. Since water is required for the active processes of growth, development must wait until summer, when it is again safe to become hydrated. But how can the tree “know” when to start up and break bud?
Leaf buds and flower buds often open on very different schedules, even in the same species; and the schedules also differ between species. Most species of northern trees all leaf out at the same time, within roughly two weeks during mid-May in central Vermont and Maine, whereas forest tree flower buds open over a six-month span. Poplars bloom first, in early April, basswood flowers in July, and witch hazel in October. There are only relatively small differences between species in leaf bud opening (with quaking aspen and white birch being first; oaks and ash being last; and beech, maples, and many others being in between).
The buds of different tree species each open according to their specific local schedules, which are dictated by a complex interplay of cues involving hours of daylight, seasonal duration of cold exposure, and warmth. Warmth, as such, is not enough. For example, if sugar maples from the north are transplanted to Georgia, they won’t break bud there, because they receive insufficient chilling. Their strategy of determining whether or not winter has occurred is like that of the previously mentioned silk moth pupa, which also won’t break dormancy unless it (or at least its brain) is chilled for a sufficiently long time.
Although many trees have their primordia for both leaves and flowers packaged into the same bud (for example, apple and other Rosaceae, and viburnums) so that leafing out and flowering occur at about the same time, most of the northern forest trees allocate separate buds for leaves and flowers. This separation of buds appears to be adaptive, because it allows the plant to strategically separate its time of reproduction from the time of leafing out. It thus allows some wind-pollinated trees (the majority of northern trees) to flower a month or more before leafing out, when they can be more easily pollinated because there is less blockage of wind carrying pollen over the flowers. It allows other northern trees, such as bee-pollinated basswood, to be pollinated a month or more after the leaf buds have opened, when in late summer the bee populations have peaked and the bees will search for the flowers among the leaves. Similarly, witch hazel, blossoming in October, takes advantage of the winter months’ pollination that is available then.
Fig. 5. Leaf and flower buds of quaking aspen, as they appear from the end of summer to early January,