Summer World_ A Season of Bounty - Bernd Heinrich [95]
Just as important, of course, are the cues that cause plants to produce these key chemicals. Warming in the spring brings trees to life; but lower temperatures alone do not cause a tree to shut down and lose its leaves at the end of summer. The relatively precise timing of the leaf fall is of some importance, and it has been “calculated” by evolution through a balance of costs and benefits. Photoperiod, specifically the length of nights, is the main stimulus.
When nights become long enough, trees begin to shut down for the summer by forming a corky layer of cells between leaf and twig. This layer, the abscission layer, then blocks off the transport of materials between the branch and the tree. Chlorophyll is then no longer replaced as it breaks down with use; and as it disintegrates, the yellow and orange leaf pigments are revealed. The abscisic acid then does its job of dissolving the corky cell layer that holds the leaves to the trees, and as the connection between leaf and twig weakens, a breeze does the rest, and the leaf falls.
The appearance and disappearance of leaves may be the most conspicuous marker of the seasons and the most important events to the lives of a host of insects and birds; but to the plant, leaves are only means to an end. They provide the energy and raw materials for producing flowers, fruit, and seeds. The timing of the bloom and fruiting is critical to many animals as well. The summer schedule for the tree’s leafing is more constrained than that of the bloom, which may appear to be totally random, since it starts long before the leaves come out in some species, occurs in midsummer in others, and extends until late fall in at least one species, witch hazel. But it’s not random at all. All the deciduous trees (those that shed their leaves in the fall) that are wind-pollinated flower before the leaves come on, probably because leaves would hinder access of airborne pollen to the pistils. Wind pollination, as such, does not require early blossoming, because all the wind-pollinated conifers flower not in early spring but in the summer. Bee-pollinated flowers, such as those of black locust and basswood, flower late in the summer, when insect pollinator populations have built up. Witch hazel, which flowers in September, is pollinated by winter moths that are beginning to be active only then. Animal-pollinated trees bloom when pollinators are available, so in theory they could be pollinated all summer long, except that even in an undisturbed habitat there is competition for pollinators. Fewer pollinators are available to any one tree species if another one that blooms simultaneously draws some away to its flowers. However, divergence of species’ blooming times—so that these times space themselves out over the entire summer—is one of several solutions that reduce competition.
The timing of blooming is also tactical in part because it secondarily affects the timing of fruiting. Different species of bamboo, for example, flower and produce large seed crops not once per year, but at intervals of 60 to 100 years or more. Furthermore, when they do flower, they do so synchronously over vast areas. The naturalist George B. Schaller noted that in 1974 and 1976 umbrella bamboo—a staple of pandas—died throughout an area of 2,000 square miles in the pandas’ northern range. At least 140 of the rare pandas died. Undoubtedly, vast numbers of rodents—seed predators of the bamboo—died as well. If the bamboo were to flower and produce seed every year, the rodent populations would be permanently high and perhaps harvest all the seed produced each year. Similarly, some tree species in the woods from Maine to Vermont also time their blooming by not blooming, and thereby control the seed predator populations. In the summer of 2007, for example, the sugar maples, American ash,