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By Root 732 0

red oak, beech, white pine, and red spruce all failed to flower, and as a consequence there were almost no sugar maple seeds, acorns, or beechnuts—a collective absence of the mast that feeds a large variety of animals. White oaks, though rare, were apparently not affected. A friend told of finding one white oak tree near Wiscasset, Maine, that was “loaded” with acorns, and he saw there three raccoons and one porcupine at once. Bears, having no mast to fatten up on, depended heavily on apples, and a friend from Montpelier, Vermont, saw five of them at once in an apple orchard. Red-breasted nuthatches, who rely on conifer seed, were absent from my woods in Maine. Chipmunks, deer mice, and red squirrels are major predators of birds, eating birds’ eggs and young in the nest. Owing to the ripple effect, I expect a surge of forest bird populations in a year or two. In the meantime, during the next flowering cycle of the trees, the seeds and seedlings will also have a much higher survival rate.

Fig. 38. Witch hazel flowering in October.

The timing of the shedding of leaves is probably of even greater tactical significance to trees than the timing of the bloom. Indeed, we almost define the season by the status of the leaves, which are unfurled and then again shed more or less synchronously (relative to blooming times). The process is accomplished so flawlessly and regularly that it is easy to take the reason for granted. We scarcely ask why, much less how, whether or not, and when.

The complex process of leaf abscission has evolved in trees of very diverse, unrelated families. However, many members of these same tree families don’t shed their leaves at the end of the summer, raising the question why the others do. Larch is one apparent anomaly among the conifers. Its leaves turn golden in the fall and are all shed before winter, having served the tree about five months. The white pine sheds its leaves only after two years, so half its leaves are shed each year. Spruce and balsam fir may keep their leaves for five or six years. Most northern broad-leafed trees shed all their leaves every fall, though some of the more southern trees, like magnolia and some oaks, may keep them not just for an entire year but for five or six years. Since leaf shedding evolved numerous times, it must have offered a powerful selective advantage. But what is that advantage?

The advantage of not shedding undamaged leaves, all other things being equal, seems obvious. Leaves are solar panels, and discarding them every few months means having to make new ones later, using time and resources that would otherwise be available for growth. Resources invested in disposable leaves would be valuable for more growth and thus for survival in the fight for light that most forest trees wage against each other throughout their lives. These resources would also be valuable in obtaining a surplus of energy for fruit and seed production. All else being equal, it should be more economical to retain leaves for a whole year, or preferably for several years, than to discard leaves used only for four months and construct new ones every summer. Retaining leaves can serve the added advantage that they are then available for use during the occasional warm spells that almost invariably occur every winter. And as might be predicted from this rationale, many trees (as already mentioned) do indeed keep individual leaves for several years before finally replacing them.

One hypothesis regarding why trees shed leaves before winter is that the leaves would be or are killed by freezing and then are shed incidentally. But this hypothesis does not satisfy the ultimate, evolutionary, question. Frost intolerance of those leaves that are normally shed may be a proximal result of not experiencing freezing and therefore not having had a need to evolve frost-hardiness. By contrast, buds (which contain embryonic stems, leaves, and flowers) are frost-tolerant, even on trees that have frost-sensitive leaves.

Frost-hardiness has evolved in the leaves of many trees. Spruce and fir leaves, for