Summer World_ A Season of Bounty - Bernd Heinrich [8]
How do the buds “know” when to open? Photoperiod has a strong effect, and to try to separate the effect of photoperiod from temperature I bagged (under triple layers of black plastic) half of each of a bush of beaked hazel and speckled alder—two of the earliest-blooming woody plants. I found that the darkness did not retard the flowering times. It seemed as though the buds’ opening is, instead, strictly controlled by temperature. However, this was a very small and select sample—two species of the very earliest-flowering trees—and the leaf buds did not open.
Leaf buds bide their time through the winter, even during thaws. I am impatient. By the winter solstice (21 December), when the nights are longest, I am already anxious to see any little bit of green leaf or colored flower. So, at that time, and in subsequent weeks over the next three months, I have developed the habit of picking some twigs with leaf and flower buds. I bring them into the house, put the stems into water, and wait (and hope) for some to open and show me whether or not they are ready for summer.
In 2006, on the solstice, I brought twigs of a dozen different species of trees and shrubs into the house and set them in water on the windowsill. Then every two weeks I again brought in the same kinds of twigs, and then I noted whether or not any buds opened, or which buds opened, to try to determine whether and when a sudden warming might release the buds’ dormancy.
I had expected that the schedules of the buds’ release might roughly parallel the trees’ normal flowering-leafing schedule, even though all the buds were already preformed the previous fall. To some degree that is what happened. From my first batch of twigs brought in at the solstice, only two of the nonnative species (forsythia and ornamental cherry) opened a few flower buds. Most of the flower buds died and dried, although the twigs remained alive and some of the leaf buds finally opened in February. But alder, willow, beaked hazel, quaking aspen, red maple, and elm brought in during January opened at least some of their flower buds after only six days. And some of the same species brought into the warmth in mid-March (one to three weeks prior to normal blooming outside) also began to expand or open flower buds in about the same time, three to six days later. As in the field, however, their leaf buds remained hesitant to respond to warmth, opening only about a month later. The leaf buds of some tree species, primarily ash, red oak, and sugar maple, showed no response at all even after two months in the warmth.
The restraint in leafing out, although proximally related to potential frost damage, is ultimately probably due to the danger of snow loading that could topple the trees (as discussed later). The risks of frost injury are different for flowers and for leaves. A tree lives for many decades or centuries. It can risk losing its flowers to frost in any one year because energy saved in not fruiting that year can be invested in growth or in fruiting the next year. Losing leaves, on the other hand, results in cutting off energy inflow and stopping growth and hence a falling behind in the competitive growth race for the light.
Tree buds break dormancy owing to local stimulation; prior chilling of one bud on a lilac stem enables it to flower while a neighboring non-chilled bud remains dormant. Similarly, certain chemical vapors applied to one lilac bud will cause it to open while an adjacent untreated bud remains dormant (Denny and Stanton 1928). Therefore, presumably, if a tree is kept in a warm greenhouse all winter it will not leaf out or flower in the spring, although if one branch of it has been protruding to the outside, then that one branch alone will leaf out and bloom. Such simple experiments show that timing—when to renew life for the summer after the long winter—is not left to chance. There are active mechanisms