What do leaves have to do with the seasons? Most people mark the coming of winter by the trees becomingbare and leaves littering the ground. The trees themselves, however, begin their winter preparations long before we do.
eciduous trees, those that shed their leaves, begin to undergo physiological changes as early as mid-summer, a process known as acclimation. How do trees ‘know’ when to begin preparing for winter? The primary factor involved is day length. As the nights begin to get longer, growth inhibiting hormones like abscicic acid build to high levels into the fall.
While the amount of light is the most important factor for dormancy, it is thought that cool temperatures may be required to initiate true dormancy. The dormancy period for most deciduous trees begins in October and ends in March.
Dormancy can be seen in other ways besides the loss of leaves. During the acclimation period, trees initiate bud dormancy. A dormant bud consists of an undeveloped shoot and rudimentary leaves, all enclosed by structures called bud scales. Similar to a seed coat, bud scales protect the bud from desiccation, regulate the movement of oxygen, and provide insulation. Bud scales also contain growth-inhibiting hormones, which will be replaced by growth-stimulating hormones in the spring.
Go out into the arboretum and take a look at a low-hanging branch that has lost its leaves. An ostensible swelling at the end of the branch is the terminal bud.
Most trees also possess axillary buds that are found along the length of the branch. Look for a ring that completely encircles the circumference of the branch, several inches away from the terminal bud. This ring marks the position of last year’s terminal bud, formed in preparation for last year’s winter.
You will also see leaf scars and small pores, called lenticels, which dot the length of the branch. Lenticels allow the exchange of carbon and oxygen to occur in the absence of leaves, where respiration usually occurs.
Falling leaves are not just a sign of autumn – they are a sign that the cold is already here.