Photo credit to http://www.fortticonderoga.org/
What do leaves have to do with the seasons? Most people mark the coming of winter by the time that trees become bare and leaves litter the ground. The trees themselves, however, begin their winter preparations long before we humans do.
Deciduous 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. Night-length triggers pre-dormancy – what triggers true dormancy?
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, and is visibly marked by falling leaves. It is important to note that trees don’t suddenly become dormant when their leaves fall – it is a continual process that occurs over many months.
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. Is there an ostensible swelling at the end of the branch? This is the terminal bud. In addition to a 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. How long has the branch grown this year? Are the distances between bud scars even or uneven in length? 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.
-Marie Schaedel ’15, for the Cole Student Naturalists