Breaking dormancy in trees

Most deciduous trees go through a period of inactivity known as “dormancy” during the winter months, especially in northern regions. This period of dormancy is brought about by decreasing day length (photoperiodism), cold, and sometimes by drought conditions.

During the fall, the tree begins forming winter buds and producing the hormone, abscisic acid, which causes the leaves to fall off and the tree to enter a dormant state. The concentrations of abscisic acid are highest at leaf fall, and slowly diminish in response to periods of cold. On average, a tree in the northern hemisphere needs to go through 60 days where the temperature is below 40 degrees for the abscisic acid to break down enough to allow the tree to leaf out once favorable conditions arrive. This requirement can be predicted by a tree’s hardiness zone, and that is why all the native trees in Maryland do not begin breaking bud should a temporary warming period arrive in mid-January, for example. Native trees can not be tricked out of dormancy so easily, since they have become adapted to their natural surroundings. Trees planted outside their hardiness zones may not be so wise, however. Sometimes these nonresident trees may break bud too early if they are from a southern climate and think that spring has arrived in January, or they may be waiting for a few more cold days in June if they were transplanted from a colder climate like Minnesota. Once the requirement for cold has been reached, and the abscisic acid has broken down, the buds begin producing various “growth” hormones like auxins, gibberellins, and cytokinins, which cause cellar activity to begin again, and the tree starts to break bud and to develop new leaves.

Credit: wikimedia.org - Huw Williams

Another process that the tree initiates in the late summer and fall is the conversion of excess sugars to starch, and the storage of the starch in the woody tissue and the root system. This starch is the energy reserve that trees require to function when they are dormant to break bud, and to grow new leaves in the spring. This stored starch is broken down by rising temperatures, so that when warmer temperatures arrive, the newly converted sugars help power activities inside the tree. During the late winter and early spring, the weather might be such that during the day the temperatures are warm, but they cool off to below freezing during the evening. This warming and freezing cycle causes the build up of pressure inside a tree during the day, and the reduction of pressure at night. The net effect is that at night, the suction in the roots draws water into the tree, and buildup of pressure draws water and sugars up the tree during the day, causing a phenomenon known as “sap rise.”

Most trees produce seeds in the fall right before winter arrives, which requires a period of dormancy to ensure that the new seedling does not germinate until conditions are favorable for growth and development. This dormancy inside the seed may be caused by a thick seed coat which may need to be damaged or worn away by the elements before the embryo inside can germinate; or sometimes the embryo’s growth is inhibited by hormones that require a period of cold before their activity diminishes and the work of the growth hormones begin. There are some seeds that may lay dormant for years until growth is triggered by increased sunlight or very hot temperatures. An example of this is a fire-dependent tree like the Table Mountain pine which has seeds that remain encased in its dense, spiny, serotinous cone that requires intense heat like that of a forest fire before it can open to expel the seed.

Article by FCFCDB

Nature Note for 5/2/20

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