While trees do not truly hibernate like bears or other mammals, they undergo a similar state of rest known as dormancy. Dormancy is a biological strategy that allows woody plants to survive the freezing temperatures, lack of liquid water, and low light conditions of winter. This programmed shutdown conserves energy and protects the tree’s delicate tissues from damage.
The Biological Reality: Tree Dormancy
Dormancy is an internally regulated state of developmental arrest, suspending growth even if external conditions are temporarily favorable. This differs from quiescence, which is a rest imposed by immediate, unfavorable conditions like drought or brief cold spells. Quiescence is easily broken once good growing conditions return, but true dormancy, or endodormancy, requires a prolonged internal process to be completed.
The tree’s internal clock and hormonal signals govern endodormancy, making the plant unresponsive to short-term temperature changes. This regulation prevents a tree from prematurely sprouting during a mid-winter warm-up, only to be killed by a subsequent hard freeze. During this period, the tree remains alive, but its growth-promoting hormones are suppressed.
Entering the State: Environmental Triggers and Preparation
The primary cue for a tree to begin its seasonal shutdown is the shortening of daylight hours, known as photoperiod, not temperature. Trees use specialized photoreceptors called phytochromes to sense the ratio of red light to far-red light. As days shorten in late summer and early autumn, the increasing dark period signals that winter is approaching, long before the first frost.
This signal triggers hormonal changes, particularly the production of abscisic acid (ABA), which inhibits growth. For deciduous trees, this shift initiates leaf senescence, where the tree reabsorbs valuable nutrients like nitrogen and phosphorus before the leaves are shed. Leaf drop, or abscission, seals the vulnerable points where leaves attach, preventing water loss and the entry of pathogens. Simultaneously, the tree forms protective bud scales around its shoot apical meristems, the growing points, to shield them from the environment.
Surviving the Cold: Physiological Changes During Dormancy
To survive sub-freezing temperatures, the tree undergoes cold hardening, which alters its cellular composition. The main threat from freezing is the formation of ice crystals inside the cell, which would rupture the cell membrane and destroy the tissue. To prevent this, the tree actively moves water out of its living cells and into the intercellular spaces.
This process, known as cellular dehydration, concentrates the remaining cellular fluid, effectively lowering its freezing point. The water outside the cells freezes harmlessly in the spaces between them, while the dehydrated cytoplasm remains a thick, unfrozen syrup. The tree also synthesizes cryoprotectants, such as soluble sugars, amino acids, and specialized proteins. These compounds act as a biological “antifreeze,” protecting cell structures and maintaining the fluidity of cell membranes even in cold conditions. Throughout dormancy, the tree’s metabolic rate, including photosynthesis and respiration, slows to a minimum to conserve energy reserves until spring.
Breaking the Rest: The Return to Growth
Exiting endodormancy requires the tree to satisfy a specific internal requirement, ensuring growth only resumes when the danger of a killing frost has passed. This requirement is met by accumulating a minimum number of “chilling hours,” which refers to the total time spent at cold temperatures, typically between 32°F and 45°F. This prolonged cold exposure is necessary to biochemically reset the plant’s growth-inhibiting mechanisms.
The necessary chilling hours vary widely among species, with some requiring less than 300 hours and others needing 1,000 or more. Once the chilling requirement is met, the tree transitions to ecodormancy. In this state, the tree is ready to grow but is still held in check by external environmental conditions. The final trigger for bud break and the resumption of active growth is the return of consistently warmer temperatures. This dual requirement—chilling followed by warmth—synchronizes the tree’s spring emergence with the stable conditions of the growing season.