A dormant tree has temporarily suspended visible growth and significantly reduced its metabolic activity. This physiological shutdown is a natural, cyclical adaptation allowing the organism to survive periods of predictable environmental stress. Dormancy is most commonly associated with the cold temperatures and reduced daylight of winter in temperate climates, but it can also be triggered by extreme dryness or heat. This reversible condition is distinct from death; the tree is alive but not actively growing, effectively pressing an internal “pause” button on its development.
The Biological Purpose of Dormancy
Dormancy functions as a sophisticated survival mechanism, allowing trees to endure environmental conditions that would otherwise cause serious damage or death. Freezing temperatures pose a severe threat because water within plant cells can expand and rupture the cell walls, a process avoided by entering this resting state. By ceasing growth, the tree protects its most vulnerable tissues, specifically the actively dividing cells at the tips of branches and roots, from being killed by frost.
Dormancy also mitigates the risk of desiccation, or extreme water loss, when water uptake from frozen ground is difficult or impossible. Deciduous trees shed their leaves, which are the primary sites for water evaporation, eliminating a major source of water loss. This conserves the tree’s internal moisture and energy reserves, synchronizing the tree’s life cycle with annual climatic rhythms until favorable conditions return.
The Internal Mechanisms of Dormancy
The transition into dormancy is not simply a reaction to cold but a proactive, internally regulated process initiated by environmental cues. The most important external trigger is photoperiod, the gradual decrease in daylight hours as summer turns to autumn. Specialized light-sensing proteins perceive this shortening day length, initiating a cascade of internal physiological changes that prepare the tree for the coming cold.
Once triggered, the tree undergoes a profound metabolic slowdown, shifting from a growth-oriented state to a survival-oriented one. Respiration and energy use are significantly reduced, and sugars produced during the growing season are converted into starch for long-term storage in the wood and roots. This stored energy will fuel the initial burst of growth when the tree wakes up.
Hormonal changes orchestrate this internal shift, with the growth-inhibiting hormone abscisic acid (ABA) playing a dominant role. ABA levels increase dramatically, acting to suspend cell division and growth, and promote the formation of protective terminal buds. This rise in ABA simultaneously suppresses growth-promoting hormones like gibberellins (GAs), which encourage active development.
The tree’s final preparation involves developing cold hardiness to withstand freezing temperatures. This is accomplished by altering cell fluids, increasing the concentration of dissolved substances like sugars and salts to lower the freezing point, similar to antifreeze. The tree also moves water out of the cells and into the spaces between them, a deliberate dehydration that prevents the formation of lethal ice crystals inside the cells.
Identifying and Classifying Dormancy
Identifying a dormant tree is done through visual inspection of its physical features, which differentiate it from a dead specimen. A healthy dormant tree will have firm, plump buds, which are the protected growth points for the next season’s leaves and flowers. These buds are typically covered by tough, protective scales that shield the delicate tissues inside.
A common method to assess vitality is the “scratch test,” where a small patch of the outer bark on a twig is gently scraped away. If the tissue immediately beneath is bright green and moist, the tree is alive and dormant; if it is brown, dry, and brittle, the branch is likely dead. Deciduous trees display unique characteristics in their twigs, such as the arrangement of buds and the shape of leaf scars, which are used for species identification.
The state of dormancy is scientifically classified into three distinct phases to describe the physiological control of the resting period. Endodormancy, or internal rest, is the deepest stage where growth inhibition is controlled by internal signals within the bud, regardless of external conditions. A tree in endodormancy requires a specific amount of chilling hours, or accumulated time at low temperatures, to release the internal block on growth.
Once the chilling requirement is met, the tree enters Ecodormancy, or external rest, where the internal block has been lifted, but growth is still prevented by unfavorable environmental conditions, such as freezing temperatures. As soon as temperatures warm and moisture is available, the tree will break ecodormancy and resume growth. A third type, Paradormancy, is a localized form of rest where growth is suppressed by factors external to the affected bud, such as hormones produced by the terminal bud that inhibit the growth of lateral buds.