Winter presents a dual challenge for trees, combining freezing temperatures with the physiological drought caused by frozen ground. To survive the cold and lack of available liquid water, deciduous and temperate evergreen trees must undergo dormancy, pausing all growth and metabolism. This protective state requires precise timing to ensure the tree is fully prepared before the first hard frost arrives. The annual shutdown is triggered by a sequence of predictable environmental signals. These cues prompt a cascade of internal, hormonal, and biochemical changes, transforming the tree into a hardened, resting state.
The Critical Timing Cue: Decreasing Daylight (Photoperiod)
The primary signal used by trees to begin winter preparation is the shortening of the day, known as the photoperiod. Unlike temperature, which can fluctuate unpredictably, the length of the dark period is a precise indicator of the approaching season. Trees measure this lengthening of the night using specialized photoreceptor proteins, primarily the phytochromes.
Phytochromes exist in two interconvertible forms: \(P_r\), the inactive form, and \(P_{fr}\), the biologically active form. Sunlight converts \(P_r\) into \(P_{fr}\) during the day, and \(P_{fr}\) slowly reverts back to \(P_r\) during the uninterrupted darkness. As nights become longer in late summer, the darkness is sufficient to convert almost all \(P_{fr}\) back to \(P_r\). The resulting low level of active \(P_{fr}\) at dawn signals that the critical day length has been reached. This photoperiodic signal triggers hormonal shifts, including a reduction in growth hormones and an increase in abscisic acid (ABA). This initiates the cessation of active growth and the formation of protective terminal buds.
Refining the Schedule: The Role of Temperature Fluctuation
While the photoperiod sets the initial timeline for growth cessation, temperature acts as a secondary signal, fine-tuning the tree’s final readiness. Consistent cool temperatures, particularly the drop in nightly lows, accelerate the process of cold hardiness. This chilling effect guides the tree through the final stages of preparation, ensuring that biochemical defenses are fully deployed. If an unseasonably warm period occurs, the hardening process temporarily slows down, but the tree does not reverse the process. Temperature is also instrumental later in the winter, as prolonged exposure to low temperatures is required to break dormancy, preventing growth from resuming too early in the spring.
The Visible Response: Leaf Color Change and Shedding
Leaf Color Change
The hormonal changes initiated by decreasing daylight rapidly lead to the most noticeable sign of winter preparation: the dramatic display of leaf color change. This process begins with the breakdown of chlorophyll, the dominant green pigment responsible for photosynthesis. The tree resorbs valuable nutrients, such as nitrogen, from the chlorophyll molecules before they are shed. Once the green pigment fades, underlying pigments that were always present in the leaf become visible, creating the yellows and oranges of autumn. These pigments are primarily carotenoids. New pigments, the anthocyanins, are sometimes actively produced in response to bright light and cool temperatures, generating the vibrant reds and purples seen in many species.
Leaf Shedding (Abscission)
The physical act of dropping the leaf, known as abscission, is controlled by a delicate hormonal balance. During the growing season, a constant supply of the growth hormone auxin prevents shedding. As the tree prepares for dormancy, auxin production decreases while the signaling molecule ethylene increases. This hormonal shift triggers the formation of the abscission zone at the base of the leaf petiole. Enzymes dissolve the cell walls that hold the leaf to the stem, allowing the leaf to detach. Simultaneously, the tree forms a protective layer of cork cells behind the abscission zone, sealing the wound to prevent water loss and the entry of pathogens after the leaf falls.
Internal Protection: Achieving Full Dormancy
The ultimate goal of all these signals and visible changes is to achieve a deep, protective state known as endodormancy, or true rest. This final stage involves complex biochemical adjustments to prepare the tree’s living tissues for surviving extreme cold, a process called cold acclimation or hardening. The tree must protect its cells from the damage caused by ice crystal formation, which can puncture cell membranes and lead to death.
A primary strategy is the strategic dehydration of cells, which forces water into the spaces between the cells where ice can form without causing lethal damage. Simultaneously, the tree dramatically increases the concentration of non-structural carbohydrates, such as sucrose, glucose, and fructose, within its remaining cell water. These soluble sugars and other compounds, like sugar alcohols, act as natural cryoprotectants, effectively lowering the freezing point of the cytoplasm. The accumulation of these substances functions much like antifreeze, preventing the formation of destructive ice crystals inside the cell. The tree also synthesizes specialized stress proteins, such as dehydrins, which help stabilize and protect cell membranes and other cellular structures. The new terminal and lateral buds, formed earlier in the autumn, are tightly sealed with protective scales, shielding the vulnerable meristematic tissue containing next year’s growth from the harsh environment.