Pine trees and other conifers are recognized as evergreens because they retain their needles through the cold winter months, maintaining a green canopy year-round. This continuous green appearance often leads to the assumption that these trees are actively growing even when snow covers the ground. However, the presence of needles reflects a survival strategy different from deciduous trees, which shed their leaves. While pines remain outwardly green, the internal processes driving growth drastically slow down in winter, shifting the tree’s focus from expansion to survival.
Winter Dormancy: The Pause in Growth
The period of winter dormancy is a programmed physiological state in pine trees, triggered by declining temperatures and the shortening of daylight hours (photoperiod). This process allows the tree to prepare for and endure freezing conditions. During this time, active growth is put on hold, a mechanism involving ecodormancy followed by endodormancy in its buds, which requires a sustained chilling period to break.
The metabolic slowdown involves a significant reduction in enzyme activity responsible for building new tissues. Cell division and elongation, the processes that create new wood and lengthen shoots, effectively cease or slow down. Instead of allocating energy to growth, the tree redirects its resources to maintenance and the production of protective compounds. Attempting to grow new, tender tissues during a hard frost would result in immediate damage to the cells.
This cessation of growth ensures the tree conserves energy reserves stored from the previous warm season. The buds that will produce the next year’s growth are already formed and tightly sealed. While the tree’s growth machinery is paused, its survival machinery is fully engaged, preparing the cells for the extreme cold.
Physiological Mechanisms for Surviving the Cold
Survival through sub-freezing temperatures relies on biological adaptations that prevent cellular destruction from ice formation and dehydration. A primary concern for evergreens is water loss when the ground is frozen, making water uptake impossible—a condition called physiological drought. Pine trees address this challenge with a specialized needle structure.
The small surface area of the pine needle reduces the amount of water lost through transpiration. A thick, waxy outer layer, called the cuticle, further seals the needle, providing a physical barrier against drying winds and evaporation. This adaptation is necessary because the tree cannot replace lost water until the soil thaws.
Internally, the tree employs mechanisms to manage water within its cells to prevent lethal intracellular freezing. As temperatures drop, pine cells accumulate specialized molecules known as cryoprotectants, such as sugars like sucrose and raffinose. These molecules work by lowering the freezing point of the water inside the cells, acting as a natural anti-freeze.
The presence of these solutes also helps manage ice formation in the non-living spaces outside the cell walls. Cryoprotectants draw water out of the living cells and into the intercellular spaces, where ice can form with less damage to the cell machinery. This process, known as freeze-tolerance, allows the tree to survive temperatures that would ordinarily burst cell membranes.
The Annual Growth Cycle of Conifers
The winter rest period is a necessary phase that sets the stage for active growth during the warmer seasons. Conifers must accumulate a certain amount of chilling hours—a specific duration of near-freezing temperatures—before their buds can be released from deep dormancy. Once this chilling requirement is met and spring temperatures rise, the growth cycle begins anew.
The first visible sign of active growth is the swelling and expansion of the terminal and lateral buds, typically beginning in late spring or early summer. This growth utilizes the energy reserves, primarily stored starch, conserved during the winter months. The rapid burst of new shoot growth, often called “candles” on pines, results in the tree’s annual height and diameter increase.
This active growth period is when the pine tree builds the majority of its biomass for the year. Photosynthesis, which slowed significantly in winter, ramps up to full capacity to power this new growth. This process also replenishes the energy reserves needed for the next cold season, ensuring the tree maximizes its growth potential when conditions are favorable.