The growth of a tree is a process of increasing in size and mass through the production of new cells and tissues. This biological activity is not continuous; instead, tree growth is cyclic, alternating between periods of active expansion and periods of rest. The timing of this cycle is regulated by internal biological signals synchronized with external environmental conditions.
The Annual Cycle of Growth and Dormancy
The active period of tree growth typically begins in the late spring and extends through the summer months. This phase starts with a rapid initial surge known as the growth flush, where pre-formed structures within the buds expand quickly. This burst of activity is fueled by energy reserves stored in the roots and woody tissues from the previous year.
Following this initial flush, many trees enter a period of sustained growth during the warmer summer season. During this time, the tree maximizes photosynthesis, converting sunlight into carbohydrates to support continued expansion and build up energy reserves. As daylight hours begin to shorten in the late summer and early autumn, the tree prepares for its dormant phase.
Dormancy is a survival mechanism where the tree’s metabolism slows significantly to conserve energy and protect tissues from freezing temperatures. Above-ground growth ceases, buds are formed to protect the undeveloped shoots for the next year, and a process called hardening occurs. Hardening involves cellular changes, such as reducing water content and increasing sugar concentration, which enhances the tree’s cold tolerance.
This resting period lasts through the coldest part of the year. While the canopy is dormant, root growth can continue, often until the soil temperature drops too low. The storage of nutrients like nitrogen and carbohydrates in the root system and lower trunk is a major focus during the transition to dormancy, providing the energy needed to restart growth the following spring.
Environmental Triggers That Initiate Growth
The timing of when a tree starts or stops growing is dictated by external environmental cues that act as biological switches. Temperature and light are the two primary factors that signal a tree to transition between its dormant and active states.
Temperature is a major factor in two distinct ways: satisfying the cold requirement for dormancy and providing the warmth needed to break it. Many temperate trees require a specific number of cold hours, known as chilling hours, to be accumulated to fully release from deep dormancy, or endodormancy. These hours are typically calculated for temperatures between 32°F and 45°F, and they prevent the tree from breaking bud during an unseasonable warm spell in mid-winter.
Once the chilling requirement is met, the tree enters a secondary state of rest, or ecodormancy, where growth is held back only by the low ambient temperature. The tree then needs a certain accumulation of heat, often measured in growing degree days, to initiate bud burst and growth. For most species, this requires a sustained period of warmer air and soil temperatures above a minimum threshold, which varies by species.
Photoperiod, the relative length of day and night, acts as a reliable calendar signal, especially in autumn. As days shorten, trees sense the reduction in light duration, which triggers hormonal changes like the production of abscisic acid. This hormone inhibits growth and promotes the formation of a protective terminal bud, signaling the tree to prepare for winter.
Increasing day length in the spring acts alongside warming temperatures to promote renewed growth. Water availability also limits the growth window, as drought conditions can induce premature growth cessation and dormancy even during warm periods, as a survival mechanism against desiccation.
Primary and Secondary Growth
During the active growing season, a tree simultaneously engages in two different types of expansion: primary growth and secondary growth.
Primary growth is responsible for increasing the height of the tree and the length of its branches and roots. This type of growth occurs at specialized zones of cell division called apical meristems, which are located at the very tips of the shoots and roots. The activity of the apical meristems allows the tree to extend its canopy upward and outward, competing for sunlight, while pushing its root system deeper and wider in search of water and nutrients.
Secondary growth, on the other hand, is responsible for the increase in the tree’s girth, or diameter. This thickening occurs through the lateral meristems, specifically the vascular cambium, which forms a thin ring just beneath the bark. The vascular cambium is an actively dividing layer that produces new xylem cells toward the inside of the trunk, forming wood, and new phloem cells toward the outside.
The cork cambium, another lateral meristem, also contributes to girth by producing new bark tissues on the exterior of the trunk. Both primary and secondary growth processes are most vigorous during the late spring and early summer when environmental conditions are optimal.