Let Sleeping Trees Lie

We benefit from trees year-round, regardless of the stage they’re in. Their beauty, ability to capture and store carbon, and soil-enhancing properties persist regardless of season. In the winter, deciduous trees enter dormancy—a phase where trees are alive but not actively growing. Dormancy is a survival mechanism that helps prevent water and nutrient loss while protecting trees from damage caused by freezing temperatures. Let’s explore the factors that trigger dormancy and the processes that unfold during this period of rest.

Light and Tree Dormancy

As summer turns to fall, the amount of daylight trees receive also changes. Trees are equipped with photoreceptors called phytochromes that sense light changes, triggering important hormonal responses.

• Red light (600-700 nanometers [nm]): Phytochromes activated by red light signal active growth and reproduction.
• Far-red light (700-800 nm): Phytochromes sensing far-red light reduces activity (the amount of which increases in the fall), signaling the tree to prepare for dormancy as daylight shortens.

These light cues play an important role in initiating the next step: the hormonal changes that control dormancy.

Hormonal Responses

Hormones orchestrate the transition into dormancy, with abscisic acid (ABA) taking the lead. ABA, produced in the tree’s roots, slows daily processes, helping the tree conserve energy and resources. It also influences leaf color changes and nutrient reabsorption before winter sets in. Here’s how other hormones contribute:

• Auxin: Dominant during the growing season, auxin production decreases as ABA signals the tree to conserve energy.
• Ethylene: This hormone facilitates leaf detachment by creating a protective layer between the leaf and its twig attachment point. As nutrients are drawn back into the tree, leaves fall and decompose, enriching the soil.

The coordination of these hormones ensures the tree is prepared to withstand the cold months while conserving vital resources. In conjunction with these hormones, bark insulates and protects trees from freezing and cracking during the cold months by reflecting light and dispersing heat. However, young trees are more susceptible to exploding or cracking bark because thin bark is less resistant to temperature changes.

Dormancy vs. Slowed Growth in Conifers

Interestingly, not all trees enter true dormancy. Conifers, like spruces and pines, slow their growth but retain their needles year-round, hence the term evergreens. A waxy coating on their needles helps conserve water, enabling them to photosynthesize during sunny winter days and maintain a steady energy supply.

For deciduous trees, dormancy lasts until days lengthen, and temperatures regularly stay above freezing. This happens because phytochromes sense an increase in red light and days within a certain temperature threshold accumulate. This reactivates growth hormones, gradually transitioning the tree back into its active phase.

The Growth Cycle of Trees

Understanding tree dormancy reveals the remarkable adaptability of trees and their ability to complete their growth cycle year after year in varying conditions. Once the dormancy period ends, that cycle is completed, and a new one begins. Whether shedding leaves or slowing growth, trees exhibit resilience and efficiency—a testament to their vital role in our environment.

Want to learn more about tree cycles? Read our companion piece about trees exiting dormancy, When Trees Wake Up