Seasonal Shutdown: The Biological Mechanisms Behind Tree Dormancy

As autumn settles over Northwest Indiana, the vibrant colors and falling leaves are a visible reminder that trees are transitioning into dormancy. While it may seem like trees simply “shut down,” the reality is far more complex. Within the canopy, trunk, and roots, countless biochemical and physiological changes are taking place — all designed to help the tree endure months of cold, dry, and dark conditions ahead.

1. Photoperiodism: The Internal Calendar

The first signal of seasonal change isn’t temperature — it’s day length. Trees rely on photoreceptors in their leaves to detect the shortening days of late summer and early fall. This process, known as photoperiodism, triggers hormonal changes that begin slowing growth, forming buds, and reducing photosynthetic activity.

This internal timing mechanism ensures the tree doesn’t respond prematurely to short-term temperature fluctuations. Even in an unusually warm fall, the reduction in daylight ensures that dormancy preparation proceeds on schedule.

2. Hormonal Regulation and Growth Cessation

During long days, proteins like Flowering Locus T (FT) and hormones such as gibberellins (GA) move freely, keeping buds active and growing. As daylight decreases, abscisic acid (ABA) and callose deposition block those pathways, signaling the buds to stop growing and enter dormancy until conditions improve.

Once triggered, the tree’s internal chemistry shifts dramatically. Levels of auxins and gibberellins, which promote growth, begin to decline, while abscisic acid (ABA) production increases.

ABA plays a central role in:

• Inhibiting new leaf and shoot development

• Promoting bud scale formation

• Increasing cold hardiness by altering cell membranes and water content

• Signaling the leaf abscission process (separation of leaves from twigs)

These hormonal changes are part of a coordinated effort to conserve energy and prepare tissues for freezing temperatures.

3. Nutrient Mobilization and Storage

Temperature differences between roots and shoots affect how trees move and store energy. In warmer conditions, sugars and starches flow upward to support growth, while cooler soil temperatures slow that movement and cause more carbohydrates to stay stored in the roots for winter.

Before the leaves are shed, the tree begins to reallocate essential nutrients. Nitrogen,

phosphorus, and potassium are withdrawn from leaves and stored within woody tissues and roots. At the same time, photosynthates (sugars produced from photosynthesis) are converted into starches and stored primarily in the roots and lower trunk.

These starches serve as the tree’s energy reserve — critical for respiration throughout dormancy and for fueling new leaf and root growth in early spring, before photosynthesis resumes. Trees under stress (from drought, pest injury, or defoliation) often enter dormancy with lower carbohydrate reserves, leaving them more vulnerable to dieback or slow recovery in the following year.

4. Cold Acclimation and Cellular Hardening

A Peach tree branch cut into to show discolored wood from cold injury due to late season pruning.

As temperatures drop, trees begin the process of cold acclimation — a physiological adjustment that allows living cells to survive freezing conditions.

During this process, trees:

• Replace some water in their cells with sugars and proteins that act as natural antifreeze

• Strengthen cell membranes and reduce water content in living tissues

• Harden buds and bark through lignin deposition and suberization

These changes don’t eliminate freezing but instead allow cells to withstand dehydration and ice formation outside the cell walls — a key difference between hardy and cold-injured tissue.

5. Root Activity Beneath the Surface

Even as the canopy goes dormant, roots continue to function as long as soil temperatures remain above freezing. During late fall and early winter, root systems absorb remaining nutrients and water, supporting microbial activity and continued carbohydrate storage.

This is why fall soil care — such as deep root fertilization, soil aeration, or organic amendments — can be so beneficial. Trees are still capable of taking up nutrients during this window, building stronger reserves for spring growth.

6. Dormancy: Survival Through Stillness

Maple buds protected by hardened bud scales, formed in late summer to seal and insulate the growing tissue through winter dormancy.

By mid to late fall, trees have completed most of their internal preparations. Growth ceases, buds are sealed, and the vascular system slows to a minimum metabolic rate. The tree has entered true dormancy, a suspended state that protects its living tissues until photoperiod and temperature signals indicate spring’s return.

Although growth pauses, the work of survival continues quietly beneath the bark — a testament to the resilience and adaptability of trees through the changing seasons.

Supporting Trees Through the Transition

Understanding these natural processes highlights why late-season care matters. Trees that enter dormancy healthy — with strong energy reserves, adequate nutrients, and no active stressors — are better equipped to resist winter injury and thrive in spring.

If you notice premature leaf drop, poor fall color, or thinning canopies, it may be a sign of underlying stress that can carry over into next year.

Ensure your trees are entering dormancy strong and ready for winter. Our certified arborists can assess nutrient levels, root health, and overall vigor to keep your landscape thriving year after year. Click the button below to schedule a tree health evaluation.

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