What Happens To Trees In Winter & How Do They Survive

Before winter, trees lose all their leaves, and there are several reasons for this. 

Water Conservation

Trees lose their leaves before winter to conserve water because, during winter, the ground often freezes, making it difficult for trees to absorb water through their roots. Leaves naturally lose water through tiny openings called stomata in a process known as transpiration.

If trees kept their leaves during winter, they would continue losing water but with limited ability to replace it due to the frozen soil. Trees minimize water loss and reduce the risk of dehydration during the cold, dry months by shedding their leaves.

Energy Conservation

During winter, trees lose their leaves to conserve energy because maintaining leaves takes a lot of resources. In the cold months, sunlight is scarce, and photosynthesis—the process by which trees convert sunlight into energy—slows down significantly.

Without enough sunlight, the leaves cannot produce enough energy to justify their upkeep. By dropping their leaves, trees reduce the amount of energy they need to survive. This allows them to focus on keeping their vital parts, like the trunk and roots, healthy until spring, when they can start growing again.

Protection from Damage

Leaves can catch and hold onto snow and ice, which adds extra weight to the tree's branches. This added weight can cause branches to bend, crack, or break, especially during heavy snowfalls or ice storms.

By shedding their leaves before winter, trees reduce this risk. Without leaves, there's less surface area for snow and ice to accumulate, making it easier for the branches to withstand the weight and reducing the chances of damage.

Disease and Pest Prevention

Shedding leaves before winter helps trees avoid disease and pests. During winter, the tree's defenses are lower because it is dormant, making it more vulnerable. Leaves can harbor fungi, bacteria, and insects that could harm the tree if they remain attached.

When the leaves fall, they take these potential threats with them, reducing the chances of infection or pest infestation during the cold months. This natural process helps the tree stay healthier and more resilient when growth resumes in the spring.

Nutrient Recycling

Before trees shed their leaves in the fall, they start nutrient recycling. During this process, trees break down the nutrients stored in the leaves, like nitrogen, phosphorus, and potassium, and move them back into the tree's branches, trunk, and roots.

This allows the tree to store these essential nutrients for the winter, helping it survive the cold months when it cannot produce new nutrients through photosynthesis. When the leaves finally fall, they have already given back most of their valuable nutrients to the tree, ensuring it has the resources for the next growing season.

Preparing for New Growth

When trees drop their leaves in the fall, it helps them get ready for new growth in the spring. Shedding old leaves clears space for fresh ones to emerge when the weather warms up.

This ensures the tree can start the growing season with a clean slate, benefiting from new, healthy leaves that can efficiently capture sunlight and perform photosynthesis. Essentially, by letting go of old leaves, the tree prepares itself to make the most of the upcoming growing season.

Without any leaves, their main food maker, one might wonder, what do trees do in the winter? During winter, trees enter dormancy to survive. Here is everything you need to know about dormancy.

What is Dormancy?

Dormancy in trees is a period of reduced metabolic activity in response to adverse environmental conditions, such as cold temperatures. Dormancy helps trees survive periods when environmental conditions are not conducive to growth, such as freezing temperatures or drought. 

By entering dormancy, trees can withstand extreme conditions without expending unnecessary energy. During dormancy, trees minimize their metabolic activity, reducing the need for water, nutrients, and energy. This is particularly important in winter when water and nutrient availability can be limited.

Types of Dormancy

Endodormancy

Also known as internal dormancy, this type occurs when trees have completed their growth cycle but are still unable to grow due to internal factors. It is controlled by hormonal changes within the tree, which keep it in dormancy until external conditions, like temperature, signal it is time to resume growth.

Ecodormancy

This is triggered by environmental conditions, such as temperature. Trees enter ecodormancy when external conditions are unfavorable for growth, like extreme cold or drought. Ecodormancy ends when conditions become suitable for growth.

How does Dormancy Occur

Dormancy in trees occurs through these processes:

Environmental Triggers

Environmental triggers play a crucial role in initiating dormancy in trees. As autumn approaches, shorter daylight hours and decreasing temperatures signal the approaching winter. These changes act as natural cues for trees to begin the dormancy process.

The reduction in light triggers a drop in the production of growth-promoting hormones and an increase in hormones that signal dormancy. Cooler temperatures further contribute by slowing metabolic processes and signaling the need to conserve energy. Additionally, changes in soil moisture reinforce the need to enter dormancy. 

Hormonal Changes

Hormonal changes play a crucial role in initiating and regulating dormancy in trees. As environmental signals, like shorter daylight hours and cooler temperatures, trigger the onset of dormancy, trees begin to produce hormones such as abscisic acid.

This hormone signals the tree to slow down its metabolic processes, preparing it for reduced activity. Abscisic acid helps the tree enter dormancy by inducing physiological changes that lead to leaf drop, reduced growth, and energy and resource conservation. These hormonal shifts ensure that the tree can survive the harsh conditions.

Why does Dormancy Help?

You can make a tree skip dormancy by keeping it indoors with consistent temperature and light, but this is typically harmful. Dormancy is essential for a tree’s survival, both short-term and long-term. Without this period, a tree’s lifespan can be greatly reduced.

Just as bears hibernate to manage resource scarcity in colder months, trees rely on dormancy to conserve energy and prepare for growth. This rest period helps them endure harsh conditions and ensures they can thrive when temperatures rise again. Allowing a tree to enter dormancy is crucial for maintaining its health and longevity.

Do All Trees Become Dormant?

Not all trees undergo dormancy. Deciduous trees, which shed their leaves in fall, enter dormancy to survive winter. They significantly slow their metabolic processes to conserve energy and resources during harsh conditions. Evergreens, like pines and firs, do not fully enter dormancy; instead, they continue to function at a reduced level.

Their needle-like leaves, adapted for cold, allow them to perform photosynthesis even in winter. While deciduous trees use dormancy as a survival strategy, evergreens adapt differently to winter, maintaining some activity throughout the season.

Tree dormancy can be broadly categorized into three phases: early rest, winter rest, and after-rest. Each phase involves specific physiological changes. During winter, trees are in the winter-rest phase, which helps them endure the cold.

Trees employ three primary strategies to prevent freezing during this stage: altering membrane properties, thickening cellular fluids, and entering a glassy state. 

Membrane Change

One way is by altering their membranes to make them more flexible. This flexibility allows water to move out of the cells and into the spaces between them.

The displaced water pushes against the cell walls, but as the cells shrink and take up less space, this pressure is balanced.

Thickening Cellular Fluids

Another method trees use to avoid freezing is by thickening the fluids inside their cells. As days shorten, trees convert starch into sugars that function as a natural antifreeze. These sugars concentrate in the cellular fluid, lowering the freezing point within the cells.

While the water between the cells can freeze, the increased sugar concentration helps protect the cells from damage. Additionally, cell membranes become more flexible in winter, allowing them to be compressed by expanding ice crystals without breaking.

Glass Phase

The third mechanism is known as the glass phase, where the liquid inside the cells becomes so thick it resembles a solid. This state, similar to how silica turns into glass when supercooled, represents a form of molecular suspended animation.

It is triggered by the cellular dehydration caused by the initial two mechanisms. This prevents the tree’s supercooled cell contents from forming crystals.

Freezing

All three cellular mechanisms prevent living cells from freezing, which is crucial for a tree’s survival. The tree does not have to protect all its cells from freezing, only the living ones or the phloem cells. This is important because a significant portion of a tree's trunk consists of dead cells, like xylem cells, which can freeze without harm.

Even though many above-ground cells do freeze during subfreezing temperatures, the living cells remain unfrozen and active at a reduced level. The living cells in the trunk can stay unfrozen even beside frozen dead cells.

Apart from natural mechanisms, trees have a specific feature that helps them survive winter: barks. The bark is a universal feature of all trees. When winter arrives, this bark becomes crucial in keeping the tree warm and insulated against the harsh weather.

Insulation

Bark protects trees from winter’s harsh conditions. During cold weather, bark helps to moderate temperature fluctuations by providing a buffer between the external environment and the tree's inner tissues.

It reduces the impact of extreme cold on the cambium layer, which is responsible for the tree's growth and nutrient transport. By insulating the tree, bark helps to keep the temperature of the wood and cambium more stable, preventing freezing and potential damage. 

Barrier Against Physical Damage

Bark serves as a barrier against physical damage to trees. Snow and ice can accumulate on branches, adding weight that might cause them to break. The bark helps shield the trunk and branches from these potential injuries.

Additionally, wildlife such as deer and rodents may chew on the bark for food or to sharpen their teeth, especially when other food sources are scarce in winter. The bark provides a tough, protective layer that prevents such animals from reaching the tree’s inner tissues. 

Protection from Sunscald

Sunscald occurs when a tree’s bark warms up during sunny winter days and then rapidly cools at night. This dramatic temperature change can cause the bark to crack or split, leading to damage that exposes the tree’s inner tissues to the elements.

Bark insulates the tree’s trunk from these temperature extremes. It helps to diffuse sunlight and prevent the underlying tissues from overheating. This mechanism will further protect it from issues like disease and pest infestations.

Disease and Pest Defense

Tree bark plays a crucial role in defending against diseases and pests during winter. The outer bark prevents many pathogens, such as fungi and bacteria, from penetrating the tree’s interior tissues. This will protect its vital growth tissues and reduce the risk of infections that could compromise its health.

Additionally, bark shields trees from insect pests that might otherwise invade and cause damage. Insects like borers and beetles can be particularly problematic, as they can lay eggs under the bark or feed on the tree’s tissues. 

Prevention of Water Loss

Bark plays a significant role in preventing water loss from trees during winter. As temperatures drop and the ground often freezes, trees face challenges in absorbing water from the soil. Without leaves, which are the primary sites for water loss through transpiration, the tree relies heavily on its bark to minimize evaporation.

The outer layer of bark acts as a protective barrier, reducing the amount of moisture that escapes from the tree’s internal tissues. By minimizing water loss, bark helps maintain the tree’s hydration levels.

Despite natural defense mechanisms against winter, some trees cannot withstand the harsh winter. Therefore, proper winter care is critical to protect your trees. You can protect the trees with these steps: 

Mulching

Mulching is a vital winter protection strategy for trees. Applying a layer of mulch around the tree base creates an insulating barrier that helps regulate soil temperature. This insulation prevents the soil from freezing and thawing rapidly, which can damage the tree's root system.

Additionally, mulch retains soil moisture, reducing the risk of dehydration when water is less available in winter. Organic mulches, such as wood chips or shredded leaves, also decompose over time, adding valuable nutrients to the soil.

Watering

Proper watering before winter sets in helps the tree withstand the cold. As temperatures drop, the soil can freeze, making it challenging for trees to absorb water. Ensuring trees are well-hydrated in the fall helps them enter winter with sufficient moisture reserves.

When trees have adequate water, their cells are better equipped to handle the stress of freezing temperatures. Well-hydrated trees are less likely to suffer from dehydration-related issues, such as desiccation of tissues or root damage.  It also maintains their health and resilience. 

Wraps and Barriers

Using wraps and barriers is an effective way to protect trees during winter, especially for young or delicate specimens. Tree wraps are made of burlap or specialized materials that can be wrapped around the trunk to shield it from damage such as sunscald.

For evergreen trees, these wraps also help prevent windburn, where cold, dry winds desiccate the needles, causing them to lose moisture and become brittle.

Pruning

Pruning is a critical winter preparation technique for trees, focusing on removing dead, damaged, or diseased branches. Before winter sets in, inspect the tree carefully for branches that show signs of weakness, disease, or damage. Pruning these branches helps prevent them from breaking off under the weight of snow and ice.

Additionally, removing such branches improves air circulation and light penetration within the canopy, which can be beneficial as the tree enters dormancy. Proper pruning also reduces the risk of pest and disease issues. 

Protecting Bark

You also have to protect the bark of trees during winter to prevent damage. One common damage is caused by animals like deer or rodents that chew on the bark, especially when food is scarce. Installing physical barriers like tree guards around the trunk can deter these animals from gnawing at the bark.

Additionally, the barrier can shield the bark from damage caused by snow shovels, ice, or strong winds. This protection helps maintain the integrity of the tree’s bark, which is essential for nutrient and water transport within the tree. 

Avoiding Salt Exposure

Minimizing salt exposure is crucial for protecting trees during winter. Road salt and de-icing chemicals, commonly used to manage ice and snow on roads, can leach into the soil and negatively impact trees.

Salt can disrupt the tree's ability to absorb water and essential nutrients, leading to dehydration and nutrient deficiencies. It can also alter the soil’s pH, making it less suitable for root growth. To prevent these issues, try to reduce the use of salt near trees or use alternatives like sand or gravel for traction. 

Monitoring

Monitoring your trees throughout the winter is essential for maintaining their health and addressing any potential issues promptly. Regularly inspect your trees for signs of damage, such as broken branches, bark peeling, or unusual discoloration.

Pay attention to changes in the tree’s appearance or growth patterns, which might indicate stress or disease. Additionally, check for any signs of pest activity, such as holes or chewing marks on the bark. By keeping a close eye on your trees, you can catch problems early and take action before they escalate.