Trees survive winter by entering a state of dormancy, slowing their metabolism to a fraction of its growing-season level and deploying a remarkable set of chemical and physical defenses against cold. Far from shutting down completely, trees remain biologically active throughout winter, converting stored energy, repairing internal damage, and even growing roots underground. What happens above and below the bark depends on whether a tree is deciduous or evergreen, but both types rely on months of preparation that begin well before the first frost.
How Trees Enter Dormancy
As days shorten and temperatures drop in autumn, trees begin a gradual transition into dormancy. This process has two distinct phases. The first, called endodormancy, is an internally regulated shutdown where growth is suppressed even if conditions temporarily warm up. A tree in endodormancy won’t resume growing during a January warm spell because its own internal chemistry is holding it back. To exit this phase, buds need prolonged exposure to cold, a requirement scientists call “chilling hours.” The exact amount varies by species. Apple and pear trees, for instance, rely almost entirely on accumulated cold temperatures rather than day length to gauge when endodormancy should lift.
Once a tree has logged enough chilling hours, it shifts into the second phase: ecodormancy. At this point the internal brakes are off, and only the cold weather itself prevents new growth. This is why an unusually warm late winter can trick trees into budding early, leaving them vulnerable to a late frost. The transition between these phases involves shifts in hormones, sugar metabolism, and responses to reactive oxygen species inside cells.
Why Deciduous Trees Drop Their Leaves
Leaf drop is not passive. Trees actively sever their own leaves through a controlled process at a specialized zone of cells at the base of each leaf stem. In autumn, the hormone that normally keeps leaves attached gradually declines, while other signaling chemicals rise and trigger cells in this zone to produce enzymes that dissolve the cell walls holding the leaf in place. The walls are broken down layer by layer until the leaf detaches.
This is a survival strategy, not a loss. Broad, thin leaves are efficient solar panels in summer but become liabilities in winter. They would lose enormous amounts of water through evaporation at a time when frozen soil makes it nearly impossible for roots to replace that moisture. They also catch snow and ice, adding dangerous weight to branches. By shedding leaves, deciduous trees cut their water loss dramatically and reduce their surface area against winter storms. Before the leaf falls, the tree pulls back valuable nutrients like nitrogen and phosphorus, recycling them into branches and trunk for spring.
How Trees Survive Freezing Temperatures
The biggest threat cold poses to a living cell is ice. If ice crystals form inside a cell, they puncture membranes and destroy the cell from within. Trees have evolved an elegant workaround: they allow ice to form between cells while keeping the insides of cells unfrozen.
When temperatures drop, ice typically nucleates in the spaces outside cells first, because those spaces have a larger volume and more particles that seed ice formation. As that extracellular ice forms, the solute concentration in the remaining unfrozen water outside the cell rises sharply. This pulls water out of the cell through osmosis, effectively dehydrating it. The now-concentrated fluid inside the cell has a much lower freezing point, preventing internal ice formation. In poplar trees, for example, the proportion of protective sugars in stem tissue climbs from about 13% of total sugar content at 10°C to 69% near 0°C. This massive starch-to-sugar conversion acts like a cellular antifreeze.
The most cold-hardy boreal species take this to extremes. Spruce, fir, larch, birch, and aspen in northern forests can tolerate temperatures of negative 80°C or lower. By contrast, temperate forest species generally need temperatures to stay above negative 45°C. The difference comes down to how aggressively each species dehydrates its cells and concentrates protective solutes.
What Happens Inside the Trunk
A tree’s water transport system faces its own winter hazard. The tiny tubes that carry water from roots to canopy (xylem vessels) can develop air bubbles when water inside them freezes and thaws. These air pockets, called embolisms, block water flow much like an air lock in a pipe. For trees at high elevations and northern latitudes, this isn’t a rare event. Long-term observations of timberline conifers show they experience pronounced embolism every single winter.
What’s remarkable is that these trees survive it year after year. They refill their xylem tubes each spring, restoring water transport before the growing season begins. Imaging with micro-CT scans has confirmed that winter embolism in these trees is widespread and uniform, and that recovery depends on active refilling rather than growing new tissue. This annual damage-and-repair cycle is normal for cold-climate trees. Embolism from summer drought, by contrast, is far more dangerous because the repair mechanisms don’t work as effectively during the growing season.
How Evergreens Handle Winter Differently
Evergreen conifers keep their needles year-round, which means they need different defenses against winter’s cold and dry air. Their needles are built for it. A thick waxy coating on the surface reduces water loss, and the needles of high-altitude trees develop even thicker coatings than their lowland relatives. The wax forms tubular structures in winter that become flatter and less protective by summer, matching seasonal need. Stomata, the tiny pores that exchange gases, are often recessed into grooves on the underside of needles, shielded from drying wind.
Evergreens can photosynthesize during winter, but only barely. In eastern hemlock, photosynthesis drops sharply once overnight lows fall below negative 2°C. After a night that dips to negative 8°C, photosynthetic output falls to roughly a fifth of its mild-weather rate. From December through March, carbon uptake is extremely sensitive to whether recent overnight temperatures hovered near negative 5°C versus just above freezing. On milder winter days, evergreens can still produce a small amount of energy, giving them a slight head start over deciduous trees that have no leaves at all.
What Happens Underground
Soil insulates roots from the worst of winter’s cold, and tree roots take advantage of that relative warmth. While above-ground growth stops completely, roots of temperate species can continue growing slowly at soil temperatures as low as 2 to 4°C. This winter root activity is subtle but meaningful. Roots extend into new soil, positioning themselves to absorb water and nutrients the moment spring arrives. Research has shown that warming the soil around nondormant roots in late winter can actually accelerate when buds open above ground, suggesting roots play an active role in triggering the end of dormancy.
Snow cover matters enormously for root survival. A thick snow layer acts as insulation, keeping soil temperatures relatively stable even when air temperatures plunge well below zero. In winters with little snow, soil can freeze deeply enough to damage fine root tips and reduce the tree’s ability to take up water in early spring.
The Energy Budget of a Dormant Tree
Trees spend autumn converting sugars produced during the growing season into starch, packing it into cells in the trunk, branches, and roots. As winter sets in and temperatures drop, that starch is gradually converted back into soluble sugars, serving the dual purpose of fueling minimal cellular maintenance and lowering the freezing point of cell fluids. This conversion is temperature-dependent and reversible: warming spells can trigger some re-conversion to starch, while renewed cold reverses it again.
A tree’s winter energy reserves are finite. This is one reason why several consecutive harsh winters, or a winter followed by a late frost that kills new growth, can weaken or kill a tree. Each time the tree has to rebuild leaves or repair frost damage, it draws down stores that take an entire growing season to replenish. Healthy trees with good energy reserves heading into winter are far more likely to leaf out vigorously in spring than stressed trees already running low on starch.