Silent Survivors: The Winter Life of Trees

Every tree has its enemy, few have an advocate. – J.R.R. Tolkien

This harsh winter of 2023 has been a reminder that trees also face climatic challenges such as ice storms, desiccation, snow-laden branches, and trunk-cracking freezes in their mission to add another annual ring of growth. There are many ways trees silently survive the winter. Some of these are strategies or adaptations used by many kinds of trees; others are only used by a small number of species. This article explores a few of the important ways trees endure the winter months.

Water Shutdowns and Antifreeze

In the growing season, hardwoods (a.k.a. deciduous trees) and softwoods (a.k.a. conifers or evergreens), move water from their roots up to the leaves through the simple process of suction.  Water evaporation from small pores on the undersides of leaves slowly pulls additional water up through microscopic vessels in the wood of the roots, trunk, and branches. But as the days grow shorter, colder, and drier in the fall, hardwoods respond by dropping their leaves. This ends the upward movement of water in preparation for the winter, effectually draining their pipes before freezing temperatures would otherwise cause cell-damaging ice crystals. Even though most softwoods retain their leaves (needles) for several years, those growing in cold regions stop water flow for the winter by closing their leaf pores.  Softwood needles contain fewer pores than hardwood leaves and are also coated with wax, which prevents them from drying out in the cold.

Additionally, both hardwoods and softwoods osmotically move water out of their living cells into their intercellular (non-living) zones to confine water freezing in this area. This increases the sugar content in their cells, which lowers each cell's internal freezing point. Tree cells also produce proteins that function like antifreeze, too. Interestingly, this entire process of restricting freezing to certain areas and employing antifreeze compounds is like that used by hibernating frogs to survive the winter season partially frozen. Northern spring peepers, eastern gray treefrogs, and wood frogs are good examples that use this strategy.

Insulation and Architecture

The important protection that bark provides trees in winter cannot be discounted. Bark is largely waterproof and prevents trees from drying out. Air pockets in bark also insulate tree trunks and branches. Generally, the older a tree, the thicker its layer of bark protection. Branch architecture is also important during the winter. The strong, upward-reaching, and flexible nature of most trees' branches allows them to easily sway and bend down to shed snow. Similarly, the young flexible branches and fine needles of softwoods have minimal surface area and shed most types of snow quickly. Occasional wet, early, or late winter storms catch hardwood trees with their leaves on – making snow shedding difficult – and can result in serious tree branch and trunk damage.

Sugar Making

Some may think trees no longer produce sugar (food) for themselves through photosynthesis in the winter. But warm winter days above 45 degrees Fahrenheit do provide such opportunities. Softwoods have a unique competitive advantage in this regard. Since nearby hardwoods are leafless and casting little shade, softwoods gain more direct sunlight, warming them and allowing photosynthesis to occur in their needles. Photosynthesis produces sugars for maintenance and growth. But hardwoods are not excluded from this adaptation. Quaking aspen stores chlorophyll (the green chemical substance that carries out photosynthesis) beneath the bark on younger trees, giving their trunks a greenish cast. This allows them to produce sugar and survive in regions where winters are especially long and cold. Other hardwoods – such as sassafras, oak, maple, yellow poplar, black cherry, black walnut, apple, and even some softwoods, including eastern white pine – position chlorophyll under the thin and tender bark of their young twigs and buds, allowing them to produce food stores in the winter as well. This chlorophyll can be easily seen by scratching off some of the thin bark on twigs and observing the next layer of green tissue.

These are just a few of the strategies and adaptations that impact tree health and longevity, allowing them to survive the winter. Trees are not completely dormant in the winter, as sometimes believed, and unless unusually destructive storms occur, they survive quite well through the cold season each year. The lives of trees depend on many critical things, and winter survival is one aspect that tree advocates find fascinating and seek to better understand.