Trees dominate the landscape in the northeast region.  The New England and New York region is the most heavily forested (by percentage of area) area in the country.  Maine is first and New Hampshire is second.

So, what do all those trees do in the winter? They freeze for one thing – or, partially at least.  Trees are about half water.  Some species like ash less so and others like white pine more so.  Regardless, the water in live trees freeze during the winter.  Don’t worry, they are constructed to withstand freezing.  But they largely go dormant when the freeze happens.  The underground parts of the tree may not completely freeze, but not a lot goes on during the winter there either.

To survive winter cold, trees begin preparations in late summer as day length shortens. Cold acclimation occurs gradually and includes a number of physiological changes in leaves, stems, and roots. And while fall color seems to get all the attention, it’s what trees do later in autumn that is the most stunning, and harder to see.

Broadleaf, deciduous trees, of course, lose their leaves in the winter to reduce water loss. Most needle-leaved trees like white pine or hemlock, known as conifers, retain needles year-round – with exceptions such as larch and bald cypress trees – only losing older, damaged needles. Needles are better at retaining water than broadleaves thanks to their small surface area and waxy outer coating that limits water loss to transpiration, the evaporation of water from leaves.

Despite all the freezing of the tree water that goes on, researchers have discovered that there are three basic ways in which living tree cells prevent freezing – not the water, but the actual plant material. One is to change their membranes during cold acclimation so that the membranes become more pliable; this allows water to migrate out of the cells and into the spaces between the cells. The relocated water exerts pressure against the cell walls, but this pressure is offset as cells shrink and occupy less space.

The second way a tree prevents its plant material from freezing is to sweeten the fluids within the living cells. Come autumn, a tree converts starch to sugars, which act as something of an antifreeze. The tree makes the starch or sugars during the growing season as the leaves conduct photosynthesis – taking carbon dioxide from the air, capturing sunlight in its chlorophyll, using water from the ground and giving off oxygen. The cellular fluid within the living cells becomes concentrated with these natural sugars, which lowers the freezing point inside the cells, while the sugar-free water between the cells is allowed to freeze. Because the cell membranes are more pliable in winter, they’re squeezed but not punctured by the expanding ice crystals.

The third coping mechanism is described as a “glass phase,” where the liquid cell contents become so viscous – between a liquid and solid phase – that they appear to be solid, a kind of “molecular suspended animation” that mimics the way silica remains liquid as it is supercooled into glass. This third mechanism is triggered by the progressive cellular dehydration that results from the first two mechanisms and allows the supercooled contents of the tree’s cells to avoid crystallizing.

So these three mechanisms keep cells from freezing even though the water in the tree freezes. A tree doesn’t have to keep all of its cells from freezing, just the living ones. This is significant, since much of a tree’s living trunk is made up of cells that are dead – this is the wood. Though it’s strange to think of these cells as dead, because they’re still involved in functions, such as sap flow, that keep the tree alive. Dead wood cells and the spaces in-between provide long microscopic straws that allows the sap to flow for maple sugaring season.

Dead cells can and do freeze, but even the lowest temperature can’t kill an already dead cell. And that’s the magic: while the overwhelming majority of a tree’s above-ground cells do indeed freeze regularly when exposed to subfreezing temperatures, the small percentage of living ones don’t. There are living cells in the trunk that remain unfrozen even though they are right next to – and at the same temperature as – dead cells that are frozen solid.

While trees have evolved amazing strategies for withstanding the winter cold, sometimes it gets so cold that trees can explode. Yes, you read that right. During spells of extreme cold or when trees haven’t had time to acclimate, the life-sustaining sap inside a tree can begin to freeze. Sap contains water so it expands when frozen, putting pressure on the bark, which can break and create an explosion. There are numerous historic and current observations of trees exploding due to extreme cold. And you sometimes hear small versions of this when you walk through the woods on a quiet really cold winter day.

Trees have other mechanisms to protect them in the winter. Bark is extremely important in protecting the tree in both summer and winter although it doesn’t prevent it from freezing as we have described above. So when you are cold and complaining about the winter, just think about what trees have to go through to survive!

The author, Charles Levesque, is President of Innovative Natural Resource Solutions, LLC, a natural resource consulting firm with offices in Maine and New Hampshire. He is also a licensed forester. He gives credit for the core of this article to Michael Snyder from his article on the same subject published in Northern Woodlands magazine.

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