Our Window on Nature

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Freeze-Dried Trees

Filed under: Trees — Lowell Christie -- February 1, 2006 @ 2:38 pm

In a normal year, most areas record their lowest temperatures for the winter right about now. ‘Tis the season for below zero windchill factors – weather cold enough to make the mercury in your thermometer curl up and hibernate until spring. But somehow, in spite of the ice, the sleet, and the snow, most living things survive.

In the animal kingdom, creatures may head south before the first frost; others stick around but grow an extra heavy coat of fur. Plants have no choice but to stay. Most wait out the weather as seeds, or as roots and bulbs buried beneath the snow. But not trees.

Perhaps because of their size and longevity, we tend to notice trees only when they do something exceptional – like change to autumn foliage or die from winterkill. But this year, wherever you go to enjoy a winter weekend, pay special attention to those trees you pass along the road.

Some kinds of trees shudder at the slightest touch of cold. Picture a palm tree in Florida, its long fronds waving in a gentle breeze. It knows a good place to spend the winter, as thousands of snowbirds are quick to agree. But move those palms up north and they turn up their roots and die.

There’s a reason why tropical trees have such a delicate nature. They contain within their cells a fatty substance that turns solid at temperatures just above freezing. As this substance hardens it expands, rupturing the cells. For such trees a lengthy cold spell proves fatal.

Few of our North American species are as touchy as the palms, however, since there aren’t too many places where you can count on a complete absence of frost. In contrast, most deciduous hardwoods can withstand temperatures that drop to as much as 40 degrees below zero. But how does a tree, which may contain more than 50 percent of its weight in water, keep from exploding when ice forms during winter?

Remember back when you were a kid and you put a bottle of soda pop in the freezer to make a carbonated slush, and then you got into trouble when you forgot to take it out? The expansion of the liquid was more than the bottle could stand – and you ended up explaining to your mother why there were glass fragments and cola all over the sirloin. And that’s at a mere zero degrees. A sugar maple or a white oak can withstand much colder temperatures without freezing at all.

We once heard a forest ranger, who happened to be standing in a snowstorm at the time, explain to us the fallacy of “freezing” temperatures. He pointed out that 32 degrees Fahrenheit isn’t water’s freezing point at all – it’s actually the melting point of ice. Although his purpose was to convince us that we weren’t as cold as we felt, his statement pointed out an apparent conflict. While it is true that ice melts at 32 degrees, under certain circumstances water can be cooled to 40 degrees below zero before it turns into ice.

Ice crystals normally need a tiny core of dust before they can start to grow, so absolutely pure water will stay liquid even when it’s supercooled – until it reaches the temperature of minus-40 degrees. At that point it freezes. Therein lies the secret of hardy trees in winter. They may contain large amounts of water in their cells, but since this water is extremely pure it freezes only at sub-sub-zero temperatures. In addition, some trees even contain antifreeze. Many tree species (such as the maple) contain sugar in their sap, and the presence of sugar lowers the freezing temperature of the tree by several additional degrees.

This minus-40-degree tolerance (plus or minus a degree or two) accounts for the treeline you see on most lofty mountain peaks. During the harshest winters the temperature at treeline is about where your thermometer would burst as the mercury tried to find its way through the bottom of the scale – you guessed it, at minus 40 degrees. In the West this region is found at an altitude of about 11,000 feet. Above that level – no trees.

But elsewhere trees survive where winters are even colder. In the Arctic reaches of Canada you find common species such as paper birch, quaking aspen and blue spruce. Up there the trees have evolved another method of surviving winter – a way that protects them from exploding like firecrackers on the fourth of February.

As winter comes to these northern forests, the trees begin transferring water out of their cells. The water isn’t transpired into the air, and it isn’t drawn down into the roots; instead it is forced into the spaces between the living cells of the tree. And there it freezes.

Since they can’t avoid having their water supply turn to ice, the trees simply move the moisture to where it causes no harm. And in this frozen state they settle in to wait for spring thaw.

With the arrival of warmer weather, the wooden icicles reabsorb the water into their cells. And once again, like trees everywhere, they stand ready for a season of growth under summertime sun.


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