It’s Finally Spring! Why Didn’t the Trees Die Over the Long Winter?

by Shraddha Chakradhar

It’s been a long winter, but spring is finally in the air! Daffodils and hyacinths are peeping from the ground, the birds are busy in the bushes, and the trees are finally budding leaves! The fresh green shoots are such a welcome change from the drab brown hues we’ve become so accustomed to, although it makes you wonder: how do these trees manage to revive themselves year after year? Winter, with its drastic temperature drops, precipitation and wind, can be be unforgiving, so how do trees manage to survive the harsh environment?

The key to making it through winter is to prevent ice crystals from forming in any of the living parts of a tree— leaves, flowers, fruits, roots, and a small portion of the trunk. Trees have developed a few defense systems to protect against the formation of ice during the frigid winter months, especially in harsher climates. One of these is the shedding of leaves. Leaves are full of water-filled cells. In freezing temperatures, the water in these leaves would turn into ice, which would in turn destroy the cells. Despite millions of years of evolution, most living beings—plants and animals alike—have not developed a defense system against ice; its crystalline structure punctures through cell components. And so, every fall, in most of the northern United States, we see deciduous trees (trees like maple and oak) shed their leaves, thus eliminating one possibility of ice being formed within living cells.

“A ruptured cell is a dead cell,” said John Seiler, tree physiology expert and professor of forestry at Virginia Tech. “So anything trees can do to prevent a cell from rupturing will help ensure its survival into spring.”

Another tactic against ice, most common in conifers such as pines, firs and spruces, is the expulsion of water from living cells. These trees have needle-like leaves that hold less water than the broad, flat leaves that deciduous trees have and so don’t have to worry about shedding leaves. Instead, they begin to push water from the living cells within the trunk and branches to the spaces outside cells and replace the water with starchy sap. This sugary substance acts as a natural antifreeze, allowing for the tree’s protection against temperatures well below freezing. Of course, in northern parts of the country, some deciduous trees combine shedding leaves with this method of forming sugary sap, which is how we can enjoy maple syrup!

But what happens to the trees in parts of Alaska, Canada and the Arctic tundra?Or trees in the lower 48 during a particularly brutal winter? Apart from shedding leaves and stocking up on sap, some trees are capable of triggering a survival mechanism that seems to defy the laws of physics: supercooling. Water that stays in liquid form at temperatures as cool as -40ºC (where the temperature is also -40ºF) is considered supercooled. This is because of something called nucleation. If you’ve ever witnessed the beginning of water freezing over, you know that water begins to freeze at the edges of objects it touches. The perimeter of an ice cube tray, around the rocks in a pond, and so on. That’s because these foreign materials—the plastic of the tray, the rock surface—act like an anchor for the first frozen crystal, which then helps anchor consequent crystals. But without any of these anchors, or nucleating points, the water exists as a homogeneous mixture incapable of freezing exactly at 0ºC. It freezes closer to the -40ºC mark.

Trees like maple and elm are capable of triggering this supercooling effect when faced with particularly brutal temperatures. How they do this is by getting rid of any nucleating points. The shape of the cells are changed to prevent any foreign surfaces that could possibly act as an anchor to begin the freezing process.

While trees do employ these various defense mechanisms to protect against the harmful effects of ice, there are instances in which they succumb. Frost cracks (pictured below) are formed when ice builds up in the plumbing system of trees, known as the xylem. The xylem is largely non-living, so there is no major damage to the life of the tree, but much like freezing pipes bursting due to expanding ice, the trunk or branches which house xylem can burst if the ice outweighs the wood.

“It sounds like a gunshot when it happens,” said Frank Telewski, professor of plant biology at Michigan State University. “The tree cracks open under the pressure and exposes part of the trunk to the outside environment.”

While the frost crack in itself isn’t fatal, the exposure to the environment upon cracking leaves the few living cells within the trunk or branches susceptible to ice. Much like self-healing wounds in humans, trees form a layer of callus layer that often seals the crack and ensures the survival of the rest of the tree.

What’s also interesting about these defenses against ice is that they also prevent the trees from drying out during the winter. But the coast isn’t always clear: some trees have to worry about winter burn.

“It happens during those rare sunny days in winter,” explained Seiler. “The little water left in trees starts to evaporate because of the sun, but more water can’t be pulled up from the roots because the soil is frozen.” The result is a reddish-brown hue in coniferous trees. Entire stretches of trees in the Rockies, for instance, are susceptible to this condition, known as red belt disease.

With the exception of frost cracks and red belt disease, winters are usually fairly stable. An ice storm may cause damage due to the sheer weight of the ice that forms around trees, but snow serves to insulate the roots and cause little damage.

All these defense mechanisms are especially amazing considering that trees, and most plants in general, have no way of regulating temperature. So the temperature of the air around them is often the temperature within them. But thousands of years of evolution has allowed all kinds of trees to weather the harshest of winters. Dendrologists (scientists who study trees) have conducted experiments transplanting trees native to warm climates to colder climates. The health of these trees is often compromised, since they rely on defense mechanisms that they have long discarded. But when the days start to get longer and the ground thaws to let water flow through the trees, even these immigrant trees, along with their native cousins, finally come out of dormancy and signal to the world that they are ready for yet another season.

Originally published April 2014 on Beacon Reader.

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