January 29, 2012 Ice and Plants – A Tricky Balance
Plants contend with snow and cold winter temperatures with a variety of different strategies. Unlike some animals of the West, they cannot migrate to warmer environs.
Many herbaceous plants do the next best thing to migrating: they shed all of their above ground parts and seek a safe place to spend winter beneath the soil. Aspens, maples, birches, alders and other deciduous trees protect themselves from winter temperatures by dropping their leaves. Native conifers, except for larches, are evergreen and so they have internal functions or physiological adaptations that help them get through the winter months ahead.
The two most common stresses among trees and shrubs of the north country are the ability to withstand low temperatures and drying-up or desiccation.
In order to prepare for winter, leaves of Northern Hemisphere plants begin to recognize the diminishing length of daylight in August. Certain plant hormones are released to slow and then eventually stop all growth. The first frost of the autumn prepares woody plants for the impending onslaught of winter. In addition, plants experience a water stress which further prepares them for the chilly months ahead.
Trees are now able to deal with freezing temperatures and the controlled formation of ice. The exact location of ice within the tree is very important. Most of the cells within trees are non-living, because their role is to conduct water during the growing season and provide mechanical support or stability. There are, however, living cells within the roots, branches, trunk and evergreen needles which are very important for storing food and kick-starting spring growth. It’s these cells where the exact formation of ice is a life or death matter.
The initial formation of ice occurs outside the living plant cell in a small space within the cell wall. All the water that isn’t bonded to other molecules inside the cell is exported to the space in the cell wall. When ice forms in the cell wall it attracts water to its crystals. The living part of the cell is protected by an elastic cell membrane and the remaining cell sap can withstand temperatures as low as minus 67 degrees Fahrenheit (minus 55 degrees Celsius). If, for any reason, the cell membrane becomes ruptured or if too much water is exported into the cell wall the cell sap will become toxic and the cell will die.
Exposed evergreen needles face the greatest water loss problems under bright sunshine and calm winter days. The needles are warmed to above-freezing temperatures and the air is dry, creating atmospheric suction or a call for water from its needles. The tree is faced with a problem: It’s loosing water in its winterized needles and must replace it.
The trunk being darker and warming above freezing like the needles is able to supply minimal amounts of stored water from the cell walls. This becomes a tricky balancing act. On a day such as this, trees prefer even the slightest of breeze, because that cools the leaf surface and prevents any moisture loss and subsequent demand for replacement water.
Heavy snow loads, particularly on the Coastal Pacific Northwest Mountains, can cause entire trees to bend. A 40-foot Pacific silver fir can accumulate a mass of snow and ice nearly 20 inches thick, weighing 6,600 pounds or more than 3 tons.
Exposed areas are subjected to blowing ice which can remove foliage or cause freezing injury and create deep pits eventually wearing away tree bark. Mountain winds, especially during the winter, shape trees and the treeline forests. Some high elevation trees actually resemble a broomstick with windswept branches and trunks with only a mop-head or cluster of foliage at their top.
Browsing activities of mammals create further winter-stress problems for plants.
Yet despite all the harsh winter environmental conditions, our coastal, subalpine, interior and northerly forests of western North America are hardy and able to live for hundreds and sometimes thousands of years, stoically facing months of winter.
Earth Dr Reese Halter is an award-winning Science Communicator: Voice for Ecology and distinguished conservation biologist at California Lutheran University. His latest books are The Insatiable Bark Beetle and The Incomparable Honeybee
Text © by Dr Reese Halter 2012. All rights reserved.
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