Plants in Winter

The recent cold snap in the Rockies makes one wonder how some of the local trees survive these frigid temperatures. Small plants can spend the winter protected from the cold temperatures below a heavy insulating layer of snow. Those plants that must remain above the surface have developed a process referred to as  hardening which allows them to resist the effects of freezing. Hardening is a gradual process allowing trees to become resistant to increasingly cold temperatures. Plants that don't undergo hardening will usually die when the temperatures drop suddenly.

The shortening days of autumn triggers an end to summer growth and the beginning to hardening. A three step process, step one occurs when a hardening factor, probably a plant hormone, is transferred from the leaves to the wintering stem. As the temperatures drop, the non-living tissues of the plant freeze without affecting its living tissues. During stage one, the cell membrane also becomes more permeable. As water outside of the cell freezes, water is drawn from out of the cell leaving it slightly dehydrated. This dehydration increases the ratio of chemicals to water within the cell and in turn, decreases its freezing point. By the end of stage one, trees can survive temperatures in the range of -5° to -10°C.

The next step is only triggered when freezing temperatures and frosts occur. This is a chemical stage which will increase the temperature resistance to even lower levels. As an example, proteins within the cell may be protected by replacing water with carbohydrates which have a lower freezing point. In addition, the cells membranes change to protect further against dropping temperatures. By the time stage two is complete, the trees are freeze proofed down to approximately -30°C.

Little is known of stage three hardening as it is rarely encountered. It only occurs under extremely cold temperatures and there is a large variation between plant species. Once the temperature drops to between -30° and
-50°C, a few more changes take place. In the balsam poplar, a special type of freezing has been noted. This freezing occurs when the cells freeze, but the ice does not take on the form of sharp crystals which would kill the cell. Stage three hardening doesn't last long and is often lost after only one mild day, but may explain why some trees have been known to survive temperatures as low as -80°C.

Chinook winds have been known to cause problems with sudden increases in temperature followed by equally sudden drops. Lodgepole pines often exhibit an affliction known as 'red belt'. This occurs when the temperature warms suddenly and the tree begins to wake up. However with the roots still frozen, the water in the needles cannot be replaced and the needles turn red and fall off. In extreme cases, whole mountainsides will appear red. Lodgepole pine may become rare in heavily chinooked locations due to the harsh affects of these rapid temperature changes. Chinooks may also affect the trees hardening resistance. As temperatures rise, the trees begin to de-harden. When the temperature drops at the end of the chinook, cell death may occur.

Like our mountain wildlife, the area’s plants must be well adapted to survive the long unpredictable winters we've all come to know and love.