January 2007 © Janet Davis
Freezing rain has been falling since the early hours of the morning. The mercury has plunged to a record low for this date and the radio warns us that exposed flesh can freeze in mere minutes. A blizzard is on its way.
Brrrrrr, we think, as we bundle ourselves up just to walk the short distance to the car.
But spare a thought for the poor plants in our gardens. What about that yew under the front window with the icicles hanging from its needled branches? The evergreen rhododendron with its leaves curled into tight little rolls? The magnolia with its furry, brown buds? How do they protect themselves from the extremes of winter weather?
Although plants are built of cells and viscous fluids just
as animals are, they are at a great disability when it comes to seasonal
environmental stress. A man, a marmot or
a moth can remove themselves from harm’s way, but plants are stationary. The yew needles must submit to sheets of
freezing rain. The magnolia buds must
hold tight against a blizzard. The rhodendron leaves must brave Arctic
winds.
Yet nature has armed plants with remarkable defence mechanisms to help them
withstand the stresses of both heat and cold.
Genetics
Most important is a plant’s inherited genetic adaptability to seasonal change. In
woody plants that must survive winter, this is called cold
acclimation. Plants vary in their degree
of adaptability, of course, depending on their locality or ecoregion. A rhododendron native to the subarctic tundra
(Zone 1) such as Lapland rosebay, Rhododendron
lapponicum, will survive far
lower winter temperatures than the American rosebay rhododendron, Rhodendron maximum native to the cool,
moist
Supercooling and
Dehydration
Woody plants may also employ two metabolic genetic
strategies to help them deal with freezing temperatures. The first is avoidance, i.e. prevention of the formation of ice crystals in
cells through the encoding of genes for supercooling
proteins that protect intracellular tissues to a maximum low temperature of
-40 F (-40C). Plants that must survive even lower temperatures, like the 
proteins. Dehydrins cause the plant to evacuate water
from cellular protoplasm into intracellular spaces where ice crystals can form
without damaging the plant. This
concentration of solutes, including sugars and amino acids, within the cells lowers
the freezing point and acts as a kind of anti-freeze. Provided this happens in a timely fashion
after the plant has acclimated in fall, provided winter temperatures do not
exceed the plant’s maximum low temperatures, and provided the plant is not lured
out of dormancy by prematurely warm spring temperatures that plunge again
later, there should be no tissue damage to the plant.
Thermonasty
Nastic movements are plant movements triggered by an external factor such as cold, heat, light or humidity. Unlike the nerve-generated movements of animals, plants “move” through changes in the internal cellular pressure of their parts. In extreme cold, the leaves of many evergreen rhododendrons exhibit thermonastic movement, curling the upper surface of the leaf inwards and pointing it down toward the ground, thus minimizing exposure to freezing temperatures and also reducing winter-burn caused by winter sunshine. It has been shown that rhododendron species indigenous to mild climates do not exhibit thermonasty.
Leaf Size & Composition
Reduction of leaf size is an important adaptation of conifers, many of which are indigenous to the northern boreal forest zone where cold, dry winters are a fact of life. Trees must invest abundant energy to produce their leaves, but the lean, nutrient-poor soil, the harsh atmospheric conditions and the short growing season of the northern forest do not favor an annual leaf cycle, as with deciduous trees. Therefore, depending on the species, conifers retain their oldest leaves for 2 years (white pine) to 45 years (Great Basin bristlecone pine) – that is, they stay “evergreen” (with the exception of tamaracks which drop their needles in fall), while hoarding nutients in their foliage While a full leaf canopy allows a tree to begin photosynthesizing early in spring and stay photosynthetically active later in autumn, it also invites dessication and freeze damage in winter. Thus, conifers have adapted by producing small, narrow, needle-like leaves (spruces, pines, firs, hemlocks, yews) or scaled leaves (cedar, cypress), reducing the surface area to reduce transpiration of water and the risk of freezing. They also have fewer stomata (pores) in the needles than deciduous leaves.
Evergreen conifers and broadleafed evergreens also protect their leaves from dessication in winter by covering them with a new layer of cuticular wax each summer.
Winter Buds
Those furry winter buds on the magnolia and the shiny, dark-brown ones on the horsechestnut are more than mere cold season finery. Their overlapping scales form a watertight, protective covering for the embryonic leaf and flower shoots that will emerge in spring. The buds of conifers are covered with protective wax or pitch, giving them extra insulation against winter weather.
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These adaptations help trees and shrubs withstand the worst extremes of winter
weather. But they’re no guarantee
against that once-in-century ice storm that tears jagged branches from sturdy
trunks; the atypical January cold front that sets record lows while confounding
a tree’s genetic definition of “normal”; or that sudden spring freeze that
pierces newly-awakened cellular protoplasm with lethal ice crystals.
They are merely the best that nature can do.