(Note: A version of this essay will appear in an upcoming issue of “Chinquapin: The Newsletter of the Southern Appalachian Botanical Society,” for which George Ellison writes a quarterly Botanical Excursions column.)
Several weeks ago, the nighttime temperature dropped below 10-degrees Fahrenheit at our place. The next day my wife, Elizabeth, and I spent most of our time in house on the North Carolina side of the Smokies, feeding wood into the two woodstoves in our living and kitchen areas. Out the back windows, we could see birds foraging around the feeders. Through the front windows, across the little creek that flows through our property, rhododendrons drooped their leaves like forlorn sheep, indicating beyond all doubt that the first really cold snap of the winter was upon us.
Animals make it through the cold by generating warmth from food, movement, shelter, or contrived means like fire. Plants, on the other hand, have devised a series of ingenious devices that allow them to survive in potentially lethal conditions.
All plants in upland or northern environments face the double-edged dilemma of low temperature stress and lack of moisture in winter. Most opt to lay low: annuals survive as over-wintering seeds; biennials produce low-growing, first-year plants protected by leaf litter or a blanket of snow; herbaceous perennials die back completely and over-winter as dormant corms or regenerative root stock; and broadleaved deciduous trees, shrubs, and various vines shed their leaves and assume other protective measures. Come spring, these plants really have to hustle to do their thing and produce seed or fruit during the growing season.
Evergreens have “chosen” the other fork in the evolutionary path. They tough winter out with their foliage intact so as to obtain a head start when the growing season arrives. For this group of plants, photosynthesis can continue longer in the fall and begin earlier in the coming year; indeed, keeping their leaves (or needles) actually helps these plants survive since they can use them in photosynthesis on mild winter days. Come spring, energy that would otherwise be channeled into producing leaves is saved for direct reproductive efforts.
Additional strategies allow evergreens to weather the drying winds and freezing temperatures of winter. Conifers have needlelike leaves that expose less surface to cold drying winds than broader leaves. Their needles, stems, and roots are filled with “botanical antifreeze” in the form of resinous chemicals. Conical shapes minimize buildups of snow or ice.
Other evergreens have developed thick leaves with waxy coats to cut down on evaporation. These tend to be shrubby or ground hugging. In order to avoid having their leaf cells ruptured by frost, water is channeled to spaces between the cells where expansion does less damage. And finally, the sugar content of the cells is increased to lower their freezing points.
Individual evergreen species often have their own distinctive over-wintering devices. Everyone has observed how rhododendron leaves curl and droop in extreme cold. Drooping (a dormant posture also assumed during periods of drought) lessens exposure to wind, while curling temporarily shields and closes off air-circulation pores (stomata) on the undersides of the leaves.
In Life in the Cold: An Introduction to Winter Ecology (Hanover NH: University Press of New England, 1987), Peter J. Marchand provides interesting information on this topic. I was especially interested in the tables Marchand presents that provide the freezing (“killing temperature”) for various tree species. He makes the point that most species have adapted to the cold by adjusting their freezing tolerance so that it closely matches the minimum temperature at their northern range limit.
For instance, live oak, a southern tree of the Gulf Coast and lower Atlantic coastal plain will tolerate temperatures down to 15 or so degrees Fahrenheit. Eastern redbud, a tree that ranges northward from the southern states to the Great Lakes, dies when subjected to temperatures approaching minus 31 degrees.
Other deciduous trees that range into the higher elevations here in the Smokies like northern red oak and yellow birch withstand temperatures in the minus 35 to 49 degree range. Red spruce and Fraser fir, trees that grow in our highest elevations above 6,000 feet, could make it in temperatures below minus 80 degrees.
Some tree species have adapted especially for the regions in which they find themselves. For instance, sycamores in Mississippi will die in the event of temperatures below minus 4 degrees, while sycamores in Minnesota will tolerate minus 40 degrees. White pine shows a similar range of adaptability.
The lesson in this for Elizabeth and I has been to prepare for the harshest winter weather situation we might reasonably expect to encounter here in the Smokies by cutting our wood early, keeping it good and dry, and hunkering down as quietly as possible by the woodstove while it burns.