Curious Nature: The evergreen advantage
Vail, CO, Colorado
Now that winter is finally here, residents of Eagle County are probably bundling up, drinking hot beverages, huddling by the fire and turning the heat up in their homes.
Many of us are familiar with the myriad of adaptations local animals have to cope with the cold. From warm coats and shelters to hibernation and migration, animals have many different strategies for surviving the cold.
But what about plants? Plants face many of the same problems animals do in winter. In cold temperatures they risk water freezing in their cells and rupturing their cell walls. Shorter days and a lower sun angle make it harder for even the evergreens to produce food through photosynthesis. This problem is compounded by a lack of available liquid water. So, if they don’t grow thick winter coats, they can’t fly south and they can’t shiver or huddle together to generate heat, what do plants do to overcome all these obstacles?
Like many animals, all plants in the mountains reduce their activity levels in winter. Deciduous (or leafy) trees shed their leaves and reduce or nearly stop photosynthesis. Coniferous (evergreen) trees however, retain their needles and continue to photosynthesize at a reduced rate. Some specially adapted mountain species, like quaking aspen, have evolved to photosynthesize through their bark even after their leaves have fallen. So, by reducing their metabolism and rate of photosynthesis, many plants become partially dormant in winter, reducing their need for limited resources like sunlight and water.
Plants that live in cold temperatures and at high elevations also need to cope with subfreezing temperatures, strong winds and blowing snow. To avoid damage to their cells from water expanding as it freezes, many mountain plants have developed an especially elastic plasma membrane which lines the interior of their cell walls. This membrane is able to flex and be harmlessly pushed out of the way even as ice forming in between cells ruptures the rigid cell walls. Different species can tolerate different temperatures. While species indigenous to the southeastern U.S. like live oak can only tolerate temperatures as low as minus 8 degrees Celsius, quaking aspen and paper birch can survive temperatures below minus 80 degrees Celsius!
While deciduous trees like aspen and birch can survive incredibly cold temperatures, evergreens still have a number of advantages over deciduous trees in winter. While they are unable to photosynthesize most of the winter, retaining their needles allows them to produce food later in fall and earlier in spring than their broad-leafed counterparts.
The thick, waxy cuticle that coats evergreen needles also offers protection from wind and blowing snow and a barrier to water loss through evaporation. Finally, the shape and structure of evergreen trees and shrubs helps them to cope with wind, blowing snow and snow buildup. The conical silhouette typical of evergreen trees allows them to shed snow from their branches much like a steep roof.
Many evergreens also have certain flexibility in the way they grow that most deciduous trees lack. Some species, like juniper, can grow either as trees or as shrubs so that individual plants growing in habitats exposed to wind can reduce their profile by remaining close to the ground. Shrubs like juniper, kinnikinnick and Oregon grape can even absorb some ultraviolet light that penetrates the snowpack, allowing them to take advantage of the protective blanket of snow while still performing photosynthesis.
Plants in the High Rockies face many of the same challenges that animals do, as well as a few of their own. Fortunately, they have a well-developed suite of adaptations to survive, and even thrive, in the harsh winter environment.
For more in-depth information on plants and other creatures in winter, I highly recommend reading “Life in the Cold,” by Peter J. Marchand.
Pete Wadden is a natural science educator at Walking Mountains Science Center. When not reflecting with awe upon the incredible adaptations of plants in the Rockies, he enjoys eating them, using them as windbreaks and burning them for fuel.
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