The unseasonably warm weather early this month was an opportune time to take a peak at the quiet world beneath the blanket of snow, as the snow unceremoniously melted away. The patches of green grass in the yard revealed plants were still alive despite the short days and previously frosty temperatures. Even with freezing temperatures for most of the winter and only a few precious months of good sunlight, plants survive and indeed thrive in Alaska. How do they do it?
Plants at northern latitudes have a variety of strategies for coping with frigid winter temperatures. First, many plants have physical growth forms suited to harsh conditions. Plants of the alpine and tundra have long since mastered some of these tricks. For example, plants often grow low to the ground, ducking the wind and soaking up the sun reflected off the ground. As snow piles up, their stature keeps them hidden from the cold wind and blowing snow.
Some alpine plants, such as woolly lousewort, have dense hairs that create a layer of still air near the plant which reduces wind chill. Crowberry and low-bush cranberry have thick, waxy, evergreen leaves that help avoid freezing injury and desiccation. In the north, widely branching growth forms, which are useful for collecting sunlight, must be balanced by the need to avoid exposure. High in the alpine many plants grow in small, protected clusters of microhabitats and don’t present a high wind profile.
Even folks lacking a green thumb know that plants need water. Because most water is frozen during the winter and therefore unavailable, dehydration can be a major problem. Freezing can disrupt the continuous column of sap in the plant stem, so that there is little or no transport of water from the roots. Many boreal trees, shrubs and herbaceous vegetation must therefore have adaptations that help minimize water loss in the winter months.
The formation of ice within plant cells is another source of winter trouble. As ice crystals form they can shred cellular components and internal structures. Remember all those high school biology terms nucleus, chloroplast, cell membrane? The plant needs these to function and grow; ice crystals can totally derail this delicate cellular machinery.
So, even hiding under a pile of snow, plant tissues can still encounter temperatures well below freezing. How do they avoid damage? Like insects, amphibians and fish that inhabit subfreezing habitats in the winter, plants mostly shut down. Growth, reproduction and dispersal give way to dormancy. During the winter, plants may either avoid freezing or make adjustments to survive the freezing process. Herbaceous plants that are perennials or biennials can achieve this feat by overwintering as a low-lying rosette or as below ground structures, like tulip bulbs, storing their sugars and nutrients away from the coldest temperatures. In the spring, stems and leaves then sprout up from the protected parts. In the case of annual plants, the vegetative parts die back each year, and the seeds take on the burden of survival and overwintering to propagate new plants each spring.
To avoid freezing damage, plants usually go through a physiological acclimation process in the fall. Triggered by short daylength (or photoperiod) and cold temperatures, this process is collectively called “winter hardening.” During hardening, water is pumped out of the cells, and carbohydrates are accumulated and stored. Soluble sugars are especially important, because they act as antifreeze, reducing the temperature at which the solution in intracellular spaces freezes.
Some plants, such as cold hardy cereal crops like wheat and rye,
can produce special proteins that bind to ice crystals to inhibit further ice formation. These amazing adaptations and others help plants avoid or survive freezing and cumulatively determine the cold-hardiness of a plant.
An early frost can cause serious damage to plant tissues and even death if the cold hardening process has not been completed.
Woody plants are perennials and have special strategies for winter survival. Deciduous trees, such as balsam poplar and birch, retrieve energy-rich compounds from their leaves in autumn, as the green chlorophyll pigments break down. The other pigments in the leaf, such as yellow carotenes, then begin to show, and fall colors burst forth. After the leaves turn color, they begin to drop off until only a naked and winter-hardy tree remains.
Spruce and mountain hemlock trees are evergreen, as are some herbaceous plants like bunchberry and pyrola. Evergreens forego this silly business of dropping leaves and regrowing them anew each year. The evergreen mode provides the important benefit of allowing the plant to quickly resume photosynthesis as soon as temperatures and photoperiod come online in the spring. Evergreens go through cold acclimation, transport water out of their cells and make sugary antifreeze to weather the cold winter, just like herbaceous plants.
As an undergraduate student I worked one summer on effects of global warming on the only two vascular plant species in Antarctica. I was astonished at how well these plants thrived in the growth chamber, at temperatures just above freezing. The plants were not only surviving but also doing photosynthesis and growing at these frigid temperatures.
The plants of the alpine, tundra and boreal forest of the north are no less ingenious. Plants have an amazing variability that allows them to survive in even the most marginal of environments.
So the next time you bumble out of the house with layers so thick that you resemble a beach ball with arms and legs, take time to appreciate the plants around you and marvel at their tremendous natural ability to survive.
Caleb Slemmons was an intern with the Kenai efuge this past summer. He is presently overwintering near Ninilchik.
Previous Refuge Notebook articles can be viewed on the refuge Web site at http://kenai.fws.gov/.
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