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Winter Wonderings…
How do evergreens stay green?


Pine trees, hemlocks and rhododendrons really stand out during the winter months because they stay green all year long. But how are they able to keep their leaves when most of the other plants in the forest have lost theirs?

Evergreen shrubs and trees in northern climates face many challenges during winter. Cold temperatures are actually the least critical problem. Many plants can carry out photosynthesis with temperatures at or a few degrees below freezing.

The biggest problem for plants in winter is desiccation or drying out. When the ground freezes, plant roots are unable to draw liquid water from the soil. When the plant becomes dehydrated, the plant cell membranes lose their shape and rupture, killing portions of the plant. Cell tissues can also die if ice crystals form inside cell membranes.

Evergreen plants in West Virginia have many different ways to survive the cold and dry winters. Many evergreens have tough, leathery leaves or needles that are resistant to drying out. Some plants can actively concentrate chemicals that work like antifreeze in their cell fluids. For other plants, simply being buried in a snowbank can help reduce water loss. Snow provides moisture as well as shelter from drying winds. However, being buried under a thick layer of ice for too long may kill a plant because toxic concentrations of compounds like carbon dioxide and ethanol build up in the closed air space beneath the ice.

--John Beckman

Do insects hibernate?

Insects survive the winter by using a variety of methods to protect their cells from freezing and being killed. One common mechanism is the production of large amounts of glycerol, a natural antifreeze, in an insect’s blood. As much as a quarter of the body weight of a “winter-hardy” insect may be made of glycerol which has the effect of greatly reducing the freezing point.

Other insects have means to partially control the freezing so that it occurs in the spaces outside the cells, protecting the cell walls. And almost all insects have behaviors that cause them to seek out protective sites for wintering, such as under sheltering debris or in similar insulated areas where temperature extremes are moderated.

Also, almost all insects that overwinter go into diapause, a state of arrested development and growth. This condition can only be “broken” if certain environmental conditions are met, such as a period of chilling has passed or a critical day length is perceived. Diapause has the protective effect of preventing many insects from prematurely emerging and losing winter hardiness during a warm spell.

Source: Colorado State University Cooperative Extension Service

Why Don’t Fish Freeze?

Although humans would never be able to withstand a long period of time in cold water (remember the Titanic?), many freshwater fish survive because of special adaptations that help with internal thermal regulation. Fish, in contrast to warm-blooded mammals and birds which maintain a constant internal body temperature, are cold-blooded animals. Their internal body temperature equals that of the environment around them.

Freshwater fish avoid freezing because they are hyperosmotic, meaning they have a higher internal concentration of salts than that of their surrounding freshwater environment. This is important because it takes heavier or more concentrated liquids a longer time to freeze than less concentrated, just as it takes saltwater longer to freeze than freshwater.

To avoid getting too cold, fish use either behavioral habits to move from one water mass or area to another in search of warmer water, or physiological means to keep at the proper temperature. In the wintertime you may have experienced fish stacked up in one spring-fed pool. Spring water is approximately 50 degrees and provides a thermal refuge from colder water.

Of course, all these methods won’t help unless part of the fish’s environment remains unfrozen.

--Nanci Bross-Fregonara

 

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