January Nature Notes: Winter Green

          Why do some leafy plants seem to ignore winter and remain green throughout the coldest months outdoors? How do they survive? In Illinois, a few species of ferns—including the aptly named Christmas fern (Polystichum acrostichoides) remain green even during the harshest of winter weather. But the greenery can be deceptive, plant biologists explain, since photosynthesis and other warm-weather plant functions, including the exchange of carbon dioxide with oxygen, slow down or shut down altogether in sub-freezing weather. During the icy winter, water in fern leaves is replaced by sugar “antifreeze” that protects the fern but also inhibits photosynthesis. Thus, the evergreen ferns, along with evergreen trees and those holiday favorites, mistletoe and holly, appear to be functioning as green plants do in summer when they are actually in a semi-dormant winter state.          

But why bother to stay green in winter? Over the ages, plants developed adaptive strategies as they evolved and competed for life in specific climates. In cold climates, staying green throughout winter allows certain plants to take advantage of the early hints of spring warmth to conduct photosynthesis. By remaining green and ready for action, plants on the forest floor can stock up on energy before the forest canopy of summer shades sunlight—and long after leaves have fallen in autumn.          

Why don’t hungry browsers eat the only available green plants in winter? Distasteful or unpleasant substances within many winter evergreens discourage browsers. But not always. White-tailed deer, for example, are known to sample ferns during the cold, hungry days of late winter. White pine seedlings also get nibbled or eaten completely. Nature is filled with all sorts of imperfect adaptations—but life goes on. 

By: Joe McFarland