Saturday, December 20, 2008

Flagged trees on Mt. Bond, west of Mt. Washington

Photo courtesy of the author

In and of themselves, low temperatures do not necessarily threaten conifers, but they do injure plants by increasing rime ice buildup. On cold days when the air is full of moisture, water droplets in fog freeze directly onto plants, rocks, and manmade structures, forming beautiful and feathery ice sculptures—rime. Directed by wind, rime ice tends to build up on the windward side of trees, exacerbating needle loss by providing a brittle and broad surface for the wind to blow against.
On Mt. Washington, winter winds are so strong and rime ice is so common that almost all exposed plants perish. A few trees, including Balsam Fir, Black Spruce, and Paper Birch survive by growing horizontally rather than vertically, forming thick mats that are extremely difficult to penetrate, for humans or competitors. These small tree communities are very flexible, which protects them from summer winds, and they are also so low that snow covers them completely in the winter. Thus the wind never scours them, nor can rime ice freeze to their needles (in the case of the conifers), which would prevent them from beginning photosynthesis as soon as the snow melts and the growing season begins. Determining average winter snow accumulation around these communities is easy, even in the summer, because the trees can only grow as high as the snow cover.

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