In situ experimental freezing produces symptoms of winter injury in red spruce foliage.

Two mechanisms have been proposed to explain winter injury to needles of red spruce (Picea rubens Sarg.): (1) desiccation, which is characterized by net loss of foliar water from the needle to the environment, with cell injury resulting from dehydration; and (2) freezing, which is characterized by direct injury to cells resulting from intracellular or extracellular ice formation during exposure to low temperature. To compare the separate and combined effects of freezing and desiccation, branches of a mature red spruce at 1160 m were (a) experimentally frozen in situ to -50 degrees C; (b) cut and tied in their original orientation and allowed to desiccate passively; or (c) both frozen in situ and cut and tied in their original orientation. Needle water content, electrolyte leakage (an index of cell injury), and needle color were monitored for 60 days after treatment. Freezing resulted in immediate increases in electrolyte leakage, rapid water loss, and reddening necrosis of needles similar to that of naturally injured needles. Cutting resulted in more gradual water loss, no significant changes in electrolyte loss until severe desiccation had occurred, and a change in the color of the needles to a dull green. Because freezing produced reddening necrosis, a key symptom of winter injury, whereas desiccation did not, we conclude that freezing is probably the primary cause of winter injury in red spruce, and that desiccation is a secondary effect.