WINTER DESICCATION AND SOLAR-RADIATION IN RELATION TO RED SPRUCE DECLINE IN THE NORTHERN APPALACHIANS

Exposure to direct solar radiation was correlated with needle death within individual high-elevation red spruce (Picea rubens Sarg.) shoots following winter injury episodes at six sites in 1980 and 1989. We observed extensive visible needle damage to red spruce in northern New England between March and May of 1989 and determined that it was preceded by desiccation. In an independent growth chamber experiment, red spruce needles were heated to above freezing when exposed to strong illumination in calm subfreezing air. Rapid needle cooling occurred when the radiation load was suddenly reduced at the end of each light period, and needles desiccated severely within 10 days. These separate observations are consistent with three hypotheses: injury results from (i) desiccation, (ii) rapid needle cooling, (iii) freezing injury caused by a reduction in cold hardiness due to solar heating. These three mechanisms are not necessarily mutually exclusive.