Effects of ambient levels of airborne chemicals on freezing resistance of red spruce foliage

Abstract

Repeated, severe winter injury incited and sustained the decline of red spruce ( Picea rubens Sarg.) in the mountain forests of the northeast. Some experiments have shown that acidic mist comparable to ambient cloud water reduces frost tolerance of red spruce seedlings. To help bridge the gap between seedling studies and mature spruce growing under field conditions, we excluded airborne chemicals from the foliage of red spruce 75 years old or more, growing on Whiteface Mountain, New York in cloud-immersed forests at 1170 m. Polyethylene and Teflon chambers with multi-stage filters excluded ambient cloud water and charcoal-reactive gases (e.g. ozone) in different combinations from the distal meter of exposed branches in the upper crown. Three months of exclusion in the summer of 1988 improved several needle characteristics, including midwinter cold tolerance. Resistance to freezing injury measured experimentally in midwinter was a good predictor of the degree of natural winter injury which occurred in late winter. Artificial freezing of shoots in situ reproduced the symptoms of naturally occurring winter injury. Thus, freezing injury appears to be important for the development of the type of winter injury observed at Whiteface Mt. In experimental trees which sustained natural winter injury, exposure to charcoal-filtered air and deionized water mist during the preceding summer improved resistance to freezing injury compared with ambient air and cloud water. The loss of freezing resistance attributed to ambient conditions was about 10°C, sufficient to increase substantially the risk of winter freezing damage.