AMBIENT CLOUD DEPOSITION REDUCES COLD TOLERANCE OF RED SPRUCE SEEDLINGS

Laboratory cold-tolerance assessments were conducted between September 1989 and March 1990 on red spruce (Picea rubens Sarg.) seedlings exposed to combinations of ozone (O3) and cloud water for two growing seasons on the summit of Whitetop Mountain, Virginia. Chamber treatments consisted of exposure to (i) ambient O3 and cloud water, (ii) ambient O3 and no cloud water, or (iii) reduced O3 and no cloud water. Ambient-air field plots were included to assess possible chamber effects. Red spruce seedlings exposed to ambient O3 and cloud water, either in chambers or field plots, were generally least cold tolerant, and differences were significant in October, November, December, and January. During 3 of those months, seedlings exposed to ambient O3 and cloud water in chambers were significantly less cold tolerant than either seedlings exposed to reduced O3 and no cloud water or seedlings exposed to ambient O3 only. There were no definitive effects of O3 in any month. The reduction in cold tolerance of seedlings exposed to O3 and cloud water appeared to result from the negative influences of acidic cloud deposition, rather than O3. This is the first conclusive evidence that ambient cloud deposition can impair the development of cold tolerance and maximum cold tolerance achieved in midwinter for red spruce growing in its native habitat. The 3 to 5-degrees-C reduction in cold tolerance associated with exposure to ambient cloud water in this study may be sufficient to explain the winter injury observed frequently in northern montane red spruce stands over the past 4 decades.