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Responses of Northern U.S. Forests to Environmental Change
ISBN 0-387-98900-5

Chapter 6: Physiological and Environmental Causes of Freezing Injury in Red Spruce

Paul G. Schaberg and Donald H. DeHayes

There has been a dramatic increase in the incidence of freezing injury to northern red spruce forests, leading to forest decline, over the past 4 decades. It is likely that the increased injury is the result of human-induced environmental changes: acid deposition and altered weather patterns. The average temperature at which freezing injures current-year needles is between -40 and -45^oC. Red spruce is much more susceptible to freezing injury than the co-occurring species balsam fir.

Many environmental factors have been studied for their potential to reduce cold tolerance. Ozone does not appear to reduce foliar cold tolerance or increase susceptibility to freezing injury. CO₂ enrichment and N nutrition may interact to influence cold tolerance but the evidence is not clear. It is virtually certain that rapid freezing stress explains localized injury concentrated on sun-exposed branches, but it is unlikely that rapid freezing explains injury on shaded foliage or region-wide injury events. Long winter thaws (4-5 days) cause reductions in cold tolerance of up to 14^oC, a response unique to red spruce. Numerous studies have shown that short-term nitrogen additions either have no impact on freezing tolerance or may improve hardiness. Sulfur additions appear to have some impact on autumn hardiness, but no impact on winter cold tolerance. Aluminum has many effects on red spruce physiology, but no apparent effect on cold tolerance. Calcium loss due to acid deposition affects red spruce health, and may have indirect effects on cold tolerance. The strongest evidence for an environmental effect on freezing injury is that exposure to acid cloud water increases the risk of foliar freezing injury by reducing the cold tolerance of current-year foliage from 5 to 12^oC. Of all the possible combinations of factors, interactions between acid mist and thaw have the greatest potential to increase the risk of freezing injury during winter, but this hypothesis has not been fully tested.

The specific mechanisms causing winter injury appear to include perturbations of winter temperatures on a species with limited genetic potential for cold tolerance, coupled with alterations to the structure and function of plasma membrane-associated calcium in mesophyll cells. Because of the strong linkage of this mechanism to acidic precipitation, there is great concern that membrane integrity of other species may also be affected, but only red spruce exhibits visible damage symptoms because of its unique susceptibility to winter injury.

(a) Relationship between critical temperatures derived from laboratory freezing experiments with 36 red spruce trees and quantitative estimates of foliar winter injury of the same trees recorded in a provenance test of red spruce near Colebrook, NH.

(b) Comparison of relative electrolyte leakage (REL) and image analysis of needle reddening (Red/100).

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