• Forest decline;
  • heat injury;
  • winter injury;
  • winter hardiness;
  • ozone;
  • air pollution


The death of many mature red spruce (Picea rubens Sarg.) trees in the northeastern United States has sparked controversy over the roles natural factors and air pollutants play. The goal of this work was to investigate the effects of long-term ozone exposures on development of injury and, in particular, the expression of winter injury in red spruce. Red spruce seedlings were exposed to elevated levels of ozone during two growing seasons, starting just after budbreak, and continuing into the period of early frosts. Trees spent both winter periods under ambient conditions of light and temperature. No visible lesions on foliage that could be attributed to ozone developed during the growing seasons. Yellow and brown needles appeared each winter, with much more injury in the second year. However, in the spring after two seasons of ozone exposure, there was no significant effect of ozone on the overall appearance of the entire population of seedlings. Samples were taken for histology and ultrastructure approximately monthly. Ozone treatments increased mesophyll cell disruption after frosts in early winter in the first year, but not in the second. There was no correlation between histological injury in the autumn and visible symptoms in the spring, indicating that there may be different aspects of the winter injury syndrome. In the second summer from 20 to 40%, of all mesophyll cells were damaged. This damage was not related to ozone treatment, and probably resulted from high summer temperatures.