The influence of ozone on the winter hardiness of Norway spruce [Picea abies (L.) Karst.]

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Abstract
Three‐year‐old clonal saplings of Picea abies (L.) Karst. were exposed in chambers to 240μg O3 m‐3 for 70 d, 6hd‐1 and then hardened for 14 d at 4°C day/night. The winter hardiness of trees was tested by subjecting excised shoots, bearing previous and current year's needles, to night temperatures of –6, –9, –12, –15 and – 18°C in a computer‐controlled freezer. No visible injury was observed during the fumigation, hardening period or on the current year's needles of any of the clones. However, during post‐freezing recovery in a mist spray unit four of the clones exposed to ozone developed extensive visible injury on the previous year's needles, in the form of severe, uniform necrosis. The affected needles began to fall prematurely. Induced chlorophyll fluorescence was significantly reduced by O3 in two of the clones that showed post‐freezing injury. This suggested that O3‐induced membrane injury increased the sensitivity of needles to freezing. One clone (no. 4076), which exhibited no effect of ozone on fluorescence kinetics, showed severe ozone‐induced injury of the previous year's needles, developing about 40 d after fumigation independent of temperature. Previous year's needles of O3‐treated plants of this clone showed significantly greater rates of euticular transpiration, so the O3 may have induced gradual desiccation of the needles. The restriction of damage to the older needles of certain clones of Norway spruce is consistent with symptoms of forest decline affecting high‐altitude forests on acid soils. Results indicate that O3 predisposes trees to freezing injury, winter desiccation and drought, and it is proposed that such interactions are important factors contributing to the decline of high‐altitude forests of Norway spruce in Europe.