Abstract
The development and physiological consequences of ozone‐induced visible injury was investigated in native populations of Jeffrey pine (Pinus jeffreyi) that were exposed to chronic levels of anthropogenic ozone. Stand structure analyses demonstrated that the expression of visible ozone injury symptoms within self‐regenerating populations of Jeffrey pine was highly variable. Of the 975 trees surveyed, 90% exhibited some degree of visible injury and 10% were classified as resistant to ozone. Needles of the most sensitive trees developed a chlorotic mottle characteristic of ozone injury 1 year after their initiation and prematurely abscised in their third year (normal needle retention is 5–6 years). Average needle retention was not different between diameter size categories but was the most variable within the smallest size category and the least variable in the largest size category. Gas exchange measurements indicated a negative correlation between photosynthetic rate and needle surface area covered by chlorotic mottle. Chlorophyll fluorescence kinetics of the current‐year needles did not differ between symptomatic and asymptomatic trees but did differ between the oldest needles, suggesting an uncontrolled physiological decline in needles about to abscise in sensitive trees. The high degree of variability of ozone‐induced visible injury coupled with the reduction of physiological capacity associated with visible injury suggest that mixed conifer forests growing in polluted regions could potentially undergo shifts in community structure if sensitive Jeffrey pine individuals were to experience differential mortality as a result of ozone exposure. Intraspecific variation in ozone sensitivity may potentially lead to increased population tolerance to oxidative air pollutants, but long‐term population analyses will be required to address genetic changes in response to ozone stress.
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