STAND CHARACTERISTICS OF OZONE-STRESSED POPULATIONS OF PINUS-JEFFREYI(PINACEAE) - EXTENT, DEVELOPMENT, AND PHYSIOLOGICAL CONSEQUENCES OF VISIBLE INJURY

The development and physiological consequences of ozone-induced visibl e 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 som e degree of visible injury and 10% were classified as resistant to ozo ne. Needles of the most sensitive trees developed a chlorotic mottle c haracteristic of ozone injury 1 year after their initiation and premat urely abscised in their third year (normal needle retention is 5-6 yea rs). 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 mea surements indicated a negative correlation between photosynthetic rate and needle surface area covered by chlorotic mottle. Chlorophyll fluo rescence kinetics of the current-year needles did not differ between s ymptomatic and asymptomatic trees but did differ between the oldest ne edles, suggesting an uncontrolled physiological decline in needles abo ut to abscise in sensitive trees. The high degree of variability of oz one-induced visible injury coupled with the reduction of physiological capacity associated with visible injury suggest that mixed conifer fo rests growing in polluted regions could potentially undergo shifts in community structure if sensitive Jeffrey pine individuals were to expe rience differential mortality as a result of ozone exposure. Intraspec ific variation in ozone sensitivity may potentially lead to increased population tolerance to oxidative air pollutants, but long-term popula tion analyses will be required to address genetic changes in response to ozone stress.