Author's manuscript for published article. See Additional Links field for link to published version. / Ethylene production by different needle age classes was characterized using a mercuric perchlorate traps in natural populations of two ponderosa pine varieties (Pinus ponderosa var. arizonica [Engelm] Shaw and var. ponderosa Dougl. ex Laws.) and Jeffery pine (Pinus leffrevi Grev. and Balf.). All ozone -exposed populations contained individuals which were symptomatic and asymptomatic with respect to visible ozone injury. Ethylene production of different needle age classes was also characterized in Pinus ponderosa var. ponderosa seedlings grown in open top ozone fumigation chambers. Older age class needles produce more ethylene than younger age class needles. Needles of both P. ponderosa var. ponderosa and P. jeffreyi exhibiting ozone injury in the field produced significantly (p >0.05) higher levels of ethylene than asymptomatic conspecifics. Seedlings exposed to highest treatment level of ozone in the fumigation study produced the highest levels of ethylene followed by fumigation with medium and low ozone concentrations and carbon filtered air. These data indicate that measurement of ethylene in conifer needles as a measure of stress needs to be calibrated for needle age class. It also suggests that the sensitivity of a tree to ozone injury may be regulated by the inherent ability of the individual to produce ethylene.