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Abstract The photosynthetic characteristics of Cycas micronesica K.D. Hill leaves aged 2 to 24 months were determined in cultivated plants. Net photosynthesis (ACO2) was measured as net carbon dioxide exchange. Light-saturated ACO2 declined as leaves aged; that for the oldest leaves was approximately 75% of that for the youngest leaves in the study.The photosynthetic photon ux at which ACO2 reached capacity was about 1550 μmol m'2 s'1 for 2-month-old leaves and 1350 μmol m'2 s'1 for 22-month-old leaves. These results indicate that photosynthetic capacity of C. micronesica leaves remains high for at least two years after leaf emergence. However, these results retain relevance only at the landscape evel in the absence of any changes in severity of leaf damage by cycad leaf consumers already on Guam and the absence of new alien leaf consumers becoming established on Guam. The number of hours during the photoperiod that the sustained photosynthetic capacity is achievable also declines with leaf age for several reasons. The contribution of these older leaves to the plant™s carbon economy is inuenced by the amount of shade imposed by younger leaf cohorts and by daily ambient sun exposure.