Seasonal patterns of leaf photosynthetic capacity and conductance were determined for deciduous hardwood tree species in natural habitats in northern lower Michigan. Leaves of bigtooth aspen and red oak at the top of the canopy had higher maximum CO2 Exchange Rate (CER) (10-15 umol/m2/s) than leaves of sugar maple, red maple, red oak, and beech growing in the understory (4-5 umol/m2/s). In all leaves, CER measured at light-saturation increased to a maximum near the completion of leaf expansion in early June, was constant until mid-September, and then rapidly declined until leaf death. A similar pattern was seen for CER measured in low light (1.5% full sun). Respiration rate in the dark was highest in young leaves and decreased during leaf expansion; a relatively constant rate was then maintained for the rest of the leaf lifespan. The seasonal pattern of the initial slope of the light response of CER paralleled the pattern of light-saturated CER. The initial slope in midsummer ranged from values of 37 to 44 umol/mol for species in the understory to 51 and 56 umol/mol for red oak and bigtooth aspen, respectively, at the top of the canopy. Leaf conductance was constant throughout most of leaf lifespan, with some decline occurring in autumn. Leaves at the top of the canopy had higher conductances for water vapor (2-5 mm/s) than leaves in the understory (1-2 mm/s). All species maintained leaf intercellular CO2 mole (ci) fractions near 200 uL/L until autumn, when ci increased during leaf senescence.