DECLINE OF PHOTOSYNTHETIC CAPACITY WITH LEAF AGE IN RELATION TO LEAF LONGEVITIES FOR 5 TROPICAL CANOPY TREE SPECIES

The effect of leaf aging on photosynthetic capacities was examined for upper canopy leaves of five tropical tree species in a seasonally dry forest in Panama. These species varied in mean leaf longevity between 174 and 315 d, and in maximum leaf life span between 304 and 679 d. T he light-saturated CO2 exchange rates of leaves produced during the pr imary annual leaf flush measured at 7-8 mo of age were 33-65% of the r ates measured at 1-2 mo of age for species with leaf life span of < 1 yr. The negative regression slopes of photosynthetic capacity against leaf age were steeper for species with shorter maximum leaf longevity. In all species, regression slopes were less steep than the slopes pre dicted by assuming a linear decline toward the maximum leaf age (20-80 % of the predicted decline rate). Maximum oxygen evolution rates and l eaf nitrogen content declined faster with age for species with shorter leaf life spans. Statistical significance of regression slopes of oxy gen evolution rates against leaf age was strongest on a leaf mass basi s (r(2) = 0.49-0.87), followed by leaf nitrogen basis (r(2) = 0.48-0.7 7), and weakest on a leaf area basis (r(2) = 0.35-0.70).