SEASONAL LEAF PHENOTYPES IN THE CANOPY OF A TROPICAL DRY FOREST - PHOTOSYNTHETIC CHARACTERISTICS AND ASSOCIATED TRAITS

We evaluated the hypothesis that photosynthetic traits differ between leaves produced at the beginning (May) and the end (November-December) of the rainy season in the canopy of a seasonally dry forest in Panam a. Leaves produced at the end of the wet season were predicted to have higher photosynthetic capacities and higher water-use efficiencies th an leaves produced during the early rainy season. Such seasonal phenot ypic differentiation may be adaptive, since leaves produced immediatel y preceding the dry season are likely to experience greater light avai lability during their lifetime due to reduced cloud cover during the d ry season. We used a construction crane for access to the upper canopy and sampled 1- to 2-month-old leaves marked in monthly censuses for s ix common tree species with various ecological habits and leaf phenolo gies. Photosynthetic capacity was quantified as light- and CO2-saturat ed oxygen evolution rates with a leaf-disk oxygen electrode in the lab oratory (O-2max) and as light-saturated CO2 assimilation rates of inta ct leaves under ambient CO2 (A(max)). In four species, pre-dry season leaves had significantly higher leaf mass per unit area. In these four species, O-2max and A(max) per unit area and maximum stomatal conduct ances were significantly greater in pre-dry season leaves than in earl y wet season leaves, In two species, A(max) for a given stomatal condu ctance was greater in pre-dry season leaves than in early wet season l eaves, suggesting a higher photosynthetic water-use efficiency in the former. Photosynthetic capacity per unit mass was not significantly di fferent between seasons of leaf production in any species. In both ear ly wet season and pre-dry season leaves? mean photosynthetic capacity per unit mass was positively correlated with nitrogen content per unit mass both within and among species. Seasonal phenotypic differentiati on observed in canopy tree species is achieved through changes in leaf mass per unit area and increased maximum stomatal conductance rather than by changes in nitrogen allocation patterns.