Seasonal variation in rates of photosynthesis of a transitional tropical forest of Mato Grosso, Brazil

Measurements of the leaf photosynthetic light response, water potential, and specific leaf area of two tropical transitional forest tree species, Tovomita schomburgkii and Brosimum lactescens, were quantified to establish how these properties are affected by seasonal variations in rainfall and leaf canopy position. The study was conducted at a site near Sinop Mato Grosso, Brazil, which is located within the ecotone of savanna and tropical rain forest. Understory T. schomburgkii leaves showed no significant difference in water potential ('I'L) between the dry season (June-Sept.) and the wet season (Dec.-Feb.); however mid-and upper-canopy leaves of both species exhibited significantly lower water potential during the dry season. Both species exhibited seasonal variations in light saturated photosynthesis (P m), dark respiration (Rd), and maximum stomatal conductance (gmax) which were positively correlated with precipitation. Internal leaf C02 concentrations (Ci) were consistently lower during the dry season suggesting that declines in stomatal conductance of C02 caused decreased levels of maximum photosynthesis. During the wet season P m and RI of both species were significantly negatively correlated with specific leaf area (SLA) but during the dry season this correlation was only observed in T. schomburgkii. These results suggest that seasonal drought has important implications for the physiology of semi-deciduous Amazonian forest trees and that the seasonal trends ofPm, RJ, and gmax are more comparable to savanna than to tropical rain forest. Keywords: photosynthesis, seasonality, tropical transitional forest, Tovomita schomburgkii, Brosimum lactescens