Seasonal variation in the leaf gas exchange of tropical forest trees in the rain forest–savanna transition of the southern Amazon Basin

Abstract

The photosynthetic light response of Amazonian semi-deciduous forest trees of the rain forest–savanna transition near Sinop Mato Grosso, Brazil was measured between July 2000 and September 2003 to test the hypothesis that the photosynthetic capacity of trees acclimated to different growth light environments will decline during the dry season. Maximum photosynthesis (A_max) and stomatal conductance (g_max) were significantly higher during the wet season; however, the physiological response to drought was not a clear function of growth light environment. For some species, such as Psychotria sp. growing in the mid-canopy, internal leaf CO₂ concentration (C_i) was >30% lower during the dry season suggesting that declines in A_max were caused in part by stomatal limitations to CO₂ diffusion. For other species, such as Brosimum lactescens growing at the top of the canopy, Tovomita schomburgkii growing in the mid-canopy, and Dinizia excelsa growing in the understorey, dry season C_i declined by <20% suggesting that factors independent of CO₂ diffusion were more important in limiting A_max. Dry-season declines in g_max appeared to be important for maintaining a more consistent leaf water potential for some species (T. schomburgkii and D. excelsa) but not others (Psychotria sp.). These results indicate that while seasonal drought exerts an important limitation on the physiological capacity of semi-deciduous Amazonian forest trees, the mechanism of this limitation may differ between species.