Photosynthetic gas exchange in eastern Amazonian primary rain forest and pasture ecosystems

Tropical rain forest ecosystems are an important component of the global carbon cycle. Knowledge of how carbon moves into and out of this biome is needed in order to better understand the global carbon cycle. As in other terrestrial ecosystems, carbon fluxes between the tropical rain forest and the atmosphere depends on the interaction between climatic patterns and ecophysiological properties of the vegetation. This dissertation evaluates ecophysiological properties related to leaf-level photosynthetic gas exchange and its relationships with the environment.; Although annual rates of precipitations are high in the Amazon, the eastern portion of the basin shows a marked reduction in precipitation from July to November. Yet, no clear influence of season was found over ecophysiological properties related to leaf-level gas exchange. While some species showed a significant decrease in carbon photosynthetic assimilation rates under saturating light during the dry season, others showed no significant changes or even increased assimilation rates during the dry season. Such patterns were similar for other parameters associated to photosynthetic gas exchange, indicating that season do not have a marked effect over the potential carbon assimilation on Amazonian primary forest and pasture ecosystems.; The structure of the canopy in the primary forest ecosystem had a strong influence on gas exchange characteristics of plants species. The physical nature of forest canopies creates gradients of light availability, temperature, relative humidity and wind speed. Accordingly, parameters related to photosynthesis showed marked relationships with the relative position within the canopy profile where the leaves were located, indicating that photosynthetic characteristics were driven by environmental conditions.; The response of plants to the environment follow basic physiological rules allowing different species to be grouped based on ecophysiological similarities, therefore reducing complexity of ecosystems with high species diversity. The ecophysiological parameters evaluated in this study were used to characterize six a priori plant functional groups. Significant differences were observed among the plant functional groups, indicating that the parameters evaluated were appropriated for ecophysiological characterization of the vegetation.