The sensitivity of Diel CO2 and H2O vapor exchange of a tropical transitional forest to seasonal variation in meteorology and water availability

Measurements of ecosystem gas exchange, meteorology, and hydrology ( rainfall and soil moisture) were used to assess the seasonal patterns of, and controls on, average diel (24 h) net ecosystem CO2 exchange (NEE), evapotranspiration (E), and bulk canopy water vapor conductance (G(c)) of a tropical transitional (ecotonal) forest in the Brazilian Amazon. Diel trends in E and NEE were almost completely explained by the diel variation in photosynthetic photon flux density (Q(PPFD)), and while the Q(PPFD) response of E varied little over the annual cycle, the Q(PPFD) response of NEE declined substantially during the dry season, and the magnitude of hysteresis in the NEE - Q(PPFD) response increased as well. The magnitude of the residuals for the Q(PPFD) versus NEE response was significantly negatively correlated with total monthly rainfall and surface soil moisture and positively correlated with the maximum daily temperature and atmospheric vapor pressure deficit ( V). Average daily Gc was also significantly correlated with average daily V ( r= - 0.72) and soil moisture (r = 0.62), suggesting strong stomatal control of NEE during drought. However, drought reduced ecosystem CO2 efflux relatively more than CO2 assimilation, suggesting that the seasonal variation in NEE was largely driven by seasonal variation in respiration. When compared with other tropical forests, seasonality in NEE was negatively correlated with annual rainfall and positively correlated with dry-season length. The relatively high sensitivity of NEE to seasonal variation in climate and water availability has profound implications for C cycling dynamics under novel climates resulting from climate and/or land-use change in the Amazon basin.