Temporal patterns of net CO2 exchange for a tropical semi-deciduous forest of the southern Amazon Basin

Background/Question/Methods: The carbon (C) cycling of tropical ecosystems has received considerable attention over the last decade; however, interactions between climate variation and tropical forest net ecosystem CO₂ exchange (NEE) are still uncertain.  To reduce this uncertainty, and assess the biophysical controls on NEE, we measured the NEE of a 25-28 m tall, mature tropical semi-deciduous forest located near Sinop Mato Grosso, Brazil over a 3-year period.  The study period encompassed warm and dry, cool and wet, and cool and dry climate conditions, and based on previous research, we hypothesized that the semi-deciduous forest would be a net source of CO₂ to the atmosphere during dry periods. 

Results/Conclusions: Using time series of the enhanced vegetation index (EVI), a NEE-light-use model, and path-analysis we found that the estimated quantum yield (a’) optimal rate of gross primary production (F_GPP,opt), and daytime ecosystem respiration (F_RE,day) were directly affected by seasonal and interannual variations in monthly EVI, precipitation, photosynthetically active radiation (PAR), and the atmospheric vapor pressure deficit (VPD).  However, indirect effects of precipitation on F_GPP,opt were stronger than direct effects because an increase in precipitation also lead to an increase in the EVI and a decrease in the VPD.  Daytime rates of net CO₂ uptake and nighttime rates of net CO₂ efflux were significantly higher during the wet season and in cooler and wetter years; however, seasonal and interannual variations in average daily NEE were small because CO₂ gain and loss were both reduced during times of drought.  Interestingly, net CO₂ uptake was often higher during the dry season because respiration was limited more by seasonal drought than canopy photosynthesis.  Thus, while warmer and drier conditions associated with anthropogenic climate change and El Niño will significantly affect CO₂ gain and loss processes of semi-deciduous tropical forests the overall effect of climate change on net ecosystem CO₂ exchange is complex and will depend on changes in temperature interact with changes in rainfall.