forest ecosystems sequester large amounts of atmospheric co 2 , and the contribution from seasonally dry tropical forests is not negligible. thus, the objective of this study was to quantify and evaluate the seasonal and annual patterns of co 2 exchanges in the Caatinga biome, as well as to evaluate the ecosystem condition as carbon sink or source during years. in addition, we analyzed the climatic factors that control the seasonal variability of gross primary production (Gpp), ecosystem respiration (R eco) and net ecosystem co 2 exchange (nee). Results showed that the dynamics of the components of the co 2 fluxes varied depending on the magnitude and distribution of rainfall and, as a consequence, on the variability of the vegetation state. Annual cumulative NEE was significantly higher (p < 0.01) in 2014 (−169.0 g C m −2) when compared to 2015 (−145.0 g C m −2) and annual NEP/GPP ratio was 0.41 in 2014 and 0.43 in 2015. Global radiation, air and soil temperature were the main factors associated with the diurnal variability of carbon fluxes. Even during the dry season, the NEE was at equilibrium and the Caatinga acted as an atmospheric carbon sink during the years 2014 and 2015. CO 2 concentration has a high interannual variability due to its absorption by terrestrial ecosystems (carbon sinks) 1-5. However, despite this variability, data show a systematic increase in CO 2 throughout the years 6,7. In South America, the Amazon forest is an example of a terrestrial carbon sink (considering its 20-year mean behavior), although it has occasionally behaved as CO 2-neutral or even a carbon source in the last years 8. Interannual variability and trends in CO 2 sinks are controlled by different biogeographic regions. The annual mean behavior of sinks is controlled mainly by highly productive lands, such as wet tropical forests (i.e. the Amazon forest) 5. On the other hand, semiarid environments control the global scale trends observed in the last few decades 9,10. Despite its prominent role, there is still much to be studied and investigated regarding CO 2 exchanges in these regions, which are still much less understood than wet forests or croplands 5,10. According to the literature 10 , gaps in understanding CO 2 exchanges in these environments have limited our ability to understand and predict interannual and decadal variations on global scale carbon cycle. There are a few inherent difficulties when quantifying CO 2 exchanges in semiarid environments, such as the rapid expansion of some of its areas due to climate change and anthropic activities 11,12. Studies show that some regions in South America are becoming more arid, such as the Amazon 13,14 ; the Brazilian semiarid region, dominated by the Caatinga biome, which is a seasonally dry tropical forest (SDTF) 15-17 and the Cerrado, which is a Brazilian savanna-type vegetation 18 .
