The caatinga dry tropical forest: A highly efficient carbon sink in South America

Seasonally Dry Tropical Forests (SDTFs) may act as considerable carbon sinks, regulating the atmospheric and terrestrial carbon storage and fluxes with implications for local, regional and global climates. In the Caatinga, an endemic Brazilian SDTF, the research on the magnitude of the CO₂ sink is still incipient. To address this gap, we conducted a comprehensive analysis of observed CO₂ fluxes using the Eddy Covariance technique across the Caatinga, quantifying and assessing the seasonal and interannual variations in CO₂ exchange under contrasting soil and climatic conditions. In our study we estimated whether the Caatinga functioned as a net source or sink of carbon at five sites during years with varying rates of rainfall. Results showed that the dynamics of CO₂ flux components varied based on the spatio-temporal distribution and magnitude of rainfall and the corresponding variations in vegetation cover. Average annual accumulated Gross Primary Productivity ranged from 1167 g C m⁻² in the Crystalline area to 2018 g C m⁻² in the Agreste ecotone. Average annual Net Ecosystem Exchange was -775 g C m⁻² (-7.7 t C ha⁻¹). The Caatinga exhibited higher carbon use efficiency (CUE) compared to other dry forests in arid and semi-arid regions worldwide and to South American ecosystems, including the Amazon, as documented by FLUXNET2015 eddy covariance datasets. CUE values ranged from 0.31 in the Crystalline area to 0.58 in the Agreste ecotone. These findings provide robust, measurement-based evidence that the Caatinga is a highly efficient carbon sink, substantially contributing to atmospheric CO₂ absorption and mitigating the growth rate of atmospheric CO₂ concentration.