Amazon rainforest ecosystem exchange of CO ₂ and H ₂ O through turbulent understory ejections

Abstract. We investigate the role of short-term variability in the mean ecosystem exchange of carbon dioxide and water vapour. Specifically, we focus on quantifying how the intermittent turbulent exchange at the forest–atmosphere interface – characterised by sweeps, ejections, and outward/inward interactions – contributes to the mean exchange. To this end, we analyse observations of high-resolution (isotopic) flux measurements taken at 25 m above the forest canopy at the Amazon Tall Tower Observatory (ATTO) during the dry season. We identify short-term turbulent eddies that eject carbon dioxide and water vapour from the understory (0–15 m) into the atmosphere. The H2O ejected from the understory is shown to be depleted in deuterium (2H) by 10 ‰ compared to H2O originating from the top canopy. We show that this matches the depleted water isotopic compositions found in understory leaf and soil samples. The diurnal cycle of the net ecosystem exchange (NEE) of CO2 is presented as a function of the sweeping and ejection motions and understory flux contributions. Understory contributions average 1.4 % of NEE, but they can reach up to 20 %. In exploring the connection between intermittent canopy turbulence and cloud passages, we found a weak relationship (r=0.027) between cloud passages and ejections, without a predominant influence of large clouds. These findings deepen our understanding of the gas exchange of the Amazon rainforest, which may ultimately allow decision-makers to incorporate policies that can prevent the region's transition from a carbon sink to a source.