Amazon rainforest ecosystem exchange of CO₂ and H₂O through turbulent understory ejections

Abstract. We investigate the role of short-term variability on the mean ecosystem exchange of carbon dioxide and water vapor. Specifically, we focus on quantifying how the intermittent turbulent exchange at the forest-atmosphere interphase – characterized by sweeps, ejections and outward/inward interactions – contributes to the mean exchange. To this end we analyze 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 vapor 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 vapor 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 reach up to 20 %. In exploring the connection between intermittent canopy turbulence and cloud passages, we found a weak but coherent temporal 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 is urgent for predicting and possibly preventing the regions transition from a carbon sink to a source.