Abstract
Leaf-cutter ant nests are biogeochemical hot spots where ants live and import vegetation to grow fungus. Metabolic activity and (in wet tropical forests) soil gas flux to the nest may result in high nest CO2 concentrations if not adequately ventilated. Wind-driven ventilation mitigates high CO2 concentrations in grasslands, but little is known about exchange for forest species faced with prolonged windless conditions. We studied Atta cephalotes nests located under dense canopy (leaf area index > 5) in a wet tropical rainforest in Costa Rica, where wind events are infrequent. We instrumented nests with thermocouples and flow-through CO2 sensing chambers. The results showed that CO2 concentrations exiting leaf-cutter ant nests follow a diel pattern with higher values at night. We developed an efflux model based on pressure differences that evaluated the observed CO2 diel pattern in terms of ventilation by (1) free convection (warm, less dense air rises out the nest more prominently at night) and (2) episodic wind-forced convection events providing occasional supplemental ventilation during daytime. Average greenhouse gas emissions were estimated through nest vents at about 78 kg CO2eq nest−1 year−1. At the ecosystem level, leaf-cutter ant nest vents accounted for 0.2% to 1% of total rainforest soil emissions. In wet, clayey tropical soils, leaf-cutter ant nests act as free convection-driven conduits for exporting CO2 and other greenhouse gases produced within the nest (fungus and ant respiration, refuse decay), and by roots and soil microbes surrounding the nest. This allows A. cephalotes nests to be ventilated without reliable wind conditions.
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Data availability
Data and code are available at https://doi.org/10.6084/m9.figshare.7789601. Detailed information about how to build the customized monitoring devices is available from the authors on request.
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Acknowledgements
We thank Adrián Pinto-Tomás, Allan Artavia-León, and the rest of the Pinto-Tomás laboratory at the Universidad de Costa Rica for allowing us to participate in their nest excavations. We also thank Samantha Young, Spencer McDermont, and Timothy Barahona for their help building the electronic devices; our colleagues of the “Atta team” Amanda C. Swanson, Ana Grace Alvarado, Catalina Murillo, Deo Lachman, Emma Aronson, Jane Zelikova, Jon Botthoff, Luitgard Schwendenmann, Michael F. Allen, Odemaris Carrasquillo‐Quintana, Philip Rundel, Shaquetta Johnson, Steven Oberbauer, and Yorelyz Rodríguez‐Reyes for all our joint efforts; Stephen C. Hart, Carlos de la Rosa, and the staff of La Selva Biological Station for their logistical support.
Funding
This work was conducted with permits granted by the “Comisión Institucional de Biodiversidad” (Institutional Biodiversity Committee, University of Costa Rica; resolution VI-8315-2014) and authorized by La Selva Biological Station, and it was supported by the National Science Foundation under Collaborative Awards DEB-1442537, 1442568, 1442622, 1442714, 1624623, and 1624658.
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ASFB led the work as part of his doctoral dissertation, and TCH and DD participated with essential contributions to the experiment conceptualization, custom devices design and fabrication, model development, and revising the manuscript. DD and ASFB conducted the field measurements.
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Communicated by Stefan Scheu.
We present a mass balance model to describe CO2 fluxes from massive nests of leaf-cutter ants and designed flow-through chambers to continuously monitor them. Our study revealed a clear diel CO2 emission pattern driven by free convection and sporadic wind-forced convection. At the ecosystem scale, nests are responsible for up to 1% of the total forest soil greenhouse gas emissions, which is impressive for a single ant species that thrives under changing climate and fragmented landscapes.
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Fernandez-Bou, A.S., Dierick, D. & Harmon, T.C. Diel pattern driven by free convection controls leaf-cutter ant nest ventilation and greenhouse gas emissions in a Neotropical rain forest. Oecologia 192, 591–601 (2020). https://doi.org/10.1007/s00442-020-04602-2
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DOI: https://doi.org/10.1007/s00442-020-04602-2