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Summary: Trees are known to emit methane (CH4) and nitrous oxide (N2O), with tropical wetland trees being considerable CH4 sources. Little is known about CH4 and especially N2O exchange of trees growing in tropical rain forests under nonflooded conditions.We determined CH4 and N2O exchange of stems of six dominant tree species, cryptogamic stem covers, soils and volcanic surfaces at the start of the rainy season in a 400‐yr‐old tropical lowland rain forest situated on a basaltic lava flow (Réunion Island). We aimed to understand the unknown role in greenhouse gas fluxes of these atypical tropical rain forests on basaltic lava flows.The stems studied were net sinks for atmospheric CH4 and N2O, as were cryptogams, which seemed to be co‐responsible for the stem uptake. In contrast with more commonly studied rain forests, the soil and previously unexplored volcanic surfaces consumed CH4. Their N2O fluxes were negligible.Greenhouse gas uptake potential by trees and cryptogams constitutes a novel and unique finding, thus showing that plants can serve not only as emitters, but also as consumers of CH4 and N2O. The volcanic tropical lowland rain forest appears to be an important CH4 sink, as well as a possible N2O sink. [ABSTRACT FROM AUTHOR]

Tropical forests are considered a natural sink for methane (CH4) and a natural source ofnitrous oxide (N2O), both important greenhouse gases (GHGs). To date, forest ecosystemexchange of CH4 and N2O has been mostly estimated based on GHGs exchange atthe soil–atmosphere interface only. However, trees of various climatic zones areknown to emit CH4 and N2O into the atmosphere. Recent research revealed tropicalwetland trees as considerable sources of CH4. Nevertheless, there is little known aboutCH4 and N2O exchange capacity of tropical trees growing under „non-flooded"conditions. We determined CH4 and N2O exchange of soil and stems of mostly endemic tree species(Syzygium borbonicum, Doratoxylon apetalum, Antirhea borbonica, Homaliumpaniculatum, Mimusops balata, Labourdonnaisia calophylloides) in a tropical lowland rainforest on lava flow of La Reunion Island in the South Western Indian Ocean. Weinvestigated (1) whether the tree stems exchange CH4 and N2O with the atmosphere, (2)how the tree fluxes contribute to the forest GHGs exchange, and (3) whether thetropical rain forest is a source or sink for CH4 and N2O at the beginning of the rainseason. The experiment was performed in Mare-Longue Nature Reserve (21˚ 21’S, 55˚ 45’E) inOctober-November 2018. The studied forest is situated on 400 years old pahoehoe basalticlava flow covered with irregular and thin soil layer. Fluxes of CH4 and N2O in mature treestems (n=24) and soil (n=24) were measured using non-steady-state chamber systems and aportable FTIR gas analyser. The stems of studied tree species were net sinks for both CH4 (-15.1 ± 2.2 μg CH4 m−2stem area h−1, mean ± s.e.) and N2O (-3.1 ± 0.8 μg N2O m−2 h−1). Such uptake potentialfor CH4 and N2O by tropical tree species represents a novel and unique finding which is incontrast to current limited studies presenting tropical trees as CH4 emitters. The soil was asignificant net CH4 sink (-79.5 ± 11.5 μg CH4 m−2 soil area h−1). However, the soil mightindicate also potential for CH4 emission under high soil water content (e.g. due to extremeprecipitation events), as one small-scaled wet soil area was characterized by CH4 emissions(192 ± 117 μg CH4 m−2 h−1). The soil N2O fluxes showed a high spatial heterogeneityincluding both N2O emissions and uptake (net flux -0.18 ± 1.61 μg N2O m−2h−1). Concluded, the studied tropical tree species were net sinks for CH4 and N2O. The tropicallowland rain forest situated on a lava flow seems to be a net sink for CH4 and toplay only a minor role in the global N2O exchange at the beginning of the rainseason. Acknowledgement This research was supported by the Czech Science Foundation (17-18112Y) and EUHorizon 2020 research and innovation programme (654182). Logistical support wasprovided by the Mare-Longue research station, funded by the POE, Reunion National Parkand OSU-Reunion. We thank Dr. Claudine Ah-Peng for scientific and organization help, andYoan Benoit, Pierre Stamenoff and Leszek Dariusz Laptaszynski for technical andorganization support. [ABSTRACT FROM AUTHOR]