Insights into the daily emissions and consumptions of methane and nitrous oxide from tropical tree stem surfaces

Methane (CH4) and nitrous oxide (N2O), critical climate-forcing trace gases, are rising sharply in the atmosphere. The estimate for their natural emissions remains uncertain because of mismatches between bottom-up (from flux measurements to process-based models) and top-down (from satellites to inversion models) approaches. Besides soils, trees can exchange CH4 and N2O with the atmosphere (either emitting or taking up), potentially acting as conduits for transporting large amounts of soil-produced CH4 and N2O. However, tree CH4 and N2O fluxes are not included yet in models, mainly because of a lack of detailed understanding of the sources and drivers of their temporal variation. This holds particularly true in tropical forests where high-frequency measurements are rare.We first hypothesized that, although on well-aerated upland forest soils, trees contribute to ecosystem CH4 and N2O fluxes, with seasonal CH4 and N2O flux dynamics being reversed between the stems and soil. Second, we postulated that, at the daily scale, circadian rhythms of tree physiology affect stem CH4 and N2O fluxes. We investigated these hypotheses by examining the high-temporal-resolution fluxes of CH4 and N2O of three tropical trees. We measured fluxes from their stems (and adjacent soils) with an automated chamber system and tree and environmental variables (i.e. tree growth, sap flux density, tree water deficit, stem and soil temperature, stem and soil water content) over 20 months in a tropical forest, in French Guiana.Long-term series of stem CH4 and N2O fluxes not only revealed that stems emitted CH4 and consumed N2O but also that flux variability was greater between tree individuals than seasons. We also found that stem CH4 and N2O fluxes were not linked to soil fluxes and exhibited diurnal patterns, with stems being greater emitters of CH4 and lower consumers of N2O in the early morning than at midday. CH4 and N2O peaks at sunrise suggest that gases accumulated in the stem at night when there was no gas flow and were vented out as soon as the pressurized flow started, following daily tree water dynamics. These preliminary results showed clear evidence of the need for continuous CH4 and N2O flux measurements in tropical forests to disentangle tree-mediated CH4 and N2O transport; more study is required to determine the relative importance of wood microbial organisms and tree traits for regulating gas transport through their aboveground parts.