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.