Researchers from Southern Cross University, together with a colleague from NASA’s Goddard Space Flight Center and the University of Maryland, carried out four field campaigns in a subtropical Australian rainforest, measuring methane fluxes from both tree stems and forest soils. Their study plots were deliberately positioned along a moisture gradient: a valley floor plot where water lingers, a lower slope plot, and an upper slope plot where the ground drains freely. This design allowed the team to isolate one of the most important controls on methane cycling in any landscape, the availability of oxygen in the soil.

The underlying science is a contest between two microbial communities. In aerated soils, methanotrophic bacteria consume methane from the atmosphere, oxidizing it for energy and acting as a biological filter that removes this potent greenhouse gas before it can accumulate. But when soils become saturated, oxygen disappears, and a different group of microbes, the methanogenic archaea, takes over, producing methane as a byproduct of anaerobic decomposition. Whether a forest soil is a net sink or a net source of methane depends on which of these processes dominates, and that, in turn, depends heavily on soil moisture.

The study’s findings show that this dominance can flip with the seasons. On the valley floor, the researchers documented a remarkable transition: at the end of the dry season, the soil absorbed methane at a rate of about 151 micromoles per square meter per day, acting as a modest sink. But by the end of the wet season, the same soil had transformed into a powerful methane source, emitting around 830 micromoles per square meter per day. That is a swing of nearly a thousand micromoles per square meter per day driven almost entirely by changing water conditions, a magnitude of seasonal variability that underscores how misleading single-visit field measurements can be when estimating a landscape’s true greenhouse gas budget.

The sloped plots told a steadier story. On the lower slope, soils absorbed methane at an average rate of 135 micromoles per square meter per day, while the upper slope soils absorbed slightly more, at 156 micromoles per square meter per day. Crucially, these drier positions maintained their methane-consuming function throughout the year, remaining reliable sinks regardless of season. Because upland forests cover vast areas of the planet, this finding reinforces the significance of well-drained forest soils as one of the biological world’s most important natural defenses against methane accumulation in the atmosphere.

But the soil was only half the investigation. In recent years, scientists have increasingly recognized that trees themselves can act as conduits for methane, drawing dissolved gas up from waterlogged soils through their vascular systems and venting it from their stems, or hosting methane-producing microbes within their own tissues. Whether these stem emissions are significant enough to undermine the methane-removal service provided by upland forest soils has remained one of the field’s most pressing open questions.

Across most of the study site, the answer was reassuring. In the sloped plots, tree stems emitted negligible amounts of methane, and their tiny emissions offset less than one percent of the methane being consumed by the surrounding soils. In other words, the upland rainforest kept its carbon credentials intact: soils continued to scrub methane from the atmosphere, and the trees did little to undo that work. This is an important benchmark for global models, which must decide how much attention to give tree stem fluxes in dry upland settings, and it suggests that in such environments the soil sink comfortably dominates.

The valley floor was another matter. At the end of the wet season, when the saturated soil was already releasing methane in earnest, the trees added substantially to the problem. Stem emissions there contributed an extra 27 percent on top of the soil source, effectively amplifying an already large methane flux. The mechanism is intuitive: when soil air spaces fill with water, methane produced below ground can escape upward more readily through the aerenchyma and transport tissues of trees than through the waterlogged soil itself, making trees the path of least resistance for gas trying to reach the atmosphere.

Among the more striking discoveries were two individual trees that the researchers describe as high emitters. These outliers vented methane at rates two hundred and three hundred times higher than neighboring trees of comparable size on the same plots. Such extreme individual variability has been noted in other forest systems, but documenting it in a subtropical rainforest highlights a persistent challenge for field scientists and modelers alike: a small number of anomalous trees can disproportionately influence plot-level emissions estimates, particularly if they cluster in wetter microsites. Understanding what makes certain trees such efficient methane conduits, whether it is their rooting depth, stem anatomy, associated microbial communities, or proximity to methane-rich soil layers, is now a priority for follow-up work.

Statistically, the study found that both tree stem and soil methane fluxes correlated significantly and positively with soil moisture, confirming the moisture gradient as the master variable governing methane exchange in this ecosystem. This relationship has implications well beyond one Australian forest. As climate change alters rainfall patterns, intensifying both droughts and deluges in many subtropical regions, the moisture status of forest soils will shift accordingly, and with it the delicate balance between methane consumption and production. Periods of unusual wetness could temporarily convert upland landscapes that normally function as methane sinks into net emitters, while prolonged drying could expand the spatial footprint of the sink.

The research also carries lessons for how greenhouse gas inventories should be constructed. Because the valley floor transitioned between sink and source within a single year, any sampling campaign that visits a site only once, in either the wet or the dry season, risks capturing a snapshot that badly misrepresents the annual picture. The authors emphasize the high spatial and temporal heterogeneity of tree and soil methane fluxes in upland forests, a heterogeneity that demands repeated, seasonally distributed measurements across topographic gradients if regional and global methane budgets are to be trustworthy.

For the broader public, the takeaway is both sobering and hopeful. Sobering, because even pristine rainforests are not immune to climate feedbacks: wetter soils and methane-venting trees can tip natural ecosystems into contributing to the very problem they help mitigate. Hopeful, because the study confirms that the drier, extensive portions of subtropical rainforest landscapes remain steadfast methane sinks year-round, quietly removing a powerful greenhouse gas from the air at rates that matter globally. Protecting these forests, and the complex moisture gradients within them, preserves not only biodiversity and carbon storage but also this often-overlooked methane-scrubbing service.

The work, conducted with support from the Australian Research Council, the Hermon Slade Foundation, and other funders, adds a valuable subtropical data point to a global dataset still dominated by temperate and boreal measurements. As the search intensifies for natural systems that help regulate atmospheric methane, this study makes clear that the answer lies in the ground as much as in the canopy, and that the ground’s verdict changes with the weather.

Subject of Research: Seasonal methane fluxes from tree stems and soils along a soil moisture gradient in a subtropical Australian rainforest

Article Title: Seasonal changes in tree stem and soil methane fluxes along a soil moisture gradient in a subtropical Australian rainforest

Article References: Dittmann, J., Maher, D. T., Johnston, S. G., Das, A., Padilla-Montalvo, J. A., Stovall, A. E. L., & Jeffrey, L. C. (2026). Seasonal changes in tree stem and soil methane fluxes along a soil moisture gradient in a subtropical Australian rainforest. Biogeochemistry. https://doi.org/10.1007/s10533-026-01371-7

Image Credits: AI Generated

DOI: 10.1007/s10533-026-01371-7

Keywords: methane flux, rainforest, soil moisture, tree stems, greenhouse gases, methanotrophs, methanogenesis, soil sink, biogeochemistry, subtropical forest, seasonal variation, climate change