Wetland trees are a potential methane sink during dry soil conditions

The contribution of trees to the wetland methane (CH₄) budget remains highly uncertain. The water table level is an essential driver for stem CH₄ emissions, which vary across seasons and soil hydrological conditions. Exceptionally dry conditions are increasingly affecting forests because of climate change, and wetland trees can potentially switch from CH₄ sources to sinks. CH₄ production and oxidation potentially occur in the oxic/anoxic microsites within wetland tree stems, as in soil, balancing the net stem CH₄ emissions to the atmosphere. Yet, the microbial ecology behind these processes is still vastly unexplored, and understanding the role of microbial ecology is essential to predict stem CH₄ emission patterns.

Our study focused on characterising stem CH₄ fluxes using semi-rigid static chambers and assessing CH₄ oxidation and production activities through gas-enriched incubations in two forested wetland ecosystems: a temperate wetland in Flitwick Moor (UK) and tropical peat swamp forests in the Sebangau Forest (Kalimantan, Indonesia), both experiencing lower water table levels than previous years during the same period. Targeted tree species were measured at multiple height intervals and were Alnus glutinosa and Betula pubescens in Flitwick Moor and Shorea balangeran and Xylopia fusca in the Sebangau Forest, the same tree species that were investigated in earlier studies at these sites. DNA analysis from bark, wood, and soil involved two-step PCR and sequencing targeting the 16S rRNA gene, complemented by whole shotgun metagenomics (WGS) to explore the microbial composition and CH₄-cycling microorganisms.

Results from Flitwick Moor and the Sebangau Forest showed significantly reduced stem CH₄ emissions (<50 µg m^-2 hr^-1) compared to earlier studies, with trees adopting an upland-like behaviour, displaying heterogenous fluxes with no clear axial pattern or relation to wood properties, as well as CH₄ uptake. There was evidence of CH₄ oxidation in trees of both ecosystems in the range of 7-47 µg m^-3 hr^-1. The aerobic and facultative anaerobic bacteria population dominated in tree tissues, and the same number of methanotrophic genera were present in soil and trees, suggesting that microbial groups were recruited from the soil. In A. glutinosa tree tissues a significant positive relation existed between the CH₄ oxidising bacteria relative abundance and the oxidation activity. The methanotrophic fraction represented up to 5% of the bacteria in wood, confirming the hypothesis that methanotrophs are ubiquitous in trees of different ecosystems.

CH₄-cycling microorganisms are likely to adapt to a soilborne-CH₄ gradient up the tree stems; the reduced stem CH₄ fluxes in this study resulted from dry soil conditions and potentially from microbial oxidation inside the stem. Conversely, the small proportion of CH₄-cycling microorganisms compared to other microbial groups likely reflected the reduced stem CH₄ fluxes along the soil-tree continuum. The ratio of CH₄-cycling microorganisms might vary across seasons and different hydrological conditions; further long-term studies in forested wetlands will help elucidate the interplay between CH₄-cycling microorganisms and stem CH₄ fluxes and the importance of trees in balancing CH₄ emissions in potentially drier future scenarios.