Methane Exchange and Microbial Functional Potential in Forest Tree Tissues

Methane (CH₄) is a potent greenhouse gas, and microorganisms play a crucial role in its cycling. While soil microbial processes are well studied, the microbial basis of CH₄ production and oxidation within tree tissues remains poorly understood. Trees play an active role in forest CH₄ exchange, yet studies on tree-associated microbial contributions are only beginning to emerge.  In this study, we aimed to assess the abundance of CH₄-cycling genes in shoots (leaves and terminal branches) and wood cores of four tree categories: European beech (Fagus sylvatica), European hornbeam (Carpinus betulus), birch (Betula pendula and Betula pubescens), and Norway spruce (Picea abies) along a transect spanning temperate to subarctic regions. We assessed CH₄ exchange through shoot incubation experiments and measured internal CH₄ concentrations in stem wood. Targeted metagenomic approach was used to analyze the relative abundance of CH₄-cycling genes. Our study revealed that among shoots, birch, spruce and beech showed potential CH₄ emissions, while hornbeam indicated potential CH₄ consumption in the incubation study. Beech had the highest internal stem wood CH₄ concentration, and hornbeam the lowest when compared to the ambient concentration. Metagenomic analysis confirmed the presence of key methanogen and methanotroph genes in both tissues. Soluble CH₄ monooxygenase gene (mmoX) were most abundant in birch shoots and spruce shoots. In addition, CH₄ exchanges showed strong positive correlation with shoot ammonia, whereas CH₄ concentration on stem wood showed strong positive association with particulate CH₄ monooxygenase (pmoA) and methanogen-to-methanotroph gene ratio. These findings provide new insights into tree microbiome and its contribution to CH₄ exchange in forest ecosystem.