Does stem wood methanogenic and methanotrophic activity drive spatial patterns in methane emissions of mature European beech?

Trees are known to be methane (CH₄) sources and sinks. However, frequently observed large spatial variability in stem CH₄ fluxes makes the estimation of net forest ecosystem CH₄ exchange difficult. This variability refers not only to large intraspecies variability, but also to variability across vertical stem profiles.

European beech (Fagus sylvatica) is a native and widely grown tree species in upland forests of Central and Southeast Europe. Our previous study detected high spatial variability in stem CH₄ emissions among twenty beech trees, which could not be explained by soil CH₄ turnover.

We aimed to investigate whether the high variability in beech stem CH₄ emissions can be explained by CH₄ production and consumption in the studied trees’ stem wood. In August-September 2024, we measured CH₄ exchange of eleven mature beech individuals (0.4 m above ground) and of adjacent soil in a temperate montane forest of White Carpathians, Czech Republic, using static chamber systems and spectroscopic gas analysis (FTIR technology). By five trees, stem CH₄ fluxes were additionally measured along vertical stem profiles up to 2 m above ground (i.e. in three heights). The measurements were followed by wood core sampling in these profiles for further investigation of potential for CH₄ production through methanogenesis and consumption through methanotrophy using incubation of wood samples.

The stem CH₄ exchange showed large variation, ranging from CH₄ uptake to CH₄ emission (from -14.0 to +279 μg CH₄ m^-2 h^-1), whereas the soil was a net CH₄ sink with less variation (-41.4 ± 3.5 μg CH₄ m^-2 h^-1). Fourteen days of incubation showed CH₄ production in 34% of total tested wood cores. These cores originated from individuals and stem heights with increased CH₄ emissions. The net incubation bottle headspace increase of CH₄ was linear (R² > 0.7) with values of 0.1 ± 0.02 μg CH₄ cm^-3 h^-1. During 25 days of incubation under anoxic conditions with labelled ¹³C-CH₄ in the headspace, the increase in ⁴⁵CO₂/⁴⁴CO₂ ratio was used to monitor oxidation of labelled CH₄. Significant net increase in this ratio was detected in several bottles. Interestingly, wood cores with the highest methanogenesis rates showed also faster increase in ⁴⁵CO₂/⁴⁴CO₂ ratio (p<0.001). This suggests that high CH₄ production rates in these cores positively influence the community of methanotrophs which are either more abundant and/or more active in these cores. Whether methanotrophic community is represented by anaerobic methanotrophic archaea or by aerobic methanotrophs adapted to hypoxic conditions will be assessed by DNA analysis. Statistical analysis will investigate the relationships between CH₄ fluxes, production and consumption to determine the fate of tree-derived CH₄, a significant greenhouse gas.

 

Acknowledgement

This research was supported by the Ministry of Education, Youth and Sports of CR within the programs LU - INTER-EXCELLENCE II [LUC23162] and CzeCOS [LM2023048], and project AdAgriF -Advanced methods of greenhouse gases emission reduction and sequestration in agriculture and forest landscape for climate change mitigation [CZ.02.01.01/00/22_008/0004635]. VT was supported by the Czech Science Foundation (23-07434O).