5:15 PM - 6:45 PM
[BCG04-P07] Spatiotemporal variability of methane fluxes from tree stems in an upland forest
Keywords:seasonality, CH4 flux, temperate forest, stem emissions
Methane (CH4) is the second most important anthropogenic greenhouse gas after CO2, contributing to 20% of the global warming. Tree methane emissions have been found to notably amplify wetland forest emissions and slightly diminish the carbon sink capacity of upland forests. In upland forests, low emission events can occur, potentially impacting global cycles due to their substantial global footprint.
Initially, tree stems were considered only as straw in which the CH4 produced by methanogenic archaea in the anoxic soil layers was transported to the atmosphere, when the CH4 concentration in the soil is higher than atmospheric concentrations. In this case, stem emissions decrease with height and emission are more significant when there is some methane production in the soil, i.e. when soil water content is high, mainly from late autumn to early spring. However, those relationships with soil parameters were not always found, Machacova et al. (2023) by studying annual variability of methane emissions showed no correlation between soil parameters and CH4 emissions but a correlation with stem CO2 effluxes while Epron et al. (2022) pointed out a relation between air temperature and CH4 emissions. Methanogenesis is not limited to soil, methanogenic archaea could also produce CH4 in tree stems (Covey et al., 2012; Wang et al., 2016). However, the presence of high CH4 concentrations in tree stems does not necessarily result in methane emission, as various factors control CH4 oxidation, diffusion, and transport. Additionally, methane production can also occur on the bark by abiotic photochemical processes that decay cell wall polysaccharides through reactive oxygen species (ROS). Production (methanogenesis) co-occurs with oxidation (methanotrophy) along the soil-stem-atmosphere continuum, defining the net stem flux and determining whether trees act as CH4 sinks or sources.
The aim of this study was to characterise the seasonal variability of CH4 fluxes from tree stems in an upland forest. Our objectives were to determine whether tree stems emit CH4, whether these methane fluxes vary spatially and/or temporally, and to identify the factors involved. To do this, stem emissions at 4 different heights, up to 4 m, were measured manually for 3 common temperate species (Carpinus betulus, Fagus sylvatica and Quercus petraea) in a 60-year-old natural regenerated forest on more than ten different dates from April 2023 to December 2023. To explain the temporal variability of methane fluxes, the data were coupled with meteorological data measured on the site. The origin of the methane production was determined on wood cores incubated in anoxic conditions and compared to methane concentrations measured within tree and to the methane emissions.
Over the sampling period, CH4 fluxes were lower than those reported in previous studies in upland forest and not significantly different from 0 (0.09 ± 0.06 µg CH4 m-2 h -1), displaying notable variability ranging from - 9.1 to 10.1 µg CH4 m-2 h -1. Nevertheless, a temporal variation independently of the species was observed with emissions in early spring and late autumn, while no significant methane production occurred during the summer period. There was an over-emission of methane on the base of the stem in early spring and late autumn when the water table was present, suggesting a soil origin at this height.
The low amount of methane emitted during the season indicates for the sampled trees either low methane production in the stem and/or oxidation of the methane produced before to be emitted. Incubation of wood cores, which is still in progress, should provide an answer to this question.
Initially, tree stems were considered only as straw in which the CH4 produced by methanogenic archaea in the anoxic soil layers was transported to the atmosphere, when the CH4 concentration in the soil is higher than atmospheric concentrations. In this case, stem emissions decrease with height and emission are more significant when there is some methane production in the soil, i.e. when soil water content is high, mainly from late autumn to early spring. However, those relationships with soil parameters were not always found, Machacova et al. (2023) by studying annual variability of methane emissions showed no correlation between soil parameters and CH4 emissions but a correlation with stem CO2 effluxes while Epron et al. (2022) pointed out a relation between air temperature and CH4 emissions. Methanogenesis is not limited to soil, methanogenic archaea could also produce CH4 in tree stems (Covey et al., 2012; Wang et al., 2016). However, the presence of high CH4 concentrations in tree stems does not necessarily result in methane emission, as various factors control CH4 oxidation, diffusion, and transport. Additionally, methane production can also occur on the bark by abiotic photochemical processes that decay cell wall polysaccharides through reactive oxygen species (ROS). Production (methanogenesis) co-occurs with oxidation (methanotrophy) along the soil-stem-atmosphere continuum, defining the net stem flux and determining whether trees act as CH4 sinks or sources.
The aim of this study was to characterise the seasonal variability of CH4 fluxes from tree stems in an upland forest. Our objectives were to determine whether tree stems emit CH4, whether these methane fluxes vary spatially and/or temporally, and to identify the factors involved. To do this, stem emissions at 4 different heights, up to 4 m, were measured manually for 3 common temperate species (Carpinus betulus, Fagus sylvatica and Quercus petraea) in a 60-year-old natural regenerated forest on more than ten different dates from April 2023 to December 2023. To explain the temporal variability of methane fluxes, the data were coupled with meteorological data measured on the site. The origin of the methane production was determined on wood cores incubated in anoxic conditions and compared to methane concentrations measured within tree and to the methane emissions.
Over the sampling period, CH4 fluxes were lower than those reported in previous studies in upland forest and not significantly different from 0 (0.09 ± 0.06 µg CH4 m-2 h -1), displaying notable variability ranging from - 9.1 to 10.1 µg CH4 m-2 h -1. Nevertheless, a temporal variation independently of the species was observed with emissions in early spring and late autumn, while no significant methane production occurred during the summer period. There was an over-emission of methane on the base of the stem in early spring and late autumn when the water table was present, suggesting a soil origin at this height.
The low amount of methane emitted during the season indicates for the sampled trees either low methane production in the stem and/or oxidation of the methane produced before to be emitted. Incubation of wood cores, which is still in progress, should provide an answer to this question.