The contribution of trees to ecosystem methane emissions in a temperate forested wetland

Dry and wet periods determine stem and soil greenhouse gas fluxes in a northern drained peatland forest

Greenhouse gas (GHG) fluxes from peatland soils are relatively well studied, whereas tree stem fluxes have received far less attention. Simultaneous year-long measurements of soil and tree stem GHG fluxes in northern peatland forests are scarce, as previous studies have primarily focused on the growing season. We determined the seasonal dynamics of tree stem and soil CH4, N2O and CO2 fluxes in a hemiboreal drained peatland forest. Gas samples for flux calculations were manually collected from chambers at different heights on Downy Birch (Betula pubescens) and Norway Spruce (Picea abies) trees (November 2020-December 2021) and analysed using gas chromatography. Environmental parameters were measured simultaneously with fluxes and xylem sap flow was recorded during the growing season. Birch stems played a greater role in the annual GHG dynamics than spruce stems. Birch stems were net annual CH4, N2O and CO2 sources, while spruce stems constituted a CH4 and CO2 source but a N2O sink. Soil was a net CO2 and N2O source, but a sink of CH4. Temporal dynamics of stem CH4 and N2O fluxes were driven by isolated emissions' peaks that contributed significantly to net annual fluxes. Stem CO2 efflux followed a seasonal trend coinciding with tree growth phenology. Stem CH4 dynamics were significantly affected by the changes between wetter and drier periods, while N2O was more influenced by short-term changes in soil hydrologic conditions. We showed that CH4 emitted from tree stems during the wetter period can offset nearly half of the soil sink capacity. We presented for the first time the relationship between tree stem GHG fluxes and sap flow in a peatland forest. The net CH4 flux was likely an aggregate of soil-derived and stem-produced CH4. A dominating soil source was more evident for stem N2O fluxes.

How tree traits modulate tree methane fluxes: A review

Trees can play different roles in the regulation of fluxes of methane (CH4), a greenhouse gas with a warming potential 83 times greater than that of carbon dioxide. Forest soils have the greatest potential for methane uptake compared to other land uses. In addition to their influence on soil CH4 fluxes, trees can act directly as a source or sink of CH4, by transporting CH4 produced in the soil and harbouring the key microorganisms involved in CH4 production and consumption (methanogens and methanotrophs). Tree CH4 fluxes can vary between species characterized by different traits that influence transport and modify the availability of CH4 reaction substrates as well as the habitat for methanogens and methanotrophs. Despite their important role in modulating CH4 fluxes from forest ecosystems, the identity and role of tree traits influencing these fluxes are poorly consolidated in the literature. The objectives of this paper are to 1) Review the functional traits of trees associated with their role in the regulation of CH4 emissions; 2) Assess the importance of inter-specific variability in CH4 fluxes via a global analysis of tree methane fluxes in the literature. Our review highlights that differences in CH4 fluxes between tree species and individuals can be explained by a diversity of traits influencing CH4 transport and microbial production of CH4 such as wood density and secondary metabolites. We propose a functional classification for trees based on the key traits associated with a function in CH4 emissions. We identified the fast-growing species with low wood density, species adapted to flood and species vulnerable to rot as functional groups which can be net sources of CH4 in conditions favorable to CH4 production. The global analysis further demonstrated the importance of taxonomy, with other factors such as land type and season in explaining variability in tree CH4 fluxes.

Aerobic methane production in Scots pine shoots is independent of drought or photosynthesis

Shoot-level emissions of aerobically produced methane (CH4) may be an overlooked source of tree-derived CH4, but insufficient understanding of the interactions between their environmental and physiological drivers still prevents the reliable upscaling of canopy CH4 fluxes. We utilised a novel automated chamber system to continuously measure CH4 fluxes from the shoots of Pinus sylvestris (Scots pine) saplings under drought to investigate how canopy CH4 fluxes respond to the drought-induced alterations in their physiological processes and to isolate the shoot-level production of CH4 from soil-derived transport and photosynthesis. We found that aerobic CH4 emissions are not affected by the drought-induced stress, changes in physiological processes, or decrease in photosynthesis. Instead, these emissions vary on short temporal scales with environmental drivers such as temperature, suggesting that they result from abiotic degradation of plant compounds. Our study shows that aerobic CH4 emissions from foliage are distinct from photosynthesis-related processes. Thus, instead of photosynthesis rates, it is more reliable to construct regional and global estimates for the aerobic CH4 emission based on regional differences in foliage biomass and climate, also accounting for short-term variations of weather variables such as air temperature and solar radiation.

Rapid bark-mediated tree stem methane transport occurs independently of the transpiration stream in Melaleuca quinquenervia

Tree stem methane emissions are important components of lowland forest methane budgets. The potential for species-specific behaviour among co-occurring lowland trees with contrasting bark characteristics has not been investigated. We compare bark-mediated methane transport in two common lowland species of contrasting bark characteristics (Melaleuca quinquenervia featuring spongy/layered bark with longitudinally continuous airspaces and Casuarina glauca featuring hard/dense common bark) through several manipulative experiments. First, the progressive cutting through M. quinquenervia bark layers caused exponential increases in methane fluxes (c. 3 orders of magnitude); however, sapwood-only fluxes were lower, suggesting that upward/axial methane transport occurs between bark layers. Second, concentrated methane pulse-injections into exposed M. quinquenervia bark, revealed rapid axial methane transport rates (1.42 mm s-1 ), which were further supported through laboratory-simulated experiments (1.41 mm s-1 ). Laboratory-simulated radial CH4 diffusion rates (through bark) were c. 20-times slower. Finally, girdling M. quinquenervia stems caused a near-instantaneous decrease in methane flux immediately above the cut. By contrast, girdling C. glauca displayed persistent, though diminished, methane fluxes. Overall, the experiments revealed evidence for rapid 'between-bark' methane transport independent from the transpiration stream in M. quinquenervia, which facilitates diffusive axial transport from the rhizosphere and/or sapwood sources. This contrasts with the slower, radial 'through-bark' diffusive-dominated gas transportation in C. glauca.

Methane and nitrous oxide emissions and related microbial communities from mangrove stems on Qi'ao Island, Pearl River Estuary in China

Mangrove forests, crucial carbon-rich ecosystems, are increasingly vulnerable to soil carbon loss and greenhouse gas (GHG) emissions due to human disturbance. However, the contribution of mangrove trees to GHG emissions remains poorly understood. This study monitored CO2, CH4, and N2O fluxes from the stems of two mangrove species, native Kandelia obovata (KO) and exotic Sonneratia apetala (SA), at three heights (0.7 m, 1.2 m, and 1.7 m) during the dry winter period on Qi'ao Island, Pearl River Estuary, China. Heartwood samples were analyzed to identify potential functional groups related to gas fluxes. Our study found that tree stems acted as both sinks and sources for N2O (ranging from -9.49 to 28.35 μg m-2 h-1 for KO and from -6.73 to 28.95 μg m-2 h-1 for SA) and CH4. SA exhibited significantly higher stem CH4 flux (from -26.67 to 97.33 μg m-2 h-1) compared to KO (from -44.13 to 88.0 μg m-2 h-1) (P < 0.05). When upscaled to the community level, both species were net emitters of CH4, contributing approximately 4.68 % (KO) and 0.51 % (SA) to total CH4 emissions. The decrease in stem CH4 flux with increasing height, indicates a soil source. Microbial analysis in the heartwood using the KEGG database indicated aceticlastic methanogenesis as the dominant CH4 pathway. The presence of methanogens, methanotrophs, denitrifiers, and nitrifiers suggests microbial involvement in CH4 and N2O production and consumption. Remarkably, the dominance of Cyanobacteria in the heartwood microbiome (with the relative abundance of 97.5 ± 0.6 % for KO and 99.1 ± 0.2 % for SA) implies roles in carbon and nitrogen fixation for mangroves coping with nitrogen limitation in coastal wetlands, and possibly in CH4 production. Although the present study has limitations in sampling duration and area, it highlights the significant role of tree stems in GHG emissions which is crucial for a holistic evaluation of the global carbon sequestration capability of mangrove ecosystems. Future research should broaden spatial and temporal scales to enhance the accuracy of upscaling tree stem gas fluxes to the mangrove ecosystem level.

Plant-mediated CH4 exchange in wetlands: A review of mechanisms and measurement methods with implications for modelling

Plant-mediated CH4 transport (PMT) is the dominant pathway through which soil-produced CH4 can escape into the atmosphere and thus plays an important role in controlling ecosystem CH4 emission. PMT is affected by abiotic and biotic factors simultaneously, and the effects of biotic factors, such as the dominant plant species and their traits, can override the effects of abiotic factors. Increasing evidence shows that plant-mediated CH4 fluxes include not only PMT, but also within-plant CH4 production and oxidation due to the detection of methanogens and methanotrophs attached to the shoots. Despite the inter-species and seasonal differences, and the probable contribution of within-plant microbes to total plant-mediated CH4 exchange (PME), current process-based ecosystem models only estimate PMT based on the bulk biomass or leaf area index of aerenchymatous plants. We highlight five knowledge gaps to which more research efforts should be devoted. First, large between-species variation, even within the same family, complicates general estimation of PMT, and calls for further work on the key dominant species in different types of wetlands. Second, the interface (rhizosphere-root, root-shoot, or leaf-atmosphere) and plant traits controlling PMT remain poorly documented, but would be required for generalizations from species to relevant functional groups. Third, the main environmental controls of PMT across species remain uncertain. Fourth, the role of within-plant CH4 production and oxidation is poorly quantified. Fifth, the simplistic description of PMT in current process models results in uncertainty and potentially high errors in predictions of the ecosystem CH4 flux. Our review suggest that flux measurements should be conducted over multiple growing seasons and be paired with trait assessment and microbial analysis, and that trait-based models should be developed. Only then we are capable to accurately estimate plant-mediated CH4 emissions, and eventually ecosystem total CH4 emissions at both regional and global scales.

The hidden roots of wetland methane emissions

Wetlands are the largest natural source of methane (CH4 ) globally. Climate and land use change are expected to alter CH4 emissions but current and future wetland CH4 budgets remain uncertain. One important predictor of wetland CH4 flux, plants, play an important role in providing substrates for CH4 -producing microbes, increasing CH4 consumption by oxygenating the rhizosphere, and transporting CH4 from soils to the atmosphere. Yet, there remain various mechanistic knowledge gaps regarding the extent to which plant root systems and their traits influence wetland CH4 emissions. Here, we present a novel conceptual framework of the relationships between a range of root traits and CH4 processes in wetlands. Based on a literature review, we propose four main CH4 -relevant categories of root function: gas transport, carbon substrate provision, physicochemical influences and root system architecture. Within these categories, we discuss how individual root traits influence CH4 production, consumption, and transport (PCT). Our findings reveal knowledge gaps concerning trait functions in physicochemical influences, and the role of mycorrhizae and temporal root dynamics in PCT. We also identify priority research needs such as integrating trait measurements from different root function categories, measuring root-CH4 linkages along environmental gradients, and following standardized root ecology protocols and vocabularies. Thus, our conceptual framework identifies relevant belowground plant traits that will help improve wetland CH4 predictions and reduce uncertainties in current and future wetland CH4 budgets.

Effects of birch encroachment, water table and vegetation on methane emissions from peatland microforms in a rewetted bog

This study investigated the influence of vegetation and microforms on methane (CH4) balances of a rewetted bog in north-west Germany. The two study sites are in close proximity on the same former peat extraction area, one dominated by Sphagnum-mosses and the other one by a dense Betula pubescens stand with a high Eriophorum vaginatum cover. The contribution of microforms (hummocks/hollows) to CH4 emissions and the effect of Betula encroachment has been studied. Transparent and opaque chambers were used to measure CH4 fluxes every 3-4 weeks during daytime for one year. For the estimation of annual balances, three methods were compared and the method using water level and soil temperature as explanatory variables was selected. Fluxes were scaled to the site level. The annual emissions per site are and 7.1 ± 1.5 g CH4-C m-2 year-1 at the treed site and 36.1 ± 3.5 g CH4-C m-2 year-1 at the open site, mainly controlled by higher water levels. Highest annual emissions originated from hollows at the open site, but in the vegetation period, hummock emissions tend to be higher. At the tree site, emission differences between the microforms were less pronounced. There were no differences between fluxes from transparent and opaque chambers.

Model of methane transport in tree stems: Case study of sap flow and radial diffusion

The transport processes of methane (CH4 ) in tree stems remain largely unknown, although they are critical in assessing the whole-forest CH4 dynamics. We used a physically based dynamic model to study the spatial and diurnal dynamics of stem CH4 transport and fluxes. We parameterised the model using data from laboratory experiments with Pinus sylvestris and Betula pendula and compared the model to experimental data from a field study. Stem CH4 flux in laboratory and field conditions were explained by the axial advective CH4 transport from soil with xylem sap flow and the radial CH4 diffusion through the stem conditions. Diffusion resistance caused by the bark permeability did not significantly affect gas transport or stem CH4 flux in the laboratory experiments. The role of axial diffusion of CH4 in trees was unresolved and requires further studies. Due to the transit time of CH4 in the stem, the diurnal dynamics of stem CH4 fluxes can deviate markedly from the diurnal dynamics of sap flow.

Plant phenology and species-specific traits control plant CH4 emissions in a northern boreal fen

Aerenchymatic transport is an important mechanism through which plants affect methane (CH₄ ) emissions from peatlands. Controlling environmental factors and the effects of plant phenology remain, however, uncertain. We identified factors controlling seasonal CH₄ flux rate and investigated transport efficiency (flux rate per unit of rhizospheric porewater CH₄ concentration). We measured CH₄ fluxes through individual shoots of Carex rostrata, Menyanthes trifoliata, Betula nana and Salix lapponum throughout growing seasons in 2020 and 2021 and Equisetum fluviatile and Comarum palustre in high summer 2021 along with water-table level, peat temperature and porewater CH₄ concentration. CH₄ flux rate of C. rostrata was related to plant phenology and peat temperature. Flux rates of M. trifoliata and shrubs B. nana and S. lapponum were insensitive to the investigated environmental variables. In high summer, flux rate and efficiency were highest for C. rostrata (6.86 mg m^-2  h^-1 and 0.36 mg m^-2  h^-1 (μmol l^-1 )^-1 , respectively). Menyanthes trifoliata showed a high flux rate, but limited efficiency. Low flux rates and efficiency were detected for the remaining species. Knowledge of the species-specific CH₄ flux rate and their different responses to plant phenology and environmental factors can significantly improve the estimation of ecosystem-scale CH₄ dynamics in boreal peatlands.

Methane emission from stems of European beech (Fagus sylvatica) offsets as much as half of methane oxidation in soil

Trees are known to be atmospheric methane (CH₄ ) emitters. Little is known about seasonal dynamics of tree CH₄ fluxes and relationships to environmental conditions. That prevents the correct estimation of net annual tree and forest CH₄ exchange. We aimed to explore the contribution of stem emissions to forest CH₄ exchange. We determined seasonal CH₄ fluxes of mature European beech (Fagus sylvatica) stems and adjacent soil in a typical temperate beech forest of the White Carpathians with high spatial heterogeneity in soil moisture. The beech stems were net annual CH₄ sources, whereas the soil was a net CH₄ sink. High CH₄ emitters showed clear seasonality in their stem CH₄ emissions that followed stem CO₂ efflux. Elevated CH₄ fluxes were detected during the vegetation season. Observed high spatial variability in stem CH₄ emissions was neither explicably by soil CH₄ exchange nor by CH₄ concentrations, water content, or temperature studied in soil profiles near each measured tree. The stem CH₄ emissions offset the soil CH₄ uptake by up to 46.5% and on average by 13% on stand level. In Central Europe, widely grown beech contributes markedly to seasonal dynamics of ecosystem CH₄ exchange. Its contribution should be included into forest greenhouse gas flux inventories.

Soil CO2 and CH4 fluxes from different forest types in tropical peat swamp forest

Information on temporal and spatial variations in soil greenhouse gas (GHG) fluxes from tropical peat forests is essential to predict the influence of climate change and estimate the effects of land use on global warming and the carbon (C) cycle. To obtain such basic information, soil carbon dioxide (CO2) and methane (CH4) fluxes, together with soil physicochemical properties and environmental variables, were measured at three major forest types in the Maludam National Park, Sarawak, Malaysia, for eight years, and their relationships were analyzed. Annual soil CO2 fluxes ranged from 860 to 1450 g C m⁻2 yr⁻1 without overall significant differences between the three forest sites, while soil CH4 fluxes, 1.2-10.8 g C m⁻2 yr⁻1, differed. Differences in GHG fluxes between dry and rainy seasons were not necessarily significant, corresponding to the extent of seasonal variation in groundwater level (GWL). The lack of significant differences in soil CO2 fluxes between the three sites could be attributed to set-off between the negative and positive effects of the decomposability of soil organic matter as estimated by pyrophosphate solubility index (PSI) and GWL. The impact of El-Niño on annual CO2 flux also varied between the sites. The variation in soil CH4 fluxes from the three sites was enhanced by variations in temperature, GWL, PSI, and soil iron (Fe) content. A positive correlation was observed between the annual CH4 flux and GWL at only one site, and the influence of soil properties was more pronounced at the site with the lowest GWL and the highest PSI. Variation in annual CH4 fluxes was controlled more strongly by temperature where GWL was the highest and GWL and plant growth fluctuations were the least. Inter-annual variations in soil CO2 and CH4 fluxes confirmed the importance of long-term monitoring of these at multiple sites supporting different forest types.

Spatiotemporal patterns and drivers of stem methane flux from two poplar forests with different soil textures

In forest ecosystems, the majority of methane (CH4) research focuses on soils, whereas tree stem CH4 flux and driving factors remain poorly understood. We measured the in situ stem CH4 flux using the static chamber-gas chromatography method at different heights in two poplar (Populus spp.) forests with separate soil textures. We evaluated the relationship between stem CH4 fluxes and environmental factors with linear mixed models and estimated the tree CH4 emission rate at the stand level. Our results showed that poplar stems were a net source of atmospheric CH4. The mean stem CH4 emission rates were 97.51 ± 6.21 μg·m-2·h-1 in Sihong and 67.04 ± 5.64 μg·m-2·h-1 in Dongtai. The stem CH4 emission rate in Sihong with clay loam soils was significantly higher (P < 0.001) than that in Dongtai with sandy loam soils. The stem CH4 emission rate also showed a seasonal variation, minimum in winter and maximum in summer. The stem CH4 emission rate generally decreased with increasing sampling height. Although the differences in CH4 emission rates between stem heights were significant in the annual averages, these differences were driven by differences observed in the summer. Stem CH4 emission rates were significantly and positively correlated with air temperature (P < 0.001), relative humidity (P < 0.001), soil water content (P < 0.001) and soil CH4 flux (P < 0.001). At these sites, the soil emitted CH4 to the atmosphere in summer (mainly from June to September) but absorbed CH4 from the atmosphere during the other season. At the stand level, tree CH4 emissions accounted for 2-35.4% of soil CH4 uptake. Overall, tree stem CH4 efflux could be an important component of the forest CH4 budget. Therefore, it is necessary to conduct more in situ monitoring of stem CH4 flux to accurately estimate the CH4 budget in the future.

Methane emissions from trees planted on a closed landfill site

Trees have morphological adaptations that allow methane (CH₄) generated below ground to bypass oxidation in aerobic surface soils. This natural phenomenon however has not been measured in a landfill context where planted trees may alter the composition and magnitude of CH₄ fluxes from the surface. To address this research gap, we measured tree stem and soil greenhouse gas (GHG) emissions (CH₄ and CO₂) from a closed UK landfill and comparable natural site, using an off-axis integrated cavity output spectroscopy analyser and flux chambers. Analyses showed average CH₄ stem fluxes from the landfill and non-landfill sites were 31.8 ± 24.4 µg m^-2 h^-1 and -0.3 ± 0.2 µg m^-2 h^-1, respectively. The landfill site showed seasonal patterns in CH₄ and CO₂ stem emissions, but no significant patterns were observed in CH₄ and CO₂ fluxes at different stem heights or between tree species. Tree stem emissions accounted for 39% of the total CH₄ surface flux (7% of the CO₂); a previously unknown contribution that should be included in future carbon assessments.

Methane emissions from forested closed landfill sites: Variations between tree species and landfill management practices

Trees in natural and managed environments can act as conduits for the transportation of methane (CH₄) from below ground to the atmosphere, bypassing oxidation in aerobic surface soils. Tree stem emissions from landfill sites exhibit large temporal and spatial variability in temperate environments and can account for approximately 40% of the total surface CH₄ flux. Emission variability was further investigated in this study by measuring CH₄ and CO₂ fluxes from landfill sites with different management strategies and varying tree species over a 7-month period. Stem and soil measurements were obtained using flux chambers and an off-axis integrated cavity output spectroscopy analyser. Analysis showed average stem and soil CH₄ emissions varied significantly (p < 0.01) between landfills with different management practices. On average, tree stem CH₄ fluxes from sites with no clay cap but gas extraction, clay cap and gas extraction, and no clay cap and no gas extraction were 1.4 ± 0.4 μg m^-2 h^-1, 47.2 ± 19.0 μg m^-2 h^-1, and 111.9 ± 165.1 μg m^-2 h^-1, respectively. There was no difference in stem CH₄ fluxes between species at each site, suggesting environmental conditions (waterlogging) and site age had a greater influence on both stem and soil fluxes. These results highlight the importance of management practices, and the resultant environmental conditions, in determining CH₄ emissions from historic landfill sites.

Insights into the mechanism of diurnal variations in methane emission from the stem surfaces of Alnus japonica

Recent studies have suggested that in certain environments, tree stems emit methane (CH₄ ). This study explored the mechanism of CH₄ emission from the stem surfaces of Alnus japonica in a riparian wetland. Stem CH₄ emission rates and sap flux were monitored year-round, and fine-root anatomy was investigated. CH₄ emission rates were estimated using a closed-chamber method. Sap flux was measured using Granier-type thermal dissipation probes. Root anatomy was studied using both optical and cryo-scanning electron microscopy. CH₄ emissions during the leafy season exhibited a diurnally changing component superimposed upon an underlying continuum in which the diurnal variation was in phase with sap flux. We propose a model in which stem CH₄ emission involves at least two processes: a sap flux-dependent component responsible for the diurnal changes, and a sap flux-independent component responsible for the background continuum. The contribution ratios of the two processes are season-dependent. The background continuum possibly resulted from the diffusive transport of gaseous CH₄ from the roots to the upper trunk. Root anatomy analysis indicated that the intercellular space of the cortex and empty xylem cells in fine roots could serve as a passageway for transport of gaseous CH₄ .

Solar radiation drives methane emissions from the shoots of Scots pine

Plants are recognized as sources of aerobically produced methane (CH₄ ), but the seasonality, environmental drivers and significance of CH₄ emissions from the canopies of evergreen boreal trees remain poorly understood. We measured the CH₄ fluxes from the shoots of Pinus sylvestris (Scots pine) and Picea abies (Norway spruce) saplings in a static, non-steady-state chamber setup to investigate if the shoots of boreal conifers are a source of CH₄ during spring. We found that the shoots of Scots pine emitted CH₄ and these emissions correlated with the photosynthetically active radiation. For Norway spruce, the evidence for CH₄ emissions from the shoots was inconclusive. Our study shows that the canopies of evergreen boreal trees are a potential source of CH₄ in the spring and that these emissions are driven by a temperature-by-light interaction effect of solar radiation either directly or indirectly through its effects on tree physiological processes.

Long-term dynamics of soil, tree stem and ecosystem methane fluxes in a riparian forest

The carbon (C) budgets of riparian forests are sensitive to climatic variability. Therefore, riparian forests are hot spots of C cycling in landscapes. Only a limited number of studies on continuous measurements of methane (CH₄) fluxes from riparian forests is available. Here, we report continuous high-frequency soil and ecosystem (eddy-covariance; EC) measurements of CH₄ fluxes with a quantum cascade laser absorption spectrometer for a 2.5-year period and measurements of CH₄ fluxes from tree stems using manual chambers for a 1.5 year period from a temperate riparian Alnus incana forest. The results demonstrate that the riparian forest is a minor net annual sink of CH₄ consuming 0.24 kg CH₄-C ha^-1 y^-1. Soil water content is the most important determinant of soil, stem, and EC fluxes, followed by soil temperature. There were significant differences in CH₄ fluxes between the wet and dry periods. During the wet period, 83% of CH₄ was emitted from the tree stems while the ecosystem-level emission was equal to the sum of soil and stem emissions. During the dry period, CH₄ was substantially consumed in the soil whereas stem emissions were very low. A significant difference between the EC fluxes and the sum of soil and stem fluxes during the dry period is most likely caused by emission from the canopy whereas at the ecosystem level the forest was a clear CH₄ sink. Our results together with past measurements of CH₄ fluxes in other riparian forests suggest that temperate riparian forests can be long-term CH₄ sinks.

Non-flooded riparian Amazon trees are a regionally significant methane source

Inundation-adapted trees were recently established as the dominant egress pathway for soil-produced methane (CH₄) in forested wetlands. This raises the possibility that CH₄ produced deep within the soil column can vent to the atmosphere via tree roots even when the water table (WT) is below the surface. If correct, this would challenge modelling efforts where inundation often defines the spatial extent of ecosystem CH₄ production and emission. Here, we examine CH₄ exchange on tree, soil and aquatic surfaces in forest experiencing a dynamic WT at three floodplain locations spanning the Amazon basin at four hydrologically distinct times from April 2017 to January 2018. Tree stem emissions were orders of magnitude larger than from soil or aquatic surface emissions and exhibited a strong relationship to WT depth below the surface (less than 0). We estimate that Amazon riparian floodplain margins with a WT < 0 contribute 2.2-3.6 Tg CH₄ yr^-1 to the atmosphere in addition to inundated tree emissions of approximately 12.7-21.1 Tg CH₄ yr^-1. Applying our approach to all tropical wetland broad-leaf trees yields an estimated non-flooded floodplain tree flux of 6.4 Tg CH₄ yr^-1 which, at 17% of the flooded tropical tree flux of approximately 37.1 Tg CH₄ yr^-1, demonstrates the importance of these ecosystems in extending the effective CH₄ emitting area beyond flooded lands. This article is part of a discussion meeting issue 'Rising methane: is warming feeding warming? (part 2)'.

Source or sink? A study on the methane flux from mangroves stems in Zhangjiang estuary, southeast coast of China

Mangrove ecosystems are an important component of "blue carbon". However, it is not clear whether the stems play roles in the CH₄ budget of mangrove ecosystems. This study investigated the CH₄ emission from mangrove stems and its potential driving factors. We set up six sample plots in the Zhangjiang Estuary National Mangrove Nature Reserve, where Kandelia obovata, Avicennia marina and Aegiceras corniculata are the main mangrove tree species. Soil properties such as total carbon content, redox potential and salinity were determined in each plot. The dynamic chamber method was used to measure mangrove stems and soil CH₄ fluxes. Combined field survey results with Principal Component Analysis (PCA) of soil properties, we divided the six plots into two sites (S1 and S2) to perform statistical analyses of stem CH₄ fluxes. Then the CH₄ fluxes from mangrove tree stems and soil were further scaled up to the ecosystem level through the mapping model. Under different backgrounds of soil properties, salinity and microbial biomass carbon were the main factors modified soil CH₄ fluxes in the two sites, and further affected the stem CH₄ fluxes of mangroves. The soil of both sites are sources of CH₄, and the soil CH₄ emission of S2 was about twice higher than that of S1. Results of upscaling model showed that mangrove stems in S1 were CH₄ sinks with -105.65 g d^-1. But stems in S2 were CH₄ sources around 1448.24 g d^-1. Taken together, our results suggested that CH₄ emission from mangrove soils closely depends on soils properties. And mangrove stems were found to act as both CH₄ sources and CH₄ sinks depend on soil CH₄ production. Therefore, when calculating the CH₄ budget of the mangrove ecosystem, the contribution of mangrove plant stems cannot be ignored.

Diurnal Tree Stem CH4 and N2O Flux Dynamics from a Riparian Alder Forest

Tree stems play an important role in forest methane (CH4) and nitrous oxide (N2O) flux dynamics. Our paper aimed to determine the unknown diurnal variability of CH4 and N2O exchange in grey alder tree stems. The gas fluxes in tree stems and adjacent soil were measured using manual static and dynamic chamber systems with gas chromatographic and laser-spectroscopic analysis, respectively. The alder trees were predominant emitters of CH4 and N2O; however, N2O emission from stems was negligible. The soil mainly emitted N2O into the atmosphere and was both a source and sink of CH4, depending on environmental conditions. Neither the tree stems nor the riparian forest soil showed significant differences in their CH4 and N2O fluxes between the daytime and nighttime, independently of the exchange rates. In contrast to several previous studies revealing a diurnal variability of greenhouse gas fluxes from tree stems, our investigation did not show any clear daytime–nighttime differences. On the other hand, we found quite clear seasonal dynamics initiated by changing environmental conditions, such as temperature and soil water conditions and tree physiological activity. Our results imply a transport role of tree stems for soil-produced CH4 and N2O rather than the production of these gases in tree tissues, even though this cannot be excluded.

Short-lived peaks of stem methane emissions from mature black alder (Alnus glutinosa (L.) Gaertn.) - Irrelevant for ecosystem methane budgets?

Tree stems can be a source of the greenhouse gas methane (CH4). However, assessments of the global importance of stem CH4 emissions are complicated by a lack of research and high variability between individual ecosystems. Here, we determined the contribution of emissions from stems of mature black alder (Alnus glutinosa (L.) Gaertn.) to overall CH4 exchange in two temperate peatlands. We measured emissions from stems and soils using closed chambers in a drained and an undrained alder forest over 2 years. Furthermore, we studied the importance of alder leaves as substrate for methanogenesis in an incubation experiment. Stem CH4 emissions were short-lived and occurred only during times of inundation at the undrained site. The drained site did not show stem emissions and the soil acted as a small CH4 sink. The contribution of stem emissions to the overall CH4 budget was below 0.3% in both sites. Our results show that mature black alder can be an intermittent source of CH4 to the atmosphere. However, the low share of stem CH4 emissions in both investigated stands indicates that this pathway may be of minor relative importance in temperate peatlands, yet strongly depend on the hydrologic regime.

Decomposition of black pine (Pinus nigra J. F. Arnold) deadwood and its impact on forest soil components

Deadwood decomposition is a complex and dynamic process with large implications for biogeochemical cycling of carbon (C) and nitrogen (N) in forest soil and litter. Moreover, it affects functional and structural diversity of fungal and bacterial communities in these components. Mesocosms with deadwood blocks at progressive decay classes were set in a black pine forest and incubated for 28 months in the field with the aim to assess the impact of deadwood decomposition on i) CO₂, CH₄ and N₂O fluxes; ii) C and N pools and allocation among deadwood, litter and soil; iii) the fungal and bacterial structural diversity and activity. CO₂, CH₄ and N₂O fluxes from deadwood were monitored throughout the field incubation; deadwood biomass loss and decay rate for each decay class were calculated. The stock of C and N, enzyme activities, fungal and bacterial communities in deadwood, litter fractions (fresh, fragmented and humified) and soil at two depths were measured. Emissions of CO₂ and CH₄ increased over the deadwood decomposition advancement and the decay reached the maximum rates in the last decomposition classes. N₂O fluxes were low and showed either production (prevalent in the first year) or consumption. Independent of the decay class, 20% of C stored in deadwood was lost as CO₂ in the atmosphere, whereas 32% was transferred to the fragmented and humified litter fractions in the last decay class. A corresponding increase of cellulose and hemicellulose degrading enzymes was found in deadwood, also favored by substrates accessibility through fragmentation and successional changes in fungal and bacterial communities. Deadwood, litter fractions and soil components were clearly distinguished in terms of chemical and microbiological properties and activities. Fragmented and humified litter fractions were the only components responsive to the advanced stage of deadwood decomposition, being directly affected by the physical redistribution of fragmented organic matter.

Multiple processes contribute to methane emission in a riparian cottonwood forest ecosystem

Methane emission from trees may partially or completely offset the methane sink in upland soils, the only process that has been regularly included in methane budgets for forest ecosystems. Our objective was to analyze multiple biogeochemical processes that influence the production, oxidation and transport of methane in a riparian cottonwood ecosystem and its adjacent river. We combined chamber flux measurements on tree stems, forest soil and the river surface with eddy covariance measurements of methane net ecosystem exchange. In addition, we tested whether methanogens were present in cottonwood stems, shallow soil layers and alluvial groundwater. Average midday peak in net methane emission measured by eddy covariance was c. 12 nmol m^-2  s^-1 . The average uptake of methane by soils (0.87 nmol m^-2  s^-1 ) was largely offset by tree stem methane emission (0.75 nmol m^-2  s^-1 ). There was evidence of methanogens in tree stems but not in shallow soil. Growing season (May-September) cumulative net methane emission (17.4 mmol CH₄  m^-2 ) included methane produced in cottonwood stems and methane input to the nocturnal boundary layer from the forest and the adjacent river. The multiple processes contributing to methane emission illustrated the linked nature of these adjacent terrestrial and aquatic ecosystems.

Larix decidua and additional light affect the methane balance of forest soil and the abundance of methanogenic and methanotrophic microorganisms

Due to the activity of methane-oxidizing bacteria, forest soils are usually net sinks for the greenhouse gas methane (CH4). Despite several hints that CH4 balances might be influenced by vegetation, there are only few investigations dealing with this connection. Therefore, we studied this soil-plant-microbe interaction by using mesocosm experiments with forest soil and Larix decidua, a common coniferous tree species within the Alps. Gas measurements showed that the presence of L. decidua significantly reduced CH4 oxidation of the forest soil by ∼10% (-0.95 µmol m-2 h-1 for soil vs -0.85 µmol m-2 h-1 for soil plus L. decidua) leading to an increased net CH4 balance. Increased light intensity was used to intensify the influence of the plant on the soil's CH4 balance. The increase in light intensity strengthened the effect of the plant and led to a greater reduction of CH4 oxidation. Besides, we examined the impact of L. decidua and light on the abundance of methanogens and methanotrophs in the rhizosphere as compared with bulk soil. The abundance of both methane-oxidizing bacteria and methanogenic archaea was significantly increased in the rhizosphere compared with bulk soil but no significant response of methanogens and methanotrophs upon light exposure was established.

Impact of forest plantation on methane emissions from tropical peatland

Tropical peatlands are a known source of methane (CH4 ) to the atmosphere, but their contribution to atmospheric CH4 is poorly constrained. Since the 1980s, extensive areas of the peatlands in Southeast Asia have experienced land-cover change to smallholder agriculture and forest plantations. This land-cover change generally involves lowering of groundwater level (GWL), as well as modification of vegetation type, both of which potentially influence CH4 emissions. We measured CH4 exchanges at the landscape scale using eddy covariance towers over two land-cover types in tropical peatland in Sumatra, Indonesia: (a) a natural forest and (b) an Acacia crassicarpa plantation. Annual CH4 exchanges over the natural forest (9.1 ± 0.9 g CH4  m-2  year-1 ) were around twice as high as those of the Acacia plantation (4.7 ± 1.5 g CH4  m-2  year-1 ). Results highlight that tropical peatlands are significant CH4 sources, and probably have a greater impact on global atmospheric CH4 concentrations than previously thought. Observations showed a clear diurnal variation in CH4 exchange over the natural forest where the GWL was higher than 40 cm below the ground surface. The diurnal variation in CH4 exchanges was strongly correlated with associated changes in the canopy conductance to water vapor, photosynthetic photon flux density, vapor pressure deficit, and air temperature. The absence of a comparable diurnal pattern in CH4 exchange over the Acacia plantation may be the result of the GWL being consistently below the root zone. Our results, which are among the first eddy covariance CH4 exchange data reported for any tropical peatland, should help to reduce the uncertainty in the estimation of CH4 emissions from a globally important ecosystem, provide a more complete estimate of the impact of land-cover change on tropical peat, and develop science-based peatland management practices that help to minimize greenhouse gas emissions.

Short-term flooding increases CH4 and N2O emissions from trees in a riparian forest soil-stem continuum

One of the characteristics of global climate change is the increase in extreme climate events, e.g., droughts and floods. Forest adaptation strategies to extreme climate events are the key to predict ecosystem responses to global change. Severe floods alter the hydrological regime of an ecosystem which influences biochemical processes that control greenhouse gas fluxes. We conducted a flooding experiment in a mature grey alder (Alnus incana (L.) Moench) forest to understand flux dynamics in the soil-tree-atmosphere continuum related to ecosystem N₂O and CH₄ turn-over. The gas exchange was determined at adjacent soil-tree-pairs: stem fluxes were measured in vertical profiles using manual static chambers and gas chromatography; soil fluxes were measured with automated chambers connected to a gas analyser. The tree stems and soil surface were net sources of N₂O and CH₄ during the flooding. Contrary to N₂O, the increase in CH₄ fluxes delayed in response to flooding. Stem N₂O fluxes were lower although stem CH₄ emissions were significantly higher than from soil after the flooding. Stem fluxes decreased with stem height. Our flooding experiment indicated soil water and nitrogen content as the main controlling factors of stem and soil N₂O fluxes. The stems contributed up to 88% of CH₄ emissions to the stem-soil continuum during the investigated period but soil N₂O fluxes dominated (up to 16 times the stem fluxes) during all periods. Conclusively, stem fluxes of CH₄ and N₂O are essential elements in forest carbon and nitrogen cycles and must be included in relevant models.

Methane emissions from tree stems in neotropical peatlands

Neotropical peatlands emit large amounts of methane (CH₄ ) from the soil surface, but fluxes from tree stems in these ecosystems are unknown. In this study we investigated CH₄ emissions from five tree species in two forest types common to neotropical lowland peatlands in Panama. Methane from tree stems accounted for up to 30% of net ecosystem CH₄ emissions. Peak CH₄ fluxes were greater during the wet season when the water table was high and temperatures were lower. Emissions were greatest from the hardwood tree Campnosperma panamensis, but most species acted as emitters, with emissions declining exponentially with height along the stem for all species. Overall, species identity, stem diameter, water level, soil temperature and soil CH₄ fluxes explained 54% of the variance in stem CH₄ emissions from individual trees. On the landscape level, On the landscape level, the high emissions from C. panamensis forests resulted in that they emitted at 340 kg CH₄  d^-1 during flooded periods despite their substantially lower areal cover. We conclude that emission from tree stems is an important emission pathway for CH₄ flux from Neotropical peatlands, and that these emissions vary strongly with season and forest type.

Comparison of methane emissions among invasive and native mangrove species in Dongzhaigang, Hainan Island

The strength of methane (CH₄) source of mangroves is not well understood, especially when including all CH₄ pathways in consideration. This study measured CH₄ fluxes by five pathways (sediments, pneumatophores, water surface, leaves, and stems) from four typical mangrove forests, including Kandelia candel without pneumatophores and three species with pneumatophores: Sonneratia apetala, Laguncularia racemosa and Bruguiera gymnorhiza-Bruguiera sexangula. The CH₄ fluxes from sediments were 4.82±1.46mgCH₄m^-2h^-1 for K. candel and 1.36±0.17mgCH₄m^-2h^-1 for the other three with pneumatophores. Among the three communities with pneumatophores, S. apetala community had significantly greater emission rate than the other two (P<0.05). Pneumatophores in S. apetala were found to significantly decrease CH₄ emission from sediments (P<0.01), while those in B. gymnorhiza-B. sexangula were significantly increase it (P<0.05). CH₄ fluxes from waters were 3.48±1.11mgCH₄m^-2h^-1, with the highest emission rate in the K. candel community for the duck farming. Leaves of mangroves except for those of K. candel were a weak CH₄ daytime sink, but stems were a weak source. The total 72ha of mangroves in the Changning river basin emitted about 8.10Gg CH₄ per year, with a weighted emission rate of about 1.29mgCH₄m^-2h^-1. Our results suggested that mangroves are only a small methane source to atmosphere with great contribution from sediments and waters, only slight contribution from leaves and stems. Pneumatophores of different mangrove species played different roles in CH₄ fluxes from sediments.

Are methane emissions from mangrove stems a cryptic carbon loss pathway? Insights from a catastrophic forest mortality

Growing evidence indicates that tree-stem methane (CH₄ ) emissions may be an important and unaccounted-for component of local, regional and global carbon (C) budgets. Studies to date have focused on upland and freshwater swamp-forests; however, no data on tree-stem fluxes from estuarine species currently exist. Here we provide the first-ever mangrove tree-stem CH₄ flux measurements from  >50 trees (n = 230 measurements), in both standing dead and living forest, from a region suffering a recent large-scale climate-driven dieback event (Gulf of Carpentaria, Australia). Average CH₄ emissions from standing dead mangrove tree-stems was 249.2 ± 41.0 μmol m^-2  d^-1 and was eight-fold higher than from living mangrove tree-stems (37.5 ± 5.8 μmol m^-2  d^-1 ). The average CH₄ flux from tree-stem bases (c. 10 cm aboveground) was 1071.1 ± 210.4 and 96.8 ± 27.7 μmol m^-2  d^-1 from dead and living stands respectively. Sediment CH₄ fluxes and redox potentials did not differ significantly between living and dead stands. Our results suggest both dead and living tree-stems act as CH₄ conduits to the atmosphere, bypassing potential sedimentary oxidation processes. Although large uncertainties exist when upscaling data from small-scale temporal measurements, we estimated that dead mangrove tree-stem emissions may account for c. 26% of the net ecosystem CH₄ flux.

Methane emissions from tree stems: a new frontier in the global carbon cycle

Tree stems from wetland, floodplain and upland forests can produce and emit methane (CH₄ ). Tree CH₄ stem emissions have high spatial and temporal variability, but there is no consensus on the biophysical mechanisms that drive stem CH₄ production and emissions. Here, we summarize up to 30 opportunities and challenges for stem CH₄ emissions research, which, when addressed, will improve estimates of the magnitudes, patterns and drivers of CH₄ emissions and trace their potential origin. We identified the need: (1) for both long-term, high-frequency measurements of stem CH₄ emissions to understand the fine-scale processes, alongside rapid large-scale measurements designed to understand the variability across individuals, species and ecosystems; (2) to identify microorganisms and biogeochemical pathways associated with CH₄ production; and (3) to develop a mechanistic model including passive and active transport of CH₄ from the soil-tree-atmosphere continuum. Addressing these challenges will help to constrain the magnitudes and patterns of CH₄ emissions, and allow for the integration of pathways and mechanisms of CH₄ production and emissions into process-based models. These advances will facilitate the upscaling of stem CH₄ emissions to the ecosystem level and quantify the role of stem CH₄ emissions for the local to global CH₄ budget.

Methanogenic Archaea dominate mature heartwood habitats of Eastern Cottonwood (Populus deltoides)

While recent reports demonstrate that the direct emission of methane from living tree trunks may be a significant terrestrial emission source, there has been debate whether tree emissions are due to transport from soils or produced in the wood environment itself. Reports of methanogens from wood of trees were prominent in the literature 40 years ago but have not been revisited with molecular ecology approaches. We examined communities associated with Populus deltoides using rRNA gene sequence analyses and how these vary with tree and wood properties. Our data indicate that wood environments are dominated by anaerobic microbiomes. Methanogens are prominent in heartwood (mean 34% relative abundance) compared to sapwood environments (13%), and dominant operational taxonomic units (OTUs) were classified as the Methanobacterium sp. Members of the Firmicutes phylum comprised 39% of total sequences and were in 42% greater abundance in sapwood over heartwood niches. Tree diameter was the strongest predictor of methanogen abundance, but wood moisture content and pH were also significant predictors of taxon abundance and overall community composition. Unlike microbiomes of the soil, rhizosphere and phyllosphere, wood associated communities are shaped by unique environmental conditions and may be prominent and overlooked sources of methane emissions in temperate forest systems.

Methane production and emissions in trees and forests

Contents Summary 35 I. Introduction 36 II. Tree CH₄ fluxes 36 III. Tree emissions of soil-produced CH₄ 40 IV. Tree-produced CH₄ 42 V. Trees in forest CH₄ budgets 44 VI. Conclusions 46 Acknowledgements 48 Author contributions 48 References 48 SUMMARY: Forest ecosystem methane (CH₄ ) research has focused on soils, but trees are also important sources and sinks in forest CH₄ budgets. Living and dead trees transport and emit CH₄ produced in soils; living trees and dead wood emit CH₄ produced inside trees by microorganisms; and trees produce CH₄ through an abiotic photochemical process. Here, we review the state of the science on the production, consumption, transport, and emission of CH₄ by living and dead trees, and the spatial and temporal dynamics of these processes across hydrologic gradients inclusive of wetland and upland ecosystems. Emerging research demonstrates that tree CH₄ emissions can significantly increase the source strength of wetland forests, and modestly decrease the sink strength of upland forests. Scaling from stem or leaf measurements to trees or forests is limited by knowledge of the mechanisms by which trees transport soil-produced CH₄ , microbial processes produce and oxidize CH₄ inside trees, a lack of mechanistic models, the diffuse nature of forest CH₄ fluxes, complex overlap between sources and sinks, and extreme variation across individuals. Understanding the complex processes that regulate CH₄ source-sink dynamics in trees and forests requires cross-disciplinary research and new conceptual models that transcend the traditional binary classification of wetland vs upland forest.

Hydrogen peroxide is involved in methane-induced tomato lateral root formation

Pharmacological and molecular evidence reveals a novel role of methane (CH₄) gas in root organogenesis, the induction of lateral root (LR) formation, and this response might require hydrogen peroxide (H₂O₂) synthesis. Although plants can produce CH₄ and release this to atmosphere, the beneficial role(s) of CH₄ are not fully elucidated. In this study, the fumigation with CH₄ not only increased NADPH oxidase activity and H₂O₂ production, but also induced tomato lateral root primordial formation and thereafter LR development. However, exogenously applied argon and nitrogen failed to influence LR formation. Above responses triggered by CH₄ were sensitive to the removal of endogenous H₂O₂ with dimethylthiourea (DMTU; a membrane-permeable scavenger of H₂O₂), suggesting the hypothesis that CH₄'s effect on LR formation could be mediated by endogenous H₂O₂. Diphenylene iodonium (DPI) inhibition of the H₂O₂ generating enzyme NADPH oxidase attenuated H₂O₂ synthesis and impaired LR formation in response to CH₄, confirming the requirement of NADPH oxidase-dependent H₂O₂. Meanwhile, the alterations of endogenous H₂O₂ concentrations failed to influence CH₄ production in tomato seedlings. Molecular evidence revealed that CH₄-induced SlCDKA1, SlCYCA2;1, and SlCYCA3;1 transcripts, and -decreased SlKRP2 mRNA were impaired by DMTU or DPI. Contrasting changes in LR formation-related miR390a and miR160 transcripts and their target genes, including SlARF4 and SlARF16, were observed. Together, our pharmacological and molecular evidence suggested the requirement of H₂O₂ synthesis in CH₄-triggered tomato LR formation, partially via the regulation of cell cycle regulatory genes, miRNA-, and tasiRNA-modulated gene expression.

Impact of vegetation on the methane budget of a temperate forest

Upland forest soils are known to be the main biological sink for methane, but studies have shown that net methane uptake of a forest ecosystem can be reduced when methane emissions by vegetation are considered. We estimated the methane budget of a young oak plantation by considering tree stems but also the understorey vegetation. Automated chambers connected to a laser-based gas analyser, on tree stems, bare soil and soil covered with understorey vegetation, recorded CH₄ fluxes for 7 months at 3 h intervals. Tree stem emissions were low and equated to only 0.1% of the soil sink. Conversely, the presence of understorey vegetation increased soil methane uptake. This plant-driven enhancement of CH₄ uptake occurred when the soil was consuming methane. At the stand level, the methane budget shifted from -1.4 ± 0.4 kg C ha^-1 when we upscaled data obtained only on bare soil, to -2.9 ± 0.6 kg C ha^-1 when we considered soil area that was covered with understorey vegetation. These results indicate that aerenchymatous plant species, which are known to reduce the methane sink in wetlands, actually increase soil methane uptake two-fold in an upland forest by enhancing methane and oxygen transport and/or by promoting growth of methanotrophic populations.

Methane emission from feather moss stands

Data from remote sensing and Eddy towers indicate that forests are not always net sinks for atmospheric CH₄ . However, studies describing specific sources within forests and functional analysis of microorganisms on sites with CH₄ turnover are scarce. Feather moss stands were considered to be net sinks for carbon dioxide, but received little attention to their role in CH₄ cycling. Therefore, we investigated methanogenic rates and pathways together with the methanogenic microbial community composition in feather moss stands from temperate and boreal forests. Potential rates of CH₄ emission from intact moss stands (n = 60) under aerobic conditions ranged between 19 and 133 pmol CH₄ h^-1 gdw^-1 . Temperature and water content positively influenced CH₄ emission. Methanogenic potentials determined under N₂ atmosphere in darkness ranged between 22 and 157 pmol CH₄ h^-1 gdw^-1 . Methane production was strongly inhibited by bromoethane sulfonate or chloroform, showing that CH₄ was of microbial origin. The moss samples tested contained fluorescent microbial cells and between 10⁴ and 10⁵ copies per gram dry weight moss of the mcrA gene coding for a subunit of the methyl CoM reductase. Archaeal 16S rRNA and mcrA gene sequences in the moss stands were characteristic for the archaeal families Methanobacteriaceae and Methanosarcinaceae. The potential methanogenic rates were similar in incubations with and without methyl fluoride, indicating that the CH₄ was produced by the hydrogenotrophic rather than aceticlastic pathway. Consistently, the CH₄ produced was depleted in ¹³ C in comparison with the moss biomass carbon and acetate accumulated to rather high concentrations (3-62 mM). The δ¹³ C of acetate was similar to that of the moss biomass, indicating acetate production by fermentation. Our study showed that the feather moss stands contained active methanogenic microbial communities producing CH₄ by hydrogenotrophic methanogenesis and causing net emission of CH₄ under ambient conditions, albeit at low rates.

Neotropical peatland methane emissions along a vegetation and biogeochemical gradient

Tropical wetlands are thought to be the most important source of interannual variability in atmospheric methane (CH4) concentrations, yet sparse data prevents them from being incorporated into Earth system models. This problem is particularly pronounced in the neotropics where bottom-up models based on water table depth are incongruent with top-down inversion models suggesting unaccounted sinks or sources of CH4. The newly documented vast areas of peatlands in the Amazon basin may account for an important unrecognized CH4 source, but the hydrologic and biogeochemical controls of CH4 dynamics from these systems remain poorly understood. We studied three zones of a peatland in Madre de Dios, Peru, to test whether CH4 emissions and pore water concentrations varied with vegetation community, soil chemistry and proximity to groundwater sources. We found that the open-canopy herbaceous zone emitted roughly one-third as much CH4 as the Mauritia flexuosa palm-dominated areas (4.7 ± 0.9 and 14.0 ± 2.4 mg CH4 m-2 h-1, respectively). Emissions decreased with distance from groundwater discharge across the three sampling sites, and tracked changes in soil carbon chemistry, especially increased soil phenolics. Based on all available data, we calculate that neotropical peatlands contribute emissions of 43 ± 11.9 Tg CH4 y-1, however this estimate is subject to geographic bias and will need revision once additional studies are published.

Temperate forest methane sink diminished by tree emissions

Global budgets ascribe 4-10% of atmospheric methane (CH₄ ) sinks to upland soils and have assumed until recently that soils are the sole surface for CH₄ exchange in upland forests. Here we report that CH₄ is emitted from the stems of dominant tree species in a temperate upland forest, measured using both the traditional static-chamber method and a new high-frequency, automated system. Tree emissions averaged across 68 observations on 17 trees from May to September were 1.59 ± 0.88 μmol CH₄  m^-2  stem h^-1 (mean ± 95% confidence interval), while soils adjacent to the trees consumed atmospheric CH₄ at a rate of -4.52 ± 0.64 μmol CH₄  m^-2  soil h^-1 (P < 0.0001). High-frequency measurements revealed diurnal patterns in the rate of tree-stem CH₄ emissions. A simple scaling exercise suggested that tree emissions offset 1-6% of the growing season soil CH₄ sink and may have briefly changed the forest to a net CH₄ source.

Methane emissions from the trunks of living trees on upland soils

Upland forests are traditionally thought to be net sinks for atmospheric methane (CH4 ). In such forests, in situ CH4 fluxes on tree trunks have been neglected relative to soil and canopy fluxes. We measured in situ CH4 fluxes from the trunks of living trees and other surfaces, such as twigs and soils, using a static closed-chamber method, and estimated the CH4 budget in a temperate upland forest in Beijing. We found that the trunks of Populus davidiana emitted large quantities of CH4 during July 2014-July 2015, amounting to mean annual emissions of 85.3 and 103.1 μg m(-2)  h(-1) on a trunk surface area basis on two replicate plots. The emission rates were similar in magnitude to those from tree trunks in wetland forests. The emitted CH4 was derived from the heartwood of trunks. On a plot or ecosystem scale, trunk CH4 emissions were equivalent to c. 30-90% of the amount of CH4 consumed by soils throughout the year, with an annual average of 63%. Our findings suggest that wet heartwoods, regardless of rot or not, occur widely in living trees on various habitats, where CH4 can be produced.

Pinus sylvestris as a missing source of nitrous oxide and methane in boreal forest

Boreal forests comprise 73% of the world's coniferous forests. Based on forest floor measurements, they have been considered a significant natural sink of methane (CH4) and a natural source of nitrous oxide (N2O), both of which are important greenhouse gases. However, the role of trees, especially conifers, in ecosystem N2O and CH4 exchange is only poorly understood. We show for the first time that mature Scots pine (Pinus sylvestris L.) trees consistently emit N2O and CH4 from both stems and shoots. The shoot fluxes of N2O and CH4 exceeded the stem flux rates by 16 and 41 times, respectively. Moreover, higher stem N2O and CH4 fluxes were observed from wet than from dry areas of the forest. The N2O release from boreal pine forests may thus be underestimated and the uptake of CH4 may be overestimated when ecosystem flux calculations are based solely on forest floor measurements. The contribution of pine trees to the N2O and CH4 exchange of the boreal pine forest seems to increase considerably under high soil water content, thus highlighting the urgent need to include tree-emissions in greenhouse gas emission inventories.