Trees are major conduits for methane egress from tropical forested wetlands

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.

Global atmospheric methane uptake by upland tree woody surfaces

Methane is an important greenhouse gas1, but the role of trees in the methane budget remains uncertain2. Although it has been shown that wetland and some upland trees can emit soil-derived methane at the stem base3,4, it has also been suggested that upland trees can serve as a net sink for atmospheric methane5,6. Here we examine in situ woody surface methane exchange of upland tropical, temperate and boreal forest trees. We find that methane uptake on woody surfaces, in particular at and above about 2 m above the forest floor, can dominate the net ecosystem contribution of trees, resulting in a net tree methane sink. Stable carbon isotope measurement of methane in woody surface chamber air and process-level investigations on extracted wood cores are consistent with methanotrophy, suggesting a microbially mediated drawdown of methane on and in tree woody surfaces and tissues. By applying terrestrial laser scanning-derived allometry to quantify global forest tree woody surface area, a preliminary first estimate suggests that trees may contribute 24.6-49.9 Tg of atmospheric methane uptake globally. Our findings indicate that the climate benefits of tropical and temperate forest protection and reforestation may be greater than previously assumed.

Microbial Communities in Standing Dead Trees in Ghost Forests are Largely Aerobic, Saprophytic, and Methanotrophic

Standing dead trees (snags) are recognized for their influence on methane (CH4) cycling in coastal wetlands, yet the biogeochemical processes that control the magnitude and direction of fluxes across the snag-atmosphere interface are not fully elucidated. Herein, we analyzed microbial communities and fluxes at one height from ten snags in a ghost forest wetland. Snag-atmosphere CH4 fluxes were highly variable (- 0.11-0.51 mg CH4 m-2 h-1). CH4 production was measured in three out of ten snags; whereas, CH4 consumption was measured in two out of ten snags. Potential CH4 production and oxidation in one core from each snag was assayed in vitro. A single core produced CH4 under anoxic and oxic conditions, at measured rates of 0.7 and 0.6 ng CH4 g-1 h-1, respectively. Four cores oxidized CH4 under oxic conditions, with an average rate of - 1.13 ± 0.31 ng CH4 g-1 h-1. Illumina sequencing of the V3/V4 region of the 16S rRNA gene sequence revealed diverse microbial communities and indicated oxidative decomposition of deadwood. Methanogens were present in 20% of the snags, with a mean relative abundance of < 0.0001%. Methanotrophs were identified in all snags, with a mean relative abundance of 2% and represented the sole CH4-cycling communities in 80% of the snags. These data indicate potential for microbial attenuation of CH4 emissions across the snag-atmosphere interface in ghost forests. A better understanding of the environmental drivers of snag-associated microbial communities is necessary to forecast the response of CH4 cycling in coastal ghost forest wetlands to a shifting coastal landscape.

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.

Quantifying the fluxes of carbon loss from an undrained tropical peatland ecosystem in Indonesia

Conservation of undrained tropical peatland ecosystems is critical for climate change mitigation as they store a tremendous amount of soil carbon that is preserved under anoxic water-logged conditions. Unfortunately, there are too few measurements of carbon fluxes from these ecosystems to estimate the climate change mitigation potential from such conservation efforts. Here, we measured carbon dioxide (CO2) and methane (CH4) fluxes as well as fluvial organic carbon export over the peat swamp forest within an undrained tropical peatland landscape in East Kalimantan, Indonesia. Our measurements throughout one year (Oct 2022-Sep 2023) showed that despite its water-logged condition, peat and water overlying the swamp forest on average emits 11.02 ± 0.49 MgCO2 ha-1 yr-1 of CO2 and 0.58 ± 0.04 MgCO2e ha-1 yr-1 of CH4. Further, the fluvial organic carbon export contributes to additional carbon loss of 1.68 ± 0.06 MgCO2e ha-1 yr-1. Our results help improve the accuracy of carbon accounting from undrained tropical peatlands, where we estimated a total carbon loss of 13.28 ± 0.50 MgCO2e ha-1 yr-1. Nevertheless, the total carbon loss reported from our sites is about half than what is reported from the drained peatland landscapes in the region and resulted in a larger onsite carbon sink potential estimate compared to other undrained peat swamp forests. Together, these findings indicate that conserving the remaining undrained peatland ecosystems in Indonesia from drainage and degradation is a promising natural climate solution strategy that avoids significant carbon emissions.

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.

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.

Methane emissions from tree stems - current knowledge and challenges: an introduction to a Virtual Issue

This article is a Commentary on the Virtual Issue ‘Methane emissions from tree stems – current knowledge and challenges’ that includes the following papers: Barba et al. (2019), Bréchet et al. (2021), Covey & Megonigal (2019), Feng et al. (2022), Flanagan et al. (2021), Jeffrey et al. (2019, 2021, 2023), Kohl et al. (2019), Machacova et al. (2021a,b, 2023), Megonigal et al. (2020), Pangala et al. (2013, 2014), Pitz & Megonigal (2017), Plain et al. (2019), Putkinen et al. (2021), Sjögersten et al. (2020), Takahashi et al. (2022), Tenhovirta et al. (2022), Wang et al. (2016), and Yip et al. (2018). Access the Virtual Issue at www.newphytologist.com/virtualissues.

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.

CH4 and N2 O emissions from smallholder agricultural systems on tropical peatlands in Southeast Asia

There are limited data for greenhouse gas (GHG) emissions from smallholder agricultural systems in tropical peatlands, with data for non-CO2 emissions from human-influenced tropical peatlands particularly scarce. The aim of this study was to quantify soil CH4 and N2 O fluxes from smallholder agricultural systems on tropical peatlands in Southeast Asia and assess their environmental controls. The study was carried out in four regions in Malaysia and Indonesia. CH4 and N2 O fluxes and environmental parameters were measured in cropland, oil palm plantation, tree plantation and forest. Annual CH4 emissions (in kg CH4 ha-1  year-1 ) were: 70.7 ± 29.5, 2.1 ± 1.2, 2.1 ± 0.6 and 6.2 ± 1.9 at the forest, tree plantation, oil palm and cropland land-use classes, respectively. Annual N2 O emissions (in kg N2 O ha-1  year-1 ) were: 6.5 ± 2.8, 3.2 ± 1.2, 21.9 ± 11.4 and 33.6 ± 7.3 in the same order as above, respectively. Annual CH4 emissions were strongly determined by water table depth (WTD) and increased exponentially when annual WTD was above -25 cm. In contrast, annual N2 O emissions were strongly correlated with mean total dissolved nitrogen (TDN) in soil water, following a sigmoidal relationship, up to an apparent threshold of 10 mg N L-1 beyond which TDN seemingly ceased to be limiting for N2 O production. The new emissions data for CH4 and N2 O presented here should help to develop more robust country level 'emission factors' for the quantification of national GHG inventory reporting. The impact of TDN on N2 O emissions suggests that soil nutrient status strongly impacts emissions, and therefore, policies which reduce N-fertilisation inputs might contribute to emissions mitigation from agricultural peat landscapes. However, the most important policy intervention for reducing emissions is one that reduces the conversion of peat swamp forest to agriculture on peatlands in the first place.

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.

Modeled production, oxidation, and transport processes of wetland methane emissions in temperate, boreal, and Arctic regions

Wetlands are the largest natural source of methane (CH₄ ) to the atmosphere. The eddy covariance method provides robust measurements of net ecosystem exchange of CH₄ , but interpreting its spatiotemporal variations is challenging due to the co-occurrence of CH₄ production, oxidation, and transport dynamics. Here, we estimate these three processes using a data-model fusion approach across 25 wetlands in temperate, boreal, and Arctic regions. Our data-constrained model-iPEACE-reasonably reproduced CH₄ emissions at 19 of the 25 sites with normalized root mean square error of 0.59, correlation coefficient of 0.82, and normalized standard deviation of 0.87. Among the three processes, CH₄ production appeared to be the most important process, followed by oxidation in explaining inter-site variations in CH₄ emissions. Based on a sensitivity analysis, CH₄ emissions were generally more sensitive to decreased water table than to increased gross primary productivity or soil temperature. For periods with leaf area index (LAI) of ≥20% of its annual peak, plant-mediated transport appeared to be the major pathway for CH₄ transport. Contributions from ebullition and diffusion were relatively high during low LAI (<20%) periods. The lag time between CH₄ production and CH₄ emissions tended to be short in fen sites (3 ± 2 days) and long in bog sites (13 ± 10 days). Based on a principal component analysis, we found that parameters for CH₄ production, plant-mediated transport, and diffusion through water explained 77% of the variance in the parameters across the 19 sites, highlighting the importance of these parameters for predicting wetland CH₄ emissions across biomes. These processes and associated parameters for CH₄ emissions among and within the wetlands provide useful insights for interpreting observed net CH₄ fluxes, estimating sensitivities to biophysical variables, and modeling global CH₄ fluxes.

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.

On the prediction of methane fluxes from pristine tropical peatland in Sarawak: application of a denitrification-decomposition (DNDC) model

Tropical peatlands have high potential function as a major source of atmospheric methane (CH₄) and can contribute to global warming due to their large soil carbon stock, high groundwater level (GWL), high humidity and high temperature. In this study, a process-based denitrification-decomposition (DNDC) model was used to simulate CH₄ fluxes in a pristine tropical peatland in Sarawak. To test the accuracy of the model, eddy covariance tower datasets were compared. The model was validated for the year 2014, which showed the good performance of the model for simulating CH₄ emissions. The monthly predictive ability of the model was better than the daily predictive ability, with a determination coefficient (R²) of 0.67, model error (ME) of 2.47, root mean square error (RMSE) of 3.33, mean absolute error (MAE) of 2.92 and mean square error (MSE) of 11.08. The simulated years of 2015 and 2016 showed the good performance of the DNDC model, although under- and overestimations were found during the drier and rainy months. Similarly, the monthly simulations for the year were better than the daily simulations for the year, showing good correlations at R² at 0.84 (2015) and 0.87 (2016). Better statistical performance in terms of monthly ME, RMSE, MAE and MSE at - 0.11, 3.38, 3.05 and 11.45 for 2015 and - 1.14, 5.28, 4.93 and 27.83 for 2016, respectively, was also observed. Although the statistical performance of the model simulation for daily average CH₄ fluxes was lower than that of the monthly average, we found that the results for total fluxes agreed well between the observed and the simulated values (E = 6.79% and difference = 3.3%). Principal component analysis (PCA) showed that CH₄, GWL and rainfall were correlated with each other and explained 41.7% of the total variation. GWL was found to be relatively important in determining the CH₄ fluxes in the naturally inundated pristine tropical peatland. These results suggest that GWL is an essential input variable for the DNDC model for predicting CH₄ fluxes from the pristine tropical peatland in Sarawak on a monthly basis.

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.

Methane Contributions of Different Components of Kandelia candel–Soil System under Nitrogen Supplementation

Kandelia candel is the most widely distributed tree species on the southeast coast of China and is also the main afforestation tree species along the coastal wetland. In recent years, inorganic nitrogen pollution has become increasingly severe, and investigating the effects of nitrogen input on methane emissions in Kandelia candel–soil systems has become significant from a global change perspective. However, the effect of nitrogen input on methane emissions in coastal wetland systems is still uncertain. The field tidal environment is complex and varied, and thus it is difficult to accurately control the amount of nitrogen in the system. Therefore, in order to accurately assess the effects of different concentrations of foreign nitrogen input on methane emission fluxes in a Kandelia candel–soil system, we use indoor tidal simulation experimental devices and design two simulation systems with and without plant planting to explore the difference of methane emission flux in this system under five nitrogen input concentrations: N0 (0 g N·m−2·a−1), N1 (5 g N·m−2·a−1), N2 (10 g N·m−2·a−1), N3 (20 g N·m−2·a−1), and N4 (30 g N·m−2·a−1). The results showed that: (1) The introduction of Kandelia candel promoted methane emissions in coastal wetland ecosystem. Under each nitrogen application concentration, the mean CH4 emission flux in the planting group was 42.98%, 65.59%, 40.87%, 58.93% and 39.23% higher than that in the non-planting group, respectively. (2) Nitrogen input significantly promoted methane emissions in both planted and non-planted environments, and the promoting effect showed as follows: N4 > N3 > N2 > N1 > N0. (3) After the introduction of Kandelia candel, the contribution of Kandelia candel and soil microorganisms to methane emissions was different under different concentrations of nitrogen addition. The contribution rate of Kandelia candel to CH4 emission flux of Kandelia candel–soil system ranged from 10.74% to 60.25%, with an average contribution rate of 37.30%. The changed soil microbes contributed 39.75% to 89.26% to the CH4 emission flux in the Kandelia candel–soil system, with an average contribution rate of 62.60%. Under N3 nitrogen application concentration, the emission flux of plant was the largest, which was significantly higher than that of the soil microbial pathway; at other concentrations, the methane emissions from the soil microbial pathway were greater than that of the plant pathway, and the contribution rate to the plant–soil system reached 60.25%. The results of this study provide an important basis for improving the estimation accuracy of carbon emissions in coastal waters and formulating policies for the restoration and protection of coastal wetlands.

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)'.

Restoration impacts on rates of denitrification and greenhouse gas fluxes from tropical coastal wetlands

Forested coastal wetlands are globally important systems sequestering carbon and intercepting nitrogen pollution from nutrient-rich river systems. Coastal wetlands that have suffered extensive disturbance are the target of comprehensive restoration efforts. Accurate assessment of restoration success requires detailed mechanistic understanding of wetland soil biogeochemical functioning across restoration chrono-sequences, which remains poorly understood for these sparsely investigated systems. This study investigated denitrification and greenhouse gas fluxes in mangrove and Melaleuca forest soils of Vietnam, using the ¹⁵N-Gas flux method. Denitrification-derived N₂O was significantly higher from Melaleuca than mangrove forest soils, despite higher potential rates of total denitrification in the mangrove forest soils (8.1 ng N g^-1 h^-1) than the Melaleuca soils (6.8 ng N g^-1 h^-1). Potential N₂O and CO₂ emissions were significantly higher from the Melaleuca soils than from the mangrove soils. Disturbance and subsequent recovery had no significant effect on N biogeochemistry except with respect to the denitrification product ratio in the mangrove sites, which was highest from the youngest mangrove site. Potential CO₂ and CH₄ fluxes were significantly affected by restoration in the mangrove soils. The lowest potential CO₂ emissions were observed in the mid-age plantation and potential CH₄ fluxes decreased in the older forests. The mangrove system, therefore, may remove excess N and improve water quality with low greenhouse gas emissions, whereas in Melaleucas, increased N₂O and CO₂ emissions also occur. These emissions are likely balanced by higher carbon stocks observed in the Melaleuca soils. These mechanistic insights highlight the importance of ecosystem restoration for pollution attenuation and reduction of greenhouse gas emissions from coastal wetlands. Restoration efforts should continue to focus on increasing wetland area and function, which will benefit local communities with improved water quality and potential for income generation under future carbon trading.

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.

Redox Heterogeneity Entangles Soil and Climate Interactions

Interactions between soils and climate impact wider environmental sustainability. Soil heterogeneity intricately regulates these interactions over short spatiotemporal scales and therefore needs to be more finely examined. This paper examines how redox heterogeneity at the level of minerals, microbial cells, organic matter, and the rhizosphere entangles biogeochemical cycles in soil with climate change. Redox heterogeneity is used to develop a conceptual framework that encompasses soil microsites (anaerobic and aerobic) and cryptic biogeochemical cycling, helping to explain poorly understood processes such as methanogenesis in oxygenated soils. This framework is further shown to disentangle global carbon (C) and nitrogen (N) pathways that include CO2, CH4, and N2O. Climate-driven redox perturbations are discussed using wetlands and tropical forests as model systems. Powerful analytical methods are proposed to be combined and used more extensively to study coupled abiotic and biotic reactions that are affected by redox heterogeneity. A core view is that emerging and future research will benefit substantially from developing multifaceted analyses of redox heterogeneity over short spatiotemporal scales in soil. Taking a leap in our understanding of soil and climate interactions and their evolving influence on environmental sustainability then depends on greater collaborative efforts to comprehensively investigate redox heterogeneity spanning the domain of microscopic soil interfaces.

Simultaneous tree stem and soil greenhouse gas (CO2 , CH4 , N2 O) flux measurements: a novel design for continuous monitoring towards improving flux estimates and temporal resolution

Tree stems and soils can act as sources and sinks for the greenhouse gases (GHG) carbon dioxide (CO₂ ), methane (CH₄ ), and nitrous oxide (N₂ O). Since both uptake and emission capacities can be large, especially in tropical rainforests, accurate assessments of the magnitudes and temporal variations of stem and soil GHG fluxes are required. We designed a new flexible stem chamber system for continuously measuring GHG fluxes in a French Guianese rainforest. Here, we describe this new system, which is connected to an automated soil GHG flux system, and discuss measurement uncertainty and potential error sources. In line with findings for soil GHG flux estimates, we demonstrated that lengthening the stem chamber closure time was required for accurate estimates of tree stem CH₄ and N₂ O flux but not tree stem CO₂ flux. The instrumented stem was a net source of CO₂ and CH₄ and a weak sink of N₂ O. Our experimental setup operated successfully in situ and provided continuous tree and soil GHG measurements at a high temporal resolution over an 11-month period. This automated system is a major step forward in the measurement of GHG fluxes in stems and the atmosphere concurrently with soil GHG fluxes in tropical forest ecosystems.

Isotopic evidence for axial tree stem methane oxidation within subtropical lowland forests

Knowledge regarding mechanisms moderating methane (CH₄ ) sink/source behaviour along the soil-tree stem-atmosphere continuum remains incomplete. Here, we applied stable isotope analysis (δ¹³ C-CH₄ ) to gain insights into axial CH₄ transport and oxidation in two globally distributed subtropical lowland species (Melaleuca quinquenervia and Casuarina glauca). We found consistent trends in CH₄ flux (decreasing with height) and δ¹³ C-CH₄ enrichment (increasing with height) in relation to stem height from ground. The average lower tree stem δ¹³ C-CH₄ (0-40 cm) of Melaleuca and Casuarina (-53.96‰ and -65.89‰) were similar to adjacent flooded soil CH₄ ebullition (-52.87‰ and -62.98‰), suggesting that stem CH₄ is derived mainly by soil sources. Upper stems (81-200 cm) displayed distinct δ¹³ C-CH₄ enrichment (Melaleuca -44.6‰ and Casuarina -46.5‰, respectively). Coupled 3D-photogrammetry with novel 3D-stem measurements revealed distinct hotspots of CH₄ flux and isotopic fractionation on Melaleuca, which were likely due to bark anomalies in which preferential pathways of gas efflux were enhanced. Diel experiments revealed greater δ¹³ C-CH₄ enrichment and higher oxidation rates in the afternoon, compared with the morning. Overall, we estimated that c. 33% of the methane was oxidised between lower and upper stems during axial transport, therefore potentially representing a globally significant, yet previously unaccounted for, methane sink.

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.

Trees as net sinks for methane (CH4 ) and nitrous oxide (N2 O) in the lowland tropical rain forest on volcanic Réunion Island

Trees are known to emit methane (CH₄ ) and nitrous oxide (N₂ O), with tropical wetland trees being considerable CH₄ sources. Little is known about CH₄ and especially N₂ O exchange of trees growing in tropical rain forests under nonflooded conditions. We determined CH₄ and N₂ O exchange of stems of six dominant tree species, cryptogamic stem covers, soils and volcanic surfaces at the start of the rainy season in a 400-yr-old tropical lowland rain forest situated on a basaltic lava flow (Réunion Island). We aimed to understand the unknown role in greenhouse gas fluxes of these atypical tropical rain forests on basaltic lava flows. The stems studied were net sinks for atmospheric CH₄ and N₂ O, as were cryptogams, which seemed to be co-responsible for the stem uptake. In contrast with more commonly studied rain forests, the soil and previously unexplored volcanic surfaces consumed CH₄ . Their N₂ O fluxes were negligible. Greenhouse gas uptake potential by trees and cryptogams constitutes a novel and unique finding, thus showing that plants can serve not only as emitters, but also as consumers of CH₄ and N₂ O. The volcanic tropical lowland rain forest appears to be an important CH₄ sink, as well as a possible N₂ O sink.

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.

Effects of Plant and Soil Amendment on Remediation Performance and Methane Mitigation in Petroleum-Contaminated Soil

Petroleum-contaminated soil is considered among the most important potential anthropogenic atmospheric methane sources. Additionally, various rhizoremediation factors can affect methane emissions by altering soil ecosystem carbon cycles. Nonetheless, greenhouse gas emissions from soil have not been given due importance as a potentially relevant parameter in rhizoremediation techniques. Therefore, in this study we sought to investigate the effects of different plant and soil amendments on both remediation efficiencies and methane emission characteristics in dieselcontaminated soil. An indoor pot experiment consisting of three plant treatments (control, maize, tall fescue) and two soil amendments (chemical nutrient, compost) was performed for 95 days. Total petroleum hydrocarbon (TPH) removal efficiency, dehydrogenase activity, and alkB (i.e., an alkane compound-degrading enzyme) gene abundance were the highest in the tall fescue and maize soil system amended with compost. Compost addition enhanced both the overall remediation efficiencies, as well as pmoA (i.e., a methane-oxidizing enzyme) gene abundance in soils. Moreover, the potential methane emission of diesel-contaminated soil was relatively low when maize was introduced to the soil system. After microbial community analysis, various TPH-degrading microorganisms (Nocardioides, Marinobacter, Immitisolibacter, Acinetobacter, Kocuria, Mycobacterium, Pseudomonas, Alcanivorax) and methane-oxidizing microorganisms (Methylocapsa, Methylosarcina) were observed in the rhizosphere soil. The effects of major rhizoremediation factors on soil remediation efficiency and greenhouse gas emissions discussed herein are expected to contribute to the development of sustainable biological remediation technologies in response to global climate change.

Paludiculture as a sustainable land use alternative for tropical peatlands: A review

Peatlands cover approximately 4.2 million km² of terrestrial land surface and store up to 700 Pg of terrestrial carbon. Preserving the carbon stocks in peatland is therefore crucial for climate change mitigation. Under natural conditions, peatland carbon storage is maintained by moist peat conditions, which decreases decomposition and encourages peat formation. However, conversion of peatlands to drainage-based agriculture in the form of industrial plantations and smallholder farming has resulted in globally significant greenhouse gas emissions. Paludiculture, loosely conceptualized as biomass production on wet peatlands with the potential to maintain carbon storage, is proposed as a sustainable, non-drainage-based agriculture alternative for peatland use. However, while the concept of paludiculture was developed in temperate ecoregions, its application in the tropics is poorly understood. In this review, we examine common definitions of paludiculture used in literature to derive key themes and future directions. We found three common themes: ecosystem services benefits of paludiculture, hydrological conditions of peatlands, and vegetation selection for planting. Ambiguities surrounding these themes have led to questions on whether paludiculture applications are sustainable in the context of carbon sequestration in peat soil. This review aims to evaluate and advance current understanding of paludiculture in the context of tropical peatlands, which is especially pertinent given expanding agriculture development into Central Africa and South America, where large reserves of peatlands were recently discovered.

Post-fire carbon dynamics in the tropical peat swamp forests of Brunei reveal long-term elevated CH4 flux

Tropical peatlands hold about 15%-19% of the global peat carbon (C) pool of which 77% is stored in the peat swamp forests (PSFs) of Southeast Asia. Nonetheless, these PSFs have been drained, exploited for timber and land for agriculture, leading to frequent fires in the region. The physico-chemical characteristics of peat, as well as the hydrology of PSFs are affected after a fire, during which the ecosystem can act as a C source for decades, as C emissions to the atmosphere exceed photosynthesis. In this work, we studied the longer-term impact of fires on C cycling in tropical PSFs, hence we quantified the magnitude and patterns of C loss (CO₂ , CH₄ and dissolved organic carbon) and soil-water quality characteristics in an intact and a degraded burnt PSF in Brunei Darussalam affected by seven fires over the last 40 years. We used natural tracers such as ¹⁴ C to investigate the age and sources of C contributing to ecosystem respiration (R_eco ) and CH₄ , while we continuously monitored soil temperature and water table (WT) level from June 2017 to January 2019. Our results showed a major difference in the physico-chemical parameters, which in turn affected C dynamics, especially CH₄ . Methane effluxes were higher in fire-affected areas (7.8 ± 2.2 mg CH₄  m^-2  hr^-1 ) compared to the intact PSF (4.0 ± 2.0 mg CH₄  m^-2  hr^-1 ) due to prolonged higher WT and more optimal methanogenesis conditions. On the other hand, we did not find significant differences in R_eco between burnt (432 ± 83 mg CO₂  m^-2  hr^-1 ) and intact PSF (359 ± 76 mg CO₂  m^-2  hr^-1 ). Radiocarbon analysis showed overall no significant difference between intact and burnt PSF with a modern signature for both CO₂ and CH₄ fluxes implying a microbial preference for the more labile C fraction in the peat matrix.

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.

Evaluation of vegetation communities, water table, and peat composition as drivers of greenhouse gas emissions in lowland tropical peatlands

Tropical peatlands are globally important source of greenhouse gases to the atmosphere, but data on carbon fluxes from these ecosystems is limited due to the logistical challenges of measuring gas fluxes in these ecosystems. Proposals to overcome the difficulties of measuring gas carbon fluxes in the tropics include remote sensing (top-down) approaches. However, these require information on the effect of vegetation communities on carbon dioxide (CO₂) and methane (CH₄) fluxes from the peat surface (bottom-up). Such information will help reducing the uncertainty in current carbon budgets and resolve inconsistencies between the top-down and bottom-up estimates of gas fluxes from tropical peatlands. We investigated temporal and spatial variability of CO₂ and CH₄ fluxes from tropical peatlands inhabited by two contrasting vegetation communities (i.e., mixed forest and palm swamp) in Panama. In addition, we explored the influence of peat chemistry and nutrient status (i.e., factorial nitrogen (N) and phosphorus (P) addition) on greenhouse gas fluxes from the peat surface. We found that: i) CO₂ and CH₄ fluxes were not significantly different between the two vegetation communities, but did vary temporally across an annual cycle; ii) precipitation rates and peat temperature were poor predictors of CO₂ and CH₄ fluxes; iii) nitrogen addition increased CH₄ fluxes at the mixed forests when the water table was above the peat surface, but neither nitrogen nor phosphorus affected gas fluxes elsewhere; iv) gas fluxes varied significantly with the water table level, with CO₂ flux being 80% greater at low water table, and CH₄ fluxes being 81% higher with the water table above the surface. Taken together, our data suggested that water table is the most important control of greenhouse gas emissions from the peat surface in forested lowland tropical peatlands, and that neither the presence of distinct vegetation communities nor the addition of nutrients outweigh such control.

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.

Interactions of sulfur and methane-oxidizing bacteria in tropical estuarine sediments

The bacterial oxidation of sulfur and methane is central to the biogeochemical processes in sediments such as the tropical mangrove sediments. However, there is a lacuna of information on the seasonal interactions including the influence of monsoons which is a major driver of seasonal change, on sulfur-oxidizing bacteria (SOB) and methane-oxidizing bacteria (MOB), their activity and the environmental variables. To understand these interactions, the analysis was carried out on sediment samples that were sampled monthly for a year from Chorao mangrove, Goa, southwest coast of India. SOB (3.8×10⁵CFU g^-1) and MOB (0.90×10⁵CFU g^-1) had maximum average abundance in the surface sediments in the post-monsoon and monsoon season, respectively. The mean sulfur-oxidation activity (SOA) of 2.63 mM day^-1 and methane-oxidation activity (MOA) of 110.94 mM day^-1 were highest in surface sediments during the post-monsoon season. Generally, the activity of SOB and MOB in surface sediments of post-monsoon was 2.2 times(×) and 2.8× respectively higher than that in the monsoon season. Among the environmental parameters analyzed, protein and sulfide concentrations significantly (p < 0.001) influenced SOA and MOA, respectively. There was a significant difference in SOA (p < 0.003) and MOA (p < 0.036) in surface sediments between the monsoon and the post-monsoon season. During the monsoon season, when the system is a sink of terrestrial/anthropogenic material, the interrelationship of SOB with MOA (r = 0.617, p < 0.001) and SOB with SOA (r = 0.489, p < 0.05) aids in maintaining the homeostasis of the system.

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.