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.

Covey KR ，Megonigal JP 《-》

Tree stem bases are sources of CH(4) and N(2) O in a tropical forest on upland soil during the dry to wet season transition.

Tropical forests on upland soils are assumed to be a methane (CH4 ) sink and a weak source of nitrous oxide (N2 O), but studies of wetland forests have demonstrated that tree stems can be a substantial source of CH4 , and recent evidence from temperate woodlands suggests that tree stems can also emit N2 O. Here, we measured CH4 and N2 O fluxes from the soil and from tree stems in a semi-evergreen tropical forest on upland soil. To examine the influence of seasonality, soil abiotic conditions and substrate availability (litter inputs) on trace greenhouse gas (GHG) fluxes, we conducted our study during the transition from the dry to the wet season in a long-term litter manipulation experiment in Panama, Central America. Trace GHG fluxes were measured from individual stem bases of two common tree species and from soils beneath the same trees. Soil CH4 fluxes varied from uptake in the dry season to minor emissions in the wet season. Soil N2 O fluxes were negligible during the dry season but increased markedly after the start of the wet season. By contrast, tree stem bases emitted CH4 and N2 O throughout the study. Although we observed no clear effect of litter manipulation on trace GHG fluxes, tree species and litter treatments interacted to influence CH4 fluxes from stems and N2 O fluxes from stems and soil, indicating complex relationships between tree species traits and decomposition processes that can influence trace GHG dynamics. Collectively, our results show that tropical trees can act as conduits for trace GHGs that most likely originate from deeper soil horizons, even when they are growing on upland soils. Coupled with the finding that the soils may be a weaker sink for CH4 than previously thought, our research highlights the need to reappraise trace gas budgets in tropical forests.

Welch B ，Gauci V ，Sayer EJ 《-》

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.

Pitz S ，Megonigal JP 《-》

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.

Wang ZP ，Gu Q ，Deng FD ，Huang JH ，Megonigal JP ，Yu Q ，Lü XT ，Li LH ，Chang S ，Zhang YH ，Feng JC ，Han XG ... -  《-》

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.

Barba J ，Bradford MA ，Brewer PE ，Bruhn D ，Covey K ，van Haren J ，Megonigal JP ，Mikkelsen TN ，Pangala SR ，Pihlatie M ，Poulter B ，Rivas-Ubach A ，Schadt CW ，Terazawa K ，Warner DL ，Zhang Z ，Vargas R ... -  《-》