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 ... -  《-》

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 《-》

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.

Han M ，Feng H ，Peng C ，Lei X ，Xue J ，Malghani S ，Ma X ，Song X ，Wang W ... -  《-》

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 《-》

Large emissions from floodplain trees close the Amazon methane budget.

Pangala SR ，Enrich-Prast A ，Basso LS ，Peixoto RB ，Bastviken D ，Hornibrook ERC ，Gatti LV ，Marotta H ，Calazans LSB ，Sakuragui CM ，Bastos WR ，Malm O ，Gloor E ，Miller JB ，Gauci V ... -  《-》