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

Abstract

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 CH 4 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.

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

“…Within upland forests, the typically well drained soils have historically been considered net methane sinks. Although the upwards transfer of methane from methane‐rich groundwater is possible, most studies now suggest that in situ methanogenic production dominates upland tree emissions (Zeikus & Henning, 1975; Yip et al ., 2019; Flanagan et al ., 2021) and increased internal tree stem methane concentrations are related to heartwood rot, water content, and/or saprotrophic fungi (Zeikus & Ward, 1974; Covey et al ., 2012; Lenhart et al ., 2012; Wang et al ., 2017). Recent discoveries that some upland forest tree stems can emit methane from dry soils suggests that, usually, the net methane sink capacity of upland forest biomes may be diminished (Megonigal & Guenther, 2008; Machacova et al ., 2016; Wang et al ., 2016; Maier et al ., 2018).…”

Bark-dwelling methanotrophic bacteria decrease methane emissions from trees

“…Although the importance of MOB within wetland soil and water is well documented [23][24][25][26][27][28] , their possible role within trees has yet to be characterised. Methanogenic archaea have been identified within the heartwood and sapwood of several lowland tree species [29][30][31][32] , but the operational taxonomic units of methanotrophic families were exceedingly rare 30 and their influence on tree stem methane emissions remains unquantified. Until now, it is unclear if bark may provide a habitat for MOB.…”

Topography-based statistical modelling reveals high spatial variability and seasonal emission patches in forest floor methane flux

“…TWI was calculated as a natural logarithm of the ratio between the specific catchment area (contributing area per unit contour length) and tangent of the local slope. The upslope catchment area was calculated using multiple flow direction algorithms (Freeman, 1991;Schwanghart and Kuhn, 2010), and local slope was calculated using adjacent points in the DEM. The calculations were made with TopoToolbox.…”

“…The upland forest CH 4 emission estimates are partly based on observations above forest canopies (Flanagan et al, 2020;Mikkelsen et al, 2011;Shoemaker et al, 2014). They further raise the question whether these CH 4 emissions originate only from the forest floor or whether trees, which have also been reported to emit CH 4 (e.g.…”