Spatiotemporal variability and origin of CO₂ and CH₄ tree stem fluxes in an upland forest

Abstract

The exchange of multiple greenhouse gases (i.e., CO2 and CH4) between tree stems and the atmosphere represents a knowledge gap in the global carbon cycle. Stem CO2 and CH4 fluxes vary across time and space and are unclear, which are their individual or shared drivers. Here we measured CO2 and CH4 fluxes at different stem heights combining manual (biweekly; n = 678) and automated (hourly; n > 38,000) measurements in a temperate upland forest. All trees showed CO2 and CH4 emissions despite 20% of measurements showing net CH4 uptake. Stem CO2 fluxes presented clear seasonal trends from manual and automated measurements. Only automated measurements captured the high temporal variability of stem CH4 fluxes revealing clear seasonal trends. Despite that temporal integration, the limited number of automated chambers made stand‐level mean CH4 fluxes sensitive to “hot spots,” resulting in mean fluxes with high uncertainty. Manual measurements provided better integration of spatial variability, but their lack of temporal variability integration hindered the detection of temporal trends and stand‐level mean fluxes. These results highlight the potential bias of previous studies of stem CH4 fluxes solely based on manual or automated measurements. Stem height, temperature, and soil moisture only explained 7% and 11% of the stem CH4 flux variability compared to 42% and 81% for CO2 (manual and automated measurements, respectively). This large unexplained variability, in combination with high CH4 concentrations in the trees' heartwood, suggests that stem CH4 fluxes might be more influenced by gas transport and diffusivity through the wood than by drivers of respiratory CO2 flux, which has crucial implications for developing process‐based ecosystem models. We postulate that CH4 is likely originated within tree stems because of lack of a consistent vertical pattern in CH4 fluxes, evidence of CH4 production in wood incubations, and low CH4 concentration in the soil profile but high concentrations within the trees' heartwood.

Methane emissions may be driven by hydrogenotrophic methanogens inhabiting the stem tissues of poplar

“…Several previous studies have also indicated that methanogens are present within poplar trees (such as P. alba, P. deltoides, and P. angustifolia) and promote generating CH 4 (Zeikus & Ward, 1974;Yip et al, 2019;Flanagan et al, 2021). Barba et al (2021) recently postulated that stem CH 4 flux in upland trees is likely due to the production in situ through comparing manual and automated measurements, laboratory incubations, and stem CH 4 concentrations. However, we found the stem CH 4 emissions to occur over the entire measurement period (Fig.…”

“…Barba et al . (2021) recently postulated that stem CH 4 flux in upland trees is likely due to the production in situ through comparing manual and automated measurements, laboratory incubations, and stem CH 4 concentrations. However, we found the stem CH 4 emissions to occur over the entire measurement period (Fig.…”

“…(1) CH 4 concentrations are high within heartwood (Zeikus & Ward, 1974;Covey et al, 2012;Barba et al, 2021); (2) heartwood CH 4 concentration or water content are correlated with stem emission rates (Wang et al, 2017); (3) there are no notable associations between stem CH 4 emissions and soil variables (Pitz & Megonigal, 2017;Warner et al, 2017); and (4) methanogenic archaea have been found to inhabit the heartwood (Yip et al, 2019).…”

“…The notion that stem CH 4 emissions originate in the soil is supported by the following observations: (1) CH 4 concentrations are high in deep soils close to the stem being measured (Koehler et al ., 2012; Maier et al ., 2018); (2) CH 4 emission rates decrease with stem sampling height (Wang et al ., 2016; Barba et al ., 2019b; Jeffrey et al ., 2020b; Sjögersten et al ., 2020); and (3) stem and soil CH 4 fluxes are correlated (Machacova et al ., 2016; Pangala et al ., 2017). In contrast, some studies have suggested that CH 4 may be produced within the heartwood based on the following observations: (1) CH 4 concentrations are high within heartwood (Zeikus & Ward, 1974; Covey et al ., 2012; Barba et al ., 2021); (2) heartwood CH 4 concentration or water content are correlated with stem emission rates (Wang et al ., 2017); (3) there are no notable associations between stem CH 4 emissions and soil variables (Pitz & Megonigal, 2017; Warner et al ., 2017); and (4) methanogenic archaea have been found to inhabit the heartwood (Yip et al ., 2019).…”

Methane and Carbon Dioxide Fluxes in a Temperate Tidal Salt Marsh: Comparisons Between Plot and Ecosystem Measurements

“…Additionally, fluxes of GPP and Reco are estimated as a result of a model based on temperature or light (Lasslop et al., 2012; Reichstein et al., 2005), and are not directly measured which introduces additional uncertainties. While both chambers and EC based approaches have their own benefits and limitations, the paired use of methods can be an especially powerful tool for reconciling differences between approaches (Barba et al., 2021; Fox et al., 2008). Specifically, including a data subset with higher spatiotemporal integration from EC (i.e., daily averaged data) can serve as a target benchmark to compare other methods, thus helping to bridge the gap between different measurement scales (e.g., plot‐ecosystem) (Hill, Barba, et al., 2021; Lucas‐Moffat et al., 2018).…”