Automated measurements of greenhouse gases fluxes from tree stems and soils: magnitudes, patterns and drivers

Abstract

Tree stems exchange CO 2 , CH 4 and N 2 O with the atmosphere but the magnitudes, patterns and drivers of these greenhouse gas (GHG) fluxes remain poorly understood. Our understanding mainly comes from static-manual measurements, which provide limited information on the temporal variability and magnitude of these fluxes. We measured hourly CO 2 , CH 4 and N 2 O fluxes at two stem heights and adjacent soils within an upland temperate forest. We analyzed diurnal and seasonal variability of fluxes and biophysical drivers (i.e., temperature, soil moisture, sap flux). Tree stems were a net source of CO 2 (3.80 ± 0.18 µmol m −2 s −1 ; mean ± 95% CI) and CH 4 (0.37 ± 0.18 nmol m −2 s −1 ), but a sink for N 2 O (−0.016 ± 0.008 nmol m −2 s −1 ). Time series analysis showed diurnal temporal correlations between these gases with temperature or sap flux for certain days. CO 2 and CH 4 showed a clear seasonal pattern explained by temperature, soil water content and sap flux. Relationships between stem, soil fluxes and their drivers suggest that CH 4 for stem emissions could be partially produced belowground. High-frequency measurements demonstrate that: a) tree stems exchange GHGs with the atmosphere at multiple time scales; and b) are needed to better estimate fluxes magnitudes and understand underlying mechanisms of GHG stem emissions.

Longitudinal Gradients in Tree Stem Greenhouse Gas Concentrations Across Six Pacific Northwest Coastal Forests

“…Studies measuring emissions have also observed higher CH 4 fluxes in summer compared to winter months. For example, CH 4 emissions from trees in temperate upland forests were found to be higher during the growing season (Barba, Poyatos, & Vargas, ; Wang et al, ). Several other studies have observed higher summer CH 4 fluxes in temperate forested wetlands (Pangala et al, ; Terazawa et al, ), but this trend was not ubiquitous; for example, one tree in the Terazawa et al () study had stable CH 4 fluxes throughout the year compared to an order of magnitude difference for other trees.…”

“…Roots can similarly mediate the transport of below-ground N 2 O to tree stems (Machacova et al, 2016;Pihlatie et al, 2005;Wen et al, 2017). Although plant adaptations for increased O 2 diffusion in anoxic or water-logged environments (such as aerenchyma and lenticels) can facilitate root-mediated transport of soil-produced GHGs (Pangala et al, 2013), this pathway has also been found in plants without these structures (Barba, Poyatos, & Vargas, 2019;Machacova et al, 2016;Pitz & Megonigal, 2017;Pitz et al, 2018).…”

Are methane emissions from mangrove stems a cryptic carbon loss pathway? Insights from a catastrophic forest mortality

“…It is also plausible that a portion of the CH 4 flux from dead stems originates aboveground due to the chemical degradation of plant tissue (Keppler et al, 2006), in situ methanogenesis from organic matter decomposition (Covey et al, 2012;Wang et al, 2017), or CH 4 from saprotrophic fungi (Mukhin & Voronin, 2008;Lenhart et al, 2012). However, most studies conclude that the bulk of living tree stem CH 4 emissions originate from the rhizosphere and are either transported upwards through the roots via nonpressurized (diffusion) and/or pressurized processes (xylem and sap flow) (Maier et al, 2018;Barba et al, 2019b). Considering that the dead trees would no longer contain active root systems and the flux rates always decreased upwards along the dead stems (Figs 2, S4), this strongly suggests a belowground CH 4 origin with CH 4 transported via passive diffusion and stem degasification increasing with height (Pitz & Megonigal, 2017;Maier et al, 2018;Barba et al, 2019b).…”

“…However, most studies conclude that the bulk of living tree stem CH 4 emissions originate from the rhizosphere and are either transported upwards through the roots via nonpressurized (diffusion) and/or pressurized processes (xylem and sap flow) (Maier et al, 2018;Barba et al, 2019b). Considering that the dead trees would no longer contain active root systems and the flux rates always decreased upwards along the dead stems (Figs 2, S4), this strongly suggests a belowground CH 4 origin with CH 4 transported via passive diffusion and stem degasification increasing with height (Pitz & Megonigal, 2017;Maier et al, 2018;Barba et al, 2019b).…”

“…To date, very few studies have assessed CH 4 fluxes from standing dead tree stems (Carmichael et al, 2014;Warner et al, 2017) and/or saline wetlands. Furthermore, there is no consensus for the biophysical mechanisms transporting CH 4 along the soil-tree-atmosphere continuum (Barba et al, 2019a), with debate continuing as to whether trees: act as passive pipes (diffusion) connecting the rhizosphere to the atmosphere; participate as active pipes (via xylem flow); or produce CH 4 internally in the heartwood (Covey et al, 2012;Barba et al, 2019b). Tree species, tree size, ecological adaptations, seasonality and hydrogeophysical context all likely play a role in the production and pathways of tree stem CH 4 flux (Keppler et al, 2006;McLeod et al, 2008;Pangala et al, 2017;Pitz et al, 2018).…”