Tree stems are a net source of CH₄ and N₂O in a hemiboreal drained peatland forest during the winter period

Abstract

Nutrient-rich northern peatlands are often drained to enhance forest productivity, turning peatland soils into sinks of methane (CH4) and sources of nitrous oxide (N2O). However, further attention is needed on CH4 and N2O dynamics during the winter period to fully understand the spatio-temporal variability of fluxes. Besides soil, tree stems can also emit CH4 and N2O. However, stem contribution is not considered in most biogeochemical models. We determined the temporal dynamics of winter-time CH4 and N2O fluxes in a drained peatland forest by simultaneously measuring stem and soil fluxes and exploring the relationships between gas fluxes and soil environmental parameters. During sampling (October 2020–May 2021), gas samples from Downy Birch (Betula pubescens) and Norway Spruce (Picea abies) trees were collected from different tree heights using manual static chambers and analysed using gas chromatography. Soil CH4 and N2O concentrations were measured using an automated dynamic soil chamber system. 
Tree stems were a net source of CH4 and N2O during the winter period. The origin of stem CH4 emissions was unclear, as stem and soil CH4 fluxes had opposite flux directions, and the irregular vertical stem flux profile did not indicate a connection between stem and soil fluxes. Stem N2O emissions may have originated from the soil, as emissions decreased with increasing stem height and were driven by soil N2O emissions and environmental parameters. Soil was a net sink for CH4, largely determined by changes in soil temperature. Soil N2O dynamics were characterised by hot moments – short periods of high emissions related to changes in soil water content. Tree stem emissions offset the soil CH4 sink by 14% and added 2% to forest floor N2O emissions. Therefore, CH4 and N2O budgets that do not incorporate stem emissions can overestimate the sink strength or underestimate the total emissions of the ecosystem.

A drained nutrient‐poor peatland forest in boreal Sweden constitutes a net carbon sink after integrating terrestrial and aquatic fluxes

“…Specifically, chamber‐derived CH 4 emissions were significantly lower and higher in the sparse and dense forest areas, respectively, compared to EC estimates. The mismatch in the dense forest might arise from local flooded areas with higher CH 4 emissions as well as additional emissions from tree stems (Ranniku et al., 2023; Vainio et al., 2022) and ditches (Peacock et al., 2021), all of which were not captured by chamber measurements. In the sparse forest, the higher chamber CH 4 emission estimate was driven by large fluxes from three of the five sampling locations (Figure S8).…”

“…This could be explained by the relatively high WTL at the sparse forest section (mean WTL = −15 cm) and the existence of locally flooded areas in the dense forest section (mean WTL = −32 cm). In addition, while previous estimates relate to soil CH 4 fluxes, our continuous (halfhourly) and ecosystem-scale measurements might have captured spatiotemporal peaks and emissions via trees (Ranniku et al, 2023;Vainio et al, 2022) and from ditches (Peacock et al, 2021). Overall, our results suggest a negligible contribution of the CH 4 flux to the annual NECB and GHG balance (accounting for its 28 times higher warming potential over a 100-year time frame; IPCC, 2022) in our drained boreal peatland forest.…”

“…81 and 28 times higher during a 20‐ and 100‐year time frame, respectively; IPCC, 2022), changes in CH 4 fluxes may strongly modify the net climate impact of peatland forestry. Yet, while previous CH 4 flux estimates in drained boreal peatland forests rely on chamber measurements of soil fluxes (Korkiakoski et al., 2017; Meyer et al., 2013; Ojanen et al., 2013), EC‐based estimates of the whole ecosystem CH 4 exchange, capturing also potentially significant emissions via trees (Ranniku et al., 2023; Vainio et al., 2022) and from ditches (Peacock et al., 2021), are not available to date.…”

Substantial uptake of nitrous oxide (N2O) by shoots of mature European beech

“…The investigation of trees' N2O exchange potential is still a young research field. Under natural field conditions, stems of different tree species, including both upland and wetland tree species, mostly have been found to be low or even negligible N2O emitters (Machacova et al, 2016(Machacova et al, , 2017a(Machacova et al, , 2019Schindler et al, 2020Schindler et al, , 2021Mander et al, 2021;Moldaschl et al, 2021;Ranniku et al, 2023). First studies from the tropical zone show results similar to those from the colder zones (Iddris et al, 2020;Soosaar et al, 2022).…”