Soil-tree-atmosphere CH4 flux dynamics of boreal birch and spruce trees during spring leaf-out

Abstract

Aims Studies on tree CH4 exchange in boreal forests regarding seasonality and role of tree canopies are rare. We aimed to quantify the contribution of boreal trees to the forest CH4 budget during spring leaf-out and to reveal the role of microbes in the CH4 exchange. Methods Methane fluxes of downy birch and Norway spruce (Betula pubescens and Picea abies) growing on fen and upland sites were measured together with soil CH4 flux, environmental variables and microbial abundances involved in the CH4 cycle. Tree CH4 fluxes were studied from three stem heights and from shoots. Results The trees emitted CH4 with higher stem emissions detected from birch and higher shoot emissions from spruce. The stem CH4 emissions from birches at the fen were high (mean 45 µg m−2 h−1), decreasing with stem height. Their dynamics followed soil temperature, suggesting the emitted CH4 originated from methanogenic activity, manifested in high mcrA gene copy numbers, in the peat soil. Methanogens were below the quantification limit in the tree tissues. Upscaled tree CH4 emissions accounted for 22% of the total CH4 emissions at the fen. Conclusions The variation in stem CH4 flux between the trees and habitats is high, and the emissions from high-emitting birches increase as the spring proceeds. The lack of detection of methanogens or methanotrophs in the aboveground plant tissues suggests that these microbes did not have a significant role in the observed tree-derived fluxes. The stem-emitted CH4 from birches at the fen is presumably produced microbially in the soil.

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

“…For example, emissions around 10 μg C m −2 h −1 have been reported from alder stems in a hemiboreal riparian forest [19], whereas average stem emissions reached around 50 μg C m −2 h −1 from birch stems and 110 μg C m −2 h −1 from alder stems during winter months in a temperate forested peatland [21]. In addition, we found spruce stem CH 4 emissions to be lower than birch emissions, similarly to boreal trees measured by Vainio et al [35]. Spatial and species-level variability in stem fluxes can been linked to hydrologic conditions in the soil [14,17], aspects of stem morphology and tree physiology, such as wood density, lenticel abundance, transpiration, sap flow rates, and wood vessel structure [22,26,27,36,37], as well as differences in microbiology in stems and soils [17,21].…”

“…This could also explain higher emissions from birch stems than from spruce, as the fine root density of Norway spruce is generally higher closer to the soil surface [43]. Thus, only birch roots may reach the deeper soil layers to facilitate CH 4 transport [35]. To further examine the origin of tree stem CH 4 emissions, information about CH 4 concentrations in the soil vertical profile and identification of methanotrophic and methanogenic bacteria in different soil layers, as well as in the tree heartwood, is needed.…”

Model of methane transport in tree stems: Case study of sap flow and radial diffusion

“…The heights of the trees were 11.3 and 8.5 m and diameters at 1.3 m height were 9.5 and 7.3 cm, respectively, for the birch and spruce. Full details of the field measurements are given in Vainio et al, (2022).…”

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).…”

“…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.…”