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Trees as methane sources: A case study of West Siberian South taiga

Within this study, we were measuring methane emission from the tree trunks, leaves and branches in the seasonally flooded forest and in the forested bogs (pine-shrub-sphagnum ecosystems or “ryams”) in south taiga zone of Western Siberia. Our results suggest that the tree trunks may act as a methane conductor from the soil to the atmosphere bypassing the methanotrophically active zones of soil. The tree methane flux depends on a trunk diameter and an ecosystem type. The average methane emission from tree trunks was 0.0061±0.0003 mg CH4·m-2·h-1 per unit of ground area. The methane emission from branches and leaves was zero.


Introduction
High-quality quantitative estimates of the CH 4 budget are crucial for predicting climate changes, managing Earth's carbon reservoirs, and understanding atmospheric chemistry [1]. The growth of the methane concentration reached up to 1% per year in the XX century. Currently several methane sources are supposed to contribute to the emission and the wetlands are the largest natural methane source [2].
However, there are some differences in CH 4 emission estimates by bottom-up models using direct flux measurements and by top-down models using the inverse modeling of tall tower and satellite data [3]. It is suggested that we may still be missing some unknown methane source.
The majority of chamber flux measurements were made from soil and excluded trees [4,5,6,7]. At the same time the role of plant-mediated methane transport from soil to the atmosphere has been well known for a long time [8,9,10,11]. Now we have a number of evidences that methane may be emitted out of and through trees [12,13,14,15]. In this paper, we studied trees in seasonally flooded forests potentially significant methane sources in Western Siberia.
The tasks of this study were to make field measurements of methane emitted out of and through tree trunks and leaves and to study the environmental controls of the methane fluxes from trees (tree diameter, chamber installation height, temperature, daytime) in various Betula pendula and Pinus sylvestris ecosystems.

Research sites
The CH 4 flux measurements from the trees and soil were carried out in Bakcharsky region of Tomsk oblast in the south taiga zone of Western Siberia during July and August 2016-2017 (figure 1). The measurements were conducted at three sites in seasonally flooded forest and two sites in forested bogs (pine-shrub-sphagnum ecosystems or "ryams"

Methods
The methane flux measurements from tree trunks were made in accordance with a chamber method modification by Siegenthaler et al. [16]. In our case, the chamber height was 2.5 cm, its volume was one liter, and the chamber walls were joined together using the universal glue "Moment" (Henkel, Germany). The gas sample volume was 5-6 ml. The exposure time varied from 8 minutes to 2 hours. The median error of individual measurements from the tree trunk was 0.008 mgCH 4 ·m -2 ·h -1 .
To measure methane emitted out of leaves and branches, the ends of branches were placed into 1-5 liter plastic bottles sealed by k-flex stopper in place of a contact. To take 5 ml samples through the kflex stopper, we used a silicone tube with inner diameter 2 mm and 12 ml syringes. Three samples were taken out when the 5-liter bottle was used and one sample in a case of the 1-liter bottle. The methane flux for the 5-liter bottle measurements were calculated by linear regression with the weights and by non-linear (exponential) regression for emission and consumption, respectively, using chamber headspace concentrations versus measuring time [17]. The methane flux for the 1-liter bottle measurements was calculated by averaging two samples from the same branch of a tree.
The methane and carbon dioxide flux measurements from the soils were made by classical chamber method (as was described in [18]) in parallel with the methane flux measurements from trees. The chambers were installed close to the measured trees.
The concentrations were analyzed no later than 18 hours after gas sampling (syringes with stoppers were stored in NaCl concentrated solution before the analysis). Gas concentrations were measured using a modified gas chromatograph KhPM-4 ("Chromatograph", Russia) with a flame ionization detector from a chromatograph LHM-80 ("Chromatograph", Russia) with 1 m stainless steel column (2.5 mm o.d.) filled with Sovpol (80-100 mesh) at 35°C with hydrogen as a carrier gas (flow rate 5 ml/min) and the loop volume of 0.5 ml. For chromatograph calibration, we used methane and air standard mix with CH 4 1.99, 5.08 and 9.84 ppm concentrations of ±0.01 ppm accuracy (National Institute for Environmental Studies, Japan). The volume of injected sample was 2-3 ml. Every gas sample from a syringe was analyzed in 3-4 replicates; the average concentration was used for the flux calculation. The methane flux from trees was calculated using the difference between the atmosphere gas concentration before the measurements and the final CH 4 concentration in the chamber headspace. The fluxes in the table 2 were calculated by averaging from 2 to 3 replications of individual flux measurements from the same place of a tree.
To analyze the data, we used Wilcoxon rank sum test in MATLAB R2009a, regression analysis and the zero comparison test using t-criteria [19]. The statistical significance P-value was 0.05.

Results
The methane fluxes measured from leaves and branches were too small (comparing to the measurement error) if they actually emit the methane (table 2). The zero comparison test showed that leaves and branches methane fluxes were indistinguishable from zero with 0.05 significance level. The methane fluxes from tree trunks were higher, and the zero comparison test indicated that fluxes could be distinguished from zero with 0.05 significance level. The results of the individual methane flux measurements from tree trunks are given in the table 3.   The linear regression analysis revealed that methane fluxes from tree trunks depended on a tree diameter (F=0.040, R 2 =0.121; figure 2.a). The influence of chamber installation height, temperature, daytime and the exposure time on methane flux was not found though more measurements are needed for reliability. The Wilcoxon rank sum test revealed that methane fluxes from Betula pendula and Pinus sylvestris trunks differed insignificantly (P=0.66) though more measurements are needed as well. The methane fluxes from tree trunks at ryams and seasonally flooded forest located nearby the Bakcharskoe bog also differed insignificantly (P=0.66). Insignificant variations in methane fluxes (P=0.73) were also observed among tree trunks of seasonally flooded forests within the drained bog and within the floodplain of the Bakchar river tributary ( figure 2.b).
It was found that methane fluxes from tree trunks tended to be higher with the decrease of methane fluxes from soils (figure 3.a) and the increase of carbon dioxide fluxes ( figure 3.b). However, detailed study is needed to confirm this dependence.

Discussion
The increase of methane concentration in soils was found at 30 cm depth as shown in the article Mochenov et al. published in this Proceedings Book. As the tree roots consume water from deeper layers, they uptake water with high dissolved methane concentration. Peat soils absorb water more intensively, so plants get less water than in mineral soils. Therefore, the transpiration will be lower in peat soils leading to lower methane release through trees. This may explain methane flux differences at the sites (figure 2.b). The ryam and seasonally flooded forest near The Bakcharskoe bog is characterized by peat soils resulting in lower methane fluxes from trees at these sites. Soils in seasonally flooded forest at the drained bog are more mineral, while the floodplain soils contain no peat. Methane fluxes from trees at these sites are considerably higher in comparison to sites with peat soils.
The high methane concentration may also explain the more intensive methane fluxes from trees with a large trunk diameter as those trees have deeper and more extended roots, so they could uptake greater amount of water with dissolved methane. Pangala et al. [15] reported that methane fluxes decrease with the tree diameter raise. However, they measured methane fluxes from trees with a larger trunk diameter (20-130 cm) and the highest methane fluxes were produced by the trees with 20-50 cm diameter. Our data showed that the maximal fluxes were released by the trees with a 10-30 cm trunk diameter. More measurements are needed from larger trees, but they were rarely found in the studied ecosystems. The trees with a larger diameter may have different factors effecting CH 4 flux.
When the methane emits through the trees, the methane flux from soils may decrease due to one more methane release way diminishing the methane amount in the source. At the same time, the increase of CO 2 fluxes along with the methane flux increase from tree may be caused by microbiological activity.
The data from our study are mostly in a good correspondence with the data from the other studies (table 4). The highest CH 4 flux from trees was found at the tropical forested peatland [15] and the lowest CH 4 flux was found at the boreal coniferous forest [20]. The latitude effect on methane fluxes from trees might be caused by the plant transpiration increase with the temperature rise resulting in a greater methane release through the trees.

Conclusion
We may conclude that the tree trunks in periodically flooded forests may act as a methane conductor from the soil to the atmosphere bypassing the methanotrophically active zones of soil. The methane fluxes from lives and branches are too small and indistinguishable from zero while the methane fluxes from tree trunks are higher, distinguish from zero and depend on tree diameter and ecosystem type. There was found no dependence of methane fluxes from tree trunks and chamber installation height, temperature, daytime and tree species. The methane fluxes from trees varied from 0.0013±0.0018 to 0.0323±0.0009. The methane flux from trees per a unit of ground area was 0.0061±0.0003. The positive correlation of methane fluxes from trees and carbon dioxide flux from soil were found while the correlation between methane fluxes from trees and methane flux from soil was negative.