In northern peatlands, particularly those that experience seasonal snow-cover, there is a tendency for researchers to only measure greenhouse gas (GHG) fluxes during the growing season. However, it has recently been shown that non-growing season CH4 fluxes can be a considerable component of the annual carbon balance (Treat et al., 2018). Thus, ignoring non-growing season fluxes can lead to incorrect assessments of annual emissions. This means that large-scale (national and international) upscalings of GHG emissions will be biased, and the complete role these ecosystems play in climatic warming will be poorly constrained.
Furthermore, forest fires are a pervasive component in boreal ecosystems and are likely to become larger and more frequent with changing climate (Flannigan et al., 2009). In this case study, Dr. Peacock and Dr. Granath wanted to see if there were differences in non-growing season GHG fluxes between burned and unburned mires and to understand if future changes in fire regimes may alter GHG emissions from forest mires.
The goal of the study was to compare non-growing season GHG emissions from different vegetation types in burned and unburned parts of a forest mire. The study area was a 1.5 km walk from the nearest road, and the burned and unburned sites were approximately 400 m apart. This meant that Dr. Peacock and Dr. Granath needed to cover a lot of ground on foot to collect their data. And because the data were to be collected before the start of the growing season, they needed a system that was not only highly portable, but also able to withstand colder temperatures.
The eosAC soil gas flux chamber paired with a Picarro GasScouter (G4301 CO2, CH4 and H2O analyzer) provided an excellent solution. The ability to log data and control the system using a tablet and wear the GasScouter as a backpack, combined with the clip-on handle for the eosAC made the system highly portable and well suited to the rough terrain.
The study site is located in Norra Lunsen Nature Reserve (59.788, 17.699), approximately 50 km north of Stockholm, Sweden. It is a boreal forest, with mean annual temperature of 6.5°C and annual precipitation of 576 mm. The survey was conducted in March, the first month of the year when monthly average temperature is above freezing (Figure 1). Lunsen Forest is predominantly Scots Pine forest (with some spruce and birch) with areas of open and forested mires.
Two small mires were chosen for measurements, which were 1.5 km into the forest from the nearest road. The burned and unburned sites were approximately 400 m apart (Figure 2) and of similar character before the fire (treed with open wetter areas, dominated by Sphagnum, Eriphorum and shrubs like Rhododendron). Burning occured in 2017 as a result of a forest fire.
At each site the chamber was deployed on areas of mire dominated by different vegetation/microhabitat: Eriphorum sedges, Sphagnum moss, and open water pools. At the burned site, fire had removed surface vegetation, and thus Polytrichum moss had colonised parts of the site. Therefore fluxes from Polytrichum were also measured at this site. The aim of this approach was to see whether fluxes differed between vegetation/ microhabitat types.
To capture fluxes across a range of air temperatures and track the early spring warming, they measured the sampling plots on three occasions: March 3th, 17th and 24th (the red dots in Figure 1). Measurements were taken in triplicate at each sampling location during each deployment, for a total of approximately 60 measurements.
When data from both sites and all sampling occasions were combined, CH4 emissions from sedges were high compared to other microhabitat/vegetation types (Figure 3), with a maximum flux of 154 mg m-2 d-1. This could be because sedges possess aerenchymatous tissue providing a direct pathway for transport of CH4 from the anoxic zone of the peat to the atmosphere (Greenup et al., 2000). These plants release labile root exudates which can stimulate CH4 production (Ström et al., 2012), which could be causing or contributing to the elevated CH4 emissions observed.
Mean CH4 fluxes from all sampling plots were highest at the end of March when daily mean temperatures were greatest (Figure 4). This is unsurprising, as warmer temperatures are favourable for methanogenesis.
Mean CH4 fluxes were approximately three times larger at the burned site compared to the unburned site (Figure 5). The response of peatland CH4 emissions to wildfire is complex and difficult to predict (Davidson et al., 2019), but post-fire increases in CH4 have been observed and attributed to reductions in methanotrophic bacteria (Danilova et al., 2015) and changing vegetation composition (Grau-Andrés et al., 2019).
Carbon dioxide fluxes changed with vegetation type/microhabitat and were highest from open water (Figure 6). Larger CH4 fluxes were expected from the open water surfaces, as these pools generally have high emissions. Instead, the pools were large sources of CO2.
Overnight temperatures dipped below freezing at various points during the sampling campaign (Figure 1), which could have resulted in pool sediments that were too cold for methane production. However, CH4 emissions from the sedges (Figure 3) indicate that production is occurring. It is possible that deeper in the peat, the vegetation and soil act as insulators, keeping temperatures higher and more stable. If so, the sedges can transport CH4 produced in the sediments to the atmosphere. Alternatively, the water may simply be keeping the sediments cold, preventing CH4 production.
There was no apparent pattern with CO2 flux and sampling date (Figure 7) but emissions were lower at the burned site (Figure 8).
Although higher emissions were expected from sedges, the magnitude of CH4 fluxes (10 times larger than Sphagnum) was surprising. The results show that non-growing season CH4 emissions in boreal mires need to be considered in sampling campaigns, at least from areas of sedges which act as hotspots for CH4 release. Furthermore, the results indicate that GHG emissions from small forest mires may be altered following wildfire; although Dr. Peacock and Dr. Granath measured larger CH4 fluxes at the burned site these were counteracted by lower CO2 fluxes.
The sites for this study were challenging to access, requiring a 500 m traverse through pathless tangled forest and across flooded wetlands. The portability of the GasScouter and eosAC chamber allowed Dr Peacock and Dr. Granath to easily cross difficult ground. The ability of analyser and chamber to function well in cold conditions meant they could easily and reliably collect non-growing season flux measurements.
Thanks to Mike Peacock of SLU and Gustaf Granath of Uppsala University in Sweden for sharing their photos, performing the measurements and analyzing the data associated with this study.
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