Wetlands are the largest natural source of atmospheric methane. Here, we assess controls on methane flux using a database of approximately 19 000 instantaneous measurements from 71 wetland sites located across subtropical, temperate, and northern high latitude regions. Our analyses confirm general controls on wetland methane emissions from soil temperature, water table, and vegetation, but also show that these relationships are modified depending on wetland type (bog, fen, or swamp), region (subarctic to temperate), and disturbance. Fen methane flux was more sensitive to vegetation and less sensitive to temperature than bog or swamp fluxes. The optimal water table for methane flux was consistently below the peat surface in bogs, close to the peat surface in poor fens, and above the peat surface in rich fens. However, the largest flux in bogs occurred when dry 30-day averaged antecedent conditions were followed by wet conditions, while in fens and swamps, the largest flux occurred when both 30-day averaged antecedent and current conditions were wet. Drained wetlands exhibited distinct characteristics, e.g. the absence of large flux following wet and warm conditions, suggesting that the same functional relationships between methane flux and environmental conditions cannot be used across pristine and disturbed wetlands. Together, our results suggest that water table and temperature are dominant controls on methane flux in pristine bogs and swamps, while other processes, such as vascular transport in pristine fens, have the potential to partially override the effect of these controls in other wetland types. Because wetland types vary in methane emissions and have distinct controls, these ecosystems need to be considered separately to yield reliable estimates of global wetland methane release.

Northern peatlands provide important global and regional ecosystem services (carbon storage, water storage, and biodiversity). However, these ecosystems face increases in the severity, areal extent and frequency of climate-mediated (e.g. wildfire and drought) and land-use change (e.g. drainage, flooding and mining) disturbances that are placing the future security of these critical ecosystem services in doubt. Here, we provide the first detailed synthesis of autogenic hydrological feedbacks that operate within northern peatlands to regulate their response to changes in seasonal water deficit and varying disturbances. We review, synthesize and critique the current process-based understanding and qualitatively assess the relative strengths of these feedbacks for different peatland types within different climate regions. We suggest that understanding the role of hydrological feedbacks in regulating changes in precipitation and temperature are essential for understanding the resistance, resilience and vulnerability of northern peatlands to a changing climate. Finally, we propose that these hydrological feedbacks also represent the foundation of developing an ecohydrological understanding of coupled hydrological, biogeochemical and ecological feedbacks.

[1] Growing season CH 4 fluxes were monitored over a two year period following the start of ecosystem-scale manipulations of water table position and surface soil temperatures in a moderate rich fen in interior Alaska. The largest CH 4 fluxes occurred in plots that received both flooding (raised water table position) and soil warming, while the lowest fluxes occurred in unwarmed plots in the lowered water table treatment. A combination of treatment and soil hydroclimate variables explained more than 70% of the variation in lntransformed CH 4 fluxes, with mean daily water table position representing the strongest predictor. We used quantitative PCR of the a-subunit of mcr operon to explore the influence of soil climate manipulations on methanogen abundances. Methanogen abundances were greatest in warmed plots, and showed a positive relationship with mean daily CH 4 fluxes. Our results show that water table manipulations that led to soil inundation (flooding) had a stronger effect on CH 4 fluxes than water table drawdown. Seasonal CH 4 fluxes increased by 80-300% under the combined wetter and warmer soil climate treatments. Thus, while warming is expected to increase CH 4 emissions from Alaskan wetlands, higher water table positions caused by increases in precipitation or disturbances such as permafrost thaw that lead to thermokarst and flooding in wetlands will stimulate CH 4 emissions beyond the effects of soil warming alone. Consequently, we argue that modeling the effects of climate change on Alaskan wetland CH 4 emissions needs to consider the interactive effects of soil warming and water table position on CH 4 production and transport.

[1] Northern peatlands are significant stocks of terrestrial soil carbon, and it has been predicted that warmer temperatures and lower water tables resulting from climate change will convert these ecosystems into sources for atmospheric carbon dioxide (CO 2 ). However, these predictions do not consider the potential for hydrologically induced ecological succession or the spatial variability of carbon accumulation rates between different microforms in peatlands. To address these issues, the vegetation community was described, and the rates of gross ecosystem photosynthesis (GEP), ecosystem respiration (R tot ) and net ecosystem CO 2 exchange were determined along poor fen microtopographic gradients at a control site and at a site which experienced a water table drawdown of $20 cm 8 years prior to the study (drained). Sampling plots within these sites were classified as microforms of hummocks, lawns, or hollows. The coverage of Sphagnum moss declined on drained hummocks, drained lawns were invaded by sedges, and hollows shifted from open water plots at the control site to Sphagnum-dominated plots with sparse vascular plant cover at the drained site. As a result, R tot was significantly greater at the drained site at all microforms while maximum rates of GEP declined at drained hummocks and were enhanced at drained lawns and hollows compared to similar control microforms. These results suggest that predictions about the response of northern peatland carbon exchange to climate change must consider the interaction between ecology and hydrology and the differential responses of microforms related to their initial ecohydrological conditions.

Measurement of the spatial variability of CH4 emissions and net CO2 ecosystem exchange were made in a boreal peatland in northern Sweden in the summers of 1992 and 1993. Variability was monitored at the microscale (hummocks and hollows), mesoscale (ridges, lawns and pools), and macroscale (landforms) to assess the role of peatland topography on the magnitude and variability of the fluxes. The general trend is for topographically lower areas, such as hollows, pools, or peatland margins, to have higher CH4 emissions and lower CO2 uptake than the adjacent topographically higher areas such as hummocks, ridges, and plateaus. However, the greatest difference occurs at the microtopographic scale because the maximum differences in water table position and temperature occur at the microscale. The CH4 flux at the margins of the peatland was three to four times greater than that observed at the central plateau sites. Net CO2 uptake was also greatest at the margin sites. A combined meso‐macro topographic model (MMF) was used to estimate the total peatland CH4 and CO2 exchange. The model indicates that a failure to measure the exchange of carbon from peatland pools can result in a large overestimate of the total peatland NEE and therefore carbon accumulation rates.

No abstract

The boreal biome is characterised by extensive wildfires that frequently burn into the thick organic soils found in many forests and wetlands. Previous studies investigating surface fuel consumption generally have not accounted for variation in the properties of organic soils or how this affects the severity of fuel consumption. We experimentally altered soil moisture profiles of peat monoliths collected from several vegetation types common in boreal bogs and used laboratory burn tests to examine the effects of depth-dependent variation in bulk density and moisture on depth of fuel consumption. Depth of burning ranged from 1 to 17 cm, comparable with observations following natural wildfires. Individually, fuel bulk density and moisture were unreliable predictors of depth of burning. However, they demonstrated a cumulative influence on the thermodynamics of downward combustion propagation. By modifying Van Wagner's surface fuel consumption model to account for stratigraphic changes in fuel conditions, we were able to accurately predict the maximum depth of fuel consumption for most of the laboratory burn tests. This modified model for predicting the depth of surface fuel consumption in boreal ecosystems may provide a useful framework for informing wildland fire management activities and guiding future development of operational fire behaviour and carbon emission models.

Summary 1. Abandoned cutover peatlands are persistent sources of atmospheric CO2. Net ecosystem CO2 exchange and Sphagnum net primary production of an abandoned block‐cut bog were measured in the field and in the laboratory using gas exchange techniques to determine the processes controlling CO2 exchange in these ecosystems. 2. Sphagnum net primary production was offset by peat respiration, resulting in the peatland becoming a net source of CO2 during the summer months. 3. Sphagnum photosynthesis was greatest at wet sites. In addition, sites with vascular plant cover photosynthesized at approximately twice the rate of sites where vascular plants were removed. 4. Laboratory results indicate that drying and wetting cycles negatively affect Sphagnum net primary production and net ecosystem CO2 exchange. Sphagnum and peat respiration increased 4–14‐fold upon rewetting, whereas Sphagnum photosynthesis did not recover until 20 days of saturation. 5. Synthesis and applications. This research emphasizes the importance of stable moisture availability for the growth of Sphagnum and the eventual development of a new acrotelm on the cutover bog surface. Restoration techniques must therefore include companion species and a constant moisture supply above the minimum threshold for Sphagnum mosses.