B12B-01 INVITED
Controls of Carbon Exchange in a Boreal Minerogenic Mire
Based on theories on both mire development and their response to environmental change, the current role of mires as a net carbon sink has been questioned. A rigorous evaluation of the contemporary net C-exchange in mires requires direct measurements of all relevant fluxes. We use data on carbon exchange from a boreal minerogenic oligotrophic mire (Degerö Stormyr, 64°11' N, 19°33E) to derive a contemporary carbon budget and to analyze the main controls on the C exchange. Data on the following fluxes were collected: land-atmosphere CO2 (continuous Eddy Covariance measurements, 7 years) and CH4 (static chambers during the snow free period, 4 years) exchange; DOC in precipitation; loss of TOC, CO2 and CH4 through water runoff, 4 years (continuous discharge measurement and regular C-content measurements). The annual land atmosphere exchange of CO2 (NEE) was fairly constant between years and varied between -48 – -61 gCm-2yr-1 during six out of the seven years, despite a large variation in weather combinations, the average being -53 ± 5 gCm-2yr-1. Of the net fixation of atmospheric CO2-C during the net uptake period, i.e. the growing season, approximately a third was lost during the net source period, i.e. the winter period. During the four years with measurements of methane and runoff C-export another third of the growing season uptake was lost from the mire ecosystem as methane and runoff C. While the balance between the length of the NEE uptake and the NEE loss period are most important for the annual net ecosystem carbon balance (NECB) it is central to understand the controls of the spring-summer, and the summer-autumn transitions. The onset of the net C uptake period was controlled by the interaction between the water content and the temperature of the peat moss surface. We interpret this as mainly being a control of the CO2 photosynthesis uptake by the Sphagnum mosses. The transition from being a net C sink to being a net C source is in contrast only controlled by the soil temperature. The higher the soil temperature during the months preceding the transition the earlier the mire will shift from being a C sink to become a C source. Our interpretation is that this transition is mainly controlled by the activity of the heterotrophic microorganisms. During a year with exceptional dry late summer the NEE dropped to -17 gCm-2yr-1, compared to -53±5 gCm- 2yr-1 during "normal" years. During this period the water table level was approximately 15 cm below the long-term lowest level. Data indicate that most of the reduction in NEE comes from decreased GPP while the ecosystem respiration was relatively stable between years. Including all component fluxes the mire still is a sink of atmospheric C during average weather conditions. During the years 2004 and 2005 the Net Ecosystem Balance (NECB) was -20±3.3 gCm-2yr-1. Both emission of methane and runoff export of carbon contributed significantly to the loss of carbon. During the dry year with a NEE of -17 gCm-2yr-1 the methane emission and runoff C export resulted in a NECB not different from 0.
B12B-02
Environmental and Physiographic Controls on Inter-Growing Season Variability of Carbon Dioxide and Water Vapour Fluxes in a Minerotrophic Fen
The interaction of fens with groundwater is spatially and temporally highly variable in response to meteorological conditions, resulting in frequent changes of groundwater fluxes in both vertical and lateral directions (flow reversals) across the mineral soil-peat boundary. However, despite the importance of the topographic and hydrogeological setting of fens, no study has been reported in the literature that explores a fen's atmospheric CO2 and energy flux densities under contrasting meteorological conditions in response to its physiographic setting. In our contribution we report four years of growing season eddy covariance and supporting measurements from the Canada Fluxnet-BERMS fen (formerly BOREAS southern peatland) in Saskatchewan, Canada. We first analyze hydrological data along two piezometer transects across the mineral soil-peat boundary with the objective of assessing changes in water table configuration and thus hydraulic gradients, indicating flow reversals, in response to dry and wet meteorological conditions. Next we quantify and compare growing season totals and diurnal and daily variations in evapotranspiration (ET) and net ecosystem exchange (NEE) and its component fluxes gross ecosystem productivity (GPP) and terrestrial ecosystem respiration (TER) to identify their controls with a major focus on water table depth. While ET growing season totals were similar (~ 310 mm) under dry and wet meteorological conditions, the CO2 sink- source strength of Sandhill fen varied substantially from carbon neutral (NEE = -2 [+-7] g C m-2 per growing season) under dry meteorological condition (2003) to a moderate CO2- sink with NEE ranging between 157 [+- 10] and 190 [+- 11] g C m-2 per growing season under wet meteorological conditions (2004, 2005, and 2006). Using a process-oriented ecosystem model, BEPS-TerrainLab, we investigate how different canopy components at Sandhill contribute to total ET and GPP, and thus water use efficiency, under dry and wet meteorological conditions.
B12B-03
Spatially Distributed Eddy-Covariance Measurements of Sensible and Latent Heat Fluxes at a Polygonal Tundra Site on Samoylov Island, Lena River Delta, Siberia
The landscape at Samoylov Island is characterized by permafrost and permafrost related processes that create the typical polygonal micro structures of wet tundra landscapes. These micro-morphologic structures strongly affect surface characteristics such as moisture content and vegetation cover. The polygonal structures usually consist of wet centers surrounded by elevated dry rims that vary in size and moisture content. Some structures show well developed rims and small centers, while others are dominated by wet areas. Several polygon centers consist of peat while other form small ponds. A few structures are affected by thermokarst erosion and form complex lake structures. These microscale landscape variations affect the ratio of dry and wet areas on a larger scale. This ratio is critical to sensible and latent heat exchange processes between soil surface and atmosphere. Thus, small scale variations of polygonal landscape features affect larger scale energy balance processes. This work concerns simultaneous eddy-covariance measurements of sensible and latent heat fluxes and their variations due to small scale differences in surface morphology on Samoylov Island. The first eddy-covariance station was erected in spring 2007 on the west side of the island. In summer 2008, a second, mobile eddy-covariance system was constructed to gather parallel flux measurements across the island. The first eddy-system served as the reference station. Three different measurement locations along a west-east transect across the island were established for the mobile system. The first measurement location is dominated by wet polygonal structures, while the second is characterized by dry, well-developed polygon rims. The third location is situated within an area of free water bodies such as small ponds and thermokarst lakes. The measurement period covers six weeks while the interval of data collection at each location was seven days. Hence every measurement location was measured twice by the mobile station. Both eddy-covariance systems were calibrated against each other while measurements occurred at the same location. Differences in sensible and latent heat fluxes were observed within the wet polygonal tundra site, although landscape variations are very small. In particular, these differences were observed during clear sky conditions, when the difference in the energy flux partitioning is at a maximum between dry and wet surfaces. The most evident differences in sensible heat fluxes were measured at location two, where dry surface conditions are dominant. At this location, the sensible heat flux is up to 20% higher than at the reference station. Differences in latent heat fluxes could be observed at the third measurement location, where free water bodies characterize the surrounding area. At this location, the latent heat flux is up to 30% lower compared to the reference station. The first results indicate that microscale surface variations of the polygonal tundra affect larger scale heat flux processes.
B12B-04
Recent findings in relation to wetland sources of methane at high latitudes
Wet terrestrial and limnic ecosystems at high latitudes such as wet tundra, peatlands and small lakes have long been known as significant contributors to the atmospheric methane concentration. However, the scale of this contribution in relation to other sources is still uncertain. Recently flux studies have shown changing emissions from wet tundra and lakes in this region due to processes associated with permafrost melting in particular in subarctic regions. Looking at the circumpolar North as a whole, however, the data coverage is still poor and up scaling these documented changes on the ground to compare them with atmospheric dynamics remains a difficult exercise. Nevertheless it is important to move closer towards understanding the overall impacts of climate warming in the Arctic on the functioning of the ecosystems that has a high emitting capacity as these potentially hold very strong feedback mechanisms in the climate system. Such improved understanding includes reconciling ground-based flux measurements with atmospheric dynamics and the variations in the growth rate of methane in the atmosphere. This presentation will review the most recent data on high latitude sources of methane and discuss implications for the atmospheric record and associated transport models.
B12B-05 INVITED
Peatland Ecohydrology: Water-Vegetation-Carbon Interactions in a Changing Climate
As natural sources of methane and long-term sinks of carbon dioxide, peatlands play an important role in the global carbon cycle. The position of the water table within a peatland can have a large effect on peatland- atmosphere carbon exchange. With climate models predicting enhanced evapotranspiration under climate change scenarios, and therefore a lower water-table position in peatlands, it has been suggested that peatland methane emissions will decrease while carbon dioxide emissions are expected to increase in the coming decades. This Fickian diffusion centric view of carbon exchange is overly simple and does not consider the effects of water-vegetation-carbon interactions and ecohydrological feedbacks that play an important role in peatland carbon cycling. Here, we will present research from our field and modeling studies investigating the effects of drought and disturbances such as permafrost degradation on vegetation, carbon cycling, and ecohydrological processes in northern wetlands at multiple spatial scales. At local scales, our findings show that interactions among vegetation, soil, and hydrology can lead to unexpected and often complex changes in soil environments that often have consequences for carbon fluxes. For example, our ecosystem-scale water table manipulation experiments in Quebec and Alaska have shown that sustained drought typically leads to peat subsidence, increasing bulk density and decreasing saturated hydraulic conductivity and peatland storativity. In Alaska, the decrease in porosity and water content with drought reduced seasonal ice thaw, which also limited water table drawdown. In contrast, peatlands underlain by permafrost are increasingly experiencing thermokarst and soil flooding. Changes in moss productivity post-thaw led to increased rates of organic matter accumulation, with very different hydrologic and soil properties than peat accumulated in permafrost settings. Through such changes in ecohydrology, responses in both carbon dioxide and methane emissions can differ substantially from predictions based on simple increases or decreases in water table position.
B12B-06
The Multi-annual carbon budget of a peat-covered catchment
This study estimates the complete carbon budget of a 11.4 km2 peat-covered catchment in Northern England. The budget considers both fluvial and gaseous carbon fluxes and includes estimates of particulate organic carbon (POC); dissolved organic carbon (DOC); excess dissolved organic carbon; release of methane (CH4); soil respiration of CO2; and uptake of CO2 by primary productivity. All components except CH4 were measured directly in the catchment and annual carbon budgets were calculated for the catchment between 1993 and 2005 using both extrapolation and interpolation methods. The study shows that: Over the 13 year study period the total carbon balance varied between a net sink of – 20 to - 91 Mg C / km2 / yr. The biggest component of this budget is the uptake of carbon by primary productivity (-178 Mg C / km2 / yr) and in most years the second largest component is the loss of DOC from the peat profile (+39 Mg C / km2 / yr). Direct exchanges of C with the atmosphere average -89 Mg C / km2 / yr in the catchment. Extrapolating the general findings of the carbon budget across all UK peatlands results in an approximate carbon balance of - 1.2 Tg C / yr (+/- 0.4 Pg C / yr) which is larger than previously reported values. Carbon budgets should always be reported with a clear statement of the techniques used and errors involved as this is significant when comparing results across studies.
B12B-07 INVITED
The Influence of Hydrologic Variability on Peatland Dynamics
Climate change may trigger peatlands to become a net source of greenhouse gases, thereby inducing a positive climate feedback effect. Although many studies examine peatland sensitivity to altered climate and environmental state, less is known about peatland sensitivity to hydrologic variability. In this presentation, we explore how precipitation variability affects peatland accumulation and depletion dynamics, and long term peatland accumulation (peat thickness). The analysis shows that peatland dynamics can be bistable, which results in evolving to one of two possible alternative steady states - either deep or shallow peatlands . The analysis uses a coupled peat accumulation hydrology model where precipitation is a stochastic forcing variable. We calibrate the model to represent the West Siberian Lowlands (WSL) between 55-60°N, but our general findings should be applicable to other peatland regions. We show that increasing precipitation variability may eliminate peatland bistability, and that the threshold value of variability where bistability is eliminated is highly sensitive to the average climate state. We also show that variability induces steady state peatlands, i.e. peatlands with peat thickness remain so over time, but cycle through extended periods of accumulation and depletion. This suggests that long term observational studies are necessary to correctly understand how peatlands may respond to climate change and whether they are transitioning to a new steady state. Fundamental changes to WSL 55-60°N peatland are expected as the WSL 21st century climate becomes wetter and warmer and may result in the now deep peatlands to have significant depletion and transition to a stable, shallow peatland. The transition will trigger peat decay from deep peatlands, which may translate into enhanced greenhouse gas emissions acting as a positive climate feedback effect.