ALBERT

Description: Natural peatlands are important carbon sinks and sources of methane (CH4). In contrast, drained peatlands turn from a carbon sink to a carbon source and potentially emit nitrous oxide (N2O). Rewetting of peatlands thus implies climate change mitigation. However, data about the time span that is needed for the re-establishment of the carbon sink function by restoration is scarce. We therefore investigated the annual greenhouse gas (GHG) balances of three differently vegetated bog sites 30 years after rewetting. All three vegetation communities turned out to be sources of carbon dioxide (CO2) ranging between 0.6 ± 1.43 t CO2 ha-2 yr-1 (Sphagnum-dominated vegetation) and 3.09 ± 3.86 t CO2 ha-2 yr-1 (vegetation dominated by heath). While accounting for the different global warming potential (GWP) of the three greenhouse gases, the annual GHG balance was calculated. Emissions ranged between 25 and 53 t CO2-eq ha-1 yr-1 and were dominated by large emissions of CH4 (22 up to 51 t CO2-eq ha-1 yr-1), while highest rates were found at purple moor grass (Molinia caerulea) stands. These are to our knowledge the highest CH4 emissions so far reported for bog ecosystems in temperate Europe. As the restored area was subject to large fluctuations in water table, we conclude that the high CH4 emission rates were caused by a combination of both the temporal inundation of the easily decomposable plant litter of this grass species and the plant-mediated transport through its tissues. In addition, as a result of the land use history, the mixed soil material can serve as an explanation. With regards to the long time span passed since rewetting, we note that the initial increase in CH4 emissions due to rewetting as described in the literature is not limited to a short-term period.

Description: Natural peatlands are important carbon sinks and sources of methane (CH4). In contrast, drained peatlands turn from a carbon sink to a carbon source and potentially emit nitrous oxide (N2O). Rewetting of peatlands thus potentially implies climate change mitigation. However, data about the time span that is needed for the re-establishment of the carbon sink function by restoration are scarce. We therefore investigated the annual greenhouse gas (GHG) balances of three differently vegetated sites of a bog ecosystem 30 years after rewetting. All three vegetation communities turned out to be sources of carbon dioxide (CO2) ranging between 0.6 ± 1.43 t CO2 ha−2 yr−1 (Sphagnum-dominated vegetation) and 3.09 ± 3.86 t CO2 ha−2 yr−1 (vegetation dominated by heath). While accounting for the different global warming potential (GWP) of CO2, CH4 and N2O, the annual GHG balance was calculated. Emissions ranged between 25 and 53 t CO2-eq ha−1 yr−1 and were dominated by large emissions of CH4 (22–51 t CO2-eq ha−1 yr−1), with highest rates found at purple moor grass (Molinia caerulea) stands. These are to our knowledge the highest CH4 emissions so far reported for bog ecosystems in temperate Europe. As the restored area was subject to large fluctuations in the water table, we assume that the high CH4 emission rates were caused by a combination of both the temporal inundation of the easily decomposable plant litter of purple moor grass and the plant-mediated transport through its tissues. In addition, as a result of the land use history, mixed soil material due to peat extraction and refilling can serve as an explanation. With regards to the long time span passed since rewetting, we note that the initial increase in CH4 emissions due to rewetting as described in the literature is not inevitably limited to a short-term period.

Description: Evapotranspiration (ET) is a key component of the energy and water balances in permafrost tundra, establishing hydrological conditions for the next year and controlling several aspects of the carbon cycle. Both the energy balance and hydrological conditions of the landscape surface are important drivers of how Arctic climate change will impact landscape processes, including the carbon feedback. The accurate measurement of evapotranspiration within an energy balance context therefore provides crucial information on ecosystem functioning and raises our predictive capacity for estimating the impact of climate change. In this study we report field measurements from 13 summers (2002-14) using the eddy covariance method in a lowland ice-wedge polygon landscape within Russia’s Lena River Delta. These time-series are gap-filled and extrapolated with both statistical and process-based models to generate estimates of growing season ET. We find that interannual differences – including two August periods with high ET and two with low ET – are locally driven more by changes in air temperature and vapor pressure deficit (VPD) than in land surface characteristics or radiation. Except for periods of high VPD, aerodynamic resistance was greater than canopy surface resistance. We explore predictive relationships between various land surface indicators (e.g., NDVI, LAI, LST, Growing season length) derived from remote sensing products (MODIS) to quantify local mechanisms necessary for upscaling to the Delta region. Nighttime land surface temperature (MODIS) is found to be a strong predictor of evaporative flux at weekly to monthly time scales. Contrary to expectations resulting from climate change studies, we do not see evidence of a sustained interannual trend in ET or sensible heat flux. We conclude with implications for the local energy balance and responses to changes in sea ice extent and a warming climate.

Description: Due to the logistic and technical difficulties associated with experimental work in high latitudes, long-term mea- surements of CO2 and CH4 fluxes from arctic ecosystems are still rare, and published trace gas balances often rely on measurements from one or few growing seasons. The inter-annual variability of environmental conditions such as temperature, precipitation, snow cover, and timing of snow melt can be high in the Arctic, especially for regions which are influenced by both continental and maritime climates, such as the Siberian arctic lowlands. For these ecosystems, we must also expect a great inter-annual variability in the balance of trace gases. Multi-annual data sets are needed to investigate this variability and its drivers. Here we present multi-annual late summer CO2 and CH4 flux data from the Lena River Delta in the Siberian Arctic (72˚N, 126˚E). The study site Samoylov Island is characterized by polygonal lowland tundra, a vegetation dominated by mosses and sedges, a soil complex of Glacic, Turbic and Histic Cryosols, and an active layer depth of on average 0.5 m. Seasonal flux measurements were carried out with the eddy covariance technique during the 13-year period 2002 - 2014. Within this period, CO2 flux data overlaps during 37 days (20 July - 25 August) for 12 years, and CH4 flux data overlaps during 25 days (28 July - 21 August) for 9 years. Cumulative net ecosystem CO2 exchange (NEE) during the late summer overlap period is fairly consistent for 9 out of 12 years with a CO2 uptake of 1.9 ± 0.1 mol m−2 . Three years show a clearly smaller uptake of 〈1.5 mol m−2 . A correlation analysis reveals a quadratic relationship between air tem- perature and NEE, which suggests the existence of a temperature optimum where the balance of photosynthesis and ecosystem respiration leads to maximum CO2 net uptake. Both photosynthesis and ecosystem respiration probably benefit initially from higher temperatures, however, in the highest temperature range ecosystem respiration outbal- ances photosynthesis. Median CH4 fluxes during the overlap period ranged between 36 and 64 µmol m−2 hr−1 and were found to be positively linearly correlated to the date of thaw and soil temperature at 10 cm depth in wet polygon centers. This suggests that (i) higher soil temperatures enhance CH4 production more than CH4 oxidation, and (ii) a long thaw period may allow a stronger accumulation of CH4 in soil pore space by methanogens and thus enhance transport processes which bypass oxidation (ebullition, plant-mediated transport). The obtained results indicate that the Siberian polygonal tundra will emit more greenhouse gases in a warming climate - at least on the short term. On the longer term, an adaptation of the vegetation or effects of higher evapotranspiration on the hy- drology may counteract these effects. Our findings can be used to evaluate and train deterministic climate-carbon cycle models for the circumpolar permafrost regions.