| 1. |
- Abdalla, M., et al.
(författare)
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Simulation of CO2 and Attribution Analysis at Six European Peatland Sites Using the ECOSSE Model
- 2014
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Ingår i: Water, Air and Soil Pollution. - : Springer Science and Business Media LLC. - 1573-2932 .- 0049-6979. ; 225:11, s. 2182-2182
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Tidskriftsartikel (refereegranskat)abstract
- In this study, we simulated heterotrophic CO2 (Rh) fluxes at six European peatland sites using the ECOSSE model and compared them to estimates of Rh made from eddy covariance (EC) measurements. The sites are spread over four countries with different climates, vegetation and management. Annual Rh from the different sites ranged from 110 to 540 g C m(-2). The maximum annual Rh occurred when the water table (WT) level was between -10 and -25 cm and the air temperature was above 6.2 degrees C. The model successfully simulated seasonal trends for the majority of the sites. Regression relationships (r(2)) between the EC-derived and simulated Rh ranged from 0.28 to 0.76, and the root mean square error and relative error were small, revealing an acceptable fit. The overall relative deviation value between annual EC-derived and simulated Rh was small (-1 %) and model efficiency ranges across sites from -0.25 to +0.41. Sensitivity analysis highlighted that increasing temperature, decreasing precipitation and lowering WT depth could significantly increase Rh from soils. Thus, management which lowers the WT could significantly increase anthropogenic CO2, so from a carbon emissions perspective, it should be avoided. The results presented here demonstrate a robust basis for further application of the ECOSSE model to assess the impacts of future land management interventions on peatland carbon emissions and to help guide best practice land management decisions.
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| 2. |
- Mbufong, H. N., et al.
(författare)
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Assessing the spatial variability in peak season CO2 exchange characteristics across the Arctic tundra using a light response curve parameterization
- 2014
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Ingår i: Biogeosciences. - : Copernicus GmbH. - 1726-4189. ; 11:17, s. 4897-4912
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Tidskriftsartikel (refereegranskat)abstract
- This paper aims to assess the spatial variability in the response of CO2 exchange to irradiance across the Arctic tundra during peak season using light response curve (LRC) parameters. This investigation allows us to better understand the future response of Arctic tundra under climatic change. Peak season data were collected during different years (between 1998 and 2010) using the micrometeorological eddy covariance technique from 12 circumpolar Arctic tundra sites, in the range of 64-74 degrees N. The LRCs were generated for 14 days with peak net ecosystem exchange (NEE) using an NEE-irradiance model. Parameters from LRCs represent site-specific traits and characteristics describing the following: (a) NEE at light saturation (F-csat), (b) dark respiration (Rd), (c) light use efficiency (alpha), (d) NEE when light is at 1000 mu molm(-2) s(-1) (F-c1000), (e) potential photosynthesis at light saturation (P-sat) and (f) the light compensation point (LCP). Parameterization of LRCs was successful in predicting CO2 flux dynamics across the Arctic tundra. We did not find any trends in LRC parameters across the whole Arctic tundra but there were indications for temperature and latitudinal differences within sub-regions like Russia and Greenland. Together, leaf area index (LAI) and July temperature had a high explanatory power of the variance in assimilation parameters (F-csat, F-c1000 and P-sat), thus illustrating the potential for upscaling CO2 exchange for the whole Arctic tundra. Dark respiration was more variable and less correlated to environmental drivers than were assimilation parameters. This indicates the inherent need to include other parameters such as nutrient availability, substrate quantity and quality in flux monitoring activities.
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| 3. |
- Yttri, K. E., et al.
(författare)
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Source apportionment of the summer time carbonaceous aerosol at Nordic rural background sites
- 2011
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Ingår i: Atmospheric Chemistry and Physics. - : Copernicus GmbH. - 1680-7316 .- 1680-7324. ; 11:24, s. 13339-13357
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Tidskriftsartikel (refereegranskat)abstract
- In the present study, natural and anthropogenic sources of particulate organic carbon (OCp) and elemental carbon (EC) have been quantified based on weekly filter samples of PM10 (particles with aerodynamic diameter
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| 4. |
- Dengel, S., et al.
(författare)
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Testing the applicability of neural networks as a gap-filling method using CH4 flux data from high latitude wetlands
- 2013
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Ingår i: Biogeosciences. - : Copernicus GmbH. - 1726-4189. ; 10, s. 8185-8200
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Tidskriftsartikel (refereegranskat)abstract
- Since the advancement in CH4 gas analyser technology and its applicability to eddy covariance flux measurements, monitoring of CH4 emissions is becoming more widespread. In order to accurately determine the greenhouse gas balance, high quality gap-free data is required. Currently there is still no consensus on CH4 gap-filling methods, and methods applied are still study-dependent and often carried out on low resolution, daily data. In the current study, we applied artificial neural networks to six distinctively different CH4 time series from high latitudes, explain the method and test its functionality. We discuss the applicability of neural networks in CH4 flux studies, the advantages and disadvantages of this method, and what information we were able to extract from such models. Three different approaches were tested by including drivers such as air and soil temperature, barometric air pressure, solar radiation, wind direction (indicator of source location) and in addition the lagged effect of water table depth and precipitation. In keeping with the principle of parsimony, we included up to five of these variables traditionally measured at CH4 flux measurement sites. Fuzzy sets were included representing the seasonal change and time of day. High Pearson correlation coefficients (r) of up to 0.97 achieved in the final analysis are indicative for the high performance of neural networks and their applicability as a gap-filling method for CH4 flux data time series. This novel approach which we show to be appropriate for CH4 fluxes is a step towards standardising CH4 gap-filling protocols.
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| 5. |
- Yi, Chuixiang, et al.
(författare)
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Climate control of terrestrial carbon exchange across biomes and continents
- 2010
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Ingår i: Environmental Research Letters. - : IOP Publishing. - 1748-9326. ; 5:3
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Tidskriftsartikel (refereegranskat)abstract
- Understanding the relationships between climate and carbon exchange by terrestrial ecosystems is critical to predict future levels of atmospheric carbon dioxide because of the potential accelerating effects of positive climate-carbon cycle feedbacks. However, directly observed relationships between climate and terrestrial CO2 exchange with the atmosphere across biomes and continents are lacking. Here we present data describing the relationships between net ecosystem exchange of carbon (NEE) and climate factors as measured using the eddy covariance method at 125 unique sites in various ecosystems over six continents with a total of 559 site-years. We find that NEE observed at eddy covariance sites is (1) a strong function of mean annual temperature at mid-and high-latitudes, (2) a strong function of dryness at mid-and low-latitudes, and (3) a function of both temperature and dryness around the mid-latitudinal belt (45 degrees N). The sensitivity of NEE to mean annual temperature breaks down at similar to 16 degrees C (a threshold value of mean annual temperature), above which no further increase of CO2 uptake with temperature was observed and dryness influence overrules temperature influence.
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| 6. |
- Christensen, Torben, et al.
(författare)
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Monitoring the multi year carbon balance of a subarctic palsa mire with micrometeorological techniques
- 2012
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Ingår i: Ambio. - : Springer Science and Business Media LLC. - 0044-7447 .- 1654-7209. ; 41, s. 207-217
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Tidskriftsartikel (refereegranskat)abstract
- This article reports a dataset on 8 years of monitoring carbon fluxes in a subarctic palsa mire based on micrometeorological eddy covariance measurements. The mire is a complex with wet minerotrophic areas and elevated dry palsa as well as intermediate sub-ecosystems. The measurements document primarily the emission originating from the wet parts of the mire dominated by a rather homogenous cover of Eriophorum angustifolium. The CO2/CH4 flux measurements performed during the years 2001-2008 showed that the areas represented in the measurements were a relatively stable sink of carbon with an average annual rate of uptake amounting to on average -46 g C m(-2) y(-1) including an equally stable loss through CH4 emissions (18-22 g CH4-C m(-2) y(-1)). This consistent carbon sink combined with substantial CH4 emissions is most likely what is to be expected as the permafrost under palsa mires degrades in response to climate warming.
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| 7. |
- Kross, Angela S. E., et al.
(författare)
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Phenology and its role in carbon dioxide exchange processes in northern peatlands
- 2014
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Ingår i: Journal of Geophysical Research - Biogeosciences. - 2169-8953. ; 119:7, s. 1370-1384
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Tidskriftsartikel (refereegranskat)abstract
- Ecosystem phenology plays an important role in carbon exchange processes and can be derived from continuous records of carbon dioxide (CO2) exchange data. In this study we examined the potential use of phenological indices for characterizing cumulative annual CO2 exchange in four contrasting northern peatland ecosystems. We used the approach of Jonsson and Eklundh (2004) to derive a set of phenological indices based on the daily time series of gross primary production (GPP), ecosystem respiration (R-e), and net ecosystem production (NEP) measured in the four peatland sites. The main objectives of this study were (a) to examine the variation in phenological indices across sites and (b) to determine the relationships among phenological indices, environmental conditions, and cumulative annual CO2 exchange. The phenological index used to define the "start of the growing season" showed good potential for differentiation among sites based on their average annual site GPP. Sites with earlier growing seasons had the highest average annual site GPP. The "peak CO2 exchange rate" phenological index performed best in reflecting variations among sites and for estimating annual values of GPP, R-e, and NEP (Pearson correlation coefficients ranged between 0.77 and 0.99, p<0.05 for all.). The phenological indices and annual GPP, R-e, and NEP were sensitive to winter (January-March) and summer (July-September) temperature and precipitation, but correlations, though significant, were weak.
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| 8. |
- Lund, Magnus, et al.
(författare)
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Variability in exchange of CO2 across 12 northern peatland and tundra sites
- 2010
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Ingår i: Global Change Biology. - : Wiley. - 1354-1013 .- 1365-2486. ; 16:9, s. 2436-2448
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Tidskriftsartikel (refereegranskat)abstract
- Many wetland ecosystems such as peatlands and wet tundra hold large amounts of organic carbon (C) in their soils, and are thus important in the terrestrial C cycle. We have synthesized data on the carbon dioxide (CO2) exchange obtained from eddy covariance measurements from 12 wetland sites, covering 1-7 years at each site, across Europe and North America, ranging from ombrotrophic and minerotrophic peatlands to wet tundra ecosystems, spanning temperate to arctic climate zones. The average summertime net ecosystem exchange of CO2 (NEE) was highly variable between sites. However, all sites with complete annual datasets, seven in total, acted as annual net sinks for atmospheric CO2. To evaluate the influence of gross primary production (GPP) and ecosystem respiration (R-eco) on NEE, we first removed the artificial correlation emanating from the method of partitioning NEE into GPP and R-eco. After this correction neither R-eco (P = 0.162) nor GPP (P = 0.110) correlated significantly with NEE on an annual basis. Spatial variation in annual and summertime R-eco was associated with growing season period, air temperature, growing degree days, normalized difference vegetation index and vapour pressure deficit. GPP showed weaker correlations with environmental variables as compared with R-eco, the exception being leaf area index (LAI), which correlated with both GPP and NEE, but not with R-eco. Length of growing season period was found to be the most important variable describing the spatial variation in summertime GPP and R-eco; global warming will thus cause these components to increase. Annual GPP and NEE correlated significantly with LAI and pH, thus, in order to predict wetland C exchange, differences in ecosystem structure such as leaf area and biomass as well as nutritional status must be taken into account.
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| 9. |
- Schubert, Per, et al.
(författare)
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Modeling GPP in the Nordic forest landscape with MODIS time series data-Comparison with the MODIS GPP product
- 2012
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Ingår i: Remote Sensing of Environment. - : Elsevier BV. - 0034-4257 .- 1879-0704. ; 126, s. 136-147
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Tidskriftsartikel (refereegranskat)abstract
- Satellite sensor-derived data are suitable for regional estimations of several important biophysical variables. Data with a finer spatial resolution should improve regional estimations of GPP (gross primary productivity), since they better capture the variation in a heterogeneous landscape. The main objective of this study was to investigate if MODIS 500 m reflectance data can be used to drive empirical models for regional estimations of GPP in Nordic forests. The performance of the proposed models was compared with the MODIS 1 km GPP product. Linear regression analyses were made on 8-day averages of eddy covariance GPP from three deciduous and ten coniferous sites in relation to MODIS 8-day composite data and 8-day averages of modeled incoming PPFD (photosynthetic photon flux density). Time series of EVI2 (two-band enhanced vegetation index) were calculated from MODIS 500 m reflectance data and smoothed by a curve fitting procedure. For most sites, GPP was fairly strongly to strongly related to the product of EVI2 and PPFD (Deciduous: R-2=0.45-0.86, Coniferous: R-2=0.49-0.90). Similar strengths were found between GPP and the product of EVI2 and MODIS 1 km daytime LST (land surface temperature) (R-2=0.55-0.81, 0.57-0.77) and between GPP and EVI2, PPFD and daytime LST in multiple linear regressions (R-2=0.73-0.89,0.65-0.93). One year of data was collected from all coniferous sites to derive a general empirical model for GPP versus (1) the product of EVI2 and PPFD (R-2=0.70), (2) the product of EVI2 and daytime LST (R-2=0.62) and (3) EVI2, PPFD and daytime LST (R-2=0.72). These three models were then validated at six sites for the remaining years by linearly relating eddy covariance GPP to modeled GPP, which resulted in fairly strong to strong relationships for most sites (R-2=0.49-0.91, RMSE=0.63-1.22 gC m(-2) day(-1), R-2=0.53-0.73, RMSE=0.90-1.43 gC m(-2) day(-1) R-2=0.56-0.87, RMSE=0.79-1.11 gC m(-2) day(-1)). In comparison, similar validation strengths were found for the latest collection 5.1 of the MODIS 1 km GPP product (R-2=0.59-0.88, RMSE=0.80-1.16 gC m(-2) day(-1)). The main conclusion is that the suggested empirical models driven by MODIS 500 m reflectance data can be used for regional estimations of Nordic forest GPP, while preserving a finer resolution than the MODIS 1 km GPP product. (C) 2012 Elsevier Inc. All rights reserved.
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| 10. |
- Stoy, Paul C., et al.
(författare)
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A data-driven analysis of energy balance closure across FLUXNET research sites: The role of landscape scale heterogeneity
- 2013
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Ingår i: Agricultural and Forest Meteorology. - : Elsevier BV. - 1873-2240 .- 0168-1923. ; 171, s. 137-152
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Tidskriftsartikel (refereegranskat)abstract
- The energy balance at most surface-atmosphere flux research sites remains unclosed. The mechanisms underlying the discrepancy between measured energy inputs and outputs across the global FLUXNET tower network are still under debate. Recent reviews have identified exchange processes and turbulent motions at large spatial and temporal scales in heterogeneous landscapes as the primary cause of the lack of energy balance closure at some intensively-researched sites, while unmeasured storage terms cannot be ruled out as a dominant contributor to the lack of energy balance closure at many other sites. We analyzed energy balance closure across 173 ecosystems in the FLUXNET database and explored the relationship between energy balance closure and landscape heterogeneity using MODIS products and GLOBEstat elevation data. Energy balance closure per research site (C-EBS)averaged 0.84 +/- 0.20, with best average closures in evergreen broadleaf forests and savannas (0.91-0.94) and worst average closures in crops, deciduous broadleaf forests, mixed forests and wetlands (0.70-0.78). Half-hourly or hourly energy balance closure on a percent basis increased with friction velocity (u.) and was highest on average under near-neutral atmospheric conditions. C-EBS was significantly related to mean precipitation, gross primary productivity and landscape-level enhanced vegetation index (EVI) from MODIS, and the variability in elevation, MODIS plant functional type, and MODIS EVI. A linear model including landscape-level variability in both EVI and elevation, mean precipitation, and an interaction term between EVI variability and precipitation had the lowest Akaike's information criterion value. C-EBS in landscapes with uniform plant functional type approached 0.9 and C-EBS in landscapes with uniform EVI approached 1. These results suggest that landscape-level heterogeneity in vegetation and topography cannot be ignored as a contributor to incomplete energy balance closure at the flux network level, although net radiation measurements, biological energy assimilation, unmeasured storage terms, and the importance of good practice including site selection when making flux measurements should not be discounted. Our results suggest that future research should focus on the quantitative mechanistic relationships between energy balance closure and landscape-scale heterogeneity, and the consequences of mesoscale circulations for surface-atmosphere exchange measurements. (C) 2012 Elsevier B.V. All rights reserved.
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| 11. |
- Watts, J. D., et al.
(författare)
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A satellite data driven biophysical modeling approach for estimating northern peatland and tundra CO2 and CH4 fluxes
- 2014
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Ingår i: Biogeosciences. - : Copernicus GmbH. - 1726-4189. ; 11:7, s. 1961-1980
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Tidskriftsartikel (refereegranskat)abstract
- The northern terrestrial net ecosystem carbon balance (NECB) is contingent on inputs from vegetation gross primary productivity (GPP) to offset the ecosystem respiration (R-eco) of carbon dioxide (CO2) and methane (CH4) emissions, but an effective framework to monitor the regional Arctic NECB is lacking. We modified a terrestrial carbon flux (TCF) model developed for satellite remote sensing applications to evaluate wetland CO2 and CH4 fluxes over pan-Arctic eddy covariance (EC) flux tower sites. The TCF model estimates GPP, CO2 and CH4 emissions using in situ or remote sensing and reanalysis-based climate data as inputs. The TCF model simulations using in situ data explained >70% of the r(2) variability in the 8 day cumulative EC measured fluxes. Model simulations using coarser satellite (MODIS) and reanalysis (MERRA) Records accounted for approximately 69% and 75% of the respective r(2) variability in the tower CO2 and CH4 records, with corresponding RMSE uncertainties of <= 1.3 gCm(-2) d(-1) (CO2) and 18.2 mg Cm-2 d(-1) (CH4). Although the estimated annual CH4 emissions were small (<18 gCm(-2) yr(-1)) relative to R-eco (>180 gCm(-2) yr(-1)), they reduced the across-site NECB by 23% and contributed to a global warming potential of approximately 165 +/- 128 gCO(2)eqm(-2) yr(-1) when considered over a 100 year time span. This model evaluation indi-cates a strong potential for using the TCF model approach to document landscape-scale variability in CO2 and CH4 fluxes, and to estimate the NECB for northern peatland and tundra ecosystems.
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