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  • 1. Azevedo, Olivia
    et al.
    Parker, Thomas C.
    Siewert, Matthias B.
    Subke, Jens-Arne
    Predicting Soil Respiration from Plant Productivity (NDVI) in a Sub-Arctic Tundra Ecosystem2021Ingår i: Remote Sensing, E-ISSN 2072-4292, Vol. 13, nr 13Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Soils represent the largest store of carbon in the biosphere with soils at high latitudes containing twice as much carbon (C) than the atmosphere. High latitude tundra vegetation communities show increases in the relative abundance and cover of deciduous shrubs which may influence net ecosystem exchange of CO2 from this C-rich ecosystem. Monitoring soil respiration (Rs) as a crucial component of the ecosystem carbon balance at regional scales is difficult given the remoteness of these ecosystems and the intensiveness of measurements that is required. Here we use direct measurements of Rs from contrasting tundra plant communities combined with direct measurements of aboveground plant productivity via Normalised Difference Vegetation Index (NDVI) to predict soil respiration across four key vegetation communities in a tundra ecosystem. Soil respiration exhibited a nonlinear relationship with NDVI (y = 0.202e3.508 x, p < 0.001). Our results further suggest that NDVI and soil temperature can help predict Rs if vegetation type is taken into consideration. We observed, however, that NDVI is not a relevant explanatory variable in the estimation of SOC in a single-study analysis.

  • 2.
    Cai, Zhanzhang
    et al.
    Lund University.
    Junttila, Sofia
    Lund University.
    Holst, Jutta
    Lund University.
    Jin, Hongxiao
    Lund University; Technical University of Denmark.
    Ardo, Jonas
    Lund University.
    Ibrom, Andreas
    Technical University of Denmark.
    Peichl, Matthias
    Swedish University of Agricultural Sciences, Umeå.
    Molder, Meelis
    Lund University.
    Jönsson, Per
    Malmö universitet, Institutionen för materialvetenskap och tillämpad matematik (MTM).
    Rinne, Janne
    Lund University.
    Karamihalaki, Maria
    Lund University.
    Eklundh, Lars
    Lund University.
    Modelling Daily Gross Primary Productivity with Sentinel-2 Data in the Nordic Region-Comparison with Data from MODIS2021Ingår i: Remote Sensing, E-ISSN 2072-4292, Vol. 13, nr 3, artikel-id 469Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The high-resolution Sentinel-2 data potentially enable the estimation of gross primary productivity (GPP) at finer spatial resolution by better capturing the spatial variation in a heterogeneous landscapes. This study investigates the potential of 10 m resolution reflectance from the Sentinel-2 Multispectral Instrument to improve the accuracy of GPP estimation across Nordic vegetation types, compared with the 250 m and 500 m resolution reflectance from the Moderate Resolution Imaging Spectroradiometer (MODIS). We applied linear regression models with inputs of two-band enhanced vegetation index (EVI2) derived from Sentinel-2 and MODIS reflectance, respectively, together with various environmental drivers to estimate daily GPP at eight Nordic eddy covariance (EC) flux tower sites. Compared with the GPP from EC measurements, the accuracies of modelled GPP were generally high (R-2 = 0.84 for Sentinel-2; R-2 = 0.83 for MODIS), and the differences between Sentinel-2 and MODIS were minimal. This demonstrates the general consistency in GPP estimates based on the two satellite sensor systems at the Nordic regional scale. On the other hand, the model accuracy did not improve by using the higher spatial-resolution Sentinel-2 data. More analyses of different model formulations, more tests of remotely sensed indices and biophysical parameters, and analyses across a wider range of geographical locations and times will be required to achieve improved GPP estimations from Sentinel-2 satellite data.

  • 3. Cai, Zhanzhang
    et al.
    Jönsson, Per
    Jin, Hongxiao
    Eklundh, Lars
    Performance of Smoothing Methods for Reconstructing NDVI Time-Series and Estimating Vegetation Phenology from MODIS Data2017Ingår i: Remote Sensing, E-ISSN 2072-4292, Vol. 9, nr 12Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Many time-series smoothing methods can be used for reducing noise and extracting plant phenological parameters from remotely-sensed data, but there is still no conclusive evidence in favor of one method over others. Here we use moderate-resolution imaging spectroradiometer (MODIS) derived normalized difference vegetation index (NDVI) to investigate five smoothing methods: Savitzky-Golay fitting (SG), locally weighted regression scatterplot smoothing (LO), spline smoothing (SP), asymmetric Gaussian function fitting (AG), and double logistic function fitting (DL). We use ground tower measured NDVI (10 sites) and gross primary productivity (GPP, 4 sites) to evaluate the smoothed satellite-derived NDVI time-series, and elevation data to evaluate phenology parameters derived from smoothed NDVI. The results indicate that all smoothing methods can reduce noise and improve signal quality, but that no single method always performs better than others. Overall, the local filtering methods (SG and LO) can generate very accurate results if smoothing parameters are optimally calibrated. If local calibration cannot be performed, cross validation is a way to automatically determine the smoothing parameter. However, this method may in some cases generate poor fits, and when calibration is not possible the function fitting methods (AG and DL) provide the most robust description of the seasonal dynamics.

  • 4.
    Carvajal, Gisela K.
    et al.
    Chalmers, Dept Earth & Space Sci, S-41296 Gothenburg, Sweden..
    Wåhlin, Anna K.
    Univ Gothenburg, Dept Earth Sci, S-40530 Gothenburg, Sweden..
    Eriksson, Leif E. B.
    Chalmers, Dept Earth & Space Sci, S-41296 Gothenburg, Sweden..
    Ulander, Lars M. H.
    Chalmers, Dept Earth & Space Sci, S-41296 Gothenburg, Sweden.;Swedish Def Res Agcy FOI, S-58111 Linkoping, Sweden..
    Correlation between Synthetic Aperture Radar Surface Winds and Deep Water Velocity in the Amundsen Sea, Antarctica2013Ingår i: Remote Sensing, E-ISSN 2072-4292, Vol. 5, nr 8, s. 4088-4106Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The recent observed thinning of the glacier ice shelves in the Amundsen Sea (Antarctica) has been attributed to warm deep currents, possibly induced by along-coast winds in the vicinity of the glacial ice sheet. Here, high resolution maps of wind fields derived from Synthetic Aperture Radar (SAR) data have been studied and correlated with subsurface measurements of the deep water velocities in the Amundsen Sea area. Focus is on periods with low ice coverage in 2010 and 2011. In 2010, which had comparatively low ice coverage, the results indicate a more rapid response to wind forcing in the deep currents than in 2011. The SAR wind speed maps have better spatial resolution than available reanalysis data, and higher maximum correlation was obtained with SAR data than with reanalysis data despite the lower temporal resolution. The maximum correlation was R = 0.71, in a direction that is consistent with wind-driven Ekman theory. This is significantly larger than in previous studies. The larger correlation could be due to the better spatial resolution or the restriction to months with minimum ice coverage. The results indicate that SAR is a useful complement to infer the subsurface variability of the ocean circulation in remote areas in polar oceans.

  • 5.
    de la Barreda-Bautista, Betsabe
    et al.
    School of Biosciences, University of Nottingham, Sutton Bonington, Loughborough, United Kingdom; School of Geography, University of Nottingham, University Park, Nottingham, United Kingdom.
    Boyd, Doreen S.
    School of Geography, University of Nottingham, Nottingham, United Kingdom.
    Ledger, Martha
    School of Biosciences, University of Nottingham, Sutton Bonington, Loughborough, United Kingdom.
    Siewert, Matthias B.
    Umeå universitet, Institutionen för ekologi, miljö och geovetenskap.
    Chandler, Chris
    School of Geography, University of Nottingham, Nottingham, United Kingdom.
    Bradley, Andrew V.
    Department of Chemical and Environmental Engineering, Faculty of Engineering, Nottingham Geospatial Institute, Nottingham, United Kingdom.
    Gee, David
    Department of Chemical and Environmental Engineering, Faculty of Engineering, University of Nottingham, Nottingham, United Kingdom.
    Large, David J.
    Department of Chemical and Environmental Engineering, Faculty of Engineering, Nottingham Geospatial Institute, Nottingham, United Kingdom; Department of Chemical and Environmental Engineering, Faculty of Engineering, University of Nottingham, Nottingham, United Kingdom.
    Olofsson, Johan
    Umeå universitet, Institutionen för ekologi, miljö och geovetenskap.
    Sowter, Andrew
    Terra Motion Ltd, Ingenuity Centre, Nottingham, United Kingdom.
    Sjögersten, Sofie
    School of Biosciences, University of Nottingham, Sutton Bonington, Loughborough, United Kingdom.
    Towards a Monitoring Approach for Understanding Permafrost Degradation and Linked Subsidence in Arctic Peatlands2022Ingår i: Remote Sensing, E-ISSN 2072-4292, Vol. 14, nr 3, artikel-id 444Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Permafrost thaw resulting from climate warming is threatening to release carbon from high latitude peatlands. The aim of this research was to determine subsidence rates linked to permafrost thaw in sub-Arctic peatlands in Sweden using historical orthophotographic (orthophotos), Unoccupied Aerial Vehicle (UAV), and Interferometric Synthetic Aperture Radar (InSAR) data. The orthophotos showed that the permafrost palsa on the study sites have been contracting in their areal extent, with the greatest rates of loss between 2002 and 2008. The surface motion estimated from differential digital elevation models from the UAV data showed high levels of subsidence (maxi-mum of −25 cm between 2017 and 2020) around the edges of the raised palsa plateaus. The InSAR data analysis showed that raised palsa areas had the greatest subsidence rates, with maximum subsidence rates of 1.5 cm between 2017 and 2020; however, all wetland vegetation types showed sub-sidence. We suggest that the difference in spatial units associated with each sensor explains parts of the variation in the subsidence levels recorded. We conclude that InSAR was able to identify the areas most at risk of subsidence and that it can be used to investigate subsidence over large spatial extents, whereas UAV data can be used to better understand the dynamics of permafrost degradation at a local level. These findings underpin a monitoring approach for these peatlands.

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  • 6. Junttila, Sofia
    et al.
    Kelly, Julia
    Kljun, Natascha
    Aurela, Mika
    Klemedtsson, Leif
    Lohila, Annalea
    Nilsson, Mats B.
    Rinne, Janne
    Tuittila, Eeva-Stiina
    Vestin, Patrik
    Weslien, Per
    Eklundh, Lars
    Upscaling Northern Peatland CO2 Fluxes Using Satellite Remote Sensing Data2021Ingår i: Remote Sensing, E-ISSN 2072-4292, Vol. 13, nr 4Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Peatlands play an important role in the global carbon cycle as they contain a large soil carbon stock. However, current climate change could potentially shift peatlands from being carbon sinks to carbon sources. Remote sensing methods provide an opportunity to monitor carbon dioxide (CO2) exchange in peatland ecosystems at large scales under these changing conditions. In this study, we developed empirical models of the CO2 balance (net ecosystem exchange, NEE), gross primary production (GPP), and ecosystem respiration (ER) that could be used for upscaling CO2 fluxes with remotely sensed data. Two to three years of eddy covariance (EC) data from five peatlands in Sweden and Finland were compared to modelled NEE, GPP and ER based on vegetation indices from 10 m resolution Sentinel-2 MSI and land surface temperature from 1 km resolution MODIS data. To ensure a precise match between the EC data and the Sentinel-2 observations, a footprint model was applied to derive footprint-weighted daily means of the vegetation indices. Average model parameters for all sites were acquired with a leave-one-out-cross-validation procedure. Both the GPP and the ER models gave high agreement with the EC-derived fluxes (R2 = 0.70 and 0.56, NRMSE = 14% and 15%, respectively). The performance of the NEE model was weaker (average R2 = 0.36 and NRMSE = 13%). Our findings demonstrate that using optical and thermal satellite sensor data is a feasible method for upscaling the GPP and ER of northern boreal peatlands, although further studies are needed to investigate the sources of the unexplained spatial and temporal variation of the CO2 fluxes.

  • 7. Palace, Michael
    et al.
    Herrick, Christina
    DelGreco, Jessica
    Finnell, Daniel
    Garnello, Anthony John
    McCalley, Carmody
    McArthur, Kellen
    Sullivan, Franklin
    Varner, Ruth K.
    Determining Subarctic Peatland Vegetation Using an Unmanned Aerial System (UAS)2018Ingår i: Remote Sensing, E-ISSN 2072-4292, Vol. 10, nr 9, s. 1-20, artikel-id 1498Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Rising global temperatures tied to increases in greenhouse gas emissions are impacting high latitude regions, leading to changes in vegetation composition and feedbacks to climate through increased methane (CH4) emissions. In subarctic peatlands, permafrost collapse has led to shifts in vegetation species on landscape scales with high spatial heterogeneity. Our goal was to provide a baseline for vegetation distribution related to permafrost collapse and changes in biogeochemical processes. We collected unmanned aerial system (UAS) imagery at Stordalen Mire, Abisko, Sweden to classify vegetation cover types. A series of digital image processing routines were used to generate texture attributes within the image for the purpose of characterizing vegetative cover types. An artificial neural network (ANN) was developed to classify the image. The ANN used all texture variables and color bands (three spectral bands and six metrics) to generate a probability map for each of the eight cover classes. We used the highest probability for a class at each pixel to designate the cover type in the final map. Our overall misclassification rate was 32%, while omission and commission error by class ranged from 0% to 50%. We found that within our area of interest, cover classes most indicative of underlying permafrost (hummock and tall shrub) comprised 43.9% percent of the landscape. Our effort showed the capability of an ANN applied to UAS high-resolution imagery to develop a classification that focuses on vegetation types associated with permafrost status and therefore potentially changes in greenhouse gas exchange. We also used a method to examine the multiple probabilities representing cover class prediction at the pixel level to examine model confusion. UAS image collection can be inexpensive and a repeatable avenue to determine vegetation change at high latitudes, which can further be used to estimate and scale corresponding changes in CH4 emissions.

  • 8. Peng, Zeli
    et al.
    Ding, Yinghui
    Qu, Ying
    Wang, Mengsi
    Li, Xijia
    Generating a Long-Term Spatiotemporally Continuous Melt Pond Fraction Dataset for Arctic Sea Ice Using an Artificial Neural Network and a Statistical-Based Temporal Filter2022Ingår i: Remote Sensing, E-ISSN 2072-4292, Vol. 14, nr 18Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The melt pond fraction (MPF) is an important geophysical parameter of climate and the surface energy budget, and many MPF datasets have been generated from satellite observations. However, the reliability of these datasets suffers from short temporal spans and data gaps. To improve the temporal span and spatiotemporal continuity, we generated a long-term spatiotemporally continuous MPF dataset for Arctic sea ice, which is called the Northeast Normal University-melt pond fraction (NENU-MPF), from Moderate Resolution Imaging Spectroradiometer (MODIS) data. First, the non-linear relationship between the MODIS reflectance/geometries and the MPF was constructed using a genetic algorithm optimized back-propagation neural network (GA-BPNN) model. Then, the data gaps were filled and smoothed using a statistical-based temporal filter. The results show that the GA-BPNN model can provide accurate estimations of the MPF (R2 = 0.76, root mean square error (RMSE) = 0.05) and that the data gaps can be efficiently filled by the statistical-based temporal filter (RMSE = 0.047; bias = −0.022). The newly generated NENU-MPF dataset is consistent with the validation data and with published MPF datasets. Moreover, it has a longer temporal span and is much more spatiotemporally continuous; thus, it improves our knowledge of the long-term dynamics of the MPF over Arctic sea ice surfaces.

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