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  • 1. Anav, Alessandro
    et al.
    Friedlingstein, Pierre
    Beer, Christian
    Stockholm University, Faculty of Science, Department of Applied Environmental Science (ITM).
    Ciais, Philippe
    Harper, Anna
    Jones, Chris
    Murray-Tortarolo, Guillermo
    Papale, Dario
    Parazoo, Nicholas C.
    Peylin, Philippe
    Piao, Shilong
    Sitch, Stephen
    Viovy, Nicolas
    Wiltshire, Andy
    Zhao, Maosheng
    Spatiotemporal patterns of terrestrial gross primary production: A review2015In: Reviews of geophysics, ISSN 8755-1209, E-ISSN 1944-9208, Vol. 53, no 3, p. 785-818Article, review/survey (Refereed)
    Abstract [en]

    Great advances have been made in the last decade in quantifying and understanding the spatiotemporal patterns of terrestrial gross primary production (GPP) with ground, atmospheric, and space observations. However, although global GPP estimates exist, each data set relies upon assumptions and none of the available data are based only on measurements. Consequently, there is no consensus on the global total GPP and large uncertainties exist in its benchmarking. The objective of this review is to assess how the different available data sets predict the spatiotemporal patterns of GPP, identify the differences among data sets, and highlight the main advantages/disadvantages of each data set. We compare GPP estimates for the historical period (1990-2009) from two observation-based data sets (Model Tree Ensemble and Moderate Resolution Imaging Spectroradiometer) to coupled carbon-climate models and terrestrial carbon cycle models from the Fifth Climate Model Intercomparison Project and TRENDY projects and to a new hybrid data set (CARBONES). Results show a large range in the mean global GPP estimates. The different data sets broadly agree on GPP seasonal cycle in terms of phasing, while there is still discrepancy on the amplitude. For interannual variability (IAV) and trends, there is a clear separation between the observation-based data that show little IAV and trend, while the process-based models have large GPP variability and significant trends. These results suggest that there is an urgent need to improve observation-based data sets and develop carbon cycle modeling with processes that are currently treated either very simplistically to correctly estimate present GPP and better quantify the future uptake of carbon dioxide by the world's vegetation.

  • 2.
    Bayer, Tina K.
    et al.
    Stockholm University, Faculty of Science, Department of Environmental Science and Analytical Chemistry.
    Gustafsson, Erik
    Stockholm University, Faculty of Science, Stockholm University Baltic Sea Centre.
    Brakebusch, Matthias
    Stockholm University, Faculty of Science, Department of Environmental Science and Analytical Chemistry.
    Beer, Christian
    Stockholm University, Faculty of Science, Department of Environmental Science and Analytical Chemistry.
    Future Carbon Emission From Boreal and Permafrost Lakes Are Sensitive to Catchment Organic Carbon Loads2019In: Journal of Geophysical Research - Biogeosciences, ISSN 2169-8953, E-ISSN 2169-8961, Vol. 124, no 7, p. 1827-1848Article in journal (Refereed)
    Abstract [en]

    Carbon storage, processing, and transport in freshwater systems are important components of the global carbon cycle and sensitive to global change. However, in large-scale modeling this part of the boundless carbon cycle is often lacking or represented in a very simplified way. A new process-oriented lake biogeochemical model is used for investigating impacts of changes in atmospheric CO2 concentrations and organic carbon loading from the catchment on future greenhouse gas emissions from lakes across two boreal to subarctic regions (Northern Sweden and Alaska). Aquatic processes represented include carbon, oxygen, phytoplankton, and nutrient dynamics leading to CO2 and CH4 exchanges with the atmosphere. The model is running inside a macroscale hydrological model and may be easily implemented into a land surface scheme. Model evaluation demonstrates the validity in terms of average concentration of nutrients, algal biomass, and organic and inorganic carbon. Cumulative annual emissions of CH4 and CO2, as well as pathways of CH4 emissions, also compare well to observations. Model calculations imply that lake emissions of CH4 may increase by up to 45% under the Representative Concentration Pathway 8.5 scenario until 2100, and CO2 emissions may increase by up to 80% in Alaska. Increasing organic carbon loading to the lakes resulted in a linear response in CO2 and CH4 emissions across both regions, but increases in CO2 emissions from subarctic lakes in Sweden were lower than for southern boreal lakes, probably due to the higher importance of imported vegetation-generated inorganic carbon for CO2 emission from subarctic lakes.

  • 3.
    Beer, Christian
    Stockholm University, Faculty of Science, Department of Environmental Science and Analytical Chemistry.
    Permafrost Sub-grid Heterogeneity of Soil Properties Key for 3-D Soil Processes and Future Climate Projections2016In: Frontiers in Earth Science, ISSN 2296-6463, Vol. 4, article id 81Article in journal (Refereed)
    Abstract [en]

    There are massive carbon stocks stored in permafrost-affected soils due to the 3-D soil movement process called cryoturbation. For a reliable projection of the past, recent and future Arctic carbon balance, and hence climate, a reliable concept for representing cryoturbation in a land surface model (LSM) is required. The basis of the underlying transport processes is pedon-scale heterogeneity of soil hydrological and thermal properties as well as insulating layers, such as snow and vegetation. Today we still lack a concept of how to reliably represent pedon-scale properties and processes in a LSM. One possibility could be a statistical approach. This perspective paper demonstrates the importance of sub-grid heterogeneity in permafrost soils as a pre-requisite to implement any lateral transport parametrization. Representing such heterogeneity at the sub-pixel size of a LSM is the next logical step of model advancements. As a result of a theoretical experiment, heterogeneity of thermal and hydrological soil properties alone lead to a remarkable initial sub-grid range of subsoil temperature of 2C, and active-layer thickness of 150 cm in East Siberia. These results show the way forward in representing combined lateral and vertical transport of water and soil in LSMs.

  • 4.
    Beer, Christian
    et al.
    Stockholm University, Faculty of Science, Department of Environmental Science and Analytical Chemistry.
    Porada, Philipp
    Stockholm University, Faculty of Science, Department of Environmental Science and Analytical Chemistry.
    Ekici, Altug
    Stockholm University, Faculty of Science, Department of Environmental Science and Analytical Chemistry. Bjerknes Centre for Climate Research, Norway.
    Brakebusch, Matthias
    Stockholm University, Faculty of Science, Department of Environmental Science and Analytical Chemistry.
    Effects of short-term variability of meteorological variables on soil temperature in permafrost regions2018In: The Cryosphere, ISSN 1994-0416, E-ISSN 1994-0424, Vol. 12, no 2, p. 741-757Article in journal (Refereed)
    Abstract [en]

    Effects of the short-term temporal variability of meteorological variables on soil temperature in northern high-latitude regions have been investigated. For this, a process-oriented land surface model has been driven using an artificially manipulated climate dataset. Short-term climate variability mainly impacts snow depth, and the thermal diffusivity of lichens and bryophytes. These impacts of climate variability on insulating surface layers together substantially alter the heat exchange between atmosphere and soil. As a result, soil temperature is 0.1 to 0.8 degrees C higher when climate variability is reduced. Earth system models project warming of the Arctic region but also increasing variability of meteorological variables and more often extreme meteorological events. Therefore, our results show that projected future increases in permafrost temperature and active-layer thickness in response to climate change will be lower (i) when taking into account future changes in short-term variability of meteorological variables and (ii) when representing dynamic snow and lichen and bryophyte functions in land surface models.

  • 5.
    Beer, Christian
    et al.
    Stockholm University, Faculty of Science, Department of Applied Environmental Science (ITM). Max Planck Society, Germany.
    Weber, Ulrich
    Tomelleri, Enrico
    Carvalhais, Nuno
    Mahecha, Miguel
    Reichstein, Markus
    Harmonized European Long-Term Climate Data for Assessing the Effect of Changing Temporal Variability on Land-Atmosphere CO2 Fluxes2014In: Journal of Climate, ISSN 0894-8755, E-ISSN 1520-0442, Vol. 27, no 13, p. 4815-4834Article in journal (Refereed)
    Abstract [en]

    Temporal variability of meteorological variables and extreme weather events is projected to increase in many regions of the world during the next century. Artificial experiments using process-oriented terrestrial ecosystem models make it possible to isolate effects of temporal variability from effects of gradual climate change on terrestrial ecosystem functions and the system state. Such factorial experiments require two long-term climate datasets: 1) a control dataset that represents observed and projected climate and 2) a dataset with the same long-term mean as the control dataset but with altered short-term variability. Using a bias correction method, various climate datasets spanning different periods are harmonized and then combined with the control dataset with consistent time series for Europe during 1901-2100. Then, parameters of a distribution transformation function are estimated for individual meteorological variables to derive the second climate dataset, which has similar long-term means but reduced temporal variability. The transformation conserves the number of rainy days within a month and the shape of the daily meteorological data distributions, which is important to ensure that, for example, drought duration does not modify the suitability of localized vegetation type to precipitation regimes. The median absolute difference between daily data of both datasets is 5% to 20%. On average, decadal extreme values are reduced by 2% to 35%. Driving a terrestrial ecosystem model with both climate datasets shows a general higher gross primary production under reduced temporal climate variability. This effect of climate variability on productivity demonstrates the potential of the climate datasets for studying various effects of temporal variability on ecosystem state and functions over large domains.

  • 6. Braakhekke, Maarten C.
    et al.
    Beer, Christian
    Stockholm University, Faculty of Science, Department of Applied Environmental Science (ITM). Max Planck Society.
    Schrumpf, Marion
    Ekici, Altug
    Ahrens, Bernhard
    Hoosbeek, Marcel R.
    Kruijt, Bart
    Kabat, Pavel
    Reichstein, Markus
    The use of radiocarbon to constrain current and future soil organic matter turnover and transport in a temperate forest2014In: Journal of Geophysical Research: Biogeosciences, ISSN 2169-8953, Vol. 119, no 3, p. 372-391Article in journal (Refereed)
    Abstract [en]

    We investigated the merits of radiocarbon measurements for estimating soil organic matter (SOM) turnover and vertical transport for a temperate deciduous forest in Germany. Eleven parameters, defining decomposition and transport in the soil carbon model SOMPROF, were estimated using a Bayesian approach based on organic carbon measurements and radiocarbon concentration of SOM and heterotrophic respiration. The addition of radiocarbon data had strong effects on the parameters, most importantly a reduction of the decomposition and production rate of the slowest SOM pool by an order of magnitude, and a similar reduction in advective SOM transport. The modified parameters further led to changes in the partitioning of SOM over the different model pools. The calibration results were subsequently used to perform transient soil carbon projections for the period 1901-2100. These simulations were run with parameter sets from calibrations both with and without radiocarbon. The results show an increase over time of topsoil carbon and a decrease in the subsoil, adding to a net gain overall. Near the end of the 21st century, total carbon stocks stabilize andfor the radiocarbon-constrained modelstart to decrease. However, the changes are small compared to the total stocks. The model results for the calibrations with and without radiocarbon are in general quite similar, but the latter shows notably higher heterotrophic respiration fluxes. Constraining the model with radiocarbon yielded only a small reduction of uncertainty for the total carbon stocks, while for the individual depth compartments, the uncertainty wasincreased. Key Points <list list-type=bulleted> <list-item id=jgrg20156-li-0001>SOM turnover and transport was estimated using Bayesian calibration <list-item id=jgrg20156-li-0002>Radiocarbon data strongly improved constraint of turnover rate of slow pool <list-item id=jgrg20156-li-0003>Predictive simulation showed small increase of soil C stocks in 21st century

  • 7. Carvalhais, Nuno
    et al.
    Forkel, Matthias
    Khomik, Myroslava
    Bellarby, Jessica
    Jung, Martin
    Migliavacca, Mirco
    Mu, Mingquan
    Saatchi, Sassan
    Santoro, Maurizio
    Thurner, Martin
    Weber, Ulrich
    Ahrens, Bernhard
    Beer, Christian
    Stockholm University, Faculty of Science, Department of Applied Environmental Science (ITM). Max Planck Society, Germany.
    Cescatti, Alessandro
    Randerson, James T.
    Reichstein, Markus
    Global covariation of carbon turnover times with climate in terrestrial ecosystems2014In: Nature, ISSN 0028-0836, E-ISSN 1476-4687, Vol. 514, no 7521, p. 213-+Article in journal (Refereed)
    Abstract [en]

    The response of the terrestrial carbon cycle to climate change is among the largest uncertainties affecting future climate change projections(1,2). The feedback between the terrestrial carbon cycle and climate is partly determined by changes in the turnover time of carbon in land ecosystems, which in turn is an ecosystem property that emerges from the interplay between climate, soil and vegetation type(3-6). Here we present a global, spatially explicit and observation-based assessment of whole-ecosystem carbon turnover times that combines new estimates of vegetation and soil organic carbon stocks and fluxes. We find that the overall mean global carbon turnover time is 23(4)(+7) years (95 per cent confidence interval). Onaverage, carbon resides in the vegetation and soil near the Equator for a shorter time than at latitudes north of 75 degrees north (mean turnover times of 15 and 255 years, respectively). We identify a clear dependence of the turnover time on temperature, as expected from our present understanding of temperature controls on ecosystem dynamics. Surprisingly, our analysis also reveals a similarly strong association between turnover time and precipitation. Moreover, we find that the ecosystem carbon turnover times simulated by state-of-the-art coupled climate/carbon-cycle models vary widely and that numerical simulations, on average, tend to underestimate the global carbon turnover time by 36 per cent. The models show stronger spatial relationships with temperature than do observation-based estimates, but generally do not reproduce the strong relationships with precipitation and predict faster carbon turnover in many semiarid regions. Our findings suggest that future climate/carbon-cycle feedbacks may depend more strongly on changes in the hydrological cycle than is expected at present and is considered in Earth system models.

  • 8. Castro-Morales, Karel
    et al.
    Kleinen, Thomas
    Kaiser, Sonja
    Zaehle, Sönke
    Kittler, Fanny
    Kwon, Min Jung
    Beer, Christian
    Stockholm University, Faculty of Science, Department of Environmental Science and Analytical Chemistry.
    Göckede, Mathias
    Year-round simulated methane emissions from a permafrost ecosystem in Northeast Siberia2018In: Biogeosciences, ISSN 1726-4170, E-ISSN 1726-4189, Vol. 15, no 9, p. 2691-2722Article in journal (Refereed)
    Abstract [en]

    Wetlands of northern high latitudes are ecosystems highly vulnerable to climate change. Some degradation effects include soil hydrologic changes due to permafrost thaw, formation of deeper active layers, and rising topsoil temperatures that accelerate the degradation of permafrost carbon and increase in CO2 and CH4 emissions. In this work we present 2 years of modeled year-round CH4 emissions into the atmosphere from a Northeast Siberian region in the Russian Far East. We use a revisited version of the process-based JSBACH-methane model that includes four CH4 transport pathways: plant-mediated transport, ebullition and molecular diffusion in the presence or absence of snow. The gas is emitted through wetlands represented by grid cell inundated areas simulated with a TOPMODEL approach. The magnitude of the summertime modeled CH4 emissions is comparable to ground-based CH4 fluxes measured with the eddy covariance technique and flux chambers in the same area of study, whereas wintertime modeled values are underestimated by 1 order of magnitude. In an annual balance, the most important mechanism for transport of methane into the atmosphere is through plants (61 %). This is followed by ebullition (similar to 35 %), while summertime molecular diffusion is negligible (0.02 %) compared to the diffusion through the snow during winter (similar to 4 %). We investigate the relationship between temporal changes in the CH4 fluxes, soil temperature, and soil moisture content. Our results highlight the heterogeneity in CH4 emissions at landscape scale and suggest that further improvements to the representation of large-scale hydrological conditions in the model will facilitate a more process-oriented land surface scheme and better simulate CH4 emissions under climate change. This is especially necessary at regional scales in Arctic ecosystems influenced by permafrost thaw.

  • 9. Chadburn, Sarah E.
    et al.
    Krinner, Gerhard
    Porada, Philipp
    Stockholm University, Faculty of Science, Department of Environmental Science and Analytical Chemistry.
    Bartsch, Annett
    Beer, Christian
    Stockholm University, Faculty of Science, Department of Environmental Science and Analytical Chemistry.
    Belelli Marchesini, Luca
    Boike, Julia
    Ekici, Altug
    Elberling, Bo
    Friborg, Thomas
    Hugelius, Gustaf
    Stockholm University, Faculty of Science, Department of Physical Geography.
    Johansson, Margareta
    Kuhry, Peter
    Stockholm University, Faculty of Science, Department of Physical Geography.
    Kutzbach, Lars
    Langer, Moritz
    Lund, Magnus
    Parmentier, Frans-Jan W.
    Peng, Shushi
    Van Huissteden, Ko
    Wang, Tao
    Westermann, Sebastian
    Zhu, Dan
    Burke, Eleanor J.
    Carbon stocks and fluxes in the high latitudes: using site-level data to evaluate Earth system models2017In: Biogeosciences, ISSN 1726-4170, E-ISSN 1726-4189, Vol. 14, no 22, p. 5143-5169Article in journal (Refereed)
    Abstract [en]

    It is important that climate models can accurately simulate the terrestrial carbon cycle in the Arctic due to the large and potentially labile carbon stocks found in permafrost-affected environments, which can lead to a positive climate feedback, along with the possibility of future carbon sinks from northward expansion of vegetation under climate warming. Here we evaluate the simulation of tundra carbon stocks and fluxes in three land surface schemes that each form part of major Earth system models (JSBACH, Germany; JULES, UK; ORCHIDEE, France). We use a site-level approach in which comprehensive, high-frequency datasets allow us to disentangle the importance of different processes. The models have improved physical permafrost processes and there is a reasonable correspondence between the simulated and measured physical variables, including soil temperature, soil moisture and snow. We show that if the models simulate the correct leaf area index (LAI), the standard C3 photosynthesis schemes produce the correct order of magnitude of carbon fluxes. Therefore, simulating the correct LAI is one of the first priorities. LAI depends quite strongly on climatic variables alone, as we see by the fact that the dynamic vegetation model can simulate most of the differences in LAI between sites, based almost entirely on climate inputs. However, we also identify an influence from nutrient limitation as the LAI becomes too large at some of the more nutrient-limited sites. We conclude that including moss as well as vascular plants is of primary importance to the carbon budget, as moss contributes a large fraction to the seasonal CO2 flux in nutrient-limited conditions. Moss photosynthetic activity can be strongly influenced by the moisture content of moss, and the carbon uptake can be significantly different from vascular plants with a similar LAI. The soil carbon stocks depend strongly on the rate of input of carbon from the vegetation to the soil, and our analysis suggests that an improved simulation of photosynthesis would also lead to an improved simulation of soil carbon stocks. However, the stocks are also influenced by soil carbon burial (e.g. through cryoturbation) and the rate of heterotrophic respiration, which depends on the soil physical state. More detailed below-ground measurements are needed to fully evaluate biological and physical soil processes. Furthermore, even if these processes are well modelled, the soil carbon profiles cannot resemble peat layers as peat accumulation processes are not represented in the models. Thus, we identify three priority areas for model development: (1) dynamic vegetation including (a) climate and (b) nutrient limitation effects; (2) adding moss as a plant functional type; and an (3) improved vertical profile of soil carbon including peat processes.

  • 10. Ekici, A.
    et al.
    Beer, Christian
    Stockholm University, Faculty of Science, Department of Applied Environmental Science (ITM). Max Planck Society, Germany;.
    Hagemann, S.
    Boike, J.
    Langer, M.
    Hauck, C.
    Simulating high-latitude permafrost regions by the JSBACH terrestrial ecosystem model2014In: Geoscientific Model Development, ISSN 1991-959X, E-ISSN 1991-9603, Vol. 7, no 2, p. 631-647Article in journal (Refereed)
    Abstract [en]

    The current version of JSBACH incorporates phenomena specific to high latitudes: freeze/thaw processes, coupling thermal and hydrological processes in a layered soil scheme, defining a multilayer snow representation and an insulating moss cover. Evaluations using comprehensive Arctic data sets show comparable results at the site, basin, continental and circumarctic scales. Such comparisons highlight the need to include processes relevant to high-latitude systems in order to capture the dynamics, and therefore realistically predict the evolution of this climatically critical biome.

  • 11.
    Ekici, Sait Altug
    et al.
    Stockholm University, Faculty of Science, Department of Applied Environmental Science (ITM). Max Planck Institute for Biogeochemistry, Germany.
    Chadburn, S.
    Chaudhary, N.
    Hajdu, L. H.
    Marmy, A.
    Peng, S.
    Boike, J.
    Burke, E.
    Friend, A. D.
    Hauck, C.
    Krinner, G.
    Langer, M.
    Miller, P. A.
    Beer, Christian
    Stockholm University, Faculty of Science, Department of Applied Environmental Science (ITM).
    Site-level model intercomparison of high latitude and high altitude soil thermal dynamics in tundra and barren landscapes2015In: The Cryosphere, ISSN 1994-0416, E-ISSN 1994-0424, Vol. 9, no 4, p. 1343-1361Article in journal (Refereed)
    Abstract [en]

    Modeling soil thermal dynamics at high latitudes and altitudes requires representations of physical processes such as snow insulation, soil freezing and thawing and subsurface conditions like soil water/ice content and soil texture. We have compared six different land models: JSBACH, ORCHIDEE, JULES, COUP, HYBRID8 and LPJ-GUESS, at four different sites with distinct cold region landscape types, to identify the importance of physical processes in capturing observed temperature dynamics in soils. The sites include alpine, high Arctic, wet polygonal tundra and non-permafrost Arctic, thus showing how a range of models can represent distinct soil temperature regimes. For all sites, snow insulation is of major importance for estimating topsoil conditions. However, soil physics is essential for the subsoil temperature dynamics and thus the active layer thicknesses. This analysis shows that land models need more realistic surface processes, such as detailed snow dynamics and moss cover with changing thickness and wetness, along with better representations of subsoil thermal dynamics.

  • 12. Frank, Dorothe A.
    et al.
    Reichstein, Markus
    Bahn, Michael
    Thonicke, Kirsten
    Frank, David
    Mahecha, Miguel D.
    Smith, Pete
    Van der Velde, Marijn
    Vicca, Sara
    Babst, Flurin
    Beer, Christian
    Stockholm University, Faculty of Science, Department of Environmental Science and Analytical Chemistry. Max Planck Institute for Biogeochemistry, Jena, Germany.
    Buchmann, Nina
    Canadell, Josep G.
    Ciais, Philippe
    Cramer, Wolfgang
    Ibrom, Andreas
    Miglietta, Franco
    Poulter, Ben
    Rammig, Anja
    Seneviratne, Sonia I.
    Walz, Ariane
    Wattenbach, Martin
    Zavala, Miguel A.
    Zscheischler, Jakob
    Effects of climate extremes on the terrestrial carbon cycle: concepts, processes and potential future impacts2015In: Global Change Biology, ISSN 1354-1013, E-ISSN 1365-2486, Vol. 21, no 8, p. 2861-2880Article, review/survey (Refereed)
    Abstract [en]

    Extreme droughts, heat waves, frosts, precipitation, wind storms and other climate extremes may impact the structure, composition and functioning of terrestrial ecosystems, and thus carbon cycling and its feedbacks to the climate system. Yet, the interconnected avenues through which climate extremes drive ecological and physiological processes and alter the carbon balance are poorly understood. Here, we review the literature on carbon cycle relevant responses of ecosystems to extreme climatic events. Given that impacts of climate extremes are considered disturbances, we assume the respective general disturbance-induced mechanisms and processes to also operate in an extreme context. The paucity of well-defined studies currently renders a quantitative meta-analysis impossible, but permits us to develop a deductive framework for identifying the main mechanisms (and coupling thereof) through which climate extremes may act on the carbon cycle. We find that ecosystem responses can exceed the duration of the climate impacts via lagged effects on the carbon cycle. The expected regional impacts of future climate extremes will depend on changes in the probability and severity of their occurrence, on the compound effects and timing of different climate extremes, and on the vulnerability of each land-cover type modulated by management. Although processes and sensitivities differ among biomes, based on expert opinion, we expect forests to exhibit the largest net effect of extremes due to their large carbon pools and fluxes, potentially large indirect and lagged impacts, and long recovery time to regain previous stocks. At the global scale, we presume that droughts have the strongest and most widespread effects on terrestrial carbon cycling. Comparing impacts of climate extremes identified via remote sensing vs. ground-based observational case studies reveals that many regions in the (sub-)tropics are understudied. Hence, regional investigations are needed to allow a global upscaling of the impacts of climate extremes on global carbon-climate feedbacks.

  • 13. Guanter, Luis
    et al.
    Zhang, Yongguang
    Jung, Martin
    Joiner, Joanna
    Voigt, Maximilian
    Berry, Joseph A.
    Frankenberg, Christian
    Huete, Alfredo R.
    Zarco-Tejada, Pablo
    Lee, Jung-Eun
    Moran, M. Susan
    Ponce-Campos, Guillermo
    Beer, Christian
    Stockholm University, Faculty of Science, Department of Applied Environmental Science (ITM).
    Camps-Valls, Gustavo
    Buchmann, Nina
    Gianelle, Damiano
    Klumpp, Katja
    Cescatti, Alessandro
    Baker, John M.
    Griffis, Timothy J.
    Global and time-resolved monitoring of crop photosynthesis with chlorophyll fluorescence2014In: Proceedings of the National Academy of Sciences of the United States of America, ISSN 0027-8424, E-ISSN 1091-6490, Vol. 111, no 14, p. E1327-E1333Article in journal (Refereed)
    Abstract [en]

    Photosynthesis is the process by which plants harvest sunlight to produce sugars from carbon dioxide and water. It is the primary source of energy for all life on Earth; hence it is important to understand how this process responds to climate change and human impact. However, model-based estimates of gross primary production (GPP, output from photosynthesis) are highly uncertain, in particular over heavily managed agricultural areas. Recent advances in spectroscopy enable the space-based monitoring of sun-induced chlorophyll fluorescence (SIF) from terrestrial plants. Here we demonstrate that spaceborne SIF retrievals provide a direct measure of the GPP of cropland and grassland ecosystems. Such a strong link with crop photosynthesis is not evident for traditional remotely sensed vegetation indices, nor for more complex carbon cycle models. We use SIF observations to provide a global perspective on agricultural productivity. Our SIF-based crop GPP estimates are 50-75% higher than results from state-of-the-art carbon cycle models over, for example, the US Corn Belt and the Indo-Gangetic Plain, implying that current models severely underestimate the role of management. Our results indicate that SIF data can help us improve our global models for more accurate projections of agricultural productivity and climate impact on crop yields. Extension of our approach to other ecosystems, along with increased observational capabilities for SIF in the near future, holds the prospect of reducing uncertainties in the modeling of the current and future carbon cycle.

  • 14. Kaiser, Sonja
    et al.
    Goeckede, Mathias
    Castro-Morales, Karel
    Knoblauch, Christian
    Ekici, Altug
    Kleinen, Thomas
    Zubrzycki, Sebastian
    Sachs, Torsten
    Wille, Christian
    Beer, Christian
    Stockholm University, Faculty of Science, Department of Environmental Science and Analytical Chemistry.
    Process-based modelling of the methane balance in periglacial landscapes (JSBACH-methane)2017In: Geoscientific Model Development, ISSN 1991-959X, E-ISSN 1991-9603, Vol. 10, no 1, p. 333-358Article in journal (Refereed)
    Abstract [en]

    A detailed process-based methane module for a global land surface scheme has been developed which is general enough to be applied in permafrost regions as well as wetlands outside permafrost areas. Methane production, oxidation and transport by ebullition, diffusion and plants are represented. In this model, oxygen has been explicitly incorporated into diffusion, transport by plants and two oxidation processes, of which one uses soil oxygen, while the other uses oxygen that is available via roots. Permafrost and wetland soils show special behaviour, such as variable soil pore space due to freezing and thawing or water table depths due to changing soil water content. This has been integrated directly into the methane-related processes. A detailed application at the Samoylov polygonal tundra site, Lena River Delta, Russia, is used for evaluation purposes. The application at Samoylov also shows differences in the importance of the several transport processes and in the methane dynamics under varying soil moisture, ice and temperature conditions during different seasons and on different microsites. These microsites are the elevated moist polygonal rim and the depressed wet polygonal centre. The evaluation shows sufficiently good agreement with field observations despite the fact that the module has not been specifically calibrated to these data. This methane module is designed such that the advanced land surface scheme is able to model recent and future methane fluxes from periglacial landscapes across scales. In addition, the methane contribution to carbon cycle-climate feedback mechanisms can be quantified when running coupled to an atmospheric model.

  • 15. Knoblauch, Christian
    et al.
    Beer, Christian
    Stockholm University, Faculty of Science, Department of Environmental Science and Analytical Chemistry.
    Liebner, Susanne
    Grigoriev, Mikhail N.
    Pfeiffer, Eva-Maria
    Methane production as key to the greenhouse gas budget of thawing permafrost2018In: Nature Climate Change, ISSN 1758-678X, E-ISSN 1758-6798, Vol. 8, no 4, p. 309-312Article in journal (Refereed)
    Abstract [en]

    Permafrost thaw liberates frozen organic carbon, which is decomposed into carbon dioxide (CO2) and methane (CH4). The release of these greenhouse gases (GHGs) forms a positive feedback to atmospheric CO2 and CH4 concentrations and accelerates climate change(1,2). Current studies report a minor importance of CH4 production in water-saturated (anoxic) permafrost soils(3-6) and a stronger permafrost carbon-climate feedback from drained (oxic) soils(1,7). Here we show through seven-year laboratory incubations that equal amounts of CO2 and CH4 are formed in thawing permafrost under anoxic conditions after stable CH4-producing microbial communities have established. Less permafrost carbon was mineralized under anoxic conditions but more CO2-carbon equivalents (CO2Ce) were formed than under oxic conditions when the higher global warming potential (GWP) of CH4 is taken into account(8). A model of organic carbon decomposition, calibrated with the observed decomposition data, predicts a higher loss of permafrost carbon under oxic conditions (113 +/- 58 g CO2-C kgC(-1) (kgC, kilograms of carbon)) by 2100, but a twice as high production of CO2-Ce (241 +/- 138 g CO2-Ce kgC(-1)) under anoxic conditions. These findings challenge the view of a stronger permafrost carbon-climate feedback from drained soils1,7 and emphasize the importance of CH4 production in thawing permafrost on climate-relevant timescales.

  • 16.
    Lade, Steven J.
    et al.
    Stockholm University, Faculty of Science, Stockholm Resilience Centre. The Australian National University, Australia.
    Donges, Jonathan F.
    Stockholm University, Faculty of Science, Stockholm Resilience Centre. Potsdam Institute for Climate Impact Research, Germany.
    Fetzer, Ingo
    Stockholm University, Faculty of Science, Stockholm Resilience Centre.
    Anderies, John M.
    Beer, Christian
    Stockholm University, Faculty of Science, Department of Environmental Science and Analytical Chemistry.
    Cornell, Sarah E.
    Stockholm University, Faculty of Science, Stockholm Resilience Centre.
    Gasser, Thomas
    Norberg, Jon
    Stockholm University, Faculty of Science, Stockholm Resilience Centre.
    Richardson, Katherine
    Rockström, Johan
    Stockholm University, Faculty of Science, Stockholm Resilience Centre.
    Steffen, Will
    Stockholm University, Faculty of Science, Stockholm Resilience Centre.
    Analytically tractable climate-carbon cycle feedbacks under 21st century anthropogenic forcing2018In: Earth System Dynamics, ISSN 2190-4979, E-ISSN 2190-4987, Vol. 9, no 2, p. 507-523Article in journal (Refereed)
    Abstract [en]

    Changes to climate-carbon cycle feedbacks may significantly affect the Earth system's response to greenhouse gas emissions. These feedbacks are usually analysed from numerical output of complex and arguably opaque Earth system models. Here, we construct a stylised global climate-carbon cycle model, test its output against comprehensive Earth system models, and investigate the strengths of its climate-carbon cycle feedbacks analytically. The analytical expressions we obtain aid understanding of carbon cycle feedbacks and the operation of the carbon cycle. Specific results include that different feedback formalisms measure fundamentally the same climate-carbon cycle processes; temperature dependence of the solubility pump, biological pump, and CO2 solubility all contribute approximately equally to the ocean climate-carbon feedback; and concentration-carbon feedbacks may be more sensitive to future climate change than climate-carbon feedbacks. Simple models such as that developed here also provide workbenches for simple but mechanistically based explorations of Earth system processes, such as interactions and feedbacks between the planetary boundaries, that are currently too uncertain to be included in comprehensive Earth system models.

  • 17. McGuire, A. David
    et al.
    Koven, Charles
    Lawrence, David M.
    Clein, Joy S.
    Xia, Jiangyang
    Beer, Christian
    Stockholm University, Faculty of Science, Department of Environmental Science and Analytical Chemistry.
    Burke, Eleanor
    Chen, Guangsheng
    Chen, Xiaodong
    Delire, Christine
    Jafarov, Elchin
    MacDougall, Andrew H.
    Marchenko, Sergey
    Nicolsky, Dmitry
    Peng, Shushi
    Rinke, Annette
    Saito, Kazuyuki
    Zhang, Wenxin
    Alkama, Ramdane
    Bohn, Theodore J.
    Ciais, Philippe
    Decharme, Bertrand
    Ekici, Altug
    Stockholm University, Faculty of Science, Department of Environmental Science and Analytical Chemistry.
    Gouttevin, Isabelle
    Hajima, Tomohiro
    Hayes, Daniel J.
    Ji, Duoying
    Krinner, Gerhard
    Lettenmaier, Dennis P.
    Luo, Yiqi
    Miller, Paul A.
    Moore, John C.
    Romanovsky, Vladimir
    Schädel, Christina
    Schaefer, Kevin
    Schuur, Edward A. G.
    Smith, Benjamin
    Sueyoshi, Tetsuo
    Zhuang, Qianlai
    Variability in the sensitivity among model simulations of permafrost and carbon dynamics in the permafrost region between 1960 and 20092016In: Global Biogeochemical Cycles, ISSN 0886-6236, E-ISSN 1944-9224, Vol. 30, no 7, p. 1015-1037Article in journal (Refereed)
    Abstract [en]

    A significant portion of the large amount of carbon (C) currently stored in soils of the permafrost region in the Northern Hemisphere has the potential to be emitted as the greenhouse gases CO2 and CH4 under a warmer climate. In this study we evaluated the variability in the sensitivity of permafrost and C in recent decades among land surface model simulations over the permafrost region between 1960 and 2009. The 15 model simulations all predict a loss of near-surface permafrost (within 3m) area over the region, but there are large differences in the magnitude of the simulated rates of loss among the models (0.2 to 58.8x10(3)km(2)yr(-1)). Sensitivity simulations indicated that changes in air temperature largely explained changes in permafrost area, although interactions among changes in other environmental variables also played a role. All of the models indicate that both vegetation and soil C storage together have increased by 156 to 954TgCyr(-1) between 1960 and 2009 over the permafrost region even though model analyses indicate that warming alone would decrease soil C storage. Increases in gross primary production (GPP) largely explain the simulated increases in vegetation and soil C. The sensitivity of GPP to increases in atmospheric CO2 was the dominant cause of increases in GPP across the models, but comparison of simulated GPP trends across the 1982-2009 period with that of a global GPP data set indicates that all of the models overestimate the trend in GPP. Disturbance also appears to be an important factor affecting C storage, as models that consider disturbance had lower increases in C storage than models that did not consider disturbance. To improve the modeling of C in the permafrost region, there is the need for the modeling community to standardize structural representation of permafrost and carbon dynamics among models that are used to evaluate the permafrost C feedback and for the modeling and observational communities to jointly develop data sets and methodologies to more effectively benchmark models.

  • 18.
    Porada, Philipp
    et al.
    Stockholm University, Faculty of Science, Department of Environmental Science and Analytical Chemistry.
    Ekici, Altug
    Stockholm University, Faculty of Science, Department of Environmental Science and Analytical Chemistry. University of Exeter, UK; Bjerknes Centre for Climate Research, Norway.
    Beer, Christian
    Stockholm University, Faculty of Science, Department of Environmental Science and Analytical Chemistry.
    Effects of bryophyte and lichen cover on permafrost soil temperature at large scale2016In: The Cryosphere, ISSN 1994-0416, E-ISSN 1994-0424, Vol. 10, no 5, p. 2291-2315Article in journal (Refereed)
    Abstract [en]

    Bryophyte and lichen cover on the forest floor at high latitudes exerts an insulating effect on the ground. In this way, the cover decreases mean annual soil temperature and can protect permafrost soil. Climate change, however, may change bryophyte and lichen cover, with effects on the permafrost state and related carbon balance. It is, therefore, crucial to predict how the bryophyte and lichen cover will react to environmental change at the global scale. To date, current global land surface models contain only empirical representations of the bryophyte and lichen cover, which makes it impractical to predict the future state and function of bryophytes and lichens. For this reason, we integrate a process-based model of bryophyte and lichen growth into the global land surface model JSBACH (Jena Scheme for Biosphere-Atmosphere Coupling in Hamburg). The model simulates bryophyte and lichen cover on upland sites. Wetlands are not included. We take into account the dynamic nature of the thermal properties of the bryophyte and lichen cover and their relation to environmental factors. Subsequently, we compare simulations with and without bryophyte and lichen cover to quantify the insulating effect of the organisms on the soil. We find an average cooling effect of the bryophyte and lichen cover of 2.7K on temperature in the topsoil for the region north of 50 degrees N under the current climate. Locally, a cooling of up to 5.7K may be reached. Moreover, we show that using a simple, empirical representation of the bryophyte and lichen cover without dynamic properties only results in an average cooling of around 0.5 K. This suggests that (a) bryophytes and lichens have a significant impact on soil temperature in high-latitude ecosystems and (b) a process-based description of their thermal properties is necessary for a realistic representation of the cooling effect. The advanced land surface scheme, including a dynamic bryophyte and lichen model, will be the basis for an improved future projection of land-atmosphere heat and carbon exchange.

  • 19.
    Porada, Philipp
    et al.
    Stockholm University, Faculty of Science, Department of Environmental Science and Analytical Chemistry.
    Lenton, T. M.
    Pohl, A.
    Weber, B.
    Mander, L.
    Donnadieu, Y.
    Beer, Christian
    Stockholm University, Faculty of Science, Department of Environmental Science and Analytical Chemistry.
    Poeschl, U.
    Kleidon, A.
    High potential for weathering and climate effects of non-vascular vegetation in the Late Ordovician2016In: Nature Communications, ISSN 2041-1723, E-ISSN 2041-1723, Vol. 7, article id 12113Article in journal (Refereed)
    Abstract [en]

    It has been hypothesized that predecessors of today's bryophytes significantly increased global chemical weathering in the Late Ordovician, thus reducing atmospheric CO2 concentration and contributing to climate cooling and an interval of glaciations. Studies that try to quantify the enhancement of weathering by non-vascular vegetation, however, are usually limited to small areas and low numbers of species, which hampers extrapolating to the global scale and to past climatic conditions. Here we present a spatially explicit modelling approach to simulate global weathering by non-vascular vegetation in the Late Ordovician. We estimate a potential global weathering flux of 2.8 (km(3) rock) yr(-1), defined here as volume of primary minerals affected by chemical transformation. This is around three times larger than today's global chemical weathering flux. Moreover, we find that simulated weathering is highly sensitive to atmospheric CO2 concentration. This implies a strong negative feedback between weathering by non-vascular vegetation and Ordovician climate.

  • 20.
    Porada, Philipp
    et al.
    Stockholm University, Faculty of Science, Department of Environmental Science and Analytical Chemistry.
    Pöschl, Ulrich
    Kleidon, Axel
    Beer, Christian
    Stockholm University, Faculty of Science, Department of Environmental Science and Analytical Chemistry.
    Weber, Bettina
    Estimating global nitrous oxide emissions by lichens and bryophytes with a process-based productivity model2017In: Biogeosciences, ISSN 1726-4170, E-ISSN 1726-4189, Vol. 14, no 6, p. 1593-1602Article in journal (Refereed)
    Abstract [en]

    Nitrous oxide is a strong greenhouse gas and atmospheric ozone-depleting agent which is largely emitted by soils. Recently, lichens and bryophytes have also been shown to release significant amounts of nitrous oxide. This finding relies on ecosystem-scale estimates of net primary productivity of lichens and bryophytes, which are converted to nitrous oxide emissions by empirical relationships between productivity and respiration, as well as between respiration and nitrous oxide release. Here we obtain an alternative estimate of nitrous oxide emissions which is based on a global process-based non-vascular vegetation model of lichens and bryophytes. The model quantifies photosynthesis and respiration of lichens and bryophytes directly as a function of environmental conditions, such as light and temperature. Nitrous oxide emissions are then derived from simulated respiration assuming a fixed relationship between the two fluxes. This approach yields a global estimate of 0.27 (0.19-0.35) (TgN(2)O) year(-1) released by lichens and bryophytes. This is lower than previous estimates but corresponds to about 50% of the atmospheric deposition of nitrous oxide into the oceans or 25% of the atmospheric deposition on land. Uncertainty in our simulated estimate results from large variation in emission rates due to both physiological differences between species and spatial heterogeneity of climatic conditions. To constrain our predictions, combined online gas exchange measurements of respiration and nitrous oxide emissions may be helpful.

  • 21. Reichstein, Markus
    et al.
    Bahn, Michael
    Ciais, Philippe
    Frank, Dorothea
    Mahecha, Miguel D.
    Seneviratne, Sonia I.
    Zscheischler, Jakob
    Beer, Christian
    Stockholm University, Faculty of Science, Department of Applied Environmental Science (ITM).
    Buchmann, Nina
    Frank, David C.
    Papale, Dario
    Rammig, Anja
    Smith, Pete
    Thonicke, Kirsten
    van der Velde, Marijn
    Vicca, Sara
    Walz, Ariane
    Wattenbach, Martin
    Climate extremes and the carbon cycle2013In: Nature, ISSN 0028-0836, E-ISSN 1476-4687, Vol. 500, no 7462, p. 287-295Article in journal (Refereed)
    Abstract [en]

    The terrestrial biosphere is a key component of the global carbon cycle and its carbon balance is strongly influenced by climate. Continuing environmental changes are thought to increase global terrestrial carbon uptake. But evidence is mounting that climate extremes such as droughts or storms can lead to a decrease in regional ecosystem carbon stocks and therefore have the potential to negate an expected increase in terrestrial carbon uptake. Here we explore the mechanisms and impacts of climate extremes on the terrestrial carbon cycle, and propose a pathway to improve our understanding of present and future impacts of climate extremes on the terrestrial carbon budget.

  • 22. Santoro, Maurizio
    et al.
    Beaudoin, Andre
    Beer, Christian
    Stockholm University, Faculty of Science, Department of Environmental Science and Analytical Chemistry.
    Cartus, Oliver
    Fransson, Johan B. S.
    Hall, Ronald J.
    Pathe, Carsten
    Schmullius, Christiane
    Schepaschenko, Dmitry
    Shvidenko, Anatoly
    Thurner, Martin
    Stockholm University, Faculty of Science, Department of Environmental Science and Analytical Chemistry. Max-Planck Institute for Biogeochemistry, Germany.
    Wegmueller, Urs
    Forest growing stock volume of the northern hemisphere: Spatially explicit estimates for 2010 derived from Envisat ASAR2015In: Remote Sensing of Environment, ISSN 0034-4257, E-ISSN 1879-0704, Vol. 168, p. 316-334Article in journal (Refereed)
    Abstract [en]

    This paper presents and assesses spatially explicit estimates of forest growing stock volume (GSV) of the northern hemisphere (north of 10 degrees N) from hyper-temporal observations of Envisat Advanced Synthetic Aperture Radar (ASAR) backscattered intensity using the BIOMASAR algorithm. Approximately 70,000 ASAR images at a pixel size of 0.01 degrees were used to estimate GSV representative for the year 2010. The spatial distribution of the GSV across four ecological zones (polar, boreal, temperate, subtropical) was well captured by the ASAR-based estimates. The uncertainty of the retrieved GSV was smallest in boreal and temperate forest (<30% for approximately 80% of the forest area) and largest in subtropical forest. ASAR-derived GSV averages at the level of administrative units were mostly in agreement with inventory-derived estimates. Underestimation occurred in regions of very high GSV (>300 m(3)/ha) and fragmented forest landscapes. For the major forested countries within the study region, the relative RMSE between ASAR-derived GSV averages at provincial level and corresponding values from National Forest Inventory was between 12% and 45% (average: 29%).

  • 23. Santoro, Maurizio
    et al.
    Cartus, Oliver
    Fransson, Johan E. S.
    Shvidenko, Anatoly
    McCallum, Ian
    Hall, Ronald J.
    Beaudoin, Andre
    Beer, Christian
    Stockholm University, Faculty of Science, Department of Applied Environmental Science (ITM).
    Schmullius, Christiane
    Estimates of Forest Growing Stock Volume for Sweden, Central Siberia, and Quebec Using Envisat Advanced Synthetic Aperture Radar Backscatter Data2013In: Remote Sensing, ISSN 2072-4292, E-ISSN 2072-4292, Vol. 5, no 9, p. 4503-4532Article in journal (Refereed)
    Abstract [en]

    A study was undertaken to assess Envisat Advanced Synthetic Aperture Radar (ASAR) ScanSAR data for quantifying forest growing stock volume (GSV) across three boreal regions with varying forest types, composition, and structure (Sweden, Central Siberia, and Quebec). Estimates of GSV were obtained using hyper-temporal observations of the radar backscatter acquired by Envisat ASAR with the BIOMASAR algorithm. In total, 5.310(6) km(2) were mapped with a 0.01 degrees pixel size to obtain estimates representative for the year of 2005. Comparing the SAR-based estimates to spatially explicit datasets of GSV, generated from forest field inventory and/or Earth Observation data, revealed similar spatial distributions of GSV. Nonetheless, the weak sensitivity of C-band backscatter to forest structural parameters introduced significant uncertainty to the estimated GSV at full resolution. Further discrepancies were observed in the case of different scales of the ASAR and the reference GSV and in areas of fragmented landscapes. Aggregation to 0.1 degrees and 0.5 degrees was then undertaken to generate coarse scale estimates of GSV. The agreement between ASAR and the reference GSV datasets improved; the relative difference at 0.5 degrees was consistently within a magnitude of 20-30%. The results indicate an improvement of the characterization of forest GSV in the boreal zone with respect to currently available information.

  • 24.
    Thurner, Martin
    et al.
    Stockholm University, Faculty of Science, Department of Environmental Science and Analytical Chemistry. Max Planck Institute for Biogeochemistry, Germany.
    Beer, Christian
    Stockholm University, Faculty of Science, Department of Environmental Science and Analytical Chemistry.
    Carvalhais, Nuno
    Forkel, Matthias
    Santoro, Maurizio
    Tum, Markus
    Schmullius, Christiane
    Large-scale variation in boreal and temperate forest carbon turnover rate related to climate2016In: Geophysical Research Letters, ISSN 0094-8276, E-ISSN 1944-8007, Vol. 43, no 9, p. 4576-4585Article in journal (Refereed)
    Abstract [en]

    Vegetation carbon turnover processes in forest ecosystems and their dominant drivers are far from being understood at a broader scale. Many of these turnover processes act on long timescales and include a lateral dimension and thus can hardly be investigated by plot-level studies alone. Making use of remote sensing-based products of net primary production (NPP) and biomass, here we show that spatial gradients of carbon turnover rate (k) in Northern Hemisphere boreal and temperate forests are explained by different climate-related processes depending on the ecosystem. k is related to frost damage effects and the trade-off between growth and frost adaptation in boreal forests, while drought stress and climate effects on insects and pathogens can explain an elevated k in temperate forests. By identifying relevant processes underlying broadscale patterns in k, we provide the basis for a detailed exploration of these mechanisms in field studies, and ultimately the improvement of their representations in global vegetation models (GVMs).

  • 25.
    Thurner, Martin
    et al.
    Stockholm University, Faculty of Science, Department of Environmental Science and Analytical Chemistry.
    Beer, Christian
    Stockholm University, Faculty of Science, Department of Environmental Science and Analytical Chemistry.
    Ciais, Philippe
    Friend, Andrew D.
    Ito, Akihiko
    Kleidon, Axel
    Lomas, Mark R.
    Shaun, Quegan
    Rademacher, Tim T.
    Schaphoff, Sibyll
    Tum, Markus
    Wiltshire, Andy
    Carvalhais, Nuno
    Evaluation of climate-related carbon turnover processes in global vegetation models for boreal and temperate forests2017In: Global Change Biology, ISSN 1354-1013, E-ISSN 1365-2486, Vol. 23, no 8, p. 3076-3091Article in journal (Refereed)
    Abstract [en]

    Turnover concepts in state-of-the-art global vegetation models (GVMs) account for various processes, but are often highly simplified and may not include an adequate representation of the dominant processes that shape vegetation carbon turnover rates in real forest ecosystems at a large spatial scale. Here, we evaluate vegetation carbon turnover processes in GVMs participating in the Inter-Sectoral Impact Model Intercomparison Project (ISI-MIP, including HYBRID4, JeDi, JULES, LPJml, ORCHIDEE, SDGVM, and VISIT) using estimates of vegetation carbon turnover rate (k) derived from a combination of remote sensing based products of biomass and net primary production (NPP). We find that current model limitations lead to considerable biases in the simulated biomass and in k (severe underestimations by all models except JeDi and VISIT compared to observation-based average k), likely contributing to underestimation of positive feedbacks of the northern forest carbon balance to climate change caused by changes in forest mortality. A need for improved turnover concepts related to frost damage, drought, and insect outbreaks to better reproduce observation-based spatial patterns in k is identified. As direct frost damage effects on mortality are usually not accounted for in these GVMs, simulated relationships between k and winter length in boreal forests are not consistent between different regions and strongly biased compared to the observation-based relationships. Some models show a response of k to drought in temperate forests as a result of impacts of water availability on NPP, growth efficiency or carbon balance dependent mortality as well as soil or litter moisture effects on leaf turnover or fire. However, further direct drought effects such as carbon starvation (only in HYBRID4) or hydraulic failure are usually not taken into account by the investigated GVMs. While they are considered dominant large-scale mortality agents, mortality mechanisms related to insects and pathogens are not explicitly treated in these models.

  • 26.
    Thurner, Martin
    et al.
    Stockholm University, Faculty of Science, Department of Environmental Science and Analytical Chemistry.
    Beer, Christian
    Stockholm University, Faculty of Science, Department of Environmental Science and Analytical Chemistry.
    Crowther, Thomas
    Falster, Daniel
    Manzoni, Stefano
    Stockholm University, Faculty of Science, Department of Physical Geography.
    Prokushkin, Anatoly
    Schulze, Ernst-Detlef
    Sapwood biomass carbon in northern boreal and temperate forests2019In: Global Ecology and Biogeography, ISSN 1466-822X, E-ISSN 1466-8238, Vol. 28, no 5, p. 640-660Article in journal (Refereed)
    Abstract [en]

    Aim Information on the amount of carbon stored in the living tissue of tree stems (sapwood) is crucial for carbon and water cycle applications. Here, we aim to investigate sapwood-to-stem proportions and differences therein between tree genera and derive a sapwood biomass map. Location Northern Hemisphere boreal and temperate forests. Time period 2010. Major taxa studied Twenty-five common tree genera. Methods First, we develop a theoretical framework to estimate sapwood biomass for a given stem biomass by applying relationships between sapwood cross-sectional area (CSA) and stem CSA and between stem CSA and stem biomass. These measurements are extracted from a biomass and allometry database (BAAD), an extensive literature review and our own studies. The established allometric relationships are applied to a remote sensing-based stem biomass product in order to derive a spatially continuous sapwood biomass map. The application of new products on the distribution of stand density and tree genera facilitates the synergy of satellite and forest inventory data. Results Sapwood-to-stem CSA relationships can be modelled with moderate to very high modelling efficiency for different genera. The total estimated sapwood biomass equals 12.87 +/- 6.56 petagrams of carbon (PgC) in boreal (mean carbon density: 1.13 +/- 0.58 kgC m(-2)) and 15.80 +/- 9.10 PgC in temperate (2.03 +/- 1.17 kgC m(-2)) forests. Spatial patterns of sapwood-to-stem biomass proportions are crucially driven by the distribution of genera (spanning from 20-30% in Larix to > 70% in Pinus and Betula forests). Main conclusions The presented sapwood biomass map will be the basis for large-scale estimates of plant respiration and transpiration. The enormous spatial differences in sapwood biomass proportions reveal the need to consider the functionally more important sapwood instead of the entire stem biomass in global carbon and water cycle studies. Alterations in tree species distribution, induced by forest management or climate change, can strongly affect the available sapwood biomass even if stem biomass remains unchanged.

  • 27. Thurner, Martin
    et al.
    Beer, Christian
    Stockholm University, Faculty of Science, Department of Applied Environmental Science (ITM).
    Santoro, Maurizio
    Carvalhais, Nuno
    Wutzler, Thomas
    Schepaschenko, Dmitry
    Shvidenko, Anatoly
    Kompter, Elisabeth
    Ahrens, Bernhard
    Levick, Shaun R.
    Schmullius, Christiane
    Carbon stock and density of northern boreal and temperate forests2014In: Global Ecology and Biogeography, ISSN 1466-822X, E-ISSN 1466-8238, Vol. 23, no 3, p. 297-310Article in journal (Refereed)
    Abstract [en]

    AimTo infer a forest carbon density map at 0.01 degrees resolution from a radar remote sensing product for the estimation of carbon stocks in Northern Hemisphere boreal and temperate forests. LocationThe study area extends from 30 degrees N to 80 degrees N, covering three forest biomes - temperate broadleaf and mixed forests (TBMF), temperate conifer forests (TCF) and boreal forests (BFT) - over three continents (North America, Europe and Asia). MethodsThis study is based on a recently available growing stock volume (GSV) product retrieved from synthetic aperture radar data. Forest biomass and spatially explicit uncertainty estimates were derived from the GSV using existing databases of wood density and allometric relationships between biomass compartments (stem, branches, roots, foliage). We tested the resultant map against inventory-based biomass data from Russia, Europe and the USA prior to making intercontinent and interbiome carbon stock comparisons. ResultsOur derived carbon density map agrees well with inventory data at regional scales (r(2)=0.70-0.90). While 40.715.7 petagram of carbon (PgC) are stored in BFT, TBMF and TCF contain 24.5 +/- 9.4PgC and 14.5 +/- 4.8 PgC, respectively. In terms of carbon density, we found 6.21 +/- 2.07kgC m(-2) retained in TCF and 5.80 +/- 2.21kgC m(-2) in TBMF, whereas BFT have a mean carbon density of 4.00 +/- 1.54kgC m(-2). Indications of a higher carbon density in Europe compared with the other continents across each of the three biomes could not be proved to be significant. Main conclusionsThe presented carbon density and corresponding uncertainty map give an insight into the spatial patterns of biomass and stand as a new benchmark to improve carbon cycle models and carbon monitoring systems. In total, we found 79.8 +/- 29.9PgC stored in northern boreal and temperate forests, with Asian BFT accounting for 22.1 +/- 8.3PgC.

  • 28. Van Oijen, M.
    et al.
    Balkovi, J.
    Beer, Christian
    Stockholm University, Faculty of Science, Department of Applied Environmental Science (ITM). Max Planck Society, Germany.
    Cameron, D. R.
    Ciais, P.
    Cramer, W.
    Kato, T.
    Kuhnert, M.
    Martin, R.
    Myneni, R.
    Rammig, A.
    Rolinski, S.
    Soussana, J. -F
    Thonicke, K.
    Van der Velde, M.
    Xu, L.
    Impact of droughts on the carbon cycle in European vegetation: a probabilistic risk analysis using six vegetation models2014In: Biogeosciences, ISSN 1726-4170, E-ISSN 1726-4189, Vol. 11, no 22, p. 6357-6375Article in journal (Refereed)
    Abstract [en]

    We analyse how climate change may alter risks posed by droughts to carbon fluxes in European ecosystems. The approach follows a recently proposed framework for risk analysis based on probability theory. In this approach, risk is quantified as the product of hazard probability and ecosystem vulnerability. The probability of a drought hazard is calculated here from the Standardized Precipitation-Evapotranspiration Index (SPEI). Vulnerability is calculated from the response to drought simulated by process-based vegetation models. We use six different models: three for generic vegetation (JSBACH, LPJmL, ORCHIDEE) and three for specific ecosystems (Scots pine forests: BASFOR; winter wheat fields: EPIC; grasslands: PASIM). The periods 1971-2000 and 2071-2100 are compared. Climate data are based on gridded observations and on output from the regional climate model REMO using the SRES A1B scenario. The risk analysis is carried out for similar to 18 000 grid cells of 0.25 x 0.25 degrees across Europe. For each grid cell, drought vulnerability and risk are quantified for five seasonal variables: net primary and ecosystem productivity (NPP, NEP), heterotrophic respiration (Rh), soil water content and evapotranspiration. In this analysis, climate change leads to increased drought risks for net primary productivity in the Mediterranean area: five of the models estimate that risk will exceed 15 %. The risks increase mainly because of greater drought probability; ecosystem vulnerability will increase to a lesser extent. Because NPP will be affected more than Rh, future carbon sequestration (NEP) will also be at risk predominantly in southern Europe, with risks exceeding 0.25 g Cm-2 d(-1) according to most models, amounting to reductions in carbon sequestration of 20 to 80 %.

  • 29. von Deimling, Thomas Schneider
    et al.
    Kleinen, Thomas
    Hugelius, Gustaf
    Stockholm University, Faculty of Science, Department of Physical Geography.
    Knoblauch, Christian
    Beer, Christian
    Stockholm University, Faculty of Science, Department of Environmental Science and Analytical Chemistry.
    Brovkin, Victor
    Long-term deglacial permafrost carbon dynamics in MPI-ESM2018In: Climate of the Past, ISSN 1814-9324, E-ISSN 1814-9332, Vol. 14, no 12, p. 2011-2036Article in journal (Refereed)
    Abstract [en]

    We have developed a new module to calculate soil organic carbon (SOC) accumulation in perennially frozen ground in the land surface model JSBACH. Running this offline version of MPI-ESM we have modelled long-term permafrost carbon accumulation and release from the Last Glacial Maximum (LGM) to the pre-industrial (PI) age. Our simulated near-surface PI permafrost extent of 16.9 x 10(6) km(2) is close to observational estimates. Glacial boundary conditions, especially ice sheet coverage, result in profoundly different spatial patterns of glacial permafrost extent. Deglacial warming leads to large-scale changes in soil temperatures, manifested in permafrost disappearance in southerly regions, and permafrost aggregation in formerly glaciated grid cells. In contrast to the large spatial shift in simulated permafrost occurrence, we infer an only moderate increase in total LGM permafrost area (18.3 x 10(6) km(2)) - together with pronounced changes in the depth of seasonal thaw. Earlier empirical reconstructions suggest a larger spread of permafrost towards more southerly regions under glacial conditions, but with a highly uncertain extent of non-continuous permafrost. Compared to a control simulation without describing the transport of SOC into perennially frozen ground, the implementation of our newly developed module for simulating permafrost SOC accumulation leads to a doubling of simulated LGM permafrost SOC storage (amounting to a total of similar to 150 PgC). Despite LGM temperatures favouring a larger permafrost extent, simulated cold glacial temperatures - together with low precipitation and low CO2 levels - limit vegetation productivity and therefore prevent a larger glacial SOC build-up in our model. Changes in physical and biogeochemical boundary conditions during deglacial warming lead to an increase in mineral SOC storage towards the Holocene (168 PgC at PI), which is below observational estimates (575 PgC in continuous and discontinuous permafrost). Additional model experiments clarified the sensitivity of simulated SOC storage to model parameters, affecting long-term soil carbon respiration rates and simulated ALDs. Rather than a steady increase in carbon release from the LGM to PI as a consequence of deglacial permafrost degradation, our results suggest alternating phases of soil carbon accumulation and loss as an effect of dynamic changes in permafrost extent, ALDs, soil litter input, and heterotrophic respiration.

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