Articles | Volume 23, issue 18
https://doi.org/10.5194/bg-23-6557-2026
© Author(s) 2026. This work is distributed under
the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/bg-23-6557-2026
© Author(s) 2026. This work is distributed under
the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
Research article
| 
18 Sep 2026
Research article |
| 18 Sep 2026
| 18 Sep 2026
Evaluating vegetation indices for monitoring drought and post-drought declines in European forest productivity
Department of Earth and Environmental Sciences, Lund University, Lund, Sweden
Tim Schacherl
Bioclimatology, University of Göttingen, Göttingen, Germany
Lars Eklundh
Department of Earth and Environmental Sciences, Lund University, Lund, Sweden
Hongxiao Jin
Department of Earth and Environmental Sciences, Lund University, Lund, Sweden
Anne Klosterhalfen
Bioclimatology, University of Göttingen, Göttingen, Germany
Alexander Knohl
Bioclimatology, University of Göttingen, Göttingen, Germany
Centre of Biodiversity and Sustainable Land Use, University of Göttingen, Göttingen, Germany
Natascha Kljun
Department of Earth and Environmental Sciences, Lund University, Lund, Sweden
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Earth Syst. Sci. Data, 18, 6357–6409, https://doi.org/10.5194/essd-18-6357-2026, https://doi.org/10.5194/essd-18-6357-2026, 2026
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EGUsphere, https://doi.org/10.5194/egusphere-2026-5083, https://doi.org/10.5194/egusphere-2026-5083, 2026
This preprint is open for discussion and under review for Biogeosciences (BG).
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We measured soil carbon fluxes during the first four years after a wildfire in the Swedish boreal forest. Soil CO2 emissions decreased substantially only when trees were killed by fire or by post-fire logging, but not when trees survived the fire and were left standing. Soil methane flux was not affected by fire. Logging trees already killed by fire had no additional impact on soil carbon fluxes. Post-fire forest management strategy impacted vegetation regrowth and carbon dynamics.
Janne Rinne, Patryk Łakomiec, Patrik Vestin, Joel D. White, Per Weslien, Julia Kelly, Natascha Kljun, Lena Ström, and Leif Klemedtsson
Biogeosciences, 19, 4331–4349, https://doi.org/10.5194/bg-19-4331-2022, https://doi.org/10.5194/bg-19-4331-2022, 2022
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Earth Syst. Sci. Data, 18, 6357–6409, https://doi.org/10.5194/essd-18-6357-2026, https://doi.org/10.5194/essd-18-6357-2026, 2026
Short summary
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This dataset includes monthly measurements of carbon dioxide and methane exchange between land, water, and the atmosphere from over 1000 sites in Arctic and boreal regions. It combines measurements from a variety of ecosystems, including wetlands, forests, tundra, lakes, and rivers, gathered by over 260 researchers from 1984–2024. This dataset can be used to improve and reduce uncertainty in carbon budgets in order to strengthen our understanding of climate feedbacks in a warming world.
Erica Jaakkola, Johan Lindström, Julia Kelly, Natascha Kljun, and Thomas Holst
EGUsphere, https://doi.org/10.5194/egusphere-2026-5083, https://doi.org/10.5194/egusphere-2026-5083, 2026
This preprint is open for discussion and under review for Biogeosciences (BG).
Short summary
Short summary
Wildfires are increasing in boreal forests, but little is known about how they affect natural emissions of volatile compounds from forest soils. We measured these emissions during the first four years after a major wildfire in Sweden. Emission recovery depended strongly on fire severity, stand age, and post-fire management. Early emissions were largely driven by litter from damaged trees, highlighting how wildfire and forest management can influence atmospheric chemistry and climate feedbacks.
Jesse Juhani Soininen, Kukka-Maaria Kohonen, Stavros Stagakis, Ara Cho, Betty Molinier, Pascal Rubli, Rainer Hilland, Natascha Kljun, and Leena Järvi
EGUsphere, https://doi.org/10.5194/egusphere-2026-4603, https://doi.org/10.5194/egusphere-2026-4603, 2026
This preprint is open for discussion and under review for Atmospheric Chemistry and Physics (ACP).
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Carbonyl sulfide is increasingly used to estimate how much carbon dioxide plants absorb from the atmosphere, but this is difficult in cities because human activities also emit this gas. We measured its exchange between the atmosphere and the urban area of Zurich and found that vegetation remained the main sink, while emissions from human activities reduced this uptake in densely built areas. These results improve the interpretation of urban carbon exchange.
Ida Bagus Mandhara Brasika, Pierre Friedlingstein, Stephen Sitch, Michael O'Sullivan, Maria Carolina Duran-Rojas, Emma W. Littleton, Toby R. Marthews, Timothy Hill, Wei Li, Yidi Xu, Jiaxin Zhou, Xiangzhong Luo, Ruiying Zhao, Yuanchao Fan, Dedy Sukmara, Timer Manurung, Masayuki Kondo, Alexander Knohl, Tejaswini Jaajpera, and Lisma Safitri
EGUsphere, https://doi.org/10.5194/egusphere-2026-3689, https://doi.org/10.5194/egusphere-2026-3689, 2026
This preprint is open for discussion and under review for Geoscientific Model Development (GMD).
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Oil palm is a major crop in Indonesia, but climate models often treat it as generic cropland. We developed a new way to represent oil palm in the Joint UK Land Environment Simulator. This improved how the model captures carbon stored and released by oil-palm plantations. The work helps estimate emissions from land-use change more realistically, supporting better understanding of tropical agriculture, deforestation and climate impacts.
László Haszpra, Zoltán Barcza, Anikó Kern, and Natascha Kljun
EGUsphere, https://doi.org/10.5194/egusphere-2026-1077, https://doi.org/10.5194/egusphere-2026-1077, 2026
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Ann-Kristin Kunz, Samuel Hammer, Patrick Aigner, Laura Bignotti, Lars Borchardt, Jia Chen, Julian Della Coletta, Lukas Emmenegger, Markus Eritt, Xochilt Gutiérrez, Josh Hashemi, Rainer Hilland, Christopher Holst, Armin Jordan, Natascha Kljun, Richard Kneißl, Changxing Lan, Virgile Legendre, Ingeborg Levin, Benjamin Loubet, Matthias Mauder, Betty Molinier, Susanne Preunkert, Michel Ramonet, Stavros Stagakis, and Andreas Christen
Atmos. Chem. Phys., 26, 4967–5003, https://doi.org/10.5194/acp-26-4967-2026, https://doi.org/10.5194/acp-26-4967-2026, 2026
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We present radiocarbon (14C)-based fossil fuel CO2 fluxes from relaxed eddy accumulation measurements on tall towers in the cities of Zurich, Paris, and Munich. By separating net CO2 fluxes into fossil and non-fossil components, these data reveal significant and variable contributions from human, plant, and soil respiration, as well as point-source emissions. These unique insights into CO2 flux composition offer crucial information for observation-based validation of urban emission estimates.
Biplob Dey, Toke Due Sjøgren, Peeyush Khare, Georgios I. Gkatzelis, Yizhen Wu, Sindhu Vasireddy, Martin Schultz, Alexander Knohl, Riikka Rinnan, Thorsten Hohaus, and Eva Y. Pfannerstill
Biogeosciences, 23, 1423–1457, https://doi.org/10.5194/bg-23-1423-2026, https://doi.org/10.5194/bg-23-1423-2026, 2026
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Trees release reactive gases that affect air quality and climate. We studied how these emissions from European beech and English oak change under realistic scenarios of combined and single heat and ozone stress. Heat increased emissions, while ozone reduced most of them. When stressors were combined, the effects were complex and varied by species. Machine learning identified key stress-related compounds. Our findings show that future tree stress may alter air quality and climate interactions.
Beatriz P. Cazorla, Ana Meijide, Javier Cabello, Julio Peñas, Javier Martínez-López, Rodrigo Vargas, Leonardo Montagnani, Alexander Knohl, Lukas Siebicke, Benimiano Gioli, Jiří Dušek, Ladislav Šigut, Andreas Ibrom, Georg Wohlfahrt, Eugénie Paul-Limoges, Kathrin Fuchs, Antonio Manco, Marian Pavelka, Lutz Merbold, Lukas Hörtnagl, Pierpaolo Duce, Ignacio Goded, Kim Pilegaard, and Domingo Alcaraz-Segura
Biogeosciences, 23, 1223–1243, https://doi.org/10.5194/bg-23-1223-2026, https://doi.org/10.5194/bg-23-1223-2026, 2026
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We assess whether satellite-derived Ecosystem Functional Types (EFTs) reflect spatial heterogeneity in carbon fluxes across Europe. Using Eddy Covariance data from 50 sites, we show that EFTs capture distinct Net Ecosystem Exchange dynamics and perform slightly better than Plant Functional Types (PFTs). EFTs offer a scalable, annually updatable approach to monitor ecosystem functioning and its interannual variability.
José Ángel Callejas-Rodelas, Justus van Ramshorst, Alexander Knohl, Lukas Siebicke, Dietmar Fellert, Marek Peksa, Dirk Böttger, and Christian Markwitz
Earth Syst. Sci. Data, 18, 845–874, https://doi.org/10.5194/essd-18-845-2026, https://doi.org/10.5194/essd-18-845-2026, 2026
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A dataset expanding around seventy eight site-years was compiled, harmonized and presented. The dataset consisted in eddy covariance and meteorological measurements over four pairs of agroforestry and open cropland systems, and one pair of agroforestry and open grassland system. This is the first ever dataset compiling this type of data over temperate agroforestry systems.
Rainer Hilland, Josh Hashemi, Stavros Stagakis, Dominik Brunner, Lionel Constantin, Natascha Kljun, Ann-Kristin Kunz, Betty Molinier, Samuel Hammer, Lukas Emmenegger, and Andreas Christen
Atmos. Chem. Phys., 25, 14279–14299, https://doi.org/10.5194/acp-25-14279-2025, https://doi.org/10.5194/acp-25-14279-2025, 2025
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We present a study of simultaneously measured fluxes of carbon dioxide (CO2) and co-emitted species in the city of Zurich. Flux measurements of CO2 alone cannot be attributed to specific emission sectors, such as road transport or residential heating. We present a model which uses the measured ratios of CO2 to carbon monoxide (CO) and nitrogen oxides (NOx) as well as sector-specific reference ratios, to attribute measured fluxes to their emission sectors.
José Ángel Callejas-Rodelas, Alexander Knohl, Ivan Mammarella, Timo Vesala, Olli Peltola, and Christian Markwitz
Biogeosciences, 22, 4507–4529, https://doi.org/10.5194/bg-22-4507-2025, https://doi.org/10.5194/bg-22-4507-2025, 2025
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The spatial variability of CO2 and water vapour exchanges with the atmosphere was quantified above an agroforestry system and further compared to a monocropping system using a total of four eddy covariance stations. The variability of fluxes within the agroforestry site was found to be as large as the variability between agroforestry and monocropping site, induced by the heterogeneity of the site, which highlights the need for replicated measurements above such ecosystems.
Thorge Wintz, Alexander Röll, Gustavo Brant Paterno, Florian Ellsäßer, Delphine Clara Zemp, Hendrayanto, Bambang Irawan, Alexander Knohl, Holger Kreft, and Dirk Hölscher
EGUsphere, https://doi.org/10.5194/egusphere-2025-2596, https://doi.org/10.5194/egusphere-2025-2596, 2025
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We investigated how the size and diversity of tree patches in Indonesian oil palm landscapes influence the movement of water to the atmosphere and local cooling. Our study shows that larger tree patches increase cooling mainly by supporting greater plant diversity and more complex vegetation structure. These findings suggest that expanding and diversifying tree patches can help manage microclimate and water cycling in agricultural areas.
Jacob A. Nelson, Sophia Walther, Fabian Gans, Basil Kraft, Ulrich Weber, Kimberly Novick, Nina Buchmann, Mirco Migliavacca, Georg Wohlfahrt, Ladislav Šigut, Andreas Ibrom, Dario Papale, Mathias Göckede, Gregory Duveiller, Alexander Knohl, Lukas Hörtnagl, Russell L. Scott, Jiří Dušek, Weijie Zhang, Zayd Mahmoud Hamdi, Markus Reichstein, Sergio Aranda-Barranco, Jonas Ardö, Maarten Op de Beeck, Dave Billesbach, David Bowling, Rosvel Bracho, Christian Brümmer, Gustau Camps-Valls, Shiping Chen, Jamie Rose Cleverly, Ankur Desai, Gang Dong, Tarek S. El-Madany, Eugenie Susanne Euskirchen, Iris Feigenwinter, Marta Galvagno, Giacomo A. Gerosa, Bert Gielen, Ignacio Goded, Sarah Goslee, Christopher Michael Gough, Bernard Heinesch, Kazuhito Ichii, Marcin Antoni Jackowicz-Korczynski, Anne Klosterhalfen, Sara Knox, Hideki Kobayashi, Kukka-Maaria Kohonen, Mika Korkiakoski, Ivan Mammarella, Mana Gharun, Riccardo Marzuoli, Roser Matamala, Stefan Metzger, Leonardo Montagnani, Giacomo Nicolini, Thomas O'Halloran, Jean-Marc Ourcival, Matthias Peichl, Elise Pendall, Borja Ruiz Reverter, Marilyn Roland, Simone Sabbatini, Torsten Sachs, Marius Schmidt, Christopher R. Schwalm, Ankit Shekhar, Richard Silberstein, Maria Lucia Silveira, Donatella Spano, Torbern Tagesson, Gianluca Tramontana, Carlo Trotta, Fabio Turco, Timo Vesala, Caroline Vincke, Domenico Vitale, Enrique R. Vivoni, Yi Wang, William Woodgate, Enrico A. Yepez, Junhui Zhang, Donatella Zona, and Martin Jung
Biogeosciences, 21, 5079–5115, https://doi.org/10.5194/bg-21-5079-2024, https://doi.org/10.5194/bg-21-5079-2024, 2024
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The movement of water, carbon, and energy from the Earth's surface to the atmosphere, or flux, is an important process to understand because it impacts our lives. Here, we outline a method called FLUXCOM-X to estimate global water and CO2 fluxes based on direct measurements from sites around the world. We go on to demonstrate how these new estimates of net CO2 uptake/loss, gross CO2 uptake, total water evaporation, and transpiration from plants compare to previous and independent estimates.
Justus G. V. van Ramshorst, Alexander Knohl, José Ángel Callejas-Rodelas, Robert Clement, Timothy C. Hill, Lukas Siebicke, and Christian Markwitz
Atmos. Meas. Tech., 17, 6047–6071, https://doi.org/10.5194/amt-17-6047-2024, https://doi.org/10.5194/amt-17-6047-2024, 2024
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In this work we present experimental field results of a lower-cost eddy covariance (LC-EC) system, which can measure the ecosystem exchange of carbon dioxide and water vapour with the atmosphere. During three field campaigns on a grassland and agroforestry grassland, we compared the LC-EC with a conventional eddy covariance (CON-EC) system. Our results show that LC-EC has the potential to measure EC fluxes at only approximately 25 % of the cost of a CON-EC system.
Julia Kelly, Stefan H. Doerr, Johan Ekroos, Theresa S. Ibáñez, Md. Rafikul Islam, Cristina Santín, Margarida Soares, and Natascha Kljun
EGUsphere, https://doi.org/10.5194/egusphere-2024-2016, https://doi.org/10.5194/egusphere-2024-2016, 2024
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We measured soil carbon fluxes during the first four years after a wildfire in the Swedish boreal forest. Soil CO2 emissions decreased substantially only when trees were killed by fire or by post-fire logging, but not when trees survived the fire and were left standing. Soil methane flux was not affected by fire. Logging trees already killed by fire had no additional impact on soil carbon fluxes. Post-fire forest management strategy impacted vegetation regrowth and carbon dynamics.
Yuan Yan, Anne Klosterhalfen, Fernando Moyano, Matthias Cuntz, Andrew C. Manning, and Alexander Knohl
Biogeosciences, 20, 4087–4107, https://doi.org/10.5194/bg-20-4087-2023, https://doi.org/10.5194/bg-20-4087-2023, 2023
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A better understanding of O2 fluxes, their exchange ratios with CO2 and their interrelations with environmental conditions would provide further insights into biogeochemical ecosystem processes. We, therefore, used the multilayer canopy model CANVEG to simulate and analyze the flux exchange for our forest study site for 2012–2016. Based on these simulations, we further successfully tested the application of various micrometeorological methods and the prospects of real O2 flux measurements.
Ross Petersen, Thomas Holst, Meelis Mölder, Natascha Kljun, and Janne Rinne
Atmos. Chem. Phys., 23, 7839–7858, https://doi.org/10.5194/acp-23-7839-2023, https://doi.org/10.5194/acp-23-7839-2023, 2023
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We investigate variability in the vertical distribution of volatile organic compounds (VOCs) in boreal forest, determined through multiyear measurements at several heights in a boreal forest in Sweden. VOC source/sink seasonality in canopy was explored using these vertical profiles and with measurements from a collection of sonic anemometers on the station flux tower. Our results show seasonality in the source/sink distribution for several VOCs, such as monoterpenes and water-soluble compounds.
Janne Rinne, Patryk Łakomiec, Patrik Vestin, Joel D. White, Per Weslien, Julia Kelly, Natascha Kljun, Lena Ström, and Leif Klemedtsson
Biogeosciences, 19, 4331–4349, https://doi.org/10.5194/bg-19-4331-2022, https://doi.org/10.5194/bg-19-4331-2022, 2022
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The study uses the stable isotope 13C of carbon in methane to investigate the origins of spatial and temporal variation in methane emitted by a temperate wetland ecosystem. The results indicate that methane production is more important for spatial variation than methane consumption by micro-organisms. Temporal variation on a seasonal timescale is most likely affected by more than one driver simultaneously.
Xin Yu, René Orth, Markus Reichstein, Michael Bahn, Anne Klosterhalfen, Alexander Knohl, Franziska Koebsch, Mirco Migliavacca, Martina Mund, Jacob A. Nelson, Benjamin D. Stocker, Sophia Walther, and Ana Bastos
Biogeosciences, 19, 4315–4329, https://doi.org/10.5194/bg-19-4315-2022, https://doi.org/10.5194/bg-19-4315-2022, 2022
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Identifying drought legacy effects is challenging because they are superimposed on variability driven by climate conditions in the recovery period. We develop a residual-based approach to quantify legacies on gross primary productivity (GPP) from eddy covariance data. The GPP reduction due to legacy effects is comparable to the concurrent effects at two sites in Germany, which reveals the importance of legacy effects. Our novel methodology can be used to quantify drought legacies elsewhere.
László Haszpra, Zoltán Barcza, Zita Ferenczi, Roland Hollós, Anikó Kern, and Natascha Kljun
Atmos. Meas. Tech., 15, 5019–5031, https://doi.org/10.5194/amt-15-5019-2022, https://doi.org/10.5194/amt-15-5019-2022, 2022
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A novel approach is used for the determination of greenhouse gas (GHG) emissions of small rural settlements, which may significantly differ from those of urban regions and have hardly been studied yet. Among other results, it turned out that wintertime nitrous oxide emission is significantly underestimated in the official emission inventories. Given the large number of such settlements, the underestimation may also distort the national total emission values reported to international databases.
Patryk Łakomiec, Jutta Holst, Thomas Friborg, Patrick Crill, Niklas Rakos, Natascha Kljun, Per-Ola Olsson, Lars Eklundh, Andreas Persson, and Janne Rinne
Biogeosciences, 18, 5811–5830, https://doi.org/10.5194/bg-18-5811-2021, https://doi.org/10.5194/bg-18-5811-2021, 2021
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Methane emission from the subarctic mire with heterogeneous permafrost status was measured for the years 2014–2016. Lower methane emission was measured from the palsa mire sector while the thawing wet sector emitted more. Both sectors have a similar annual pattern with a gentle rise during spring and a decrease during autumn. The highest emission was observed in the late summer. Winter emissions were positive during the measurement period and have a significant impact on the annual budgets.
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Short summary
It is vital to monitor drought-related reductions in forest productivity over large areas. We tested the ability of 12 satellite variables to capture negative drought impacts on forest productivity across 18 European forest sites. Surface temperature worked best across all forest sites and droughts. After drought, forest productivity recovered faster than some commonly-used satellite variables, meaning use of these variables may underestimate forests’ resilience to drought.
It is vital to monitor drought-related reductions in forest productivity over large areas. We...
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