Browsing by Subject "eddy-covariance"

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  • Mahnken, Mats; Cailleret, Maxime; Collalti, Alessio; Trotta, Carlo; Biondo, Corrado; D'Andrea, Ettore; Dalmonech, Daniela; Marano, Gina; Mäkelä, Annikki; Minunno, Francesco; Peltoniemi, Mikko; Trotsiuk, Volodymyr; Nadal-Sala, Daniel; Sabate, Santiago; Vallet, Patrick; Aussenac, Raphael; Cameron, David R.; Bohn, Friedrich J.; Grote, Ruediger; Augustynczik, Andrey L. D.; Yousefpour, Rasoul; Huber, Nica; Bugmann, Harald; Merganicova, Katarina; Merganic, Jan; Valent, Peter; Lasch-Born, Petra; Hartig, Florian; del Valle, Iliusi D. Vega; Volkholz, Jan; Gutsch, Martin; Matteucci, Giorgio; Krejza, Jan; Ibrom, Andreas; Meesenburg, Henning; Roetzer, Thomas; van der Maaten-Theunissen, Marieke; van der Maaten, Ernst; Reyer, Christopher P. O. (2022)
    Forest models are instrumental for understanding and projecting the impact of climate change on forests. A considerable number of forest models have been developed in the last decades. However, few systematic and comprehensive model comparisons have been performed in Europe that combine an evaluation of modelled carbon and water fluxes and forest structure. We evaluate 13 widely used, state-of-the-art, stand-scale forest models against field measurements of forest structure and eddy-covariance data of carbon and water fluxes over multiple decades across an environmental gradient at nine typical European forest stands. We test the models' performance in three dimensions: accuracy of local predictions (agreement of modelled and observed annual data), realism of environmental responses (agreement of modelled and observed responses of daily gross primary productivity to temperature, radiation and vapour pressure deficit) and general applicability (proportion of European tree species covered). We find that multiple models are available that excel according to our three dimensions of model performance. For the accuracy of local predictions, variables related to forest structure have lower random and systematic errors than annual carbon and water flux variables. Moreover, the multi-model ensemble mean provided overall more realistic daily productivity responses to environmental drivers across all sites than any single individual model. The general applicability of the models is high, as almost all models are currently able to cover Europe's common tree species. We show that forest models complement each other in their response to environmental drivers and that there are several cases in which individual models outperform the model ensemble. Our framework provides a first step to capturing essential differences between forest models that go beyond the most commonly used accuracy of predictions. Overall, this study provides a point of reference for future model work aimed at predicting climate impacts and supporting climate mitigation and adaptation measures in forests.
  • Kittler, Fanny; Heimann, Martin; Kolle, Olaf; Zimov, Nikita; Zimov, Sergei; Gockede, Mathias (2017)
    Permafrost landscapes in northern high latitudes with their massive organic carbon stocks are an important, poorly known, component of the global carbon cycle. However, in light of future Arctic warming, the sustainability of these carbon pools is uncertain. To a large part, this is due to a limited understanding of the carbon cycle processes because of sparse observations in Arctic permafrost ecosystems. Here we present an eddy covariance data set covering more than 3 years of continuous CO2 and CH4 flux observations within a moist tussock tundra ecosystem near Chersky in north-eastern Siberia. Through parallel observations of a disturbed (drained) area and a control area nearby, we aim to evaluate the long-term effects of a persistently lowered water table on the net vertical carbon exchange budgets and the dominating biogeochemical mechanisms. Persistently drier soils trigger systematic shifts in the tundra ecosystem carbon cycle patterns. Both, uptake rates of CO2 and emissions of CH4 decreased. Year-round measurements emphasize the importance of the non-growing seasonin particular the zero-curtain period in the fallto the annual budget. Approximately 60% of the CO2 uptake in the growing season is lost during the cold seasons, while CH4 emissions during the non-growing season account for 30% of the annual budget. Year-to-year variability in temperature conditions during the late growing season was identified as the primary control of the interannual variability observed in the CO2 and CH4 fluxes.
  • Foken, Thomas; Babel, Wolfgang; Munger, J. William; Gronholm, Tiia; Vesala, Timo; Knohl, Alexander (2021)
    Extensive studies are available that analyse time series of carbon dioxide and water flux measurements of FLUXNET sites over many years and link these results to climate change such as changes in atmospheric carbon dioxide concentration, air temperature and growing season length and other factors. Many of the sites show trends to a larger carbon uptake. Here we analyse time series of net ecosystem exchange, gross primary production, respiration, and evapotranspiration of four forest sites with particularly long measurement periods of about 20 years. The regular trends shown are interrupted by periods with higher or lower increases of carbon uptake. These breakpoints can be of very different origin and include forest decline, increased vegetation period, drought effects, heat waves, and changes in site heterogeneity. The influence of such breakpoints should be included in long-term studies of land-atmosphere exchange processes.