Browsing by Subject "EDDY COVARIANCE"

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  • Katul, Gabriel; Mammarella, Ivan; Grönholm, Tiia; Vesala, Timo (2018)
    Two ideas regarding the structure of turbulence near a clear air-water interface are used to derive a waterside gas transfer velocity k(L) for sparingly and slightly soluble gases. The first is that k(L) is proportional to the turnover velocity described by the vertical velocity structure function D-ww(r), where r is separation distance between two points. The second is that the scalar exchange between the air-water interface and the waterside turbulence can be suitably described by a length scale proportional to the Batchelor scale l(B) = Sc-1/2, where Sc is the molecular Schmidt number and eta is the Kolmogorov microscale defining the smallest scale of turbulent eddies impacted by fluid viscosity. Using an approximate solution to the von Karman-Howarth equation predicting D-ww(r) in the inertial and viscous regimes, prior formulations for k(L) are recovered including (i) kL = root 2/15Sc(-1/2)v(K), v(K) is the Kolmogorov velocity defined by the Reynolds number v(K)eta/nu = 1 and nu is the kinematic viscosity of water; (ii) surface divergence formulations; (iii) k(L) alpha Sc(-1/2)u(*), where u(*) is the waterside friction velocity; (iv) k(L) alpha Sc-1/2 root g nu/u(*) for Keulegan numbers exceeding a threshold needed for long-wave generation, where the proportionality constant varies with wave age, g is the gravitational acceleration; and (v) k(L) = root 2/15Sc(-1/2) (nu g beta(o)q(o))(1/4) in free convection, where q(o) is the surface heat flux and beta(o) is the thermal expansion of water. The work demonstrates that the aforementioned k(L) formulations can be recovered from a single structure function model derived for locally homogeneous and isotropic turbulence.
  • Keronen, Petri; Reissell, Anni; Chevallier, Frederic; Siivola, Erkki; Pohja, Toivo; Hiltunen, Veijo; Hatakka, Juha; Aalto, Tuula; Rivier, Leonard; Ciais, Philippe; Jordan, Armin; Hari, Pertti; Viisanen, Yrjo; Vesala, Timo (2014)
  • Kiuru, Petri; Ojala, Anne; Mammarella, Ivan; Heiskanen, Jouni; Erkkila, Kukka-Maaria; Miettinen, Heli; Vesala, Timo; Huttula, Timo (2019)
    Freshwater lakes are important in carbon cycling, especially in the boreal zone where many lakes are supersaturated with the greenhouse gas carbon dioxide (CO2) and emit it to the atmosphere, thus ventilating carbon originally fixed by the terrestrial system. The exchange of CO2 between water and the atmosphere is commonly estimated using simple wind-based parameterizations or models of gas transfer velocity (k). More complex surface renewal models, however, have been shown to yield more correct estimates of k in comparison with direct CO2 flux measurements. We incorporated four gas exchange models with different complexity into a vertical process-based physico-biochemical lake model, MyLake C, and assessed the performance and applicability of the alternative lake model versions to simulate air-water CO2 fluxes over a small boreal lake. None of the incorporated gas exchange models significantly outperformed the other models in the simulations in comparison to the measured near-surface CO2 concentrations or respective air-water CO2 fluxes calculated directly with the gas exchange models using measurement data as input. The use of more complex gas exchange models in the simulation, on the contrary, led to difficulties in obtaining a sufficient gain of CO2 in the water column and thus resulted in lower CO2 fluxes and water column CO2 concentrations compared to the respective measurement-based values. The inclusion of sophisticated and more correct models for air-water CO2 exchange in process-based lake models is crucial in efforts to properly assess lacustrine carbon budgets through model simulations in both single lakes and on a larger scale. However, finding higher estimates for both the internal and external sources of inorganic carbon in boreal lakes is important if improved knowledge of the magnitude of CO2 evasion from lakes is included in future studies on lake carbon budgets.
  • Nadal-Sala, Daniel; Grote, Ruediger; Birami, Benjamin; Lintunen, Anna; Mammarella, Ivan; Preisler, Yakir; Rotenberg, Eyal; Salmon, Yann; Tatarinov, Fedor; Yakir, Dan; Ruehr, Nadine K. (2021)
    Climate change will impact forest productivity worldwide. Forecasting the magnitude of such impact, with multiple environmental stressors changing simultaneously, is only possible with the help of process-based models. In order to assess their performance, such models require careful evaluation against measurements. However, direct comparison of model outputs against observational data is often not reliable, as models may provide the right answers due to the wrong reasons. This would severely hinder forecasting abilities under unprecedented climate conditions. Here, we present a methodology for model assessment, which supplements the traditional output-to-observation model validation. It evaluates model performance through its ability to reproduce observed seasonal changes of the most limiting environmental driver (MLED) for a given process, here daily gross primary productivity (GPP). We analyzed seasonal changes of the MLED for GPP in two contrasting pine forests, the Mediterranean Pinus halepensis Mill. Yatir (Israel) and the boreal Pinus sylvestris L. Hyytiala (Finland) from three years of eddy-covariance flux data. Then, we simulated the same period with a state-of-the-art process-based simulation model (LandscapeDNDC). Finally, we assessed if the model was able to reproduce both GPP observations and MLED seasonality. We found that the model reproduced the seasonality of GPP in both stands, but it was slightly overestimated without site-specific fine-tuning. Interestingly, although LandscapeDNDC properly captured the main MLED in Hyytiala (temperature) and in Yatir (soil water availability), it failed to reproduce high-temperature and high-vapor pressure limitations of GPP in Yatir during spring and summer. We deduced that the most likely reason for this divergence is an incomplete description of stomatal behavior. In summary, this study validates the MLED approach as a model evaluation tool, and opens up new possibilities for model improvement.
  • Barba, J; Cueva, A; Bahn, M; Barron-Gafford, GA; Bond-Lamberty, B; Hanson, PJ; Jaimes, A; Kulmala, Liisa-Maija; Pumpanen, J; Scott, RL; Wohlfahrt, G; Vargas, R (2018)
    The net ecosystem exchange (NEE) is the difference between ecosystem CO2 assimilation and CO2 losses to the atmosphere. Ecosystem respiration (R-eco), the efflux of CO2 from the ecosystem to the atmosphere, includes the soil-to-atmosphere carbon flux (i.e., soil respiration; R-soil) and aboveground plant respiration. Therefore, R-soil is a fraction of R-eco and theoretically has to be smaller than R-eco at daily, seasonal, and annual scales. However, several studies estimating R-eco with the eddy covariance technique and measuring R-soll within the footprint of the tower have reported higher R-soil than R-eco, at different time scales. Here, we compare four different and contrasting ecosystems (from forest to grasslands, and from boreal to semiarid) to test if measurements of R-eco are consistently higher than R-soil. In general, both fluxes showed similar temporal patterns, but R-eco, was not consistently higher than R-soil from daily to annual scales across sites. We identified several issues that apply for measuring NEE and measuring/upscaling R-soil that could result in an underestimation of R-eco and/or an overestimation of R-soil. These issues are discussed based on (a) nighttime measurements of NEE, (b) R-soil measurements, and (c) the interpretation of the functional relationships of these fluxes with temperature (i.e., Q(10)). We highlight that there is still a need for better integration of R-soil with eddy covariance measurements to address challenges related to the spatial and temporal variability of R-eco, and R-soil.
  • Linkosalmi, Maiju; Aurela, Mika; Tuovinen, Juha-Pekka; Peltoniemi, Mikko; Tanis, Cemal M.; Arslan, Ali N.; Kolari, Pasi; Bottcher, Kristin; Aalto, Tuula; Rainne, Juuso; Hatakka, Juha; Laurila, Tuomas (2016)
    Digital repeat photography has become a widely used tool for assessing the annual course of vegetation phenology of different ecosystems. By using the green chromatic coordinate (GCC) as a greenness measure, we examined the feasibility of digital repeat photography for assessing the vegetation phenology in two contrasting high-latitude ecosystems. Ecosystem-atmosphere CO2 fluxes and various meteorological variables were continuously measured at both sites. While the seasonal changes in GCC were more obvious for the ecosystem that is dominated by annual plants (open wetland), clear seasonal patterns were also observed for the evergreen ecosystem (coniferous forest). Daily and seasonal time periods with sufficient solar radiation were determined based on images of a grey reference plate. The variability in cloudiness had only a minor effect on GCC, and GCC did not depend on the sun angle and direction either. The daily GCC of wetland correlated well with the daily photosynthetic capacity estimated from the CO2 flux measurements. At the forest site, the correlation was high in 2015 but there were discernible deviations during the course of the summer of 2014. The year-to-year differences were most likely generated by meteorological conditions, with higher temperatures coinciding with higher GCCs. In addition to depicting the seasonal course of ecosystem functioning, GCC was shown to respond to environmental changes on a timescale of days. Overall, monitoring of phenological variations with digital images provides a powerful tool for linking gross primary production and phenology.
  • Provenzale, Maria; Ojala, Anne; Heiskanen, Jouni; Erkkila, Kukka-Maaria; Mammarella, Ivan; Hari, Pertti; Vesala, Timo (2018)
    Lakes are important actors in biogeochemical cycles and a powerful natural source of CO2. However, they are not yet fully integrated in carbon global budgets, and the carbon cycle in the water is still poorly understood. In freshwater ecosystems, productivity studies have usually been carried out with traditional methods (bottle incubations, C-14 technique), which are imprecise and have a poor temporal resolution. Consequently, our ability to quantify and predict the net ecosystem productivity (NEP) is limited: the estimates are prone to errors and the NEP cannot be parameterised from environmental variables. Here we expand the testing of a free-water method based on the direct measurement of the CO2 concentration in the water. The approach was first proposed in 2008, but was tested on a very short data set (3 days) under specific conditions (autumn turnover); despite showing promising results, this method has been neglected by the scientific community. We tested the method under different conditions (summer stratification, typical summer conditions for boreal dark-water lakes) and on a much longer data set (40 days), and quantitatively validated it comparing our data and productivity models. We were able to evaluate the NEP with a high temporal resolution (minutes) and found a very good agreement (R-2 >= 0.71) with the models. We also estimated the parameters of the productivity-irradiance (PI) curves that allow the calculation of the NEP from irradiance and water temperature. Overall, our work shows that the approach is suitable for productivity studies under a wider range of conditions, and is an important step towards developing this method so that it becomes more widely used.
  • Saunders, Matthew; Dengel, Sigrid; Kolari, Pasi; Moureaux, Christine; Montagnani, Leonardo; Ceschia, Eric; Altimir, Nuria; Lopez-Ballesteros, Ana; Maranon-Jimenez, Sara; Acosta, Manuel; Klumpp, Katja; Gielen, Bert; Op de Beeck, Maarten; Hortnagl, Lukas; Merbold, Lutz; Osborne, Bruce; Grunwald, Thomas; Arrouays, Dominique; Boukir, Hakima; Saby, Nicolas; Nicolini, Giacomo; Papale, Dario; Jones, Michael (2018)
    There are many factors that influence ecosystem scale carbon, nitrogen and greenhouse gas dynamics, including the inherent heterogeneity of soils and vegetation, anthropogenic management interventions, and biotic and abiotic disturbance events. It is important therefore, to document the characteristics of the soils and vegetation and to accurately report all management activities, and disturbance events to aid the interpretation of collected data, and to determine whether the ecosystem either amplifies or mitigates climate change. This paper outlines the importance of assessing both the spatial and temporal variability of soils and vegetation and to report all management events, the import or export of C or N from the ecosystem, and the occurrence of biotic/abiotic disturbances at ecosystem stations of the Integrated Carbon Observation System, a pan-European research infrastructure.
  • Kulmala, Liisa; Pumpanen, Jukka; Kolari, Pasi; Dengel, Sigrid; Berninger, Frank; Köster, Kajar; Matkala, Laura; Vanhatalo, Anni; Vesala, Timo; Bäck, Jaana (2019)
    We studied the inter- and intra-annual dynamics of the photosynthesis of forest floor vegetation and tree canopy in a subarctic Scots pine stand at the northern timberline in Finland. We tackled the issue using three different approaches: 1) measuring carbon dioxide exchange above and below canopy with the eddy covariance technique, 2) modelling the photosynthesis of the tree canopy based on shoot chamber measurements, and 3) upscaling the forest floor photosynthesis using biomass estimates and available information on the annual cycle of photosynthetic capacity of those species. The studied ecosystem was generally a weak sink of carbon but the sink strength showed notable year-to-year variation. Total ecosystem respiration and photosynthesis indicated a clear temperature limitation for the carbon exchange. However, the increase in photosynthetic production was steeper than the increase in respiration with temperature, indicating that warm temperatures increase the sink strength and do not stimulate the total ecosystem respiration as much in the 4-year window studied. The interannual variation in the photosynthetic production of the forest stand mainly resulted from the forest floor vegetation, whereas the photosynthesis of the tree canopy seemed to be more stable from year to year. Tree canopy photosynthesis increased earlier in the spring, whereas that of the forest floor increased after snowmelt, highlighting that models for photosynthesis in the northern area should also include snow cover in order to accurately estimate the seasonal dynamics of photosynthesis in these forests.
  • Korkiakoski, Mika; Tuovinen, Juha-Pekka; Aurela, Mika; Koskinen, Markku; Minkkinen, Kari; Ojanen, Paavo; Penttila, Timo; Rainne, Juuso; Laurila, Tuomas; Lohila, Annalea (2017)
    We measured methane (CH4) exchange rates with automatic chambers at the forest floor of a nutrient-rich drained peatland in 2011-2013. The fen, located in southern Finland, was drained for forestry in 1969 and the tree stand is now a mixture of Scots pine, Norway spruce, and pubescent birch. Our measurement system consisted of six transparent chambers and stainless steel frames, positioned on a number of different field and moss layer compositions. Gas concentrations were measured with an online cavity ring-down spectroscopy gas analyzer. Fluxes were calculated with both linear and exponential regression. The use of linear regression resulted in systematically smaller CH4 fluxes by 10-45% as compared to exponential regression. However, the use of exponential regression with small fluxes (
  • Thum, Tea; Zaehle, Sönke; Köhler, Philipp; Aalto, Tuula; Aurela, Mika; Guanter, Luis; Kolari, Pasi; Laurila, Tuomas; Lohila, Annalea; Magnani, Federico; Van der Tol, Christiaan; Markkanen, Tiina (2017)
    Recent satellite observations of sun-induced chlorophyll fluorescence (SIF) are thought to provide a large-scale proxy for gross primary production (GPP), thus providing a new way to assess the performance of land surface models (LSMs). In this study, we assessed how well SIF is able to predict GPP in the Fenno-Scandinavian region and what potential limitations for its application exist. We implemented a SIF model into the JSBACH LSM and used active leaf-level chlorophyll fluorescence measurements (Chl F) to evaluate the performance of the SIF module at a coniferous forest at Hyytiala, Finland. We also compared simulated GPP and SIF at four Finnish micrometeorological flux measurement sites to observed GPP as well as to satellite-observed SIF. Finally, we conducted a regional model simulation for the Fenno-Scandinavian region with JSBACH and compared the results to SIF retrievals from the GOME-2 (Global Ozone Monitoring Experiment-2) space-borne spectrometer and to observation-based regional GPP estimates. Both observations and simulations revealed that SIF can be used to estimate GPP at both site and regional scales. At regional scale the model was able to simulate observed SIF averaged over 5 years with r(2) of 0.86. The GOME-2-based SIF was a better proxy for GPP than the remotely sensed fA-PAR (fraction of absorbed photosynthetic active radiation by vegetation). The observed SIF captured the seasonality of the photosynthesis at site scale and showed feasibility for use in improving of model seasonality at site and regional scale.
  • Hari, Pertti; Noe, Steffen; Dengel, Sigrid; Elbers, Jan; Gielen, Bert; Kerminen, Veli-Matti; Kruijt, Bart; Kulmala, Liisa; Lindroth, Anders; Mammarella, Ivan; Petaja, Tuukka; Schurgers, Guy; Vanhatalo, Anni; Kulmala, Markku; Back, Jaana (2018)
    Photosynthesis provides carbon for the synthesis of macromolecules to construct cells during growth. This is the basis for the key role of photosynthesis in the carbon dynamics of ecosystems and in the biogenic CO2 assimilation. The development of eddy-covariance (EC) measurements for ecosystem CO2 fluxes started a new era in the field studies of photosynthesis. However, the interpretation of the very variable CO2 fluxes in evergreen forests has been problematic especially in transition times such as the spring and autumn. We apply two theoretical needle-level equations that connect the variation in the light intensity, stomatal action and the annual metabolic cycle of photosynthesis. We then use these equations to predict the photosynthetic CO2 flux in five Scots pine stands located from the northern timberline to Central Europe. Our result has strong implications for our conceptual understanding of the effects of the global change on the processes in boreal forests, especially of the changes in the metabolic annual cycle of photosynthesis.
  • Machacova, Katerina; Vainio, Elisa; Urban, Otmar; Pihlatie, Mari (2019)
    The role of trees in the nitrous oxide (N2O) balance of boreal forests has been neglected despite evidence suggesting their substantial contribution. We measured seasonal changes in N2O fluxes from soil and stems of boreal trees in Finland, showing clear seasonality in stem N2O flux following tree physiological activity, particularly processes of CO2 uptake and release. Stem N2O emissions peak during the vegetation season, decrease rapidly in October, and remain low but significant to the annual totals during winter dormancy. Trees growing on dry soils even turn to consumption of N2O from the atmosphere during dormancy, thereby reducing their overall N2O emissions. At an annual scale, pine, spruce and birch are net N2O sources, with spruce being the strongest emitter. Boreal trees thus markedly contribute to the seasonal dynamics of ecosystem N2O exchange, and their species-specific contribution should be included into forest emission inventories.
  • Pavelka, Marian; Acosta, Manuel; Kiese, Ralf; Altimir, Nuria; Bruemmer, Christian; Crill, Patrick; Darenova, Eva; Fuss, Roland; Gielen, Bert; Graf, Alexander; Klemedtsson, Leif; Lohila, Annalea; Longdoz, Bernhard; Lindroth, Anders; Nilsson, Mats; Jimenez, Sara Maranon; Merbold, Lutz; Montagnani, Leonardo; Peichl, Matthias; Pihlatie, Mari; Pumpanen, Jukka; Ortiz, Penelope Serrano; Silvennoinen, Hanna; Skiba, Ute; Vestin, Patrik; Weslien, Per; Janous, Dalibor; Kutsch, Werner (2018)
    Chamber measurements of trace gas fluxes between the land surface and the atmosphere have been conducted for almost a century. Different chamber techniques, including static and dynamic, have been used with varying degrees of success in estimating greenhouse gases (CO2, CH4, N2O) fluxes. However, all of these have certain disadvantages which have either prevented them from providing an adequate estimate of greenhouse gas exchange or restricted them to be used under limited conditions. Generally, chamber methods are relatively low in cost and simple to operate. In combination with the appropriate sample allocations, chamber methods are adaptable for a wide variety of studies from local to global spatial scales, and they are particularly well suited for in situ and laboratory-based studies. Consequently, chamber measurements will play an important role in the portfolio of the Pan-European long-term research infrastructure Integrated Carbon Observation System. The respective working group of the Integrated Carbon Observation System Ecosystem Monitoring Station Assembly has decided to ascertain standards and quality checks for automated and manual chamber systems instead of defining one or several standard systems provided by commercial manufacturers in order to define minimum requirements for chamber measurements. The defined requirements and recommendations related to chamber measurements are described here.
  • Park, Sung-Bin; Knohl, Alexander; Migliavacca, Mirco; Thum, Tea; Vesala, Timo; Peltola, Olli; Mammarella, Ivan; Prokushkin, Anatoly; Kolle, Olaf; Lavric, Jost; Park, Sang Seo; Heimann, Martin (2021)
    Climate change impacts the characteristics of the vegetation carbon-uptake process in the northern Eurasian terrestrial ecosystem. However, the currently available direct CO2 flux measurement datasets, particularly for central Siberia, are insufficient for understanding the current condition in the northern Eurasian carbon cycle. Here, we report daily and seasonal interannual variations in CO2 fluxes and associated abiotic factors measured using eddy covariance in a coniferous forest and a bog near Zotino, Krasnoyarsk Krai, Russia, for April to early June, 2013-2017. Despite the snow not being completely melted, both ecosystems became weak net CO2 sinks if the air temperature was warm enough for photosynthesis. The forest became a net CO2 sink 7-16 days earlier than the bog. After the surface soil temperature exceeded similar to 1 degrees C, the ecosystems became persistent net CO2 sinks. Net ecosystem productivity was highest in 2015 for both ecosystems because of the anomalously high air temperature in May compared with other years. Our findings demonstrate that long-term monitoring of flux measurements at the site level, particularly during winter and its transition to spring, is essential for understanding the responses of the northern Eurasian ecosystem to spring warming.
  • van der Molen, M. K.; de Jeu, R. A. M.; Wagner, W.; van der Velde, I. R.; Kolari, P.; Kurbatova, J.; Varlagin, A.; Maximov, T. C.; Kononov, A. V.; Ohta, T.; Kotani, A.; Krol, M. C.; Peters, W. (2016)
    Boreal Eurasia is a region where the interaction between droughts and the carbon cycle may have significant impacts on the global carbon cycle. Yet the region is extremely data sparse with respect to meteorology, soil moisture, and carbon fluxes as compared to e.g. Europe. To better constrain our vegetation model SiBCASA, we increase data usage by assimilating two streams of satellite-derived soil moisture. We study whether the assimilation improved SiBCASA's soil moisture and its effect on the simulated carbon fluxes. By comparing to unique in situ soil moisture observations, we show that the passive microwave soil moisture product did not improve the soil moisture simulated by SiBCASA, but the active data seem promising in some aspects. The match between SiBCASA and ASCAT soil moisture is best in the summer months over low vegetation. Nevertheless, ASCAT failed to detect the major droughts occurring between 2007 and 2013. The performance of ASCAT soil moisture seems to be particularly sensitive to ponding, rather than to biomass. The effect on the simulated carbon fluxes is large, 5-10% on annual GPP and TER, tens of percent on local NEE, and 2% on area-integrated NEE, which is the same order of magnitude as the inter-annual variations. Consequently, this study shows that assimilation of satellite-derived soil moisture has potentially large impacts, while at the same time further research is needed to understand under which conditions the satellite-derived soil moisture improves the simulated soil moisture.
  • Vainio, Elisa; Peltola, Olli; Kasurinen, Ville; Kieloaho, Antti-Jussi; Tuittila, Eeva-Stiina; Pihlatie, Mari (2021)
    Boreal forest soils are globally an important sink for methane (CH4), while these soils are also capable of emitting CH4 under favourable conditions. Soil wetness is a well-known driver of CH4 flux, and the wetness can be estimated with several terrain indices developed for the purpose. The aim of this study was to quantify the spatial variability of the forest floor CH4 flux with a topography-based upscaling method connecting the flux with its driving factors. We conducted spatially extensive forest floor CH4 flux and soil moisture measurements, complemented by ground vegetation classification, in a boreal pine forest. We then modelled the soil moisture with a random forest model using digital-elevation-model-derived topographic indices, based on which we upscaled the forest floor CH4 flux. The modelling was performed for two seasons: May–July and August–October. Additionally, we evaluated the number of flux measurement points needed to get an accurate estimate of the flux at the whole study site merely by averaging. Our results demonstrate high spatial heterogeneity in the forest floor CH4 flux resulting from the soil moisture variability as well as from the related ground vegetation. The mean measured CH4 flux at the sample points was −5.07 µmol m−2 h−1 in May–July and −8.67 µmol m−2 h−1 in August–October, while the modelled flux for the whole area was −7.42 and −9.91 µmol m−2 h−1 for the two seasons, respectively. The spatial variability in the soil moisture and consequently in the CH4 flux was higher in the early summer (modelled range from −12.3 to 6.19 µmol m−2 h−1) compared to the autumn period (range from −14.6 to −2.12 µmol m−2 h−1), and overall the CH4 uptake rate was higher in autumn compared to early summer. In the early summer there were patches emitting high amounts of CH4; however, these wet patches got drier and smaller in size towards the autumn, changing their dynamics to CH4 uptake. The mean values of the measured and modelled CH4 fluxes for the sample point locations were similar, indicating that the model was able to reproduce the results. For the whole site, upscaling predicted stronger CH4 uptake compared to simply averaging over the sample points. The results highlight the small-scale spatial variability of the boreal forest floor CH4 flux and the importance of soil chamber placement in order to obtain spatially representative CH4 flux results. To predict the CH4 fluxes over large areas more reliably, the locations of the sample points should be selected based on the spatial variability of the driving parameters, in addition to linking the measured fluxes with the parameters.