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See detailBeech phenology and productivity at the Vielsalm Terrestrial Observatory
Hurdebise, Quentin ULiege; Vincke, Caroline; De Ligne, Anne ULiege et al

Poster (2017, October 20)

Temperate forests, as other ecosystems and oceans, mitigate the accumulation in the atmosphere of CO2, the main responsible for the current climate change. It is, therefore, a necessity to understand how ... [more ▼]

Temperate forests, as other ecosystems and oceans, mitigate the accumulation in the atmosphere of CO2, the main responsible for the current climate change. It is, therefore, a necessity to understand how these forests react and will react in a changing environment. That requires long-term simultaneous monitoring of the forest productivity and phenology as well as of climatic variables. Using the 20 years dataset from the Vielsalm Terrestrial Observatory (VTO), an eddy covariance site located in a mixed forest in east Belgium and dominated by beeches, the relations between phenological and productivity indicators were analyzed. Phenological indicators (leaf out beginning, duration and ending as well as leaf fall date) were derived from the relative light transmissivity through the canopy and from the temporal dynamic of the Net Ecosystem Productivity (NEP). No trend was observed for these indicators during the last two decades, but significant relations were found between them and between them and climatic variables. Productivity indicators were derived from eddy covariance measurements (NEP), tree ring widths (growth index) and masting intensity. Growth index was significantly related to the NEP and a reverse proportionality to the masting intensity was found. The analysis of the relation between the beech stand NEP and the phenology gives us evidences that annual NEP is significantly related to the length of the carbon uptake period. A model combining the length of the carbon uptake period and the photosynthetic capacity of the ecosystem was able to predict more than two third of the interannual variability at the VTO. This work underlines the multiplicity of the relation between phenological and productivity indicators. [less ▲]

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See detailImpact of canopy aerodynamic distance spatial and temporal variability on long term eddy covariance measurements
Hurdebise, Quentin ULiege; Heinesch, Bernard ULiege; De Ligne, Anne ULiege et al

in Agricultural and Forest Meteorology (2017), 247(2017), 131-138

Understanding if and how the spatial and temporal variability of the surrounding environment affects turbulence is essential for long-term eddy covariance measurements. It requires characterizing the ... [more ▼]

Understanding if and how the spatial and temporal variability of the surrounding environment affects turbulence is essential for long-term eddy covariance measurements. It requires characterizing the surrounding environment. One way to achieve this is to analyse the canopy aerodynamic distance (Δ), which is the difference between measurement height (zm) and displacement height (d). In this work, an original method to estimate the canopy aerodynamic distance at a fine spatial (30° sectors) and temporal (one year) resolution was proposed. It was based on sensible heat cospectra analysis, calibrated on a measurement height change and validated using canopy height inventories. This method was applied to 20 years of eddy covariance measurements from the Vielsalm Terrestrial Observatory (VTO), a site located in a mixed temperate forest. The method allowed Δ spatio-temporal variability due to changes in canopy or measurement height to be detected. Relationships between Δ and turbulence statistics were then analysed: the momentum correlation coefficient (ruw) was found to be dependent on Δ, confirming that the measurements were made in the roughness sublayer of the atmospheric surface layer. In contrast, no such relationship was found sensible heat, CO2 or water vapour correlation coefficients, suggesting that the Δ variability did not affect significantly these fluxes. There were significant differences, however, between azimuthal directions, suggesting that these scalars were affected by forest heterogeneity in a different way. Various hypotheses were put forward to explain the differences and their relevance was evaluated. This study highlighted the need to consider the spatial and temporal variability of the surrounding environment in order to verify the consistency of long-term eddy covariance datasets. [less ▲]

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See detailMomentum, sensible heat and CO2 correlation coefficient: what can we learn from 20 years of eddy covariance measurements
Hurdebise, Quentin ULiege; De Ligne, Anne ULiege; Vincke, Caroline et al

Poster (2017, April 24)

Long-term data series of carbon dioxide and other gas exchanges between terrestrial ecosystems and atmosphere become more and more numerous. Long-term analyses of such exchanges require a good ... [more ▼]

Long-term data series of carbon dioxide and other gas exchanges between terrestrial ecosystems and atmosphere become more and more numerous. Long-term analyses of such exchanges require a good understanding of measurement conditions during the investigated period. Independently of climate drivers, measurements may indeed be influenced by measurement conditions themselves subjected to long-term variability due to vegetation growth or set-up changes. The present research refers to the Vielsalm Terrestrial Observatory (VTO) an ICOS candidate site located in a mixed forest (beech, silver fir, Douglas fir, Norway spruce) in the Belgian Ardenne. Fluxes of momentum, carbon dioxide and sensible heat have been continuously measured there by eddy covariance for more than 20 years. During this period, changes in canopy height and measurement height occurred. The correlation coefficients (for momemtum, sensible heat and CO2) and the normalized standard deviations measured for the past 20 years at the Vielsalm Terrestrial Observatory (VTO) were analysed in order to define how the fluxes, independently of climate conditions, were affected by the surrounding environment evolution, including tree growth, forest thinning and tower height change. A relationship between canopy aerodynamic distance and the momentum correlation coefficient was found which is characteristic of the roughness sublayer, and suggests that momentum transport processes were affected by z-d. In contrast, no relationship was found for sensible heat and CO2 correlation coefficients, suggesting that the z-d variability observed did not affect their turbulent transport. There were strong differences in these coefficients, however, between two wind sectors, characterized by contrasted stands (height differences, homogeneity) and different hypotheses were raised to explain it. This study highlighted the importance of taking the surrounding environment variability into account in order to ensure the spatio-temporal consistency of datasets. [less ▲]

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See detailCanopy aerodynamic distance (z-d) estimation and impact on eddy covariance measurements
Hurdebise, Quentin ULiege; De Ligne, Anne ULiege; Vincke, Caroline et al

in Geophysical Research Abstracts 2017 (2017, April 24)

Understanding if and how the spatial and temporal variability of the surrounding environment affects turbulence is essential for long-term eddy covariance measurements above growing and heterogeneous ... [more ▼]

Understanding if and how the spatial and temporal variability of the surrounding environment affects turbulence is essential for long-term eddy covariance measurements above growing and heterogeneous ecosystems. It requires characterizing the surrounding environment. One way to achieve this is to analyse the canopy aerodynamic distance, which is the difference between measurement height (z) and displacement height (d). In this study, twenty years of eddy covariance measurements from the Vielsalm Terrestrial Observatory, a site located in a mixed temperate forest, were used. Canopy aerodynamic distance (z-d) estimates were obtained using two micrometeorological methods: the first one, which is original so far as we know, was based on analysing sensible heat cospectra; the second one was derived from the wind speed profile equation. Canopy height estimates based on inventories were used to validate both methods. The micrometeorological methods allowed the z-d variations due to changes in canopy or measurement height to be detected. In addition, the results obtained using the two methods were well correlated, spatially and temporally, with the z-d derived from canopy height measurements. The micrometeorological approaches used could therefore be a promising tool for investigating z-d variability at a high directional and temporal resolution. Questions remain, however, particularly with regard to the variability observed that cannot be explained by canopy or measurement height variation. Forest management practices and the non-fulfilment of similarity relationships were suspected to be the main explanatory factors. [less ▲]

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See detailCarbon budget measurement over 12 years at a crop production site in the silty-loam region in Belgium
Buysse, Pauline; Bodson, Bernard ULiege; Debacq, Alain ULiege et al

in Agricultural and Forest Meteorology (2017), 246

Carbon dioxide (CO2) exchanges between crops and the atmosphere are influenced by both climatic and management drivers. In this study, a site at the Lonzée Terrestrial Observatory (candidate ICOS site) in ... [more ▼]

Carbon dioxide (CO2) exchanges between crops and the atmosphere are influenced by both climatic and management drivers. In this study, a site at the Lonzée Terrestrial Observatory (candidate ICOS site) in Belgium that had been managed for more than 70 years using conventional farming practices, was monitored over three complete rotation cycles (sugar beet/winter wheat/seed potato/winter wheat) from 2004 to 2016. Continuous eddy-covariance measurements and regular biomass samplings were performed in order to obtain the daily and seasonal Net Ecosystem Exchange (NEE), Gross Primary Productivity (GPP), Total Ecosystem Respiration (TER), Net Primary Productivity (NPP) and Net Biome Production (NBP). Meteorological data and crop management practices were also recorded. Over the 12 crop seasons, total NEE varied both between and within crop types. Crop type influenced net carbon (C) sequestration, with the seed potato crop exhibiting the smallest C fluxes. Despite differences in CO2 assimilation response to weather variables and in cumulated TER/GPP dynamics, the sugar beet, winter wheat and maize crops had similar seasonal NEE values. The NEE inter-annual variability, both between and within crop types, was explained mainly by the length of the active vegetation period, as well as the cumulated photosynthetic photon flux density and average air temperature during this period. The establishment of the C budget over the 12 years of study showed that NEE was a downward (negative) flux (−4.40 ± 0.05 kg C m−2), but NBP was an upward (positive) flux (0.99 ± 0.22 kg C m−2). That is, as soon as C exportation at harvest and C importation (manure, slimes) were included in the budget, the site behaved as a C source. The intercrop periods contributed significantly to the total C budget, and the C fluxes generated during these periods were positively related to crop residue amount and temperature. The largest uncertainties about the crop C budget were those relating to biomass measurements. Carrying out a soil C inventory would help to validate the NBP-based estimate of soil C loss. [less ▲]

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See detailSensitivity of the annual net ecosystem exchange to the cospectral model used for high frequency loss corrections at a grazed grassland site
Mamadou, Ossenatou; Gourlez de la Motte, Louis ULiege; De Ligne, Anne ULiege et al

in Agricultural and Forest Meteorology (2016), 228-229

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See detailClimatic and management drivers of CO2 exchanges by a production crop: Analysis over three successive 4-year crop rotation cycles
Buysse, Pauline ULiege; Manise, Tanguy ULiege; De Ligne, Anne ULiege et al

Poster (2016, September 27)

Carbon dioxide (CO2) exchanges between crops and the atmosphere are influenced by both climatic and crop management drivers. The investigated crop, situated at the Lonzée Terrestrial Observatory (LTO ... [more ▼]

Carbon dioxide (CO2) exchanges between crops and the atmosphere are influenced by both climatic and crop management drivers. The investigated crop, situated at the Lonzée Terrestrial Observatory (LTO, candidate ICOS site) in Belgium and managed for more than 70 years using conventional farming practices, was monitored over three complete sugar beet/winter wheat/potato/winter wheat rotation cycles from 2004 to 2016. Continuous eddy-covariance measurements and regular biomass samplings were performed in order to obtain the daily and seasonal Net Ecosystem Exchange (NEE), Gross Primary Productivity (GPP), Total Ecosystem Respiration (TER), Net Primary Productivity (NPP), and Net Biome Production (NBP). Meteorological data and crop management practices were also recorded. Over the 12 years, NEE was negative (-4.34 kg C m-2) but NBP was positive (1.05 kg C m-2), i.e. as soon as carbon exportation by harvest and carbon importation (manure, slimes) are included in the budget, the site behaves as a carbon source. At the crop rotation scale (4 years) it was quite remarkable to observe that NBP was very similar over the three rotations (0.30-0.36 kg C m-2), despite climatic and management differences between years. Crop type impacted carbon exchanges, with sugar beet and winter wheat crops leading to higher net carbon sequestration than seed potato crops. For one given crop, larger growth length and cumulated global radiation drove larger cumulated NEE. Net carbon emissions were observed during intercrops, but growing mustard during these periods reduced their rates and provided carbon residues to the soil. NBP values suggest that one sixth of the total soil organic carbon stock at LTO (6.23 ± 0.16 kg C m-2 in [0, 60] cm) would be lost in 12 years. Large uncertainties (mostly due to biomass measurements) affect NBP estimates, but still, this figure is huge and should encourage cultural practices returning carbon to the soil. [less ▲]

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See detailCanopy proximity estimation and impact on long term turbulent fluxes above a heterogeneous forest
Hurdebise, Quentin ULiege; Vincke, Caroline; De Ligne, Anne ULiege et al

Poster (2016, April 18)

With the development of eddy covariance networks like Fluxnet, ICOS or NEON, long-term data series of carbon dioxide, water vapor and other gas exchanges between terrestrial ecosystems and atmosphere will ... [more ▼]

With the development of eddy covariance networks like Fluxnet, ICOS or NEON, long-term data series of carbon dioxide, water vapor and other gas exchanges between terrestrial ecosystems and atmosphere will become more and more numerous. However, long-term analyses of such exchanges require a good understanding of measurement conditions during the investigated period. Independently of climate drivers, measurements may indeed be influenced by measurement conditions themselves subjected to long-term variability due to vegetation growth or set-up changes. The present research refers to the Vielsalm Terrestrial Observatory (VTO) where fluxes of momentum, carbon dioxide, latent and sensible heat have been continuously measured by eddy covariance during twenty years. VTO is an ICOS site installed in a mixed forest (beech, silver fir, Douglas fir, Norway spruce) in the Belgian Ardennes. A multidisciplinary approach was developed in order to investigate the spatial and temporal evolution of several site characteristics: - displacement height (d) and relative measurement height (z-d) were determined using a spectral approach that compared observed and theoretical cospectra; - turbulence statistics were analyzed in the context of Monin-Obukhov similarity theory; - tree height during the measurement period was obtained by combining tree height inventories, a LIDAR survey and tree growth models; - measurement footprint was determined by using a footprint model. A good agreement was found between the three first approaches. Results show notably that z-d was subjected to both temporal and spatial evolution. Temporal evolution resulted from continuous tree growth as well as from a tower raise, achieved in 2009. Spatial evolution, due to canopy heterogeneity, was also observed. The impacts of these changes on measurements are investigated. In particular, it was shown that they affect measurement footprint, flux spectral corrections and flux quality. All these effects must be taken into consideration in order to disentangle long-term flux evolutions due to climate or phenology from changes resulting from measurement set-up changes. [less ▲]

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See detailDimensioning IRGA gas sampling systems: laboratory and field experiments
Aubinet, Marc ULiege; Joly, Lilian; Loustau, Denis et al

in Atmospheric Measurement Techniques (2016), 9

Both laboratory and field experiments were carried out in order to define suitable configuration ranges for the gas sampling systems (GSSs) of infrared gas analyzers (IRGAs) used in eddy covariance ... [more ▼]

Both laboratory and field experiments were carried out in order to define suitable configuration ranges for the gas sampling systems (GSSs) of infrared gas analyzers (IRGAs) used in eddy covariance measurements. In the laboratory, an original dynamic calibration bench was developed in order to test the frequency attenuation and pressure drop generated by filters. In the field, three IRGAs of the same type equipped with different filters or different rain caps were installed and run and the real frequency response of the complete setup was tested. The main results are as follows. – Filters may have a strong impact on the pressure drop in the GSS and this impact increases with flow rate. – Conversely, no impact of the tested filters on cut-off frequency was found, GSSs with and without filters presenting similar cut-off frequencies. – The main limiting factor of cut-off frequency in the field was found to be the rain cap design. In addition, the impact of this design on pressure drop was also found to be noteworthy. [less ▲]

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See detailUnexpected sensitivity of the annual net ecosystem exchange to the high frequency loss corrections in a grazed grassland site in Belgium
Mamadou, Ossénatou ULiege; Gourlez de la Motte, Louis ULiege; De Ligne, Anne ULiege et al

Poster (2016)

The eddy covariance technique is widely used to measure CO2 and other gas fluxes. However, eddy covariance fluxes are affected by systematic errors that must be corrected. Among them, high frequency loss ... [more ▼]

The eddy covariance technique is widely used to measure CO2 and other gas fluxes. However, eddy covariance fluxes are affected by systematic errors that must be corrected. Among them, high frequency loss corrections are particularly important in this regard, especially when using a closed-path infrared gas analyzer. In this study, we compared three approaches to do these corrections for CO2 fluxes and evaluated their impact on the carbon balance an intensively grazed grassland site in Belgium . In the first approach, the computation of correction factors was based on the measured sensible heat cospectra (‘local’ cospectra), whereas the other two were based on theoretical models (Kaimal et al., 1972). The correction approaches were validated by comparing the nighttime eddy covariance CO2 fluxes corrected with each approach and in situ soil respiration measurements. We found that the local cospectra differed from the Kaimal theoretical shape, although the site could not be considered ‘difficult’ (i.e., fairly flat, homogeneous, low vegetation, sufficient measurement height), appearing less peaked in the inertial subrange. This difference greatly affected the correction factor, especially for night fluxes. Night fluxes measured by eddy covariance were found to be in good accord with in situ soil respiration measurements when corrected with local cospectra and to be overestimated when corrected with Kaimal cospectra. As the difference between correction factors was larger in stable than unstable conditions, this acts as a selective systematic error and has an important impact on annual fluxes. On the basis of a 4-year average, at DTO the errors reach 71-150 g C m-2 y-1 for net ecosystem exchange (NEE), 280-562 g C m-2 y-1 for total ecosystem respiration (TER) and 209-412 g C m-2 y-1 for gross primary productivity (GPP), depending on the approach used. We finally encourage site PIs to check the cospectrum shape at their sites and, if necessary, compute frequency correction factors on the basis of local cospectra rather than on Kaimal cospectra. [less ▲]

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See detailORCHIDEE-CROP (v0), a new process based Agro-Land Surface Model: model description and evaluation over Europe
Wu, Xiuchen; Vuichard, N.; Ciais, P. et al

in Geoscientific Model Development Discussions [=GMDD] (2016), 8

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See detailThe ICOS Vielsalm Terrestrial Observatory: long term greehouse gas flux measurements
Hurdebise, Quentin ULiege; De Ligne, Anne ULiege

Poster (2015, December 10)

Presentation of the Vielsalm ICOS Terrestrial Observatory: site management, data available, data access, flux temporal dynamic and flux relation with climate variables.

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See detailPhénologie des hêtres dans une forêt ardennaise: comparaison de méthodes de suivi et relations avec les flux de CO2
Hurdebise, Quentin ULiege; Vincke, Caroline; De Ligne, Anne ULiege et al

Poster (2015, November 18)

Bien caractériser la phénologie des essences forestières dans un contexte climatique changeant est indispensable. Dans cette optique, confronter les différentes sources d’informations phénologiques ... [more ▼]

Bien caractériser la phénologie des essences forestières dans un contexte climatique changeant est indispensable. Dans cette optique, confronter les différentes sources d’informations phénologiques (relevé de terrain, modélisation, capteurs de rayonnement, caméra phénologique, satellite, eddy covariance…) est une démarche essentielle. Les différences et la complémentarité de ces sources d’informations ont été mises en évidence en utilisant les données provenant d’une forêt mixte de l’Est de la Belgique, principalement composée de hêtres. Par ailleurs, ces informations ont été utilisées pour étudier l’influence de la phénologie des hêtres sur les échanges annuels de CO2 de ces derniers. [less ▲]

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See detailHigh-frequency correction method for CO2 and water vapour fluxes: an application on Walloon Terrestrial Observatories data
De Ligne, Anne ULiege

Report (2015)

High frequency concentration fluctuations are attenuated by the eddy covariance (EC) sensors system. This is due to an inability of the EC system to process eddies of small size and induces an ... [more ▼]

High frequency concentration fluctuations are attenuated by the eddy covariance (EC) sensors system. This is due to an inability of the EC system to process eddies of small size and induces an underestimation of the fluxes. The two main reasons are the fluid frictions through the inlet tube of the gas analyzer and the separation of the sensors position between the center of the anemometer and the inlet tube. A correction factor is then needed for CO2 and H2O fluxes. In this study, we describe a new procedure for the high frequency correction (HFC). This new HFC is applied for the CO2 and H2O fluxes measured at Vielsalm and Lonzée since their setup. Finally, we compare the former and updated corrected fluxes of these two sites which are part of the Terrestrial Observatories of Belgium. Both Observatories are included in the European ecosystem network of ICOS. [less ▲]

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See detailCritical periods and critical values explaining fluxes inter-annual variability in a temperate mixed forest
Hurdebise, Quentin ULiege; Vincke, Caroline; De Ligne, Anne ULiege et al

Poster (2015, June 04)

In order to explain inter-annual variability of Net Ecosystem CO2 Exchange (NEE) above a mixed temperate forest, two approaches were followed: •Detection of critical periods using the R-squared of the ... [more ▼]

In order to explain inter-annual variability of Net Ecosystem CO2 Exchange (NEE) above a mixed temperate forest, two approaches were followed: •Detection of critical periods using the R-squared of the regression between annual NEE and cumulated NEE on a mobile window. •Identification of critical values of a threshold used to decompose annual and seasonal NEE in two components. [less ▲]

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See detailIRGA GAS SAMPLING SYSTEM DIMENSIONING: LABORATORY AND FIELD EXPERIMENTS
De Ligne, Anne ULiege; Joly, Lilian; Cousin, Julien et al

Poster (2014, September 23)

The gas sampling system (GSS), which carries air from the sampling point to the IRGA, is an essential component of the eddy covariance system. It has to meet several constraints, among which minimizing ... [more ▼]

The gas sampling system (GSS), which carries air from the sampling point to the IRGA, is an essential component of the eddy covariance system. It has to meet several constraints, among which minimizing high frequency attenuation of concentration measurement and keeping pressure drop in the measurement cell in an acceptable range. Rain cup, filters, tubes and pumps are key elements of this system and need proper dimensioning. The elaboration of the ICOS protocol for IRGA required such dimensioning and optimization. Laboratory and field measurements were carried out with this aim. In the laboratory, a dynamic calibration bench was developed to investigate experimentally the pressure drop and the concentration fluctuation attenuation caused by different filters. In the field, three LI-7200 equipped with different GSS were installed and run at the Dorinne Terrestrial Observatory (ICOS-Belgium). Main experiment conclusions were that: • The shape and size of the rain cup has a critical impact on cut off frequencies • The filter porosity and size has a critical impact on pressure drop • Filter heating is necessary in order to avoid condensation and filter blocking These experiments led to the definition of the GSS functioning range that is finally proposed in the ICOS IRGA protocol. [less ▲]

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See detailICOS IN BELGIUM: WALLONIA
De Ligne, Anne ULiege

Poster (2014, September 23)

Présentation ICOS Wallonia

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See detailDocumentation on the Terrestrial Observatory of Vielsalm (OTV)
De Ligne, Anne ULiege; Manise, Tanguy ULiege

E-print/Working paper (2014)

Documentation on the Terrestrial Observatory of Vielsalm (OTV)

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See detailDocumentation on the Terrestrial Observatory of Lonzée (OTL)
De Ligne, Anne ULiege; Manise, Tanguy ULiege

E-print/Working paper (2014)

General informations on the station of Lonzée, Wallonia, Belgium.

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