References of "Agricultural and Forest Meteorology"
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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 detailMethane balance of an intensively grazed pasture and estimation of the enteric methane emissions from cattle
Dumortier, Pierre ULiege; Aubinet, Marc ULiege; Beckers, Yves ULiege et al

in Agricultural and Forest Meteorology (2017), 232

The methane turbulent fluxes of an intensively grazed pasture were measured continuously from June 2012 to December 2013 at the Dorinne Terrestrial Observatory (DTO) in Belgium. During grazing periods ... [more ▼]

The methane turbulent fluxes of an intensively grazed pasture were measured continuously from June 2012 to December 2013 at the Dorinne Terrestrial Observatory (DTO) in Belgium. During grazing periods, the fluxes were dominated by enteric fermentation and were found to be strongly related to cow stocking density. In 2013, total emission from the pasture was found between 9 and 11 g CH4 m−2, 97% of which being emitted during grazing periods. Emission per LU (livestock unit) was estimated in a non-invasive way by integrating eddy covariance fluxes over large periods and by assuming a homogeneous average cattle disposition on the pasture. This estimate was compared to the one obtained during confinement periods, where cows were confined in a small part of the pasture. The emission per LU varied between 104 and 134 g CH4 LU−1 day−1 (13 and 17 g CH4 kg DMI−1), depending on the dataset and the computation method used. Diel course was characterized by two emission peaks, one in the morning and a larger one in the afternoon. For rest periods (no cattle on the pasture), small emissions were observed (median and mean values of 0.5 and 1.5 mg CH4 m−2 day−1, respectively). [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 detailExperimental validation of footprint models for eddy covariance CO2 flux measurements above grassland by means of natural and artificial tracers
Arriga, Nicola; Rannik, Üllar; Aubinet, Marc ULiege et al

in Agricultural and Forest Meteorology (2017), 242

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See detailCarbon balance of an intensively grazed grassland in southern Belgium
Gourlez de la Motte, Louis ULiege; Jérôme, Elisabeth; Mamadou, Ossénatou et al

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

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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 detailA comparison of within-season yield prediction algorithms based on crop model behaviour analysis
Dumont, Benjamin ULiege; Basso, Bruno; Leemans, Vincent ULiege et al

in Agricultural and Forest Meteorology (2015), 204

The development of methodologies for predicting crop yield, in real-time and in response to different agro-climatic conditions, could help to improve the farm management decision process by providing an ... [more ▼]

The development of methodologies for predicting crop yield, in real-time and in response to different agro-climatic conditions, could help to improve the farm management decision process by providing an analysis of expected yields in relation to the costs of investment in particular practices. Based on the use of crop models, this paper compares the ability of two methodologies to predict wheat yield (Triticum aestivum L.), one based on stochastically generated climatic data and the other on mean climate data. It was shown that the numerical experimental yield distribution could be considered as a log-normal distribution. This function is representative of the overall model behaviour. The lack of statistical differences between the numerical realisations and the logistic curve showed in turn that the Generalised Central Limit Theorem (GCLT) was applicable to our case study. In addition, the predictions obtained using both climatic inputs were found to be similar at the inter and intra-annual time-steps, with the root mean square and normalised deviation values below an acceptable level of 10% in 90% of the climatic situations. The predictive observed lead-times were also similar for both approaches. Given (i) the mathematical formulation of crop models, (ii) the applicability of the CLT and GLTC to the climatic inputs and model outputs, respectively, and (iii) the equivalence of the predictive abilities, it could be concluded that the two methodologies were equally valid in terms of yield prediction. These observations indicated that the Convergence in Law Theorem was applicable in this case study. For purely predictive purposes, the findings favoured an algorithm based on a mean climate approach, which needed far less time (by 300-fold) to run and converge on same predictive lead time than the stochastic approach. [less ▲]

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See detailLong term observations of carbon dioxide exchange over cultivated savanna under a Sudanian climate in Benin (West Africa)
Ago, Expédit Evariste ULiege; Agbossou, Euloge Kossi; Galle, Sylvie et al

in Agricultural and Forest Meteorology (2014), 197

Turbulent CO2 exchanges between a cultivated Sudanian savanna and the atmosphere were measured during 29 months (August 2007–December 2009) by an eddy-covariance system in North-Western Benin, West Africa ... [more ▼]

Turbulent CO2 exchanges between a cultivated Sudanian savanna and the atmosphere were measured during 29 months (August 2007–December 2009) by an eddy-covariance system in North-Western Benin, West Africa. The site (Lat 9.74◦ N, Long 1.60◦ E, Alt: 449 m) is the one of three sites fitted out by the international AMMA-CATCH program. The flux station footprint area is mainly composed of herbs and crops with some sparse trees and shrubs. Fluxes data were completed by an inventory of dominating species around the tower and the meteorological measurements. Flux response to climatic and edaphic factors was studied. Water was found the main controlling factor of ecosystem dynamics: much larger uptake was found in wet than dry season. During wet season, a very clear answer of net CO2 fluxes to photosynthetic photon fluxes density (PPFD) was observed. A low limitation in response to saturation deficit and soil water variability was however observed. The total ecosystem respiration (TER) was found highly dependent on soil moisture below 0.1 m3m−3, but saturates above this threshold. The average annual carbon sequestration was 232 ± 27 gC m−2 with its inter-annual variability mainly controlled by TER. Finally, the ecosystem appeared more efficient during morning and wet season than during afternoon and dry period. [less ▲]

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See detailCharacterization of the soil CO2 production and its carbon isotope composition in forest soil layers using the flux-gradient approach
Goffin, Stéphanie; Aubinet, Marc ULiege; Maier, Martin et al

in Agricultural and Forest Meteorology (2014), 188

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See detailGlobal comparison of light use efficiency models for simulating terrestrial vegetation gross primary production based on the LaThuile database
Yuan, W.; Cai, W.; Xia, J. et al

in Agricultural and Forest Meteorology (2014), 192-193

Simulating gross primary productivity (GPP) of terrestrial ecosystems has been a major challenge in quantifying the global carbon cycle. Many different light use efficiency (LUE) models have been ... [more ▼]

Simulating gross primary productivity (GPP) of terrestrial ecosystems has been a major challenge in quantifying the global carbon cycle. Many different light use efficiency (LUE) models have been developed recently, but our understanding of the relative merits of different models remains limited. Using CO2 flux measurements from multiple eddy covariance sites, we here compared and assessed major algorithms and performance of seven LUE models (CASA, CFix, CFlux, EC-LUE, MODIS, VPM and VPRM). Comparison between simulated GPP and estimated GPP from flux measurements showed that model performance differed substantially among ecosystem types. In general, most models performed better in capturing the temporal changes and magnitude of GPP in deciduous broadleaf forests and mixed forests than in evergreen broadleaf forests and shrublands. Six of the seven LUE models significantly underestimated GPP during cloudy days because the impacts of diffuse radiation on light use efficiency were ignored in the models. CFlux and EC-LUE exhibited the lowest root mean square error among all models at 80% and 75% of the sites, respectively. Moreover, these two models showed better performance than others in simulating interannual variability of GPP. Two pairwise comparisons revealed that the seven models differed substantially in algorithms describing the environmental regulations, particularly water stress, on GPP. This analysis highlights the need to improve representation of the impacts of diffuse radiation and water stress in the LUE models. © 2014 Elsevier B.V. [less ▲]

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See detailFifty years of crop residue management have a limited impact on soil heterotrophic respiration.
Buysse, Pauline ULiege; Schnepf-Kiss, Anne-Caroline; Carnol, Monique ULiege et al

in Agricultural and Forest Meteorology (2013), 180

The impacts of crop residue management on soil microbial biomass, labile carbon and heterotrophic respiration (HR) were assessed at a long-term experimental site in the Hesbaye region in Belgium. Three ... [more ▼]

The impacts of crop residue management on soil microbial biomass, labile carbon and heterotrophic respiration (HR) were assessed at a long-term experimental site in the Hesbaye region in Belgium. Three treatments, residue export (RE), farmyard manure addition (FYM) and residue restitution after harvest (RR), have been applied continuously since 1959. The soil is a Eutric Cambisol with, in 2010, significantly different total soil organic carbon contents of 4.4, 5.1 and 5.9 kg C m-2 under the RE, RR and FYM treatments, respectively. Manual field HR measurements were carried out during the 2010 and 2012 crop seasons using a dynamic closed chamber system. Microbial biomass, labile C content and metabolic diversity of soil bacteria were assessed in spring 2012. Fifty-one years after the beginning of the treatments, residue management had a limited impact on HR. Based on daily averaged values, the treatment had a significant impact (α = 10%) in 2012 but not in 2010. Based on the individual measurement dates, the treatment impact was less obvious in 2012; with the observation of a significant impact (α = 10%) on HR in only 7% and 36.8% of the measurement dates in 2010 and 2012, respectively. Labile C and microbial biomass were significantly lower in the RE treatment than in FYM and RR. Residue management had no significant effect on cold-water extracted carbon and metabolic diversity of heterotrophic soil bacteria. The limited impact of residue management on HR could be explained by (i) the relatively low amounts of recent above-ground crop inputs, (ii) the large proportion of below-ground residues and other non-exportable above-ground residues reducing the potential differences between treatments and (iii) the relatively large spatial variability of HR. In conclusion, carbon losses due to heterotrophic respiration did not differ between RE, FYM and RR treatments in the studied soil. This contrasts with the different soil organic carbon contents observed in these three treatments after fifty years of experiment. Further investigations regarding the reduction of spatial variability and the potential roles played by organic matter protection within aggregates and biochemical composition of inputs are needed. [less ▲]

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See detailFluxes of the greenhouse gases (CO2, CH4 and N2O) above a short-rotation poplar plantation after conversion from agricultural land
Zona, Donatella; Janssens, I.A.; Aubinet, Marc ULiege et al

in Agricultural and Forest Meteorology (2013), 169

The increasing demand for renewable energy may lead to the conversion of millions of hectares into bioenergy plantations with a possible substantial transitory carbon (C) loss. In this study we report on ... [more ▼]

The increasing demand for renewable energy may lead to the conversion of millions of hectares into bioenergy plantations with a possible substantial transitory carbon (C) loss. In this study we report on the greenhouse gas fluxes (CO2, CH4, and N2O) measured using eddy covariance of a short-rotation bioenergy poplar plantation converted from agricultural fields. During the first six months after the establishment of the plantation (June–December 2010) there were substantial CO2, CH4, and N2O emissions (a total of 5.36 ± 0.52 Mg CO2eq ha−1 in terms of CO2 equivalents). Nitrous oxide loss mostly occurred during a week-long peak emission after an unusually large rainfall. This week-long N2O emission represented 52% of the entire N2O loss during one and an half years of measurements. As most of the N2O loss occurred in just this week-long period, accurately capturing these emission events are critical to accurate estimates of the GHG balance of bioenergy. While initial establishment (June–December 2010) of the plantation resulted in a net CO2 loss into the atmosphere (2.76 ± 0.16 Mg CO2eq ha−1), in the second year (2011) there was substantial net CO2 uptake (−3.51 ± 0.56 Mg CO2eq ha−1). During the entire measurement period, CH4 was a source to the atmosphere (0.63 ± 0.05 Mg CO2eq ha−1 in 2010, and 0.49 ± 0.05 Mg CO2eq ha−1 in 2011), and was controlled by water table depth. Importantly, over the entire measurement period, the sum of the CH4 and N2O losses was much higher (3.51 ± 0.52 Mg CO2eq ha−1) than the net CO2 uptake (−0.76 ± 0.58 Mg CO2eq ha−1). As water vailability was an important control on the GHG emission of the plantation, expected climate change and altered rainfall pattern could increase the negative environmental impacts of bioenergy. [less ▲]

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See detailSpatial variability of soil CO 2 efflux linked to soil parameters and ecosystem characteristics in a temperate beech forest
Ngao, J.; Epron, D.; Delpierre, N. et al

in Agricultural and Forest Meteorology (2012), 154-155

The aim of this study was to determine the amplitude and the driving factors of the spatial variability in soil CO 2 efflux in a young European beech forest. Soil CO 2 efflux was measured in 2003 and 2004 ... [more ▼]

The aim of this study was to determine the amplitude and the driving factors of the spatial variability in soil CO 2 efflux in a young European beech forest. Soil CO 2 efflux was measured in 2003 and 2004 in seven beech plots differing in terms of soil type and leaf area index. After eliminating temporal fluctuations due to soil temperature and soil water content, standardized soil CO 2 efflux varied significantly among plots over a large range given the homogeneity of the land cover type. Correlation analyses revealed that this spatial variability could not be explained by root biomass, litter C content, soil C contents, stand basal area or stem density. Conversely, very significant correlations were found with topsoil bulk density, superficial soil C/N ratio, and leaf area index. Multiple regression analysis led to a model relating standardized soil CO 2 efflux to C/N ratio and topsoil bulk density, thus explaining 87% of observed inter-plot spatial variability. This study highlighted the need to consider spatially varying soil factors such as C/N ratio and bulk density when experimental schemes are elaborated to estimate mean soil CO 2 efflux at forest scale. © 2011 Elsevier B.V. [less ▲]

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See detailSpatial fields' dispersion as a farmer strategy to reduce agro-climatic risk at the household level in pearl millet-based systems in the Sahel: A modeling perspective
Akponikpe, Pierre B. I.; Minet, Julien ULiege; Gerard, Bruno et al

in Agricultural and Forest Meteorology (2011), 151(2), 215-227

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See detailRespiration of three Belgian crops: Partitioning of total ecosystem respiration in its heterotrophic, above- and below-ground autotrophic components
Suleau, Marie ULiege; Moureaux, Christine ULiege; Dufranne, Delphine ULiege et al

in Agricultural and Forest Meteorology (2011), (151), 633-643

An experimental system combining an eddy covariance system, a micrometeorological station and soil chambers placed in planted areas and in root exclusion zones was installed during three successive years ... [more ▼]

An experimental system combining an eddy covariance system, a micrometeorological station and soil chambers placed in planted areas and in root exclusion zones was installed during three successive years in a production crop managed in a traditional way at the Lonzée experimental site (Belgium). Measurements were made successively on seed potato, winter wheat and sugar beet. The general objectives of the study were, first to evaluate the relative contributions to total ecosystem respiration (TER) of heterotrophic, above ground autotrophic and below ground autotrophic respiration over a succession of three agricultural crops (seed potato, winter wheat and sugar beet) cultivated on successive years at the same location and, secondly, to identify the driving variables of these contributions. Results showed that, during the observation periods, TER was dominated by autotrophic respiration (AR) (60–90%) and that AR was dominated by its above ground component (60–80%). HR was found to increase with temperature and to be independent of Gross Primary Production (GPP), whereas AR was driven by GPP and was mostly independent of temperature. The AR response to GPP was specific to the crop: not only AR intensity but also AR distribution between its above- (ARa) and below- (ARb) ground components were found to differ from one crop to another and, in the winter wheat, from one development stage to another. Generally, ARb contribution to AR was found larger when carbon allocation towards roots was more important. An uncertainty analysis was made and showed that the main sources of uncertainties on the estimates were the spatial variability for soil chamber measurements and uncertainties linked to the data gap filling method for eddy covariance measurements. [less ▲]

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See detailNight-time airflow in a forest canopy near a mountain crest
Sedlak, Pavel; Aubinet, Marc ULiege; Heinesch, Bernard ULiege et al

in Agricultural and Forest Meteorology (2010), 150(5), 736-744

Night-time airflow within a deep and dense canopy near the top of a mountain ridge is investigated based on measurements at Bily Kriz, Czech Republic. The site is characterized by a young Norway spruce ... [more ▼]

Night-time airflow within a deep and dense canopy near the top of a mountain ridge is investigated based on measurements at Bily Kriz, Czech Republic. The site is characterized by a young Norway spruce forest on a 13 degrees slope and the occurrence of almost exclusively upslope or downslope flows. The forest canopy reaches the ground surface. A decoupled two-layer structure of canopy flow typically develops at night. While the above-canopy flow is most frequently an upslope-directed larger-scale flow over the ridge, the lower-canopy flow is downslope (katabatic). However, the lower-canopy flow can be forced upslope when the wind speed above the canopy exceeds a well-defined limit. Less frequently, on the lee slope to the larger-scale flow, both the above-canopy and the lower-canopy flow are usually downslope, although a flow reversal in the lower canopy is also observed, accompanied with a large shear stress (friction velocity) above the canopy. The occurrence of opposing flows is not limited to sunset/sunrise transition periods. In a simplified modelling approach to the dynamics of the nocturnal lower-canopy flow decoupled from above, local equilibrium is assumed of solely two opposing driving forces - one induced by the negative buoyancy (due to radiative cooling of the canopy) and the other by the hydrodynamic pressure gradient (resulting from the larger-scale flow over the ridge) - and the canopy drag as a retarding force. The diagnostic model gives realistic values of the major driving terms for Bily Kriz, and the downslope or upslope direction and speed of the lower-canopy flow that agree well with the measurements. The model contributes to better interpretation of the experimental results, which are in accordance with recent publications on the flow patterns on forested hills. Knowledge of the lower-canopy flow behaviour and of the degree of its decoupling from the flow aloft is necessary for assessing the contribution of advection to the CO2 budget at sloping forest sites, and for analysis of the flux footprint. (C) 2010 Elsevier B.V. All rights reserved. [less ▲]

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See detailSpatiotemporal evolution of CO2 concentration, temperature, and wind field during stable nights at the Norunda forest site
Feigenwinter, Christian; Molder, Meelis; Lindroth, Anders et al

in Agricultural and Forest Meteorology (2010), 150(5), 692-701

Unusually high CO2 concentrations were frequently observed during stable nights in late summer 2006 at the CarboEurope-Integrated Project (CEIP) forest site in Norunda, Sweden. Mean CO2 concentrations in ... [more ▼]

Unusually high CO2 concentrations were frequently observed during stable nights in late summer 2006 at the CarboEurope-Integrated Project (CEIP) forest site in Norunda, Sweden. Mean CO2 concentrations in the layer below the height of the eddy-covariance measurement system at 30 m reached up to 500 mu mol mol(-1) and large vertical and horizontal gradients occurred, leading to very large advective fluxes with a high variability in size and direction. CO2 accumulation was found to build up in the second part of the night, when the stratification in the canopy sub-layer turned from stable to neutral. Largest vertical gradients of temperature and CO2 were shifted from close to the ground early in the night to the crown space of the forest late at night, decoupling the canopy sub-layer from the surface roughness layer. At the top of the canopy at 25 m CO2 concentrations up to 480 mu mol mol(-1) were observed at all four tower locations of the 3D cube setup and concentrations were still high (>400 mu mol mol(-1)) at the 100 m level of the Central tower. The vertical profiles of horizontal advective fluxes during the nights under investigation were similar and showed largest negative horizontal advection (equivalent to an additional CO2-sink) to occur in the crown space of the forest, and not, as usually expected, close to the ground. The magnitude of these fluxes was sometimes larger than 50 mu mol m(-2) s(-1)and they were caused by the large horizontal CO2 concentration gradients with maximum values of up to 1 mu mol mol(-1) m(-1). As a result of these high within canopy CO2 concentrations, the vertical advection also became large with frequent changes of direction according to the sign of the mean vertical wind component, which showed very small values scattering around zero. Inaccuracy of the sonic anemometer at such low wind velocities is the reason for uncertainty in vertical advection, whereas for horizontal advection, instrument errors were small compared to the fluxes. The advective fluxes during these nights were unusually high and it is not clear what they represent in relation to the biotic fluxes. Advection is most likely a scale overlapping process. With a control volume of about 100 m x 100 m x 30 m and the applied spatial resolution of the sensors, we obviously miss relevant information from processes in the mesoscale as well as in the turbulent scale. (C) 2009 Elsevier B.V. All rights reserved. [less ▲]

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See detailPlot-scale vertical and horizontal transport of CO2 modified by a persistent slope wind system in and above an alpine forest
Feigenwinter, Christian; Montagnani, Leonardo; Aubinet, Marc ULiege

in Agricultural and Forest Meteorology (2010), 150(5), 665-673

Data from the flux tower site Renon/Ritten, Italy, located at 1735 m. a.s.l. on a south exposed steep (11 degrees) forested alpine slope, is analyzed. In spite of the complex terrain, a persistent slope ... [more ▼]

Data from the flux tower site Renon/Ritten, Italy, located at 1735 m. a.s.l. on a south exposed steep (11 degrees) forested alpine slope, is analyzed. In spite of the complex terrain, a persistent slope wind system prevailed at the site during most of the ADVEX campaign from April to September 2005. We describe in detail how CO2 is transported parallel to the slope and how this transport affects net ecosystem exchange (NEE) in the diurnal course. The local slope wind system may be strongly modified by two different large scale synoptic situations. The "Tramontana", a persistent strong wind from the north, amplified the drainage flow during nighttime and suppressed the upslope flow above the forest canopy during daytime. Vice versa, we observed periods with continuing flow from the south, which supported the local daytime upslope flow and partly suppressed the nighttime downslope flow. This led to periods of several hours with opposite flow directions in and above the canopy. Depending on the prevailing situation, the trunk space is coupled and/or decoupled with/from the roughness sublayer above the forest canopy. In particular, vertical and horizontal mixing of CO2 was strongly dependent on the dominating wind field with essential impact on the horizontal advective flux of CO2. The most common "Local" situation, dominated by the slope wind system, showed positive horizontal and vertical advection (with typical values around 7 and 3 mu mol m(-2) s(-1), respectively) together with downslope winds at night and slightly negative horizontal advection (typical values around 2 mu mol m(-2) s(-1)) together with upslope winds during the day. This pattern was amplified at night when the wind was consistently (day and night) blowing downslope (the "Tramontana" situation) and, vice versa, attenuated during the night, when the wind was blowing permanently upslope (the "Southerlies" situation). Taking into account these advective fluxes would significantly reduce the reported annual CO2 uptake of this forest. Related effects are expected to occur at flux tower sites with similar topography and vegetation. (C) 2009 Elsevier B.V. All rights reserved. [less ▲]

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See detailDirect advection measurements do not help to solve the night-time CO2 closure problem: Evidence from three different forests
Aubinet, Marc ULiege; Feigenwinter, Christian; Heinesch, Bernard ULiege et al

in Agricultural and Forest Meteorology (2010), 150(5), 655-664

The ADVEX project involved conducting extensive advection measurements at three sites, each with a different topography. One goal of the project was to measure the [CO2] balance under night-time ... [more ▼]

The ADVEX project involved conducting extensive advection measurements at three sites, each with a different topography. One goal of the project was to measure the [CO2] balance under night-time conditions, in an attempt to improve NEE estimates. Four towers were arranged in a square around a main tower, with the sides of the square about 100 m long. Equipped with 16 sonic anemometers and [CO2] sampling points, the towers were installed to measure vertical and horizontal advection of [CO2]. Vertical turbulent fluxes were measured by an eddy covariance system at the top of the main tower. The results showed that horizontal advection varied greatly from site to site and from one wind sector to another, the highest values being reached when there were large friction velocities and fairly unstable conditions. There was less variation in vertical advection, the highest values being reached when there were low friction velocities and stable conditions. The night-time NEE estimates deduced from the mass balance were found to be incompatible with biologically driven fluxes because (i) they varied strongly from one wind sector to another and this variation could not be explained in terms of a response of the biologic flux to climate, (ii) their order of magnitude was not realistic and (iii) they still showed a trend vs. friction velocity. From a critical analysis of the measurement and data treatment we concluded that the causes of the problem are related to the representativeness of the measurement (control volume size, sampling resolution) or the hypotheses underlying the derivation of the [CO2] mass balance (ignoring the horizontal turbulent flux divergence). This suggests that the improvement of eddy flux measurements by developing an advection completed [CO2] mass balance at night would be practically difficult. (C) 2010 Elsevier B.V. All rights reserved. [less ▲]

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See detailDirect CO2 advection measurements and the night flux problem
Aubinet, Marc ULiege; Feigenwinter, Christian

in Agricultural and Forest Meteorology (2010), 150(5), 651-654

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