Eddy covariance measurements of CO2 fluxes were conducted in 2004 at the agricultural site of Lon... more Eddy covariance measurements of CO2 fluxes were conducted in 2004 at the agricultural site of Lonzée, Belgium, over a sugar beet crop. Additional measurements of biomass net primary production and leaf area index (LAI) were carried out. The response of the fluxes to climatic and non-climatic variables was analysed. Nighttime fluxes were dependent on turbulence, temperature and high soil water
Recent European heatwaves have raised interest in the impact of land cover conditions on temperat... more Recent European heatwaves have raised interest in the impact of land cover conditions on temperature extremes. At present, it is believed that such extremes are enhanced by stronger surface heating of the atmosphere, when soil moisture content is below average. However, the impact of land cover on the exchange of water and energy and the interaction of this exchange with the soil water balance during heatwaves is largely unknown. Here we analyse observations from an extensive network of flux towers in Europe that reveal a difference between the temporal responses of forest and grassland ecosystems during heatwaves. We find that initially, surface heating is twice as high over forest than over grassland. Over grass, heating is suppressed by increased evaporation in response to increased solar radiation and temperature. Ultimately, however, this process accelerates soil moisture depletion and induces a critical shift in the regional climate system that leads to increased heating. We propose that this mechanism may explain the extreme temperatures in August 2003. We conclude that the conservative water use of forest contributes to increased temperatures in the short term, but mitigates the impact of the most extreme heat and/or long-lasting events.
Sea urchins have characteristic spines that fulfil critical functions. Several studies revealed m... more Sea urchins have characteristic spines that fulfil critical functions. Several studies revealed marked spine internal heterogeneities at different structural levels despite the single-crystal character of the spines. Most of these studies did not speculate about the functional meaning of these heterogeneities. Spine heterogeneities were investigated in the sea urchin Paracentrotus lividus and their possible functional implications discussed. Spines mainly show two morphological parts: the base, made of a meshwork stereom, and the shaft, with longitudinal plain septa and a central core of meshwork stereom. Electron Backscatter Diffraction showed no difference in crystallite orientation between the two structures. Atomic Absorption Spectrometry and Energy dispersive X-ray analysis revealed that Mg was not uniformly distributed in the spine. Mg concentration is higher in the inner part of the septa than in the septum outer part. Furthermore, a cyclic pattern of Mg concentration in septa was observed. This is suggested to be linked to the spine ontogeny. Nano-and microindentation analyses revealed that the septa have higher stiffness and hardness than the meshwork stereom and that septum stiffness and hardness present different trends in longitudinal and transverse section. These mechanical heterogeneities may have an adaptive functional value.
The carbon balance of a winter wheat crop in Lonzée, Belgium, was assessed from measurements carr... more The carbon balance of a winter wheat crop in Lonzée, Belgium, was assessed from measurements carried out at different spatial and temporal scales between November 2004 and August 2005. From eddy covariance measurements, the net ecosystem exchange was found to be À0.63 kg C m À2 and resulted from the difference between gross primary productivity (GPP) (À1.58 kg C m À2 ) and total ecosystem respiration (TER) (0.95 kg C m À2 ). The impact of the u * threshold value on these fluxes was assessed and found to be very small. GPP assessment was partially validated by comparison with an estimation scaled up from leaf scale assimilation measurements. Soil respiration (SR), extrapolated from chamber measurements, was 0.52 kg C m À2 . Net primary productivity, assessed from crop sampling, was À0.83 kg C m À2 . By combining these fluxes, the autotrophic and heterotrophic components of respiration were deduced. Autotrophic respiration dominated both TER and SR. The evolution of these fluxes was analysed in relation to wheat development.
Echinoderms are known to readily incorporate metals in their calcified endoskeleton. It is curren... more Echinoderms are known to readily incorporate metals in their calcified endoskeleton. It is currently unclear if this has an impact on the skeleton function or if this can be considered as a detoxification mechanism. In the present work, populations of the sea urchin Echinus acutus and the starfish Asterias rubens were studied in stations distributed along a metal contamination gradient in a Norwegian fjord (Sørfjord). Ossicles involved in major mechanical functions -sea urchin spine and starfish ambulacral plate -were analyzed for their metal concentration (Cd, Cu, Pb and Zn) and their biometric and mechanical properties.
Three winter wheat growing seasons were compared in order to analyse crop development, CO 2 fluxe... more Three winter wheat growing seasons were compared in order to analyse crop development, CO 2 fluxes and inter-annual variability in productivity. Crop development monitoring, leaf scale measurements and continuous eddy-covariance measurements were conducted in a production crop at the Lonzée experimental site in Belgium. The 3 years were characterised by similar soil proprieties (same site), similar management (sowing, harvesting, plant protection and nitrogen application, adhering to regional standards), and the use of recommended cultivars (the most productive ones for this region).
The greenhouse gas budgets of 15 European crop sites covering a large climatic gradient and corre... more The greenhouse gas budgets of 15 European crop sites covering a large climatic gradient and corresponding to 41 site-years were estimated. The sites included a wide range of management practices (organic and/or mineral fertilisation, tillage or ploughing, with or without straw removal, with or without irrigation, etc.) and were cultivated with 15 representative crop species common to Europe. At all sites, carbon inputs (organic fertilisation and seeds), carbon exports (harvest or fire) and net ecosystem production (NEP), measured with the eddy covariance technique, were calculated. The variability of the different terms and their relative contributions to the net ecosystem carbon budget (NECB) were analysed for all site-years, and the effect of management on NECB was assessed. To account for greenhouse gas (GHG) fluxes that were not directly measured on site, we estimated the emissions caused by field operations (EFO) for each site using emission factors from the literature. The EFO were added to the NECB to calculate the total GHG budget (GHGB) for a range of cropping systems and management regimes. N 2 O emissions were calculated following the IPCC (2007) guidelines, and CH 4 emissions were estimated from the literature for the rice crop site only. At the other sites, CH 4 emissions/oxidation were assumed to be negligible compared to other contributions to the net GHGB. Finally, we evaluated crop efficiencies (CE) in relation to global warming potential as the ratio of C exported from the field (yield) to the total GHGB. On average, NEP was negative (−284 ± 228 g C m −2 year −1 ), and most cropping systems behaved as atmospheric sinks, with sink strength generally increasing with the number of days of active vegetation. The NECB was, on average, 138 ± 239 g C m −2 year −1 , corresponding to an annual loss of about 2.6 ± 4.5% of the soil organic C content, but with high uncertainty. Management strongly influenced the NECB, with organic fertilisation tending to lower the ecosystem carbon budget. On average, emissions caused by fertilisers (manufacturing, packaging, transport, storage and associated N 2 O emissions) represented close to 76% of E. Ceschia et al. / Agriculture, Ecosystems and Environment 139 (2010) 363-383 EFO. The operation of machinery (use and maintenance) and the use of pesticides represented 9.7 and 1.6% of EFO, respectively. On average, the NEP (through uptake of CO 2 ) represented 88% of the negative radiative forcing, and exported C represented 88% of the positive radiative forcing of a mean total GHGB of 203 ± 253 g C-eq m −2 year −1 . Finally, CE differed considerably among crops and according to management practices within a single crop. Because the CE was highly variable, it is not suitable at this stage for use as an emission factor for management recommendations, and more studies are needed to assess the effects of management on crop efficiency.
led to highest median increase in respiration on the 7 days timescale (+43%), which was still +15... more led to highest median increase in respiration on the 7 days timescale (+43%), which was still +15% in the 28 days comparison. On average, however, management-induced increases in respiration losses from croplands were quite moderate (typically <20% increase over 28 days).
Data sets of biometeorological measurements of ecosystem CO 2 flux, combined with harvest and man... more Data sets of biometeorological measurements of ecosystem CO 2 flux, combined with harvest and manure data from several European cropland were integrated to provide an assessment of the carbon budget. Sites encompassed different climatic regions and contrasting crop rotations. The influence of different crops and management practices was also assessed to identify some of the major factors contributing to the cropland carbon balance. Since crops are rotated and cropping periods do not always follow the calendar year, net ecosystem production (NEP) as well as net biome production (NBP) sums of full crop rotations or of at least 4 years of longer-term crop rotations and of monocultures were used. In a second analysis NBP sums were correlated to soil properties. Finally, the data were combined with additional data to derive a mean annual GHG balance for the European cropland sites under consideration.
The estimated Net Ecosystem Exchange (NEE) of CO 2 based on measurements at 17 flux sites in Euro... more The estimated Net Ecosystem Exchange (NEE) of CO 2 based on measurements at 17 flux sites in Europe for 44 cropping periods showed an average loss of 35 gC m -2 per cropping period. Where the cropping period is defined as the period after sowing or planting until harvest. The variability taken as the standard deviation these cropping periods was 249 gC m -2 . These numbers do not include lateral inputs such as the carbon content of applied manure, nor the carbon exchange out of the cropping period. Both are expected to have a major effect on the C budget of high energy summer crops such as maize.
An experimental system combining an eddy covariance system, a micrometeorological station and soi... 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
a g r i c u l t u r a l a n d f o r e s t m e t e o r o l o g y 1 4 8 ( 2 0 0 8 ) 6 6 8 -6 7 9 (J... more a g r i c u l t u r a l a n d f o r e s t m e t e o r o l o g y 1 4 8 ( 2 0 0 8 ) 6 6 8 -6 7 9 (J. Hoyaux). Abbreviations: a, quantum yield; A, net assimilation; A s , net assimilation at saturating light; D s , air saturation deficit; D l , leaf to air vapour pressure difference; EAI, ear area index; GLAI, green leaf area index; GPP, gross primary productivity; G s , gross assimilation at saturating light; PPFD, photosynthetic photon flux density; PsAI, photosynthetic area index; Q, photosynthetic photon flux density; R d , dark respiration; SAI, stem area index; T a , air temperature; T l , leaf temperature; VAI, total vegetation area index; YLAI, yellow leaf area index.
a g r i c u l t u r a l a n d f o r e s t m e t e o r o l o g y 1 4 9 ( 2 0 0 9 ) 4 0 7 -4 1 8 a ... more a g r i c u l t u r a l a n d f o r e s t m e t e o r o l o g y 1 4 9 ( 2 0 0 9 ) 4 0 7 -4 1 8 a b s t r a c t A crop managed in a traditional way was monitored over a complete sugar beet/winter wheat/potato/winter wheat rotation cycle from 2004 to 2008. Eddy covariance, automatic and manual soil chamber, leaf diffusion and biomass measurements were performed continuously in order to obtain the daily and seasonal Net Ecosystem Exchange (NEE), Gross Primary Productivity (GPP), Total Ecosystem Respiration (TER), Net Primary Productivity (NPP), autotrophic respiration, heterotrophic respiration and Net Biome Production (NBP). The results showed that GPP and TER were subjected to important inter-annual variability due to differences between crops and to climate variability. A significant impact of intercrop assimilation and of some farmer interventions was also detected and quantified. Notably, the impact of ploughing was found to be limited in intensity (1-2 mmol m À2 s À1 ) and duration (not more than 1 day). Seasonal budgets showed that, during cropping periods, the TER/GPP ratio varied between 40 and 60% and that TER was dominated mainly by the autotrophic component (65% of TER and more). Autotrophic respiration was closely related to GPP during the growth period. The whole cycle budget showed that NEE was negative and the rotation behaved as a sink of 1.59 kgC m À2 over the 4-year rotation.
A model describing the half-hourly evolution of the net ecosystem exchange of a heterogeneous for... more A model describing the half-hourly evolution of the net ecosystem exchange of a heterogeneous forest was developed. It viewed the forest as a patchwork of three homogeneous vegetation plots whose contribution varied with wind direction. The model was calibrated on eight (1997)(1998)(1999)(2000)(2001)(2002)(2003)(2004) years of measurements made at the Vielsalm experimental site in Belgium. The first 6 years were used for model calibration, the last two for validation. The model predicted the eddy flux measured by the system with a degree of performance comparable with those of other models running on the same time scale on homogeneous canopies. The model also allowed the three ecosystem behaviours to be differentiated: the beech characterised by higher carbon sequestration efficiency during the growth period; but also by a shorter growth period, the Douglas fir and the spruce/silver fir characterised by a longer growth period, with the efficiency of the former higher than the latter. The evolution with wind direction of the beech forest contribution (i.e., the relative contribution of the beech plot to the total measured flux) was also obtained and was found to be in very good agreement with footprint predictions on average. However, on a half-hourly scale the agreement between observed and predicted beech forest contributions was not so good. In particular, it was found that the predictions made by footprint models of the variations due to longitudinal footprint changes were not observed by the experimental system. #
Understanding the relationships between climate and carbon exchange by terrestrial ecosystems is ... more Understanding the relationships between climate and carbon exchange by terrestrial ecosystems is critical to predict future levels of atmospheric carbon dioxide because of the potential accelerating effects of positive climate-carbon cycle feedbacks. However, directly observed relationships between climate and terrestrial CO 2 exchange with the atmosphere across biomes and continents are lacking. Here we present data describing the relationships between net ecosystem exchange of carbon (NEE) and climate factors as measured using the eddy covariance method at 125 unique sites in various ecosystems over six continents with a total of 559 site-years. We find that NEE observed at eddy covariance sites is (1) a strong function of mean annual temperature at mid-and high-latitudes, (2) a strong function of dryness at mid-and low-latitudes, and (3) a function of both temperature and dryness around the mid-latitudinal belt (45 • N). The sensitivity of NEE to mean annual temperature breaks down at ∼16 • C (a threshold value of mean annual temperature), above which no further increase of CO 2 uptake with temperature was observed and dryness influence overrules temperature influence.
Eddy covariance measurements of CO2 fluxes were conducted in 2004 at the agricultural site of Lon... more Eddy covariance measurements of CO2 fluxes were conducted in 2004 at the agricultural site of Lonzée, Belgium, over a sugar beet crop. Additional measurements of biomass net primary production and leaf area index (LAI) were carried out. The response of the fluxes to climatic and non-climatic variables was analysed. Nighttime fluxes were dependent on turbulence, temperature and high soil water
Recent European heatwaves have raised interest in the impact of land cover conditions on temperat... more Recent European heatwaves have raised interest in the impact of land cover conditions on temperature extremes. At present, it is believed that such extremes are enhanced by stronger surface heating of the atmosphere, when soil moisture content is below average. However, the impact of land cover on the exchange of water and energy and the interaction of this exchange with the soil water balance during heatwaves is largely unknown. Here we analyse observations from an extensive network of flux towers in Europe that reveal a difference between the temporal responses of forest and grassland ecosystems during heatwaves. We find that initially, surface heating is twice as high over forest than over grassland. Over grass, heating is suppressed by increased evaporation in response to increased solar radiation and temperature. Ultimately, however, this process accelerates soil moisture depletion and induces a critical shift in the regional climate system that leads to increased heating. We propose that this mechanism may explain the extreme temperatures in August 2003. We conclude that the conservative water use of forest contributes to increased temperatures in the short term, but mitigates the impact of the most extreme heat and/or long-lasting events.
Sea urchins have characteristic spines that fulfil critical functions. Several studies revealed m... more Sea urchins have characteristic spines that fulfil critical functions. Several studies revealed marked spine internal heterogeneities at different structural levels despite the single-crystal character of the spines. Most of these studies did not speculate about the functional meaning of these heterogeneities. Spine heterogeneities were investigated in the sea urchin Paracentrotus lividus and their possible functional implications discussed. Spines mainly show two morphological parts: the base, made of a meshwork stereom, and the shaft, with longitudinal plain septa and a central core of meshwork stereom. Electron Backscatter Diffraction showed no difference in crystallite orientation between the two structures. Atomic Absorption Spectrometry and Energy dispersive X-ray analysis revealed that Mg was not uniformly distributed in the spine. Mg concentration is higher in the inner part of the septa than in the septum outer part. Furthermore, a cyclic pattern of Mg concentration in septa was observed. This is suggested to be linked to the spine ontogeny. Nano-and microindentation analyses revealed that the septa have higher stiffness and hardness than the meshwork stereom and that septum stiffness and hardness present different trends in longitudinal and transverse section. These mechanical heterogeneities may have an adaptive functional value.
The carbon balance of a winter wheat crop in Lonzée, Belgium, was assessed from measurements carr... more The carbon balance of a winter wheat crop in Lonzée, Belgium, was assessed from measurements carried out at different spatial and temporal scales between November 2004 and August 2005. From eddy covariance measurements, the net ecosystem exchange was found to be À0.63 kg C m À2 and resulted from the difference between gross primary productivity (GPP) (À1.58 kg C m À2 ) and total ecosystem respiration (TER) (0.95 kg C m À2 ). The impact of the u * threshold value on these fluxes was assessed and found to be very small. GPP assessment was partially validated by comparison with an estimation scaled up from leaf scale assimilation measurements. Soil respiration (SR), extrapolated from chamber measurements, was 0.52 kg C m À2 . Net primary productivity, assessed from crop sampling, was À0.83 kg C m À2 . By combining these fluxes, the autotrophic and heterotrophic components of respiration were deduced. Autotrophic respiration dominated both TER and SR. The evolution of these fluxes was analysed in relation to wheat development.
Echinoderms are known to readily incorporate metals in their calcified endoskeleton. It is curren... more Echinoderms are known to readily incorporate metals in their calcified endoskeleton. It is currently unclear if this has an impact on the skeleton function or if this can be considered as a detoxification mechanism. In the present work, populations of the sea urchin Echinus acutus and the starfish Asterias rubens were studied in stations distributed along a metal contamination gradient in a Norwegian fjord (Sørfjord). Ossicles involved in major mechanical functions -sea urchin spine and starfish ambulacral plate -were analyzed for their metal concentration (Cd, Cu, Pb and Zn) and their biometric and mechanical properties.
Three winter wheat growing seasons were compared in order to analyse crop development, CO 2 fluxe... more Three winter wheat growing seasons were compared in order to analyse crop development, CO 2 fluxes and inter-annual variability in productivity. Crop development monitoring, leaf scale measurements and continuous eddy-covariance measurements were conducted in a production crop at the Lonzée experimental site in Belgium. The 3 years were characterised by similar soil proprieties (same site), similar management (sowing, harvesting, plant protection and nitrogen application, adhering to regional standards), and the use of recommended cultivars (the most productive ones for this region).
The greenhouse gas budgets of 15 European crop sites covering a large climatic gradient and corre... more The greenhouse gas budgets of 15 European crop sites covering a large climatic gradient and corresponding to 41 site-years were estimated. The sites included a wide range of management practices (organic and/or mineral fertilisation, tillage or ploughing, with or without straw removal, with or without irrigation, etc.) and were cultivated with 15 representative crop species common to Europe. At all sites, carbon inputs (organic fertilisation and seeds), carbon exports (harvest or fire) and net ecosystem production (NEP), measured with the eddy covariance technique, were calculated. The variability of the different terms and their relative contributions to the net ecosystem carbon budget (NECB) were analysed for all site-years, and the effect of management on NECB was assessed. To account for greenhouse gas (GHG) fluxes that were not directly measured on site, we estimated the emissions caused by field operations (EFO) for each site using emission factors from the literature. The EFO were added to the NECB to calculate the total GHG budget (GHGB) for a range of cropping systems and management regimes. N 2 O emissions were calculated following the IPCC (2007) guidelines, and CH 4 emissions were estimated from the literature for the rice crop site only. At the other sites, CH 4 emissions/oxidation were assumed to be negligible compared to other contributions to the net GHGB. Finally, we evaluated crop efficiencies (CE) in relation to global warming potential as the ratio of C exported from the field (yield) to the total GHGB. On average, NEP was negative (−284 ± 228 g C m −2 year −1 ), and most cropping systems behaved as atmospheric sinks, with sink strength generally increasing with the number of days of active vegetation. The NECB was, on average, 138 ± 239 g C m −2 year −1 , corresponding to an annual loss of about 2.6 ± 4.5% of the soil organic C content, but with high uncertainty. Management strongly influenced the NECB, with organic fertilisation tending to lower the ecosystem carbon budget. On average, emissions caused by fertilisers (manufacturing, packaging, transport, storage and associated N 2 O emissions) represented close to 76% of E. Ceschia et al. / Agriculture, Ecosystems and Environment 139 (2010) 363-383 EFO. The operation of machinery (use and maintenance) and the use of pesticides represented 9.7 and 1.6% of EFO, respectively. On average, the NEP (through uptake of CO 2 ) represented 88% of the negative radiative forcing, and exported C represented 88% of the positive radiative forcing of a mean total GHGB of 203 ± 253 g C-eq m −2 year −1 . Finally, CE differed considerably among crops and according to management practices within a single crop. Because the CE was highly variable, it is not suitable at this stage for use as an emission factor for management recommendations, and more studies are needed to assess the effects of management on crop efficiency.
led to highest median increase in respiration on the 7 days timescale (+43%), which was still +15... more led to highest median increase in respiration on the 7 days timescale (+43%), which was still +15% in the 28 days comparison. On average, however, management-induced increases in respiration losses from croplands were quite moderate (typically <20% increase over 28 days).
Data sets of biometeorological measurements of ecosystem CO 2 flux, combined with harvest and man... more Data sets of biometeorological measurements of ecosystem CO 2 flux, combined with harvest and manure data from several European cropland were integrated to provide an assessment of the carbon budget. Sites encompassed different climatic regions and contrasting crop rotations. The influence of different crops and management practices was also assessed to identify some of the major factors contributing to the cropland carbon balance. Since crops are rotated and cropping periods do not always follow the calendar year, net ecosystem production (NEP) as well as net biome production (NBP) sums of full crop rotations or of at least 4 years of longer-term crop rotations and of monocultures were used. In a second analysis NBP sums were correlated to soil properties. Finally, the data were combined with additional data to derive a mean annual GHG balance for the European cropland sites under consideration.
The estimated Net Ecosystem Exchange (NEE) of CO 2 based on measurements at 17 flux sites in Euro... more The estimated Net Ecosystem Exchange (NEE) of CO 2 based on measurements at 17 flux sites in Europe for 44 cropping periods showed an average loss of 35 gC m -2 per cropping period. Where the cropping period is defined as the period after sowing or planting until harvest. The variability taken as the standard deviation these cropping periods was 249 gC m -2 . These numbers do not include lateral inputs such as the carbon content of applied manure, nor the carbon exchange out of the cropping period. Both are expected to have a major effect on the C budget of high energy summer crops such as maize.
An experimental system combining an eddy covariance system, a micrometeorological station and soi... 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
a g r i c u l t u r a l a n d f o r e s t m e t e o r o l o g y 1 4 8 ( 2 0 0 8 ) 6 6 8 -6 7 9 (J... more a g r i c u l t u r a l a n d f o r e s t m e t e o r o l o g y 1 4 8 ( 2 0 0 8 ) 6 6 8 -6 7 9 (J. Hoyaux). Abbreviations: a, quantum yield; A, net assimilation; A s , net assimilation at saturating light; D s , air saturation deficit; D l , leaf to air vapour pressure difference; EAI, ear area index; GLAI, green leaf area index; GPP, gross primary productivity; G s , gross assimilation at saturating light; PPFD, photosynthetic photon flux density; PsAI, photosynthetic area index; Q, photosynthetic photon flux density; R d , dark respiration; SAI, stem area index; T a , air temperature; T l , leaf temperature; VAI, total vegetation area index; YLAI, yellow leaf area index.
a g r i c u l t u r a l a n d f o r e s t m e t e o r o l o g y 1 4 9 ( 2 0 0 9 ) 4 0 7 -4 1 8 a ... more a g r i c u l t u r a l a n d f o r e s t m e t e o r o l o g y 1 4 9 ( 2 0 0 9 ) 4 0 7 -4 1 8 a b s t r a c t A crop managed in a traditional way was monitored over a complete sugar beet/winter wheat/potato/winter wheat rotation cycle from 2004 to 2008. Eddy covariance, automatic and manual soil chamber, leaf diffusion and biomass measurements were performed continuously in order to obtain the daily and seasonal Net Ecosystem Exchange (NEE), Gross Primary Productivity (GPP), Total Ecosystem Respiration (TER), Net Primary Productivity (NPP), autotrophic respiration, heterotrophic respiration and Net Biome Production (NBP). The results showed that GPP and TER were subjected to important inter-annual variability due to differences between crops and to climate variability. A significant impact of intercrop assimilation and of some farmer interventions was also detected and quantified. Notably, the impact of ploughing was found to be limited in intensity (1-2 mmol m À2 s À1 ) and duration (not more than 1 day). Seasonal budgets showed that, during cropping periods, the TER/GPP ratio varied between 40 and 60% and that TER was dominated mainly by the autotrophic component (65% of TER and more). Autotrophic respiration was closely related to GPP during the growth period. The whole cycle budget showed that NEE was negative and the rotation behaved as a sink of 1.59 kgC m À2 over the 4-year rotation.
A model describing the half-hourly evolution of the net ecosystem exchange of a heterogeneous for... more A model describing the half-hourly evolution of the net ecosystem exchange of a heterogeneous forest was developed. It viewed the forest as a patchwork of three homogeneous vegetation plots whose contribution varied with wind direction. The model was calibrated on eight (1997)(1998)(1999)(2000)(2001)(2002)(2003)(2004) years of measurements made at the Vielsalm experimental site in Belgium. The first 6 years were used for model calibration, the last two for validation. The model predicted the eddy flux measured by the system with a degree of performance comparable with those of other models running on the same time scale on homogeneous canopies. The model also allowed the three ecosystem behaviours to be differentiated: the beech characterised by higher carbon sequestration efficiency during the growth period; but also by a shorter growth period, the Douglas fir and the spruce/silver fir characterised by a longer growth period, with the efficiency of the former higher than the latter. The evolution with wind direction of the beech forest contribution (i.e., the relative contribution of the beech plot to the total measured flux) was also obtained and was found to be in very good agreement with footprint predictions on average. However, on a half-hourly scale the agreement between observed and predicted beech forest contributions was not so good. In particular, it was found that the predictions made by footprint models of the variations due to longitudinal footprint changes were not observed by the experimental system. #
Understanding the relationships between climate and carbon exchange by terrestrial ecosystems is ... more Understanding the relationships between climate and carbon exchange by terrestrial ecosystems is critical to predict future levels of atmospheric carbon dioxide because of the potential accelerating effects of positive climate-carbon cycle feedbacks. However, directly observed relationships between climate and terrestrial CO 2 exchange with the atmosphere across biomes and continents are lacking. Here we present data describing the relationships between net ecosystem exchange of carbon (NEE) and climate factors as measured using the eddy covariance method at 125 unique sites in various ecosystems over six continents with a total of 559 site-years. We find that NEE observed at eddy covariance sites is (1) a strong function of mean annual temperature at mid-and high-latitudes, (2) a strong function of dryness at mid-and low-latitudes, and (3) a function of both temperature and dryness around the mid-latitudinal belt (45 • N). The sensitivity of NEE to mean annual temperature breaks down at ∼16 • C (a threshold value of mean annual temperature), above which no further increase of CO 2 uptake with temperature was observed and dryness influence overrules temperature influence.
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Papers by Christine Moureaux