The use of the scintillation technique for monitoring seasonal water consumption of olive orchard... more The use of the scintillation technique for monitoring seasonal water consumption of olive orchards in a semi-arid region.
The simulation of gross primary production (GPP) at various spatial and temporal scales remains a... more The simulation of gross primary production (GPP) at various spatial and temporal scales remains a major challenge for quantifying the global carbon cycle. We developed a light use efficiency model, called EC-LUE, driven by only four variables: normalized difference vegetation index (NDVI), photosynthetically active radiation (PAR), air temperature, and the Bowen ratio of sensible to latent heat flux. The EC-LUE model may have the most potential to adequately address the spatial and temporal dynamics of GPP because its parameters (i.e., the potential light use efficiency and optimal plant growth temperature) are invariant across the various land cover types. However, the application of the previous EC-LUE model was hampered by poor prediction of Bowen ratio at the large spatial scale. In this study, we substituted the Bowen ratio with the ratio of evapotranspiration (ET) to net radiation, and revised the RS-PM (Remote Sensing-Penman Monteith) model for quantifying ET. Fifty-four eddy covariance towers, including various ecosystem types, were selected to calibrate and validate the revised RS-PM and EC-LUE models. The revised RS-PM model explained 82% and 68% of the observed variations of ET for all the calibration and validation sites, respectively. Using estimated ET as input, the EC-LUE model performed well in calibration and validation sites, explaining 75% and 61% of the observed GPP variation for calibration and validation sites respectively. Global patterns of ET and GPP at a spatial resolution of 0.5°latitude by 0.6°longitude during the years 2000-2003 were determined using the global MERRA dataset (Modern Era Retrospective-Analysis for Research and Applications) and MODIS (Moderate Resolution Imaging Spectroradiometer). The global estimates of ET and GPP agreed well with the other global models from the literature, with the highest ET and GPP over tropical forests and the lowest values in dry and high latitude areas. However, comparisons with observed GPP at eddy flux towers showed significant underestimation of ET and GPP due to lower net radiation of MERRA dataset. Applying a procedure to correct the systematic errors of global meteorological data would improve global estimates of GPP and ET. The revised RS-PM and EC-LUE models will provide the alternative approaches making it possible to map ET and GPP over large areas because (1) the model parameters are invariant across various land cover types and (2) all driving forces of the models may be derived from remote sensing data or existing climate observation networks.
As part of the quality assurance and quality control activities within the CarboEurope-IP network... more As part of the quality assurance and quality control activities within the CarboEurope-IP network, a comparison of eddy-covariance software was conducted. For four five-day datasets, CO2 flux estimates were calculated by seven commonly used software packages to assess the uncertainty of CO2 flux estimates due to differences in post-processing. The datasets originated from different sites representing different commonly applied instrumentation
This study reports carbon and water fluxes between the land surface and atmosphere in eleven diff... more This study reports carbon and water fluxes between the land surface and atmosphere in eleven different ecosystems types in Sub-Saharan Africa, as measured using eddy covariance (EC) technology in the first two years of the CarboAfrica network operation. The ecosystems for which data were available ranged in mean annual rainfall from 5 320 mm (Sudan) to 1150 mm (The Republic of Congo) and include a spectrum of vegetation types (or land cover) (open savannas, woodlands, croplands and grasslands). Given the shortness of the record, the EC data were analysed across the network rather than longitudinally at sites, in order to understand the driving factors for ecosystem respiration and carbon assimilation, and to reveal the different water use strategies 10 in these highly seasonal environments.
The use of the scintillation technique for monitoring seasonal water consumption of olive orchard... more The use of the scintillation technique for monitoring seasonal water consumption of olive orchards in a semi-arid region.
Aerodynamic parameters, friction velocity u * , Temperature scaling parameter T * , Monin-Obhukov... more Aerodynamic parameters, friction velocity u * , Temperature scaling parameter T * , Monin-Obhukov length L, stability correction terms for momentum Ψ m and sensible heat Ψ h , displacement height d and roughness length for momentum z 0 , were estimated from the Alpilles energy balance measurement data. Air temperature and wind speed measurement at two heights and sensible heat flux data were needed in the estimation. Application of this method to two data sets measured in different ways for the temperature gradients found that the d and z 0 estimation was applicable only when the temperature gradients were measured differentially. Sensitivity test showed that the estimated d and z 0 were sensitive to the temperature gradients. However, both of the data sets produced good estimation of u * and T * , and therefore L, Ψ m and Ψ h . The main advantage of this method is that it is applicable not only under near neutral conditions but also under strong unstable conditions. 2 2 1 z z d z d z
The directional surface temperatures over urban areas are measured using an airborne TIR camera e... more The directional surface temperatures over urban areas are measured using an airborne TIR camera equipped with wide-angle lenses. The experimental design is described and the possible sources of errors related to the instruments and the atmosphere are analysed ...
The potential of the LAS (large aperture scintillometry) method for measuring sensible heat flux ... more The potential of the LAS (large aperture scintillometry) method for measuring sensible heat flux (H) directly integrated over a two-field composite surface is evaluated. We describe a field experiment performed within the Alpilles/ReSeDa project in the south-east of France over a composite surface made up of wheat and bare soil (451 and 216 m long respectively) using two 0.15-m diameter
ABSTRACT In this study, we simulated the ecohydrological processes of a landscape that comprises ... more ABSTRACT In this study, we simulated the ecohydrological processes of a landscape that comprises of managed maritime pine stands and crops, in a spatially explicit manner. We used various spatio-temporal datasets to run the STEPS model that describes ecophysiological, biogeochemicaland hydrological processes in a tightly coupled manner. The simulated gross primary productivity (GPP) and evapotranspiration (ET) showed large spatial variability over this landscape owing to the heterogeneities in landcover, soil texture, topography and soil hydrology. Croplands exhibited higher magnitudes of GPP (200–2500 gCm−2 yr−1) and ET fluxes (150–800 mm yr−1) in comparison to other landcover types. Maize (C4) is generally cultivated in this landscape that is managed in terms of optimal availability of water and nutrients. The spatial variability was huge in the croplands due to the presence of fallow vs. cultivated lands. The maritime pine stands also showed considerable spatial variability in GPP (426 to 1320 gCm−2 yr−1) and ET (234 to 570 mm yr−1) because of their stand ages (and the LAI) and depending on the nature of their understory species compositions. It was found that the understory contributions to the stand scale annual GPP and annual ET as much as 27% and 34%, respectively. The comparison of the simulated daily values with the measurements taken at the LeBray eddy covariance site (a maritime pine stand) revealed that the model performance were reasonable for both GPP (R2= 0.92, RMSE= 0.77 gC m−2 day−1) and ET (r2=0.81, RMSE= 0.52 mm day−1). The seasonal patterns of ET fluxes were more dynamic than GPP due to the presence of distinct subcomponent processes that were uniquely governed by environmental factors.
Introducing nitrogen-fixing tree species in fast-growing eucalypt plantations has the potential t... more Introducing nitrogen-fixing tree species in fast-growing eucalypt plantations has the potential to improve soil nitrogen availability compared with eucalypt monocultures. Whether or not the changes in soil nutrient status and stand structure will lead to mixtures that out-yield monocultures depends on the balance between positive interactions and the negative effects of interspecific competition, and on their effect on carbon (C) uptake and partitioning. We used a C budget approach to quantify growth, C uptake and C partitioning in monocultures of Eucalyptus grandis (W. Hill ex Maiden) and Acacia mangium (Willd.) (treatments E100 and A100, respectively), and in a mixture at the same stocking density with the two species at a proportion of 1 : 1 (treatment MS). Allometric relationships established over the whole rotation, and measurements of soil CO(2) efflux and aboveground litterfall for ages 4-6 years after planting were used to estimate aboveground net primary production (ANPP), total belowground carbon flux (TBCF) and gross primary production (GPP). We tested the hypotheses that (i) species differences for wood production between E. grandis and A. mangium monocultures were partly explained by different C partitioning strategies, and (ii) the observed lower wood production in the mixture compared with eucalypt monoculture was mostly explained by a lower partitioning aboveground. At the end of the rotation, total aboveground biomass was lowest in A100 (10.5 kg DM m(-2)), intermediate in MS (12.2 kg DM m(-2)) and highest in E100 (13.9 kg DM m(-2)). The results did not support our first hypothesis of contrasting C partitioning strategies between E. grandis and A. mangium monocultures: the 21% lower growth (ΔB(w)) in A100 compared with E100 was almost entirely explained by a 23% lower GPP, with little or no species difference in ratios such as TBCF/GPP, ANPP/TBCF, ΔB(w)/ANPP and ΔB(w)/GPP. In contrast, the 28% lower ΔB(w) in MS than in E100 was explained both by a 15% lower GPP and by a 15% lower fraction of GPP allocated to wood growth, thus partially supporting our second hypothesis: mixing the two species led to shifts in C allocations from above- to belowground, and from growth to litter production, for both species.
We monitored seasonal variations in net primary production (NPP), estimated by allometric equatio... more We monitored seasonal variations in net primary production (NPP), estimated by allometric equations from organ dimensions, gross primary production (GPP), estimated by the eddy covariance method, autotrophic respiration (R a ), estimated by a model, and fruit production in a coconut (Cocos nucifera L.) plantation located in the sub-tropical South Pacific archipelago of Vanuatu. Net primary production of the vegetative compartments of the trees accumulated steadily throughout the year. Fruits accounted for 46% of tree NPP and showed large seasonal variations. On an annual basis, the sum of estimated NPP (16.1 Mg C ha -1 year -1 ) and R a (24.0 Mg C ha -1 year -1 ) for the ecosystem (coconut trees and herbaceous understory) closely matched GPP (39.0 Mg C ha -1 year -1 ), suggesting adequate cross-validation of annual C budget methods. However, seasonal variations in NPP + R a were smaller than the seasonal variations in GPP, and maximum tree NPP occurred 6 months after the midsummer peak in GPP and solar radiation. We propose that this discrepancy reflects seasonal variation in the allocation of dry mass to carbon reserves and new plant tissue, thus affecting the allometric relationships used for estimating NPP.
The effects of management practices on energy, water and carbon exchanges were investigated in a ... more The effects of management practices on energy, water and carbon exchanges were investigated in a young pine plantation in south-west France. In 2009-10, carbon dioxide (CO 2 ), H 2 O and heat fluxes were monitored using the eddy covariance and sap flow techniques in a control plot (C) with a developed gorse layer, and an adjacent plot that was mechanically weeded and thinned (W). Despite large differences in the total leaf area index and canopy structure, the annual net radiation absorbed was only 4% lower in plot W. We showed that higher albedo in this plot was offset by lower emitted long-wave radiation. Annual evapotranspiration (ET) from plot W was 15% lower, due to lower rainfall interception and transpiration by the tree canopy, partly counterbalanced by the larger evaporation from both soil and regrowing weedy vegetation. The drainage belowground from plot W was larger by 113 mm annually. The seasonal variability of ET was driven by the dynamics of the soil and weed layers, which was more severely affected by drought in plot C. Conversely, the temporal changes in pine transpiration and stem diameter growth were synchronous between sites despite higher soil water content in the weeded plot. At the annual scale, both plots were carbon sinks, but thinning and weeding reduced the carbon uptake by 73%: annual carbon uptake was 243 and 65 g C m −2 on plots C and W, respectively. Summer drought dramatically impacted the net ecosystem exchange: plot C became a carbon source as the gross primary production (GPP) severely decreased. However, plot W remained a carbon sink during drought, as a result of decreases in both GPP and ecosystem respiration (R E ). In winter, both plots were carbon sources, plots C and W emitting 67.5 and 32.4 g C m −2 , respectively. Overall, this study highlighted the significant contribution of the gorse layer to mass and energy exchange in young pine plantations.
D. L o u s t a u 9 P. B e r b i g i e r 9 P. R o u m a g n a c C. A r r u d a -P a c h e e o 9 J.... more D. L o u s t a u 9 P. B e r b i g i e r 9 P. R o u m a g n a c C. A r r u d a -P a c h e e o 9 J.S. D a v i d . M.I. Ferreira J.S. Pereira 9 R. Tavares A b s t r a c t The transpiration, sap flow, stomatal conductance and water relations of Pinus pinaster were deter-
Terrestrial ecosystems sequester 2.1 Pg of atmospheric carbon annually. A large amount of the ter... more Terrestrial ecosystems sequester 2.1 Pg of atmospheric carbon annually. A large amount of the terrestrial sink is realized by forests. However, considerable uncertainties remain regarding the fate of this carbon over both short and long timescales. Relevant data to address these uncertainties are being collected at many sites around the world, but syntheses of these data are still sparse. To facilitate future synthesis activities, we have assembled a comprehensive global database for forest ecosystems, which includes carbon budget variables (fluxes and stocks), ecosystem traits (e.g. leaf area index, age), as well as ancillary site information such as management regime, climate, and soil characteristics. This publicly available database can be used to quantify global, regional or biome-specific carbon budgets; to re-examine established relationships; to test emerging hypotheses about ecosystem functioning [e.g. a constant net ecosystem production (NEP) to gross primary production (GPP) ratio]; and as benchmarks for model evaluations. In this paper, we present the first analysis of this database. We discuss the climatic influences on GPP, net primary production (NPP) and NEP and present the CO 2 balances for boreal, temperate, and tropical forest biomes based on micrometeorological, ecophysiological, and biometric flux and inventory estimates. Globally, GPP of forests benefited from higher temperatures and precipitation whereas NPP saturated above either a threshold of 1500 mm precipitation or a mean annual temperature of 10 1C. The global pattern in NEP was insensitive to climate and is hypothesized to be mainly determined by nonclimatic conditions such as successional stage, management, site history, and site disturbance. In all biomes, closing the CO 2 balance required the introduction of substantial biome-specific closure terms. Nonclosure was taken as an indication that respiratory processes, advection, and non-CO 2 carbon fluxes are not presently being adequately accounted for.
1] Stable CO 2 isotope measurements are increasingly used to partition the net CO 2 exchange betw... more 1] Stable CO 2 isotope measurements are increasingly used to partition the net CO 2 exchange between terrestrial ecosystems and the atmosphere in terms of nonfoliar respiration (F R ) and net photosynthesis (F A ) in order to better understand the variations of this exchange. However, the accuracy of the partitioning strongly depends on the isotopic disequilibrium between these two gross fluxes, and a rigorous estimation of the errors on F A and F R is needed. In this study, we account for and propagate uncertainties on all terms in the mass balance and isotopic mass balance equations for CO 2 in order to get accurate estimates of the errors on F A and F R . We apply our method to a maritime pine forest in the southwest of France. Nighttime Keeling plots are used to estimate the 13 C and 18 O isotopic signature of F R (d R ), and for both isotopes the a priori uncertainty associated with this term is estimated to be around 2% at our site. Using d 13 C-CO 2 and [CO 2 ] measurements, we then show that the uncertainty on instantaneous values of F A and F R can be as large as 4 mmol m À2 s À1 . Even if we could get more accurate estimates of the net CO 2 flux, the isoflux, and the isotopic signatures of F A and F R , this uncertainty would not be significantly reduced because the isotopic disequilibrium between F A and F R is too small, around 2-3%. With d 18 O-CO 2 and [CO 2 ] measurements the uncertainty associated with the gross fluxes lies also around 4 mmol m À2 s À1 but could be dramatically reduced if we were able to get more accurate estimates of the CO 18 O isoflux and the associated discrimination during photosynthesis. This is because the isotopic disequilibrium between F A and F R is large, of the order of 12-17%. The isotopic disequilibrium between F A and F R and the uncertainty on d R vary among ecosystems and over the year. Our approach should help to choose the best strategy to study the carbon budget of a given ecosystem using stable isotopes.
Our objectives were to quantify soil respiration in a 3-year-old Eucalyptus plantation in coastal... more Our objectives were to quantify soil respiration in a 3-year-old Eucalyptus plantation in coastal Congo and to investigate both temporal and spatial variations of this major component of ecosystem respiration. Soil respiration exhibited pronounced seasonal variations that clearly reflected those of soil water content, with minimum values below 1.6 mmol m À2 s À1 at the end of the dry season in September and a maximum value of 5.6 mmol m À2 s À1 after re-wetting in December. An empirical model describing the relationship between soil respiration and soil water content predicts the seasonal variations in soil respiration reasonably well (R 2 = 0.88), even if the effects of soil temperature and soil water content may be confounded since both factors co-vary across seasons. Spatial heterogeneity of soil respiration was clearly affected by management practices with higher respiration rate in slash inter-rows which received higher amounts of detritus at the logging stage, and lower respiration rate in haulage inter-rows used for heavy vehicle traffic. Higher values of soil respiration were also recorded in the vicinity of trunks than in the middle of the inter-rows. While soil water content is the main determinant of seasonal variation of soil respiration, it poorly accounts for its spatial variability over the experimental stand, except for days with low soil water content. Soil respiration was related neither to root biomass nor to soil carbon content, but was positively correlated with both leaf and total aboveground litter (i.e. leaf, bark and woody debris). Plots exhibiting the highest soil respiration also contained the highest amounts of aboveground litter. Microbial respiration associated with litter decomposition is likely a major component of soil www.elsevier.com/locate/foreco Forest Ecology and Management 202
The use of the scintillation technique for monitoring seasonal water consumption of olive orchard... more The use of the scintillation technique for monitoring seasonal water consumption of olive orchards in a semi-arid region.
The simulation of gross primary production (GPP) at various spatial and temporal scales remains a... more The simulation of gross primary production (GPP) at various spatial and temporal scales remains a major challenge for quantifying the global carbon cycle. We developed a light use efficiency model, called EC-LUE, driven by only four variables: normalized difference vegetation index (NDVI), photosynthetically active radiation (PAR), air temperature, and the Bowen ratio of sensible to latent heat flux. The EC-LUE model may have the most potential to adequately address the spatial and temporal dynamics of GPP because its parameters (i.e., the potential light use efficiency and optimal plant growth temperature) are invariant across the various land cover types. However, the application of the previous EC-LUE model was hampered by poor prediction of Bowen ratio at the large spatial scale. In this study, we substituted the Bowen ratio with the ratio of evapotranspiration (ET) to net radiation, and revised the RS-PM (Remote Sensing-Penman Monteith) model for quantifying ET. Fifty-four eddy covariance towers, including various ecosystem types, were selected to calibrate and validate the revised RS-PM and EC-LUE models. The revised RS-PM model explained 82% and 68% of the observed variations of ET for all the calibration and validation sites, respectively. Using estimated ET as input, the EC-LUE model performed well in calibration and validation sites, explaining 75% and 61% of the observed GPP variation for calibration and validation sites respectively. Global patterns of ET and GPP at a spatial resolution of 0.5°latitude by 0.6°longitude during the years 2000-2003 were determined using the global MERRA dataset (Modern Era Retrospective-Analysis for Research and Applications) and MODIS (Moderate Resolution Imaging Spectroradiometer). The global estimates of ET and GPP agreed well with the other global models from the literature, with the highest ET and GPP over tropical forests and the lowest values in dry and high latitude areas. However, comparisons with observed GPP at eddy flux towers showed significant underestimation of ET and GPP due to lower net radiation of MERRA dataset. Applying a procedure to correct the systematic errors of global meteorological data would improve global estimates of GPP and ET. The revised RS-PM and EC-LUE models will provide the alternative approaches making it possible to map ET and GPP over large areas because (1) the model parameters are invariant across various land cover types and (2) all driving forces of the models may be derived from remote sensing data or existing climate observation networks.
As part of the quality assurance and quality control activities within the CarboEurope-IP network... more As part of the quality assurance and quality control activities within the CarboEurope-IP network, a comparison of eddy-covariance software was conducted. For four five-day datasets, CO2 flux estimates were calculated by seven commonly used software packages to assess the uncertainty of CO2 flux estimates due to differences in post-processing. The datasets originated from different sites representing different commonly applied instrumentation
This study reports carbon and water fluxes between the land surface and atmosphere in eleven diff... more This study reports carbon and water fluxes between the land surface and atmosphere in eleven different ecosystems types in Sub-Saharan Africa, as measured using eddy covariance (EC) technology in the first two years of the CarboAfrica network operation. The ecosystems for which data were available ranged in mean annual rainfall from 5 320 mm (Sudan) to 1150 mm (The Republic of Congo) and include a spectrum of vegetation types (or land cover) (open savannas, woodlands, croplands and grasslands). Given the shortness of the record, the EC data were analysed across the network rather than longitudinally at sites, in order to understand the driving factors for ecosystem respiration and carbon assimilation, and to reveal the different water use strategies 10 in these highly seasonal environments.
The use of the scintillation technique for monitoring seasonal water consumption of olive orchard... more The use of the scintillation technique for monitoring seasonal water consumption of olive orchards in a semi-arid region.
Aerodynamic parameters, friction velocity u * , Temperature scaling parameter T * , Monin-Obhukov... more Aerodynamic parameters, friction velocity u * , Temperature scaling parameter T * , Monin-Obhukov length L, stability correction terms for momentum Ψ m and sensible heat Ψ h , displacement height d and roughness length for momentum z 0 , were estimated from the Alpilles energy balance measurement data. Air temperature and wind speed measurement at two heights and sensible heat flux data were needed in the estimation. Application of this method to two data sets measured in different ways for the temperature gradients found that the d and z 0 estimation was applicable only when the temperature gradients were measured differentially. Sensitivity test showed that the estimated d and z 0 were sensitive to the temperature gradients. However, both of the data sets produced good estimation of u * and T * , and therefore L, Ψ m and Ψ h . The main advantage of this method is that it is applicable not only under near neutral conditions but also under strong unstable conditions. 2 2 1 z z d z d z
The directional surface temperatures over urban areas are measured using an airborne TIR camera e... more The directional surface temperatures over urban areas are measured using an airborne TIR camera equipped with wide-angle lenses. The experimental design is described and the possible sources of errors related to the instruments and the atmosphere are analysed ...
The potential of the LAS (large aperture scintillometry) method for measuring sensible heat flux ... more The potential of the LAS (large aperture scintillometry) method for measuring sensible heat flux (H) directly integrated over a two-field composite surface is evaluated. We describe a field experiment performed within the Alpilles/ReSeDa project in the south-east of France over a composite surface made up of wheat and bare soil (451 and 216 m long respectively) using two 0.15-m diameter
ABSTRACT In this study, we simulated the ecohydrological processes of a landscape that comprises ... more ABSTRACT In this study, we simulated the ecohydrological processes of a landscape that comprises of managed maritime pine stands and crops, in a spatially explicit manner. We used various spatio-temporal datasets to run the STEPS model that describes ecophysiological, biogeochemicaland hydrological processes in a tightly coupled manner. The simulated gross primary productivity (GPP) and evapotranspiration (ET) showed large spatial variability over this landscape owing to the heterogeneities in landcover, soil texture, topography and soil hydrology. Croplands exhibited higher magnitudes of GPP (200–2500 gCm−2 yr−1) and ET fluxes (150–800 mm yr−1) in comparison to other landcover types. Maize (C4) is generally cultivated in this landscape that is managed in terms of optimal availability of water and nutrients. The spatial variability was huge in the croplands due to the presence of fallow vs. cultivated lands. The maritime pine stands also showed considerable spatial variability in GPP (426 to 1320 gCm−2 yr−1) and ET (234 to 570 mm yr−1) because of their stand ages (and the LAI) and depending on the nature of their understory species compositions. It was found that the understory contributions to the stand scale annual GPP and annual ET as much as 27% and 34%, respectively. The comparison of the simulated daily values with the measurements taken at the LeBray eddy covariance site (a maritime pine stand) revealed that the model performance were reasonable for both GPP (R2= 0.92, RMSE= 0.77 gC m−2 day−1) and ET (r2=0.81, RMSE= 0.52 mm day−1). The seasonal patterns of ET fluxes were more dynamic than GPP due to the presence of distinct subcomponent processes that were uniquely governed by environmental factors.
Introducing nitrogen-fixing tree species in fast-growing eucalypt plantations has the potential t... more Introducing nitrogen-fixing tree species in fast-growing eucalypt plantations has the potential to improve soil nitrogen availability compared with eucalypt monocultures. Whether or not the changes in soil nutrient status and stand structure will lead to mixtures that out-yield monocultures depends on the balance between positive interactions and the negative effects of interspecific competition, and on their effect on carbon (C) uptake and partitioning. We used a C budget approach to quantify growth, C uptake and C partitioning in monocultures of Eucalyptus grandis (W. Hill ex Maiden) and Acacia mangium (Willd.) (treatments E100 and A100, respectively), and in a mixture at the same stocking density with the two species at a proportion of 1 : 1 (treatment MS). Allometric relationships established over the whole rotation, and measurements of soil CO(2) efflux and aboveground litterfall for ages 4-6 years after planting were used to estimate aboveground net primary production (ANPP), total belowground carbon flux (TBCF) and gross primary production (GPP). We tested the hypotheses that (i) species differences for wood production between E. grandis and A. mangium monocultures were partly explained by different C partitioning strategies, and (ii) the observed lower wood production in the mixture compared with eucalypt monoculture was mostly explained by a lower partitioning aboveground. At the end of the rotation, total aboveground biomass was lowest in A100 (10.5 kg DM m(-2)), intermediate in MS (12.2 kg DM m(-2)) and highest in E100 (13.9 kg DM m(-2)). The results did not support our first hypothesis of contrasting C partitioning strategies between E. grandis and A. mangium monocultures: the 21% lower growth (ΔB(w)) in A100 compared with E100 was almost entirely explained by a 23% lower GPP, with little or no species difference in ratios such as TBCF/GPP, ANPP/TBCF, ΔB(w)/ANPP and ΔB(w)/GPP. In contrast, the 28% lower ΔB(w) in MS than in E100 was explained both by a 15% lower GPP and by a 15% lower fraction of GPP allocated to wood growth, thus partially supporting our second hypothesis: mixing the two species led to shifts in C allocations from above- to belowground, and from growth to litter production, for both species.
We monitored seasonal variations in net primary production (NPP), estimated by allometric equatio... more We monitored seasonal variations in net primary production (NPP), estimated by allometric equations from organ dimensions, gross primary production (GPP), estimated by the eddy covariance method, autotrophic respiration (R a ), estimated by a model, and fruit production in a coconut (Cocos nucifera L.) plantation located in the sub-tropical South Pacific archipelago of Vanuatu. Net primary production of the vegetative compartments of the trees accumulated steadily throughout the year. Fruits accounted for 46% of tree NPP and showed large seasonal variations. On an annual basis, the sum of estimated NPP (16.1 Mg C ha -1 year -1 ) and R a (24.0 Mg C ha -1 year -1 ) for the ecosystem (coconut trees and herbaceous understory) closely matched GPP (39.0 Mg C ha -1 year -1 ), suggesting adequate cross-validation of annual C budget methods. However, seasonal variations in NPP + R a were smaller than the seasonal variations in GPP, and maximum tree NPP occurred 6 months after the midsummer peak in GPP and solar radiation. We propose that this discrepancy reflects seasonal variation in the allocation of dry mass to carbon reserves and new plant tissue, thus affecting the allometric relationships used for estimating NPP.
The effects of management practices on energy, water and carbon exchanges were investigated in a ... more The effects of management practices on energy, water and carbon exchanges were investigated in a young pine plantation in south-west France. In 2009-10, carbon dioxide (CO 2 ), H 2 O and heat fluxes were monitored using the eddy covariance and sap flow techniques in a control plot (C) with a developed gorse layer, and an adjacent plot that was mechanically weeded and thinned (W). Despite large differences in the total leaf area index and canopy structure, the annual net radiation absorbed was only 4% lower in plot W. We showed that higher albedo in this plot was offset by lower emitted long-wave radiation. Annual evapotranspiration (ET) from plot W was 15% lower, due to lower rainfall interception and transpiration by the tree canopy, partly counterbalanced by the larger evaporation from both soil and regrowing weedy vegetation. The drainage belowground from plot W was larger by 113 mm annually. The seasonal variability of ET was driven by the dynamics of the soil and weed layers, which was more severely affected by drought in plot C. Conversely, the temporal changes in pine transpiration and stem diameter growth were synchronous between sites despite higher soil water content in the weeded plot. At the annual scale, both plots were carbon sinks, but thinning and weeding reduced the carbon uptake by 73%: annual carbon uptake was 243 and 65 g C m −2 on plots C and W, respectively. Summer drought dramatically impacted the net ecosystem exchange: plot C became a carbon source as the gross primary production (GPP) severely decreased. However, plot W remained a carbon sink during drought, as a result of decreases in both GPP and ecosystem respiration (R E ). In winter, both plots were carbon sources, plots C and W emitting 67.5 and 32.4 g C m −2 , respectively. Overall, this study highlighted the significant contribution of the gorse layer to mass and energy exchange in young pine plantations.
D. L o u s t a u 9 P. B e r b i g i e r 9 P. R o u m a g n a c C. A r r u d a -P a c h e e o 9 J.... more D. L o u s t a u 9 P. B e r b i g i e r 9 P. R o u m a g n a c C. A r r u d a -P a c h e e o 9 J.S. D a v i d . M.I. Ferreira J.S. Pereira 9 R. Tavares A b s t r a c t The transpiration, sap flow, stomatal conductance and water relations of Pinus pinaster were deter-
Terrestrial ecosystems sequester 2.1 Pg of atmospheric carbon annually. A large amount of the ter... more Terrestrial ecosystems sequester 2.1 Pg of atmospheric carbon annually. A large amount of the terrestrial sink is realized by forests. However, considerable uncertainties remain regarding the fate of this carbon over both short and long timescales. Relevant data to address these uncertainties are being collected at many sites around the world, but syntheses of these data are still sparse. To facilitate future synthesis activities, we have assembled a comprehensive global database for forest ecosystems, which includes carbon budget variables (fluxes and stocks), ecosystem traits (e.g. leaf area index, age), as well as ancillary site information such as management regime, climate, and soil characteristics. This publicly available database can be used to quantify global, regional or biome-specific carbon budgets; to re-examine established relationships; to test emerging hypotheses about ecosystem functioning [e.g. a constant net ecosystem production (NEP) to gross primary production (GPP) ratio]; and as benchmarks for model evaluations. In this paper, we present the first analysis of this database. We discuss the climatic influences on GPP, net primary production (NPP) and NEP and present the CO 2 balances for boreal, temperate, and tropical forest biomes based on micrometeorological, ecophysiological, and biometric flux and inventory estimates. Globally, GPP of forests benefited from higher temperatures and precipitation whereas NPP saturated above either a threshold of 1500 mm precipitation or a mean annual temperature of 10 1C. The global pattern in NEP was insensitive to climate and is hypothesized to be mainly determined by nonclimatic conditions such as successional stage, management, site history, and site disturbance. In all biomes, closing the CO 2 balance required the introduction of substantial biome-specific closure terms. Nonclosure was taken as an indication that respiratory processes, advection, and non-CO 2 carbon fluxes are not presently being adequately accounted for.
1] Stable CO 2 isotope measurements are increasingly used to partition the net CO 2 exchange betw... more 1] Stable CO 2 isotope measurements are increasingly used to partition the net CO 2 exchange between terrestrial ecosystems and the atmosphere in terms of nonfoliar respiration (F R ) and net photosynthesis (F A ) in order to better understand the variations of this exchange. However, the accuracy of the partitioning strongly depends on the isotopic disequilibrium between these two gross fluxes, and a rigorous estimation of the errors on F A and F R is needed. In this study, we account for and propagate uncertainties on all terms in the mass balance and isotopic mass balance equations for CO 2 in order to get accurate estimates of the errors on F A and F R . We apply our method to a maritime pine forest in the southwest of France. Nighttime Keeling plots are used to estimate the 13 C and 18 O isotopic signature of F R (d R ), and for both isotopes the a priori uncertainty associated with this term is estimated to be around 2% at our site. Using d 13 C-CO 2 and [CO 2 ] measurements, we then show that the uncertainty on instantaneous values of F A and F R can be as large as 4 mmol m À2 s À1 . Even if we could get more accurate estimates of the net CO 2 flux, the isoflux, and the isotopic signatures of F A and F R , this uncertainty would not be significantly reduced because the isotopic disequilibrium between F A and F R is too small, around 2-3%. With d 18 O-CO 2 and [CO 2 ] measurements the uncertainty associated with the gross fluxes lies also around 4 mmol m À2 s À1 but could be dramatically reduced if we were able to get more accurate estimates of the CO 18 O isoflux and the associated discrimination during photosynthesis. This is because the isotopic disequilibrium between F A and F R is large, of the order of 12-17%. The isotopic disequilibrium between F A and F R and the uncertainty on d R vary among ecosystems and over the year. Our approach should help to choose the best strategy to study the carbon budget of a given ecosystem using stable isotopes.
Our objectives were to quantify soil respiration in a 3-year-old Eucalyptus plantation in coastal... more Our objectives were to quantify soil respiration in a 3-year-old Eucalyptus plantation in coastal Congo and to investigate both temporal and spatial variations of this major component of ecosystem respiration. Soil respiration exhibited pronounced seasonal variations that clearly reflected those of soil water content, with minimum values below 1.6 mmol m À2 s À1 at the end of the dry season in September and a maximum value of 5.6 mmol m À2 s À1 after re-wetting in December. An empirical model describing the relationship between soil respiration and soil water content predicts the seasonal variations in soil respiration reasonably well (R 2 = 0.88), even if the effects of soil temperature and soil water content may be confounded since both factors co-vary across seasons. Spatial heterogeneity of soil respiration was clearly affected by management practices with higher respiration rate in slash inter-rows which received higher amounts of detritus at the logging stage, and lower respiration rate in haulage inter-rows used for heavy vehicle traffic. Higher values of soil respiration were also recorded in the vicinity of trunks than in the middle of the inter-rows. While soil water content is the main determinant of seasonal variation of soil respiration, it poorly accounts for its spatial variability over the experimental stand, except for days with low soil water content. Soil respiration was related neither to root biomass nor to soil carbon content, but was positively correlated with both leaf and total aboveground litter (i.e. leaf, bark and woody debris). Plots exhibiting the highest soil respiration also contained the highest amounts of aboveground litter. Microbial respiration associated with litter decomposition is likely a major component of soil www.elsevier.com/locate/foreco Forest Ecology and Management 202
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Papers by J. Bonnefond