Recent advances in understanding the relationships between spectral reflectance of vegetation can... more Recent advances in understanding the relationships between spectral reflectance of vegetation canopies and the structural/physiological drivers of canopy-atmosphere net ecosystem carbon exchange highlight the potential for using narrowband spectral vegetation indices to spatially scale C fluxes beyond the area of a tower footprint. However, ground reference observations of narrowband spectral reflectance in support of satellite observations can be challenging to
Afin de mesurer en continu la respiration du sol, deux laboratoires de l'INRA ont développé d... more Afin de mesurer en continu la respiration du sol, deux laboratoires de l'INRA ont développé deux systèmes automatiques basés sur des designs et automatismes très similaires. Le choix de la mesure des échanges gazeux s'est porté sur les deux principes bien connus des méthodes dynamiques, à savoir, le système ouvert ou fermé (Field et al. 1989). Ces deux méthodes présentent chacune leurs avantages et leurs inconvénients et chaque équipe a fait ses choix en fonction du matériel disponible, des conditions environnementales de mesure, des budgets ou des techniques maîtrisées. La prise en compte poussée (calculs préalables, vérification a postériori) des problèmes potentiels de mesure rapportés dans la littérature a conduit à réaliser deux systèmes correctement dimensionnés.
Measurement-while-drilling of downhole pressure accurately determines annular pressure losses fro... more Measurement-while-drilling of downhole pressure accurately determines annular pressure losses from circulation and drillstring rotation and helps monitor swab and surge pressures during tripping. In early 1993, two slim-hole wells (3.4 in. and 3 in. diameter) were drilled with continuous real-time electromagnetic wave transmission of downhole temperature and annular pressure. The data were obtained during all stages of the drilling operation and proved useful for operations personnel. The use of real-time measurements demonstrated the characteristic hydraulic effects of pressure surges induced by drillstring rotation in the small slim-hole annulus under field conditions. The interest in this information is not restricted to the slim-hole geometry. Monitoring or estimating downhole pressure is a key element for drilling operations. Except in special cases, no real-time measurements of downhole annular pressure during drilling and tripping have been used on an operational basis. The h...
ABSTRACT Biogenic volatile organic compound (BVOCs) emissions play an important role in regional ... more ABSTRACT Biogenic volatile organic compound (BVOCs) emissions play an important role in regional air quality and global atmospheric chemistry. In addition, these natural VOC emissions serve important biological functions including attracting and repelling pollinators and herbivores. Some biological organisms use ambient air as a communication medium and the oxidation of these compounds brings about the concentration gradients sensed by insects and other organisms. Isoprene is the predominant BVOC emitted by vegetation and tropical forests are the dominant global source. This compound is very reactive in the atmosphere and contributes to the reactions that control tropospheric oxidant concentrations and thus the concentrations and lifetimes of longer-lived species. This paper presents a study on the seasonal variations in isoprene and some other significant BVOCs such as α-pinene, β-pinene, limonene, e-β ocimene and longifolene, measured at the Guyaflux Tower located in a wet tropical forest in Paracou French Guiana (5o16´54´´N, 52o54´44´´W), during the year of 2011, using the Relaxed Eddy Accumulation technique at approximately 20 meters high above the canopy. The results show a lower concentration of isoprene during the month of February and March which correspond to the wet season with an average of 0,545 μg/m3 and 0,341 μg/m3, respectively followed by a slight increase in middle April (still wet season) and a higher concentration later in mid-June. The same behavior was observed for α-pinene with higher concentrations for the same periods as isoprene however with a smaller increase. All the other compounds had concentrations below 1 μg/m3during the whole year. The monoterpene, e-β ocimene, was observed and is known as a stress compound but the vegetation at the site did not face any known severe stress condition such as excessive drought or flooding. Concerning the fluxes, the results showed that just a small amount of BVOCs were deposited by wet or dry process and the majority of them were released in the atmosphere. Isoprene was by far the biogenic volatile organic compound with the highest concentration and flux, followed by alpha-pinene. The lowest concentration and flux rate for all the studied compounds was observed during the months of March and late July and beginning of August indicating a lower production of those BVOCs by vegetation during those periods. Previous limited studies in Amazonia and the Congo suggested that a higher concentration and flux rate of isoprene and α-pinene should be expected during the dry season with lower emissions during the wet season, which is in relative agreement with what was observed at this tropical forest site in French Guiana. The exceptions were observed in April and June which correspond to a long wet period in which the concentration of isoprene and α-pinene increased more than it was expected for this time of the year. The observations will be compared to output from the global chemistry transport model CAM-chem, which includes the MEGAN biogenic emissions model.
The distribution and speciation of mercury (Hg) in air, rain, and surface waters from the artific... more The distribution and speciation of mercury (Hg) in air, rain, and surface waters from the artificial tropical lake of Petit-Saut in French Guiana were investigated during the 2003/04 period. In the air, total gaseous mercury (TGM) at the dam station averaged 12 2 pmol m-3 of which > 98% was gaseous elemental mercury (GEM). GEM distribution depicted a day-night cycling with high concentrations (up to 15 pmol m-3) at dawn and low concentrations (down to 5 pmol m-3) at nightfall. Reactive gaseous mercury (RGM) represented < 1 % of the GEM with a mean concentration of 4 3 fmol m-3. Diel RGM variations were negatively related to GEM. In the rain, the sum of all Hg species in the unfiltered (HgTUNF) averaged 16 12 pmol L-1. Temporal distribution of HgTUNF exhibited a pattern of high concentrations during the late dry seasons (up to 57.5 pmol L-1) and low concentrations (down to 2.7 pmol L-1) in the course of the wet seasons. Unfiltered reactive (HgRUNF), dissolved gaseous (DGM) and monomethyl (MMHgUNF) Hg constituted 20, 5 and 5 % of HgTUNF, respectively. All measured Hg species were positively related and displayed negative relationships with the pH of the rain. In the reservoir surface waters, dissolved total mercury (HgTD) averaged 3.4 1.2 pmol L-1 of which 10 % consisted of DGM. DGM showed a trend of high concentrations during the dry seasons (480 270 fmol L-1) and lower (230 130 fmol L-1) in the course of the wet seasons. Diel variations included diurnal photo-induced DGM production (of about 60 fmol L-1 h-1) coupled to minute to hour oxidation / reduction cycles (of > 100 fmol L-1 amplitude). Finally, calculated atmospheric Hg inputs to the Petit-Saut reservoir represented 14 moles yr-1 whereas DGM evasion reached 23 moles yr-1. Apportionment among forms of Hg deposition indicated that up to 75 % of the total Hg invasive flux follows the rainfall pathway.
studied trees relied on soil water below 100-cm depth during dry periods. Ψ pd was negatively cor... more studied trees relied on soil water below 100-cm depth during dry periods. Ψ pd was negatively correlated with depth of water extraction and can be taken as a rough proxy of this depth. Some trees showed considerable plasticity in their depth of water uptake, exhibiting an efficient adaptive strategy for water and nutrient resource acquisition. We did not find a strong relationship between tree dimensions and depth of water uptake. While tall trees preferentially extract water from layers below 100-cm depth, shorter trees show broad variations in mean depth of water uptake. This precludes the use of tree dimensions to parameterize functional models.
1] Methane (CH 4 ) and carbon dioxide (CO 2 ) concentrations and water-air fluxes were measured i... more 1] Methane (CH 4 ) and carbon dioxide (CO 2 ) concentrations and water-air fluxes were measured in three tropical reservoirs and their respective rivers downstream of the dams. From reservoirs, CH 4 and CO 2 flux were in the range of 3 ± 2 and 254 ± 392 mmol.m À2 .d À1 , respectively. Rivers downstream of dams were significantly enriched in CH 4 and CO 2 originating from reservoir hypolimnions. From rivers, CH 4 and CO 2 flux were in the range of 60 ± 38 and 859 ± 400 mmol.m À2 .d À1 , respectively. Despite their relatively small surfaces, rivers downstream of dams accounted for a significant fraction (9-33% for CH 4 and 7-25% for CO 2 ) of the emissions across the reservoir surfaces classically taken into account for reservoirs.
& Introduction Stem CO 2 efflux (E s ) is a significant component of total ecosystem respiration,... more & Introduction Stem CO 2 efflux (E s ) is a significant component of total ecosystem respiration, but there is only scant information on seasonal variations in E s in tropical rainforests and on the main factors explaining these variations. & Methods We conducted a comprehensive 18-month study in French Guiana to try to better understand which environmental factors contribute to seasonal variations in E s in two habitats differing in soil water conditions. & Results In both habitats, large seasonal variations in E s were observed for most trees. The main variations occurred during climatic transition periods and were consistent with seasonal variations in soil and total ecosystem respiration. & Discussion Seasonal variations in atmospheric conditions, including air temperature, did not explain seasonal variations in E s . In contrast, seasonal variations in surface soil water content clearly induced seasonal variations in E s in each habitat. Both soil drought and flooded conditions induced a decrease in E s . & Conclusion Our results emphasize the need to integrate the influence of soil water content on E s into global models that simulate the response of forest ecosystem fluxes to climate changes.
The relative contribution of gross primary production and ecosystem respiration to seasonal chang... more The relative contribution of gross primary production and ecosystem respiration to seasonal changes in the net carbon flux of tropical forests remains poorly quantified by both modelling and field studies. We use data assimilation to combine nine ecological time series from an eastern Amazonian forest, with mass balance constraints from an ecosystem carbon cycle model. The resulting analysis quantifies, with uncertainty estimates, the seasonal changes in the net carbon flux of a tropical rainforest which experiences a pronounced dry season. We show that the carbon accumulation in this forest was four times greater in the dry season than in the wet season and that this was accompanied by a 5% increase in the carbon use efficiency. This seasonal response was caused by a dry season increase in gross primary productivity, in response to radiation and a similar magnitude decrease in heterotrophic respiration, in response to drying soils. The analysis also predicts increased carbon allocation to leaves and wood in the wet season, and greater allocation to fine roots in the dry season. This study demonstrates implementation of seasonal variations in parameters better enables models to simulate observed patterns in data. In particular, we highlight the necessity to simulate the seasonal patterns of heterotrophic respiration to accurately simulate the net carbon flux seasonal tropical forest.
Recent advances in understanding the relationships between spectral reflectance of vegetation can... more Recent advances in understanding the relationships between spectral reflectance of vegetation canopies and the structural/physiological drivers of canopy-atmosphere net ecosystem carbon exchange highlight the potential for using narrowband spectral vegetation indices to spatially scale C fluxes beyond the area of a tower footprint. However, ground reference observations of narrowband spectral reflectance in support of satellite observations can be challenging to
Afin de mesurer en continu la respiration du sol, deux laboratoires de l'INRA ont développé d... more Afin de mesurer en continu la respiration du sol, deux laboratoires de l'INRA ont développé deux systèmes automatiques basés sur des designs et automatismes très similaires. Le choix de la mesure des échanges gazeux s'est porté sur les deux principes bien connus des méthodes dynamiques, à savoir, le système ouvert ou fermé (Field et al. 1989). Ces deux méthodes présentent chacune leurs avantages et leurs inconvénients et chaque équipe a fait ses choix en fonction du matériel disponible, des conditions environnementales de mesure, des budgets ou des techniques maîtrisées. La prise en compte poussée (calculs préalables, vérification a postériori) des problèmes potentiels de mesure rapportés dans la littérature a conduit à réaliser deux systèmes correctement dimensionnés.
Measurement-while-drilling of downhole pressure accurately determines annular pressure losses fro... more Measurement-while-drilling of downhole pressure accurately determines annular pressure losses from circulation and drillstring rotation and helps monitor swab and surge pressures during tripping. In early 1993, two slim-hole wells (3.4 in. and 3 in. diameter) were drilled with continuous real-time electromagnetic wave transmission of downhole temperature and annular pressure. The data were obtained during all stages of the drilling operation and proved useful for operations personnel. The use of real-time measurements demonstrated the characteristic hydraulic effects of pressure surges induced by drillstring rotation in the small slim-hole annulus under field conditions. The interest in this information is not restricted to the slim-hole geometry. Monitoring or estimating downhole pressure is a key element for drilling operations. Except in special cases, no real-time measurements of downhole annular pressure during drilling and tripping have been used on an operational basis. The h...
ABSTRACT Biogenic volatile organic compound (BVOCs) emissions play an important role in regional ... more ABSTRACT Biogenic volatile organic compound (BVOCs) emissions play an important role in regional air quality and global atmospheric chemistry. In addition, these natural VOC emissions serve important biological functions including attracting and repelling pollinators and herbivores. Some biological organisms use ambient air as a communication medium and the oxidation of these compounds brings about the concentration gradients sensed by insects and other organisms. Isoprene is the predominant BVOC emitted by vegetation and tropical forests are the dominant global source. This compound is very reactive in the atmosphere and contributes to the reactions that control tropospheric oxidant concentrations and thus the concentrations and lifetimes of longer-lived species. This paper presents a study on the seasonal variations in isoprene and some other significant BVOCs such as α-pinene, β-pinene, limonene, e-β ocimene and longifolene, measured at the Guyaflux Tower located in a wet tropical forest in Paracou French Guiana (5o16´54´´N, 52o54´44´´W), during the year of 2011, using the Relaxed Eddy Accumulation technique at approximately 20 meters high above the canopy. The results show a lower concentration of isoprene during the month of February and March which correspond to the wet season with an average of 0,545 μg/m3 and 0,341 μg/m3, respectively followed by a slight increase in middle April (still wet season) and a higher concentration later in mid-June. The same behavior was observed for α-pinene with higher concentrations for the same periods as isoprene however with a smaller increase. All the other compounds had concentrations below 1 μg/m3during the whole year. The monoterpene, e-β ocimene, was observed and is known as a stress compound but the vegetation at the site did not face any known severe stress condition such as excessive drought or flooding. Concerning the fluxes, the results showed that just a small amount of BVOCs were deposited by wet or dry process and the majority of them were released in the atmosphere. Isoprene was by far the biogenic volatile organic compound with the highest concentration and flux, followed by alpha-pinene. The lowest concentration and flux rate for all the studied compounds was observed during the months of March and late July and beginning of August indicating a lower production of those BVOCs by vegetation during those periods. Previous limited studies in Amazonia and the Congo suggested that a higher concentration and flux rate of isoprene and α-pinene should be expected during the dry season with lower emissions during the wet season, which is in relative agreement with what was observed at this tropical forest site in French Guiana. The exceptions were observed in April and June which correspond to a long wet period in which the concentration of isoprene and α-pinene increased more than it was expected for this time of the year. The observations will be compared to output from the global chemistry transport model CAM-chem, which includes the MEGAN biogenic emissions model.
The distribution and speciation of mercury (Hg) in air, rain, and surface waters from the artific... more The distribution and speciation of mercury (Hg) in air, rain, and surface waters from the artificial tropical lake of Petit-Saut in French Guiana were investigated during the 2003/04 period. In the air, total gaseous mercury (TGM) at the dam station averaged 12 2 pmol m-3 of which > 98% was gaseous elemental mercury (GEM). GEM distribution depicted a day-night cycling with high concentrations (up to 15 pmol m-3) at dawn and low concentrations (down to 5 pmol m-3) at nightfall. Reactive gaseous mercury (RGM) represented < 1 % of the GEM with a mean concentration of 4 3 fmol m-3. Diel RGM variations were negatively related to GEM. In the rain, the sum of all Hg species in the unfiltered (HgTUNF) averaged 16 12 pmol L-1. Temporal distribution of HgTUNF exhibited a pattern of high concentrations during the late dry seasons (up to 57.5 pmol L-1) and low concentrations (down to 2.7 pmol L-1) in the course of the wet seasons. Unfiltered reactive (HgRUNF), dissolved gaseous (DGM) and monomethyl (MMHgUNF) Hg constituted 20, 5 and 5 % of HgTUNF, respectively. All measured Hg species were positively related and displayed negative relationships with the pH of the rain. In the reservoir surface waters, dissolved total mercury (HgTD) averaged 3.4 1.2 pmol L-1 of which 10 % consisted of DGM. DGM showed a trend of high concentrations during the dry seasons (480 270 fmol L-1) and lower (230 130 fmol L-1) in the course of the wet seasons. Diel variations included diurnal photo-induced DGM production (of about 60 fmol L-1 h-1) coupled to minute to hour oxidation / reduction cycles (of > 100 fmol L-1 amplitude). Finally, calculated atmospheric Hg inputs to the Petit-Saut reservoir represented 14 moles yr-1 whereas DGM evasion reached 23 moles yr-1. Apportionment among forms of Hg deposition indicated that up to 75 % of the total Hg invasive flux follows the rainfall pathway.
studied trees relied on soil water below 100-cm depth during dry periods. Ψ pd was negatively cor... more studied trees relied on soil water below 100-cm depth during dry periods. Ψ pd was negatively correlated with depth of water extraction and can be taken as a rough proxy of this depth. Some trees showed considerable plasticity in their depth of water uptake, exhibiting an efficient adaptive strategy for water and nutrient resource acquisition. We did not find a strong relationship between tree dimensions and depth of water uptake. While tall trees preferentially extract water from layers below 100-cm depth, shorter trees show broad variations in mean depth of water uptake. This precludes the use of tree dimensions to parameterize functional models.
1] Methane (CH 4 ) and carbon dioxide (CO 2 ) concentrations and water-air fluxes were measured i... more 1] Methane (CH 4 ) and carbon dioxide (CO 2 ) concentrations and water-air fluxes were measured in three tropical reservoirs and their respective rivers downstream of the dams. From reservoirs, CH 4 and CO 2 flux were in the range of 3 ± 2 and 254 ± 392 mmol.m À2 .d À1 , respectively. Rivers downstream of dams were significantly enriched in CH 4 and CO 2 originating from reservoir hypolimnions. From rivers, CH 4 and CO 2 flux were in the range of 60 ± 38 and 859 ± 400 mmol.m À2 .d À1 , respectively. Despite their relatively small surfaces, rivers downstream of dams accounted for a significant fraction (9-33% for CH 4 and 7-25% for CO 2 ) of the emissions across the reservoir surfaces classically taken into account for reservoirs.
& Introduction Stem CO 2 efflux (E s ) is a significant component of total ecosystem respiration,... more & Introduction Stem CO 2 efflux (E s ) is a significant component of total ecosystem respiration, but there is only scant information on seasonal variations in E s in tropical rainforests and on the main factors explaining these variations. & Methods We conducted a comprehensive 18-month study in French Guiana to try to better understand which environmental factors contribute to seasonal variations in E s in two habitats differing in soil water conditions. & Results In both habitats, large seasonal variations in E s were observed for most trees. The main variations occurred during climatic transition periods and were consistent with seasonal variations in soil and total ecosystem respiration. & Discussion Seasonal variations in atmospheric conditions, including air temperature, did not explain seasonal variations in E s . In contrast, seasonal variations in surface soil water content clearly induced seasonal variations in E s in each habitat. Both soil drought and flooded conditions induced a decrease in E s . & Conclusion Our results emphasize the need to integrate the influence of soil water content on E s into global models that simulate the response of forest ecosystem fluxes to climate changes.
The relative contribution of gross primary production and ecosystem respiration to seasonal chang... more The relative contribution of gross primary production and ecosystem respiration to seasonal changes in the net carbon flux of tropical forests remains poorly quantified by both modelling and field studies. We use data assimilation to combine nine ecological time series from an eastern Amazonian forest, with mass balance constraints from an ecosystem carbon cycle model. The resulting analysis quantifies, with uncertainty estimates, the seasonal changes in the net carbon flux of a tropical rainforest which experiences a pronounced dry season. We show that the carbon accumulation in this forest was four times greater in the dry season than in the wet season and that this was accompanied by a 5% increase in the carbon use efficiency. This seasonal response was caused by a dry season increase in gross primary productivity, in response to radiation and a similar magnitude decrease in heterotrophic respiration, in response to drying soils. The analysis also predicts increased carbon allocation to leaves and wood in the wet season, and greater allocation to fine roots in the dry season. This study demonstrates implementation of seasonal variations in parameters better enables models to simulate observed patterns in data. In particular, we highlight the necessity to simulate the seasonal patterns of heterotrophic respiration to accurately simulate the net carbon flux seasonal tropical forest.
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