Wildfire cumulative frequency-area distributions of Mediterranean landscapes are examined for agr... more Wildfire cumulative frequency-area distributions of Mediterranean landscapes are examined for agreement with self-similar (fractal) behavior. Our results support landscape-specific restricted scaling regions of 1.5-3.5 orders of magnitude in size, which are delimited by breakpoints or 'cut-offs'. By identifying the extent of such regions in the fractal frequency-area distribution of wildfires, fractal statistics may give a deeper insight into the scale-invariant dynamics of fire spread, whereas the observed cut-offs may be related to changes in the process-pattern interactions that control wildfire propagation at the landscape scale.
The effects of water limitations on the emission of biogenic volatile organic compounds are not w... more The effects of water limitations on the emission of biogenic volatile organic compounds are not well understood. Experimental approaches studying drought effects in natural conditions are still missing. To address this question, a throughfall displacement experiment was set up in a natural forest of Quercus ilex, an evergreen Mediterranean oak emitting monoterpenes. Mature trees were exposed in 2005 and 2006 either to an additional drought, to irrigation or to natural drought (untreated control). In both years, absolute monoterpene emission rates as well as the respective standard factors of the trees exposed to normal and additional drought strongly declined during the drought periods. Monoterpene emissions were lower in year 2006 than in year 2005 (factor 2) due to a more pronounced summer drought period in this respective year. We observed a significant difference between the irrigation and additional drought or control treatment: irrigated trees emitted 82% more monoterpenes during the drought period 2006 than the trees of the other treatments. However, no significant effect on monoterpene emission was observed between normal and additional drought treatments, despite a significant effect on leaf water potential and photochemical efficiency. During the development of drought, monoterpene emissions responded exponentially rather than linearly to decreasing leaf water potential. Emissions rapidly declined when the water potential dropped be-Correspondence to: A.-V. Lavoir (avlavoir@gmail.com) low −2 MPa and photosynthesis was persistently inhibited. Monoterpene synthase activities measured in vitro showed no clear reduction during the same period. From our results we conclude that drought significantly reduces monoterpene fluxes of Mediterranean Holm oak forest into the atmosphere due to a lack of primary substrates coming from photosynthetic processes.
International Journal of Remote Sensing, Apr 27, 2007
We have tested the following assumption for a spatially complex area of natural vegetation in Sou... more We have tested the following assumption for a spatially complex area of natural vegetation in Southern France. The spectral response of a mixed pixel is a linear combination of the individual responses of its components. The test has been done with HCMM data: each pixel (500m spatial resolution) has been characterized by its components defined as physiognomical vegetation units. Results from the analysis of multitemporal HCMM data indicate that the assumption is verified for albedos and surface temperatures.
Water availability is the main factor limiting ecosystem productivity in Mediterranean ecosystems... more Water availability is the main factor limiting ecosystem productivity in Mediterranean ecosystems, and is expected to increase as a result of climate change in the Mediterranean region. Hence, understanding and anticipating the effect of water limitation on those ecosystems is of most importance for future generations. The Puechabon experimental site, located in southern France, is representative of typical Mediterranean forests, dominated by Quercus ilex. This site has been monitored at levels from leaf to ecosystem since 1984, including eddy correlation measurements since 1998 and a partial throughfall interception experiment since 2003. With this contribution, we focused on developing, calibrating and validating a process-based model (CASTANEA, Dufrêne et al., 2005) on the Puechabon site, for simulating the effect of the throughfall interception on stand gross (GPP) and net (NPP) primary productivity. Model developments at the seasonal scale included detailed representations of leaf demography and wood growth seasonal dynamics, based on detailed monitoring of leaf litter (26-year time series) and cambium dynamics (7-year time series), respectively. New biomass compartments (flowers and fruits) were developed and implemented to the original vegetative carbon module, and helped insuring a realistic representation of woody vegetative pools dynamics (as fructification can require on a given year up to 15% NPP) along the 26-year growth time series. Once improved, the model allowed us to assess the influence of water stress on leaf demography (positive covariation of leaf lifespan and drought duration) and on carbon allocation at tree level (negative covariation of leaf area with drought), based on a comparison between the control site (where the model was developed and validated) and the 6-year partial throughfall exclusion. Keywords: Quercus ilex, process-based modeling, net primary production, carbon allocation, fructification, throughfall exclusion
We studied monoterpene emissions from the evergreen oak Quercus ilex exposed to different levels ... more We studied monoterpene emissions from the evergreen oak Quercus ilex exposed to different levels of summer drought to examine the seasonal variation of emissions in the Mediterranean area and to test the role of water limitations therein. Measurements were made in seven campaigns between June and January on intact leaves of mature trees growing in two adjacent sites, in one of which the natural water supply was reduced by a ground roof. In both sites, actual emission rates as well as light- and temperature-normalized emission rates (i.e., emission factor (EF)) significantly changed during the seasons: Mean EFs increased from June to July to a broad summer maximum between 5.2 and 9.4 nmol m-2 leaf area s-1 (12-21 μg g-1 leaf dry mass h-1), dropped in October and November and reached a minimum of about 0.77 nmol m-2 s-1 (1.7 μg g-1 h-1) in January. From June to July, mean EFs of the trees with reduced water supply were significantly lower than those of trees with normal water supply. The lowered EF in summer was paralleled by lower predawn water potentials, leaf transpiration, photosynthesis and stomatal conductance with respect to control trees. The results suggest that in natural Q. ilex habitats the seasonal evolution of EF follows a marked summer-winter cycle whose shape and intensity can be modified by summer drought.
Xleasurepents carried out between 1971 and 1977 indicate the effect of rainfall on above-ground p... more Xleasurepents carried out between 1971 and 1977 indicate the effect of rainfall on above-ground plant production. The site investigated consists of steppeland on sandy soil, dominated by the perennial dwarf shrub Rhantherium suaz*eolens with associated annual species. The average precipitation in this region (180 mm), produces about 1000 kg dry matter/ha. The ARFEJ model described, uses the most pertinent characteristics of the system so that production can be predicted under the assumption that water is the limiting factor. Seven model coefficients were calculated with the data o f 1971-75. The results show (a) the considerable effect of rainfall distribution on evaporation from bare soil. The mean of this evaporation was 55 per cent (range4C69 per cent) of total evapotranspiration; (b) during the short annual period of fast growth, the ratio of dry matter to water transpired would be'about 1.26 g dry matter!kg H,O for annuals, and 1.39 g dry matterikg H,O for perennial species. Over the whole year, the acerage efficiency of water use is 0.45 g dry matterlkg H 2 0 for all species; (c) above 250 mm precipitation, the availability of nitrogen andjor phosphorus becomes the limiting factor for plant production and efficiency of water use decreases.
Les transferts d'eau dans le système sol-plante sont des éléments importants du fonctionnement de... more Les transferts d'eau dans le système sol-plante sont des éléments importants du fonctionnement des phytocénoses, notamment en zone aride où le facteur hydrique limite la production végétale. L'extrême variabilité des phénomènes climatiques, et notamment des précipitations dans les zones arides, font que les mesures du bilan hydrique des sols, réalisées sur des périodes nécessairement courtes, ne sont que quelques situations possibles dont la représentativité peut être mise en doute. Par contre, il existe d'assez longues séries d'observations météorologiques, d'où la recherche d'un modèle simple de bilan hydrique du sol, qui permettrait, après réelages sur les années observées, la simulations de celui-ci à partir des éléments climatiques et des caractéristiques des sols. On obtiendrait donc autant d'estimations que d'années d'observation des données météorologiques, avec la possibilité d'effectuer sur ces résultats une analyse fréquentielle. La station d'étude se situe dans le Centre de Recherches zootechniques de DAHRA, dépendant de l'Institut Sénégalais de Recherches Agronomiques (I.S.R.A.). Ce centre situé en zone sahélienne, à la limite sud du FERLO-sableux, couvre une superficie de 6 800 ha. Le climat est typiquement sahélien, chaud et sec. Les précipitations sont faibles, la moyenne sur 38 années~tde 492 mm avec une grande variabilitL inter annuelle (coefficient de variation 29,7 %). Elles sont concentrées en une brève période humide de 3 mois. Aout est le mois le plus arrosé avec 35,7 % des pluies annuelles, septembre 27,2 % et juillet 21,1 ::.
Measurements of total nitrogen and inorganic nitrogen in precipitation samples from NitroEurope s... more Measurements of total nitrogen and inorganic nitrogen in precipitation samples from NitroEurope sites across Europe permit the calculation of organic nitrogen concentrations and wet deposition, by difference. The contribution of organic N to total N in precipitation ranged from only a few % to around 40 % across 18 sites from northern Finland to Italy, similar to results from previous individual studies. This paper presents the absolute and relative contributions of organic N to wet N deposition across Europe, and examines seasonal trends. There were only weak correlations with other solutes in precipitation. These simple statistics indicate that sources of organic N in precipitation vary across Europe, and that no single source is responsible. The organic N contributes to total N deposition, yet this input is rarely quantified in nitrogen budgets.
Measurements were made of the actual évapotranspiration, root extraction and aboveground yield in... more Measurements were made of the actual évapotranspiration, root extraction and aboveground yield in the case of two varieties of lucerne Medicago sativa L. -Du Puits and Polderand mixtures of them in equal parts. The daily kinetics of the water potential of the leaves and the collar show that the water regimes of the two varieties are identical in a first approximation. The root systems are different. Polder has the deeper system. The regime of the binary mixture is not simply a juxtaposition of the regimes of the constituents. Two types of non-linearity were found. The first relates to water flow and the second to productivity. It is postulated that the root systems of the two varieties constituting the mixture undergo plastic deformation, leading to a more efficient resultant system. A system of this type needs, on the other hand, an allocation of photosynthates that would affect its productivity.
Wildfire cumulative frequency-area distributions of Mediterranean landscapes are examined for agr... more Wildfire cumulative frequency-area distributions of Mediterranean landscapes are examined for agreement with self-similar (fractal) behavior. Our results support landscape-specific restricted scaling regions of 1.5-3.5 orders of magnitude in size, which are delimited by breakpoints or 'cut-offs'. By identifying the extent of such regions in the fractal frequency-area distribution of wildfires, fractal statistics may give a deeper insight into the scale-invariant dynamics of fire spread, whereas the observed cut-offs may be related to changes in the process-pattern interactions that control wildfire propagation at the landscape scale.
The effects of water limitations on the emission of biogenic volatile organic compounds are not w... more The effects of water limitations on the emission of biogenic volatile organic compounds are not well understood. Experimental approaches studying drought effects in natural conditions are still missing. To address this question, a throughfall displacement experiment was set up in a natural forest of Quercus ilex, an evergreen Mediterranean oak emitting monoterpenes. Mature trees were exposed in 2005 and 2006 either to an additional drought, to irrigation or to natural drought (untreated control). In both years, absolute monoterpene emission rates as well as the respective standard factors of the trees exposed to normal and additional drought strongly declined during the drought periods. Monoterpene emissions were lower in year 2006 than in year 2005 (factor 2) due to a more pronounced summer drought period in this respective year. We observed a significant difference between the irrigation and additional drought or control treatment: irrigated trees emitted 82% more monoterpenes during the drought period 2006 than the trees of the other treatments. However, no significant effect on monoterpene emission was observed between normal and additional drought treatments, despite a significant effect on leaf water potential and photochemical efficiency. During the development of drought, monoterpene emissions responded exponentially rather than linearly to decreasing leaf water potential. Emissions rapidly declined when the water potential dropped be-Correspondence to: A.-V. Lavoir (avlavoir@gmail.com) low −2 MPa and photosynthesis was persistently inhibited. Monoterpene synthase activities measured in vitro showed no clear reduction during the same period. From our results we conclude that drought significantly reduces monoterpene fluxes of Mediterranean Holm oak forest into the atmosphere due to a lack of primary substrates coming from photosynthetic processes.
International Journal of Remote Sensing, Apr 27, 2007
We have tested the following assumption for a spatially complex area of natural vegetation in Sou... more We have tested the following assumption for a spatially complex area of natural vegetation in Southern France. The spectral response of a mixed pixel is a linear combination of the individual responses of its components. The test has been done with HCMM data: each pixel (500m spatial resolution) has been characterized by its components defined as physiognomical vegetation units. Results from the analysis of multitemporal HCMM data indicate that the assumption is verified for albedos and surface temperatures.
Water availability is the main factor limiting ecosystem productivity in Mediterranean ecosystems... more Water availability is the main factor limiting ecosystem productivity in Mediterranean ecosystems, and is expected to increase as a result of climate change in the Mediterranean region. Hence, understanding and anticipating the effect of water limitation on those ecosystems is of most importance for future generations. The Puechabon experimental site, located in southern France, is representative of typical Mediterranean forests, dominated by Quercus ilex. This site has been monitored at levels from leaf to ecosystem since 1984, including eddy correlation measurements since 1998 and a partial throughfall interception experiment since 2003. With this contribution, we focused on developing, calibrating and validating a process-based model (CASTANEA, Dufrêne et al., 2005) on the Puechabon site, for simulating the effect of the throughfall interception on stand gross (GPP) and net (NPP) primary productivity. Model developments at the seasonal scale included detailed representations of leaf demography and wood growth seasonal dynamics, based on detailed monitoring of leaf litter (26-year time series) and cambium dynamics (7-year time series), respectively. New biomass compartments (flowers and fruits) were developed and implemented to the original vegetative carbon module, and helped insuring a realistic representation of woody vegetative pools dynamics (as fructification can require on a given year up to 15% NPP) along the 26-year growth time series. Once improved, the model allowed us to assess the influence of water stress on leaf demography (positive covariation of leaf lifespan and drought duration) and on carbon allocation at tree level (negative covariation of leaf area with drought), based on a comparison between the control site (where the model was developed and validated) and the 6-year partial throughfall exclusion. Keywords: Quercus ilex, process-based modeling, net primary production, carbon allocation, fructification, throughfall exclusion
We studied monoterpene emissions from the evergreen oak Quercus ilex exposed to different levels ... more We studied monoterpene emissions from the evergreen oak Quercus ilex exposed to different levels of summer drought to examine the seasonal variation of emissions in the Mediterranean area and to test the role of water limitations therein. Measurements were made in seven campaigns between June and January on intact leaves of mature trees growing in two adjacent sites, in one of which the natural water supply was reduced by a ground roof. In both sites, actual emission rates as well as light- and temperature-normalized emission rates (i.e., emission factor (EF)) significantly changed during the seasons: Mean EFs increased from June to July to a broad summer maximum between 5.2 and 9.4 nmol m-2 leaf area s-1 (12-21 μg g-1 leaf dry mass h-1), dropped in October and November and reached a minimum of about 0.77 nmol m-2 s-1 (1.7 μg g-1 h-1) in January. From June to July, mean EFs of the trees with reduced water supply were significantly lower than those of trees with normal water supply. The lowered EF in summer was paralleled by lower predawn water potentials, leaf transpiration, photosynthesis and stomatal conductance with respect to control trees. The results suggest that in natural Q. ilex habitats the seasonal evolution of EF follows a marked summer-winter cycle whose shape and intensity can be modified by summer drought.
Xleasurepents carried out between 1971 and 1977 indicate the effect of rainfall on above-ground p... more Xleasurepents carried out between 1971 and 1977 indicate the effect of rainfall on above-ground plant production. The site investigated consists of steppeland on sandy soil, dominated by the perennial dwarf shrub Rhantherium suaz*eolens with associated annual species. The average precipitation in this region (180 mm), produces about 1000 kg dry matter/ha. The ARFEJ model described, uses the most pertinent characteristics of the system so that production can be predicted under the assumption that water is the limiting factor. Seven model coefficients were calculated with the data o f 1971-75. The results show (a) the considerable effect of rainfall distribution on evaporation from bare soil. The mean of this evaporation was 55 per cent (range4C69 per cent) of total evapotranspiration; (b) during the short annual period of fast growth, the ratio of dry matter to water transpired would be'about 1.26 g dry matter!kg H,O for annuals, and 1.39 g dry matterikg H,O for perennial species. Over the whole year, the acerage efficiency of water use is 0.45 g dry matterlkg H 2 0 for all species; (c) above 250 mm precipitation, the availability of nitrogen andjor phosphorus becomes the limiting factor for plant production and efficiency of water use decreases.
Les transferts d'eau dans le système sol-plante sont des éléments importants du fonctionnement de... more Les transferts d'eau dans le système sol-plante sont des éléments importants du fonctionnement des phytocénoses, notamment en zone aride où le facteur hydrique limite la production végétale. L'extrême variabilité des phénomènes climatiques, et notamment des précipitations dans les zones arides, font que les mesures du bilan hydrique des sols, réalisées sur des périodes nécessairement courtes, ne sont que quelques situations possibles dont la représentativité peut être mise en doute. Par contre, il existe d'assez longues séries d'observations météorologiques, d'où la recherche d'un modèle simple de bilan hydrique du sol, qui permettrait, après réelages sur les années observées, la simulations de celui-ci à partir des éléments climatiques et des caractéristiques des sols. On obtiendrait donc autant d'estimations que d'années d'observation des données météorologiques, avec la possibilité d'effectuer sur ces résultats une analyse fréquentielle. La station d'étude se situe dans le Centre de Recherches zootechniques de DAHRA, dépendant de l'Institut Sénégalais de Recherches Agronomiques (I.S.R.A.). Ce centre situé en zone sahélienne, à la limite sud du FERLO-sableux, couvre une superficie de 6 800 ha. Le climat est typiquement sahélien, chaud et sec. Les précipitations sont faibles, la moyenne sur 38 années~tde 492 mm avec une grande variabilitL inter annuelle (coefficient de variation 29,7 %). Elles sont concentrées en une brève période humide de 3 mois. Aout est le mois le plus arrosé avec 35,7 % des pluies annuelles, septembre 27,2 % et juillet 21,1 ::.
Measurements of total nitrogen and inorganic nitrogen in precipitation samples from NitroEurope s... more Measurements of total nitrogen and inorganic nitrogen in precipitation samples from NitroEurope sites across Europe permit the calculation of organic nitrogen concentrations and wet deposition, by difference. The contribution of organic N to total N in precipitation ranged from only a few % to around 40 % across 18 sites from northern Finland to Italy, similar to results from previous individual studies. This paper presents the absolute and relative contributions of organic N to wet N deposition across Europe, and examines seasonal trends. There were only weak correlations with other solutes in precipitation. These simple statistics indicate that sources of organic N in precipitation vary across Europe, and that no single source is responsible. The organic N contributes to total N deposition, yet this input is rarely quantified in nitrogen budgets.
Measurements were made of the actual évapotranspiration, root extraction and aboveground yield in... more Measurements were made of the actual évapotranspiration, root extraction and aboveground yield in the case of two varieties of lucerne Medicago sativa L. -Du Puits and Polderand mixtures of them in equal parts. The daily kinetics of the water potential of the leaves and the collar show that the water regimes of the two varieties are identical in a first approximation. The root systems are different. Polder has the deeper system. The regime of the binary mixture is not simply a juxtaposition of the regimes of the constituents. Two types of non-linearity were found. The first relates to water flow and the second to productivity. It is postulated that the root systems of the two varieties constituting the mixture undergo plastic deformation, leading to a more efficient resultant system. A system of this type needs, on the other hand, an allocation of photosynthates that would affect its productivity.
The effects of climate changes on carbon and water fluxes are quantified using a physiologically ... more The effects of climate changes on carbon and water fluxes are quantified using a physiologically multi-layer, process-based model containing a carbon allocation model and coupled with a soil model (CASTANEA). The model is first evaluated on four EUROFLUX sites using eddy covariance data, which provide estimates of carbon and water fluxes at the ecosystem scale. It correctly reproduces the diurnal fluxes and the seasonal pattern. Thereafter simulations were conducted on six French forest ecosystems representative of three climatic areas (oceanic, continental and Mediterranean areas) dominated by deciduous species (Fagus sylvatica, Quercus robur), coniferous species (Pinus pinaster, Pinus sylvestris) or sclerophyllous evergreen species (Quercus ilex). The model is driven by the results of a meteorological model (ARPEGE) following the B2 scenario of IPCC. From 1960 to 2100, the average temperature increases by 3.1 °C (30%) and the rainfall during summer decreases by 68 mm (−27%). For all the sites, between the two periods, the simulations predict on average a gross primary production (GPP) increase of 513 g(C) m−2 (+38%). This increase is relatively steep until 2020, followed by a slowing down of the GPP rise due to an increase of the effect of water stress. Contrary to GPP, the ecosystem respiration (Reco) raises at a constant rate (350 g(C) m−2 i.e. 31% from 1960 to 2100). The dynamics of the net ecosystem productivity (GPP minus Reco) is the consequence of the effect on both GPP and Reco and differs per site. The ecosystems always remain carbon sinks; however the sink strength globally decreases for coniferous (−8%), increases for sclerophyllous evergreen (+34%) and strongly increases for deciduous forest (+67%) that largely benefits by the lengthening of the foliated period. The separately quantified effects of the main variables (temperature, length of foliated season, CO2 fertilization, drought effect), show that the magnitude of these effects depends on the species and the climatic zone.
Uploads
Papers by Serge Rambal