University of Avignon

About: University of Avignon is a education organization based out in Avignon, Provence-Alpes-Côte-d'Azur, France. It is known for research contribution in the topics: Population & Speaker recognition. The organization has 1526 authors who have published 3766 publications receiving 88928 citations.

The status of transitions between cultivated fields and their boundaries : ecotones, ecoclines or edge effects?

Abstract: Since the beginning of the 20th century, the concepts of ecotones, ecoclines and edge effects have been discussed from a theoretical point of view. However, there have been very few experimental tests of these ideas, which are sometimes radically different. This study presents data from field experimental researches and determines the status of transitions between cereal fields and grazed grasslands. Five study sites were chosen in Southern France because they were included in agri-environmental schemes aimed at conserving arable weeds and dry grassland species. In total, 128 quadrats of 1 m2 were sampled on replicated transects running through transition zones. There was no significant increase of species-richness but there were changes in the botanical composition from cereal fields to grassland. These experimental results confirmed the opinion of Van der Maarel, E. (1990. Ecotones and ecoclines are different. J. Veg. Sci. 1, 135–138) that an ecotone is poorer in species than the adjacent ecosystems, as only a few species can adapt to the typical environmental factors in this zone. The transition zones studied rather reflected an edge effect than a real ecotone following the definition of Odum, E.P. (1971. Fundamentals of Ecology, 3e ed. W.B. Saunders Company, Philadelphie). In our case, when the transition zone between the two adjacent ecosystems is managed as a “constraint ecotone” following Vanpeene-Bruhier's, S. (1998. Transformations des paysages et dynamique de la biodiversite vegetale. Les ecotones, un concept cle pour l'etude des vegetations post-culturales. L'exemple de la commune d'Aussois (Savoie). These de Doctorat de l'ENGREF; CEMAGREF de Grenoble) definition, sheep grazing allowed the weed flora to colonise grassland boundaries via the gaps created by livestock trampling. These results are then discussed for the biological conservation of threatened arable weeds in agricultural landscapes.

The SPARSE model for the prediction of water stress and evapotranspiration components from thermal infra-red data and its evaluation over irrigated and rainfed wheat

Abstract: . Evapotranspiration is an important component of the water cycle, especially in semi-arid lands. A way to quantify the spatial distribution of evapotranspiration and water stress from remote-sensing data is to exploit the available surface temperature as a signature of the surface energy balance. Remotely sensed energy balance models enable one to estimate stress levels and, in turn, the water status of continental surfaces. Dual-source models are particularly useful since they allow derivation of a rough estimate of the water stress of the vegetation instead of that of a soil–vegetation composite. They either assume that the soil and the vegetation interact almost independently with the atmosphere (patch approach corresponding to a parallel resistance scheme) or are tightly coupled (layer approach corresponding to a series resistance scheme). The water status of both sources is solved simultaneously from a single surface temperature observation based on a realistic underlying assumption which states that, in most cases, the vegetation is unstressed, and that if the vegetation is stressed, evaporation is negligible. In the latter case, if the vegetation stress is not properly accounted for, the resulting evaporation will decrease to unrealistic levels (negative fluxes) in order to maintain the same total surface temperature. This work assesses the retrieval performances of total and component evapotranspiration as well as surface and plant water stress levels by (1) proposing a new dual-source model named Soil Plant Atmosphere and Remote Sensing Evapotranspiration (SPARSE) in two versions (parallel and series resistance networks) based on the TSEB (Two-Source Energy Balance model, Norman et al., 1995) model rationale as well as state-of-the-art formulations of turbulent and radiative exchange, (2) challenging the limits of the underlying hypothesis for those two versions through a synthetic retrieval test and (3) testing the water stress retrievals (vegetation water stress and moisture-limited soil evaporation) against in situ data over contrasted test sites (irrigated and rainfed wheat). We demonstrated with those two data sets that the SPARSE series model is more robust to component stress retrieval for this cover type, that its performance increases by using bounding relationships based on potential conditions (root mean square error lowered by up to 11 W m−2 from values of the order of 50–80 W m−2), and that soil evaporation retrieval is generally consistent with an independent estimate from observed soil moisture evolution.

High-Performance Liquid Chromatographic Analysis of Triterpenoids in Commercial Frankincense

Abstract: A reversed-phase high-performance liquid chromatographic procedure has been developed: it is a simple and specific method for the determination of fifteen pentacyclic triterpenic compounds (a-boswellic acid, 3-O-acetyl-a-boswellic acid, b-boswellic acid, 3-O-acetyl-b-boswellic acid, a-amyrin, b-amyrin, lupeol, 3-epi-a-amyrin, 3-epi-b-amyrin, 3-epi-lupeol, a-amyrenone, b-amyrenone, lupenone, lupeolic acid and 3-O-acetyl-lupeolic acid) found in the most commonly traded frankincense, usually called ‘‘Eritrean-type’’ olibanum In addition, the chromatographic comparison between fresh commercial resins and botanically certified ones was described in order to determine the geographical and/or the botanical origins of commercial frankincense According to previous botanical studies, it appears difficult to make an unequivocal distinction between Boswellia carteri and B sacra On the other hand, Boswellia frereana (considered as a source of high-grade frankincense) shows a characteristic chromatogram and could be unambiguously distinguished from the other producing species of commercial frankincense In a chemical point of view, Boswellia carteri and B sacra were more especially characterized by the presence of lupeolic acid, boswellic acids and their respective O-acetyl derivatives, whereas 3-epi-lupeol was the major compound in B frereana methanolic extracts