An explanation for the isotopic offset between soil and stem water in a temperate tree species

A growing number of field studies report isotopic offsets between stem water and its potential sources that prevent the unambiguous identification of plant water origin using water isotopes. We explored the causes of this isotopic offset by conducting a controlled experiment on the temperate tree species Fagus sylvatica. We measured d2H and d18O of soil and stem water from potted saplings growing on three soil substrates and subjected to two watering regimes. Regardless of substrate, soil and stem water d2H were similar only near permanent wilting point. Under moister conditions, stem water d2H was 11 ± 3 more negative than soil water d2H, coherent with field studies. Under drier conditions, stem water d2H became progressively more enriched than soil water d2H. Although stem water d18O broadly reflected that of soil water, soil stem d2H and d18O differences were correlated (r = 0.76) and increased with transpiration rates indicated by proxies. Soil stem isotopic offsets are more likely to be caused by water isotope heterogeneities within the soil pore and stem tissues, which would be masked under drier conditions as a result of evaporative enrichment, than by fractionation under root water uptake. Our results challenge our current understanding of isotopic signals in the soil plant continuum. © 2020 The Authors. New Phytologist © 2020 New Phytologist TrustThis work was supported by the French national programme EC2CO-Biohefect (RootWater), the French national research agency (projects Hydrobeech, Climbeech and Micromic within the Cluster of Excellence COTE with grant agreement ANR-10-LABX-45; project ORCA with grant agreement ANR-13-BS06-0005-01), the European Research Council (ERC) under the EU Seventh Framework Program (FP7/2007-2013, with grant agreement no. 338264, awarded to LW) and the Aquitaine Region (project Athene with grant agreement 2016-1R20301-00007218). AB also acknowledges an IdEx Bordeaux postdoctoral fellowship from the Universite de Bordeaux (contract no. 22001162)

Exacerbated fires in Mediterranean Europe due to anthropogenic warming projected with non-stationary climate-fire models

The observed trend towards warmer and drier conditions in southern Europe is projected to continue in the next decades, possibly leading to increased risk of large fires. However, an assessment of climate change impacts on fires at and above the 1.5 °C Paris target is still missing. Here, we estimate future summer burned area in Mediterranean Europe under 1.5, 2, and 3 °C global warming scenarios, accounting for possible modifications of climate-fire relationships under changed climatic conditions owing to productivity alterations. We found that such modifications could be beneficial, roughly halving the fire-intensifying signals. In any case, the burned area is robustly projected to increase. The higher the warming level is, the larger is the increase of burned area, ranging from ~40% to ~100% across the scenarios. Our results indicate that significant benefits would be obtained if warming were limited to well below 2 °C

La couverture forestière et la saisonnalité des précipitations contrôlent l'inflammabilité du sous-bois dans les forêts montagnardes des Alpes

International audienceLittle is known about the understorey flammability of European mountain forests. The aim of this study was to determine the relative effects of climate, vegetation structure and composition on the fuel-driven variation in fire spread and intensity. Fire spread and intensity were simulated under constant moisture and weather conditions for a wide range of understorey fuel parameters measured in the litter, grass and shrub layers. Simulation outputs were used to compare understorey flammability between different forest ecosystem types (FET). The FETs were characterized by using a co-inertia analysis between composition and the environment (vegetation structure and climate). The relationships between these factors, fuel properties and understorey flammability were then tested using partial regression analyses.Results The most flammable forests displayed an open canopy (dry-subalpine and open-mediterranean) and grew in areas with dry autumns and wet and cold springs. Fire spread and intensity were controlled by the trade-off between tree cover and dead (litter) and live (grass and shrub) biomass load. Fire intensity also increased as a result of seasonal precipitation patterns (differential distribution between the seasons): rainy springs enhanced biomass growth, whereas dry climates, especially in autumn, promoted shrub biomass and stimulated litter accumulation and residence (higher litterfall and lower decomposition rates). Interestingly, we found a positive relationship between fire intensity and the proportion of conifers that disappeared after accounting for tree cover, indicating that, in the Alps, the open canopy structure of needle-leaved forests makes them potentially more flammable than broad-leaved forests because of the higher load and continuity of surface fuels. Main conclusions Inter-relationships between tree cover, precipitation seasonality and species composition govern the understorey flammability of mountain forests. We also found evidence that tree cover strongly constrains fire spread by driving the amount and type of surface fuel, which suggests that land use change can have a strong influence on flammability patterns .On connaît peu l'inflammabilité du sous-étage dans les forêts de montagne européennes. Le but de cette étude était de déterminer les effets relatifs du climat, de la structure de la végétation et de la composition du combustible sur la propagation et l'intensité du feu. La propagation et l'intensité des incendies ont été simulés sous humidité constante et les conditions météorologiques pour une large gamme de paramètres de combustible (litière, herbe et arbustes). Les sorties de simulation ont été utilisés pour comparer entre les sous-bois d'inflammabilité types d'écosystèmes forestiers différents (FET). Les FET ont été caractérisés en utilisant une analyse de co-inertie entre la composition et l'environnement (structure de la végétation et le climat). Les relations entre ces facteurs, les propriétés du carburant et sous-bois inflammabilité ont ensuite été testées en utilisant des analyses de régression partielle. Les forêts les plus inflammables affichent un sous-bois clair et poussent dans les zones avec des automnes secs et humides et des printemps frais. La propagation et l'intensité des incendies ont été contrôlés par le compromis entre couvert d'arbres et de morts (litière) et en direct (herbe et arbuste) charge de la biomasse. L'intensité du feu a augmenté en raison de la configuration des précipitations saisonnières (répartition différentielle entre les saisons): printemps pluvieux amélioré la croissance de la biomasse, tandis que les climats secs, surtout en automne, favorisent la biomasse d'arbustes, l'accumulation de la litière et sont temps de résidence (litière plus élevée et la décomposition inférieure les taux). Nous avons trouvé une relation positive entre l'intensité du feu et la proportion de conifères qui a disparu après comptabilisation de la couverture forestière, ce qui indique que, dans les Alpes, la structure des forêts conifères les rend potentiellement plus inflammables que les forêts feuillues en raison de la charge plus élevée et la continuité des combustibles de surface. Les inter-relations entre le couvert forestier, les précipitations saisonnières et la composition des espèces régissent l'inflammabilité du sous-bois des forêts de montagne. Nous avons également constaté que la couverture forestière contrôle fortement le feu par la quantité et le type de combustible de surface, ce qui donne à penser que l'utilisation des terres peut avoir une forte influence sur les modèles d'inflammabilité

Caractérisation de pyrorégions dans le sud-est de la France

Efficient fire policies may rely on good knowledge of the regional variations of fire activity and of fire drivers.South-eastern France comprises a range of pyroclimates, i.e. regions with contrasted fire activity (from fireprone Mediterranean areas to mountain areas with few fires) and contrasted climate and fire weather. We testedif these pyroclimates also corresponded to specific hierarchy among environmental and human variables which drive fire activity. We used a 1973-2009 georeferenced fire database, and we computed how the landscapecompartmentalization, the fuel coverage, the human density and the fire suppression capacity varied at a 2x2 km scale. The first pyroregion regroups two maritime fire-prone mountains (Corsica and the maritime Alps) inwhich there are no clear limitation to fire activity because of high human activity (i.e. numerous ignitions), no fuel limitation, and no weather limitation. The area is especially fire-prone because the suppression capacity is low to medium, and because the compartmented landscape hinders the activity of firemen. In the second pyroregion (fire-prone Mediterranean plains and foothills), fire activity is neither weather-limited (especially during dry summers) nor fuel-limited. It is clearly controlled by fire suppression which is especially active. In the third pyroregion (cool peri-Mediterranean mountains), fire activity remains low in spite of low fire suppression capacity and high landscape compartmentalization, because human activity is low and fire weather is unfavourable on average. We discuss to which extent the present fire suppression strategy (i.e. fast, hard-hitting initial attack on all fires) is adapted to these different pyroregions. In the fire-prone maritime mountains, it would be useful to increase the fire suppression forces. In fire-prone plains and foothills, the current plan of action is well suited, but large and destructive fire may persist due to the climate change and the fuelaccumulation. In mountainous areas with low fire activity, fire suppression forces will likely have to adapt to the forecasted increase of fire activity

Combined Ring Inversion and Side Group Rotation in Geminal Diphosphoryl Substituted Pyrrolinoxyl Radicals: ESR Analysis of Chemical Exchange between Four Nonequivalent Sites

The ESR spectra of three diphosphorylated pyrrolidinoxyl radicals have been studied over a large temperature range. While for the trans 2,5-disubstituted compound 1 no line width alternation was found, for the 2,2-disubstituted compounds 2 and 3, dramatic changes in the spectra were observed as a function of the temperature. These changes were explained by a four-site chemical exchange model including both ring inversion and hindered rotation around the carbon-phosphorus bonds. For radical 3, the presence of additional 5,5-dimethyl substitution can completely block rotations around carbon-phosphorus bonds for certain ring geometries, while for other ring conformations chemical exchange still occurs through combined inversionrotation processes. An effective two-site model composed of nonequivalent sites and a superposition model composed of a pair of exchanging conformers and a pair of nonexchanging conformers were used to simulate spectrum variations. A satisfactory fit was obtained over the entire temperature range investigated. From the temperature dependence of the exchange frequencies the potential barriers for ring inversion (27 kJ/mol), for combined inversion-rotation (11 kJ/mol), and for rotation around the carbon-phosphorus bonds (14 kJ/mol) were estimated

An explanation for the isotopic offset between soil and stem water in a temperate tree species

A growing number of field studies report isotopic offsets between stem water and its potential sources that prevent the unambiguous identification of plant water origin using water isotopes. We explored the causes of this isotopic offset by conducting a controlled experiment on the temperate tree species Fagus sylvatica. We measured d2H and d18O of soil and stem water from potted saplings growing on three soil substrates and subjected to two watering regimes. Regardless of substrate, soil and stem water d2H were similar only near permanent wilting point. Under moister conditions, stem water d2H was 11 ± 3 more negative than soil water d2H, coherent with field studies. Under drier conditions, stem water d2H became progressively more enriched than soil water d2H. Although stem water d18O broadly reflected that of soil water, soil stem d2H and d18O differences were correlated (r = 0.76) and increased with transpiration rates indicated by proxies. Soil stem isotopic offsets are more likely to be caused by water isotope heterogeneities within the soil pore and stem tissues, which would be masked under drier conditions as a result of evaporative enrichment, than by fractionation under root water uptake. Our results challenge our current understanding of isotopic signals in the soil plant continuum. © 2020 The Authors. New Phytologist © 2020 New Phytologist TrustThis work was supported by the French national programme EC2CO-Biohefect (RootWater), the French national research agency (projects Hydrobeech, Climbeech and Micromic within the Cluster of Excellence COTE with grant agreement ANR-10-LABX-45; project ORCA with grant agreement ANR-13-BS06-0005-01), the European Research Council (ERC) under the EU Seventh Framework Program (FP7/2007-2013, with grant agreement no. 338264, awarded to LW) and the Aquitaine Region (project Athene with grant agreement 2016-1R20301-00007218). AB also acknowledges an IdEx Bordeaux postdoctoral fellowship from the Universite de Bordeaux (contract no. 22001162)

Genèse médiévale de l'anthroponymie moderne. Tome II-1 : Persistances du nom unique

En 1986-87, un groupe de médiévistes français s'est attaché à étudier, dans les cartulaires, quand et comment s'est élaboré le système anthroponymique moderne, prénom et patronyme, au cours des xie et xiie siècles. Après cette première phase centrée sur les points communs et les nuances régionales de cette évolution, l'enquête s'est élargie, en 1989 et 1990, aux autres pays européens qui feront l'objet de publications ultérieures et aux « résistances », dans l'espace français, à cette double dénomination. Une région résistante : la Bretagne, un mode de désignation spécifique, pour les clercs par leur fonction, pour les femmes par une relation familiale : ce sont les modalités de ces évolutions différentes qui font l'objet du volume II des Rencontres d'Azay-le-Ferron, publié aujourd'hui en deux tomes, par le même groupe de chercheurs. Le même principe, de comparaison régionale fondée sur un questionnement identique des sources, y est appliqué. Les « prénoms » des clercs sont-ils différents de ceux des laïcs ? Les femmes sont-elles identifiées par la famille où elles sont nées ou par celle à laquelle elles se sont alliées par mariage ? L'analyse de ces usages spécifiques permet de comprendre comment fonctionne, entre la norme et le concret, la désignation écrite des individus entre le xie et le xive siècle

Shelving spectroscopy of the strontium intercombination line

International audienceWe present a spectroscopy scheme for the 7-kHz-wide 689-nm intercombination line of strontium. We rely on shelving detection, where electrons are first excited to a metastable state by the spec-troscopy laser before their state is probed using the broad transition at 461 nm. As in the similar setting of calcium beam clocks, this enhances dramatically the signal strength as compared to direct saturated fluorescence or absorption spectroscopy of the narrow line. We implement shelving spectroscopy both in directed atomic beams and hot vapor cells with isotropic atomic velocities. We measure a fractional frequency instability ∼ 2 × 10 −12 at 1 s limited by technical noise-about one order of magnitude above shot noise limitations for our experimental parameters. Our work illustrates the robustness and flexibility of a scheme that can be very easily implemented in the reference cells or ovens of most existing strontium experiments, and may find applications for low-complexity clocks