Change in hydraulic traits of mediterranean Quercus ilex subjected to long-term throughfall exclusion

International audienceMediterranean tree species experience unpredictable climate environments and severe summer droughts and they may be impaired by the trend of decline in precipitation projected as a consequence of global climate change. The response of Quercus ilex to drought was studied by measuring hydraulic traits of trees growing in a mature forest subjected to partial throughfall exclusion for 6 years. We measured hydraulic conductivity, xylem vulnerability to embolism, and anatomical features in branches and roots. Xylem vulnerability to embolism was higher in the dry treatment than in the control treatment, P50 of branches was on average –3.88 ± 0.80 MPa for the control treatment compared with –3.41 ± 0.80 MPa for the dry treatment, but the difference was not statistically significant. A similar difference between treatments was observed for roots, which exhibited lower P50 values. This change of xylem vulnerability to embolism was not linked to modification of the hydraulic conductivity or vessel anatomy, which remained unaffected by the throughfall exclusion treatment. The xylem density of branches was lower in the dry treatment. The hydraulic conductivity was correlated with the mean vessel diameter of xylem, but the P50 was not. The main response of trees from the dry treatment to reduced water availability appeared to be a reduction in the transpiring leaf area, which resulted in significantly increased leaf-specific conductivit

Des moulins hydrauliques en périphérie d’Augustonemetum/Clermont-Ferrand, Puy-de-Dôme (IIe-IIIe s. ap. J.-C.)

International audienc

Étude d’un marqueur chimiotaxonomique capable d’identifier le genre Aquilaria (Thymelaeaceae)

The genus Aquilaria Lam. (Thymelaeaceae) comprises 21 tree species (The Plant List, 2013) and is mostly found in Southeast Asia. When the tree is infected (fungi, bacteria), its wood turns brownish or blackish (then called agarwood) due to the secretion of an oleoresin as a reaction to the infection. The resin is very fragrant and has been sought after and used for centuries by Buddhists, Hindus and Muslims to make incense for religious ceremonies. This oleoresin is mainly found in species of the genus Aquilaria, but also in a few species of the genera Gyrinops Gaertner and Gonystylus Teijsmann & Binnendijk. It is difficult to distinguish between these species, and this lack of taxonomic knowledge has led to over-use of the trees and endangering these endemic species listed in CITES Appendix II. We used chemotaxonomy as a discrimination tool to analyse polyphenolic molecules, which are secondary metabolites and known to act as taxonomic markers in other plants. Using a simple, effective and inexpensive analysis technique (two-dimensional thin-layer chromatography), we found the same polyphenolic molecule in the six species of the genus Aquilaria we studied, which was identified as mangiferin by chromatographic and colorimetric analysis in comparison with a mangiferin control molecule. During these analyses, we found a relationship between the genera Aquilaria and Gyrinops (botanically demonstrated) in the occasional existence of mangiferin in three species of the genus Gyrinops. We also found that a time laps of 140 years between two herbarium samples of the same species had virtually no effect on the mangiferin concentration in the plant. This study showed the merits of chemotaxonomy in seeking taxonomic markers, and the originality of colorimetric analysis of a thin-layer chromatographed plant metabolite.Le genre Aquilaria Lam. (Thymelaeaceae) comprend 21 espèces d’arbres (The Plant List, 2013) et se trouve principalement en Asie du Sud-Est. Lorsque l’arbre est infecté (champignons, bactéries), son bois devient brunâtre ou noirâtre (appelé bois d’agar) en raison de la sécrétion d’une oléorésine en réaction à l’infection. La résine est très parfumée et a été recherchée et utilisée pendant des siècles par les bouddhistes, les hindous et les musulmans pour faire de l’encens pour les cérémonies religieuses. Cette oléorésine se trouve principalement dans les espèces du genre Aquilaria, mais aussi dans quelques espèces des genres Gyrinops Gaertner et Gonystylus Teijsmann & Binnendijk. Il est difficile de faire la distinction entre ces espèces, et ce manque de connaissances taxonomiques a conduit à une surutilisation des arbres, mettant en danger ces espèces endémiques inscrites à l’Annexe II de la CITES. Nous avons utilisé la chimiotaxonomie comme outil de discrimination pour analyser les molécules polyphénoliques, métabolites secondaires, qui sont connus pour agir comme marqueurs taxonomiques dans d’autres plantes. En utilisant une technique d’analyse simple, efficace et peu coûteuse (chromatographie bidimensionnelle en couche mince), nous avons trouvé la même molécule polyphénolique dans les six espèces du genre Aquilaria étudiées, qui a été identifiée comme mangiférine par analyse colorimétrique et chromatographique en comparaison avec la mangiférine de contrôle. Au cours de ces analyses, nous avons trouvé une relation entre les genres Aquilaria et Gyrinops (botaniquement démontrée) par l’existence occasionnelle de mangiférine chez trois espèces du genre Gyrinops. Nous avons également constaté qu’un laps de temps de cent-quarante ans entre deux échantillons d’herbier de la même espèce n’avait pratiquement aucun effet sur la concentration de mangiférine dans la plante. Ces travaux ont montré les mérites de la chimiotaxonomie dans la recherche de marqueurs taxonomiques et l’originalité de l’analyse colorimétrique d’un métabolite de plante chromatographié en couche mince

Sub-chapter 2.4.2. Mediterranean ecosystems facing global change

Introduction Mediterranean-type ecosystems (MTEs) cover a small fraction of the terrestrial component of the Earth, and their functioning is driven by a recurrent summer drought concomitant with the hottest temperatures. Climate projections by general circulation models (GCM) under RCP 6.0 and 8.5 scenarios collectively forecast substantial drying and warming in regions surrounding the Mediterranean Sea, especially in summer (Schleussner et al 2016). MTEs are expected to be highly responsive ..

How reliable are methods to assess xylem vulnerability to cavitation? The issue of ‘open vessel’ artifact in oaks

International audienceThree methods are in widespread use to build vulnerability curves (VCs) to cavitation. The bench drying (BD) method is considered as a reference because embolism and xylem pressure are measured on large branches dehydrating in the air, in conditions similar to what happens in nature. Two other methods of embolism induction have been increasingly used. While the Cavitron (CA) uses centrifugal force to induce embolism, in the air injection (AI) method embolism is induced by forcing pressurized air to enter a stem segment. Recent studies have suggested that the AI and CA methods are inappropriate in long-vesselled species because they produce a very high-threshold xylem pressure for embolism (e.g., P50) compared with what is expected from (i) their ecophysiology in the field (native embolism, water potential and stomatal response to xylem pressure) and (ii) the P50 obtained with the BD method. However, other authors have argued that the CA and AI methods may be valid because they produce VCs similar to the BD method. In order to clarify this issue, we assessed VCs with the three above-mentioned methods on the long-vesselled Quercus ilex L. We showed that the BD VC yielded threshold xylem pressure for embolism consistent with in situ measurements of native embolism, minimal water potential and stomatal conductance. We therefore concluded that the BD method provides a reliable estimate of the VC for this species. The CA method produced a very high P50 (i.e., less negative) compared with the BD method, which is consistent with an artifact related to the vessel length. The VCs obtained with the AI method were highly variable, producing P50 ranging from −2 to −8.2 MPa. This wide variability was more related to differences in base diameter among samples than to differences in the length of samples. We concluded that this method is probably subject to an artifact linked to the distribution of vessel lengths within the sample. Overall, our results indicate that the CA and the AI should be used with extreme caution on long-vesselled species. Our results also highlight that several criteria may be helpful to assess the validity of a VC

Deciphering 's'marts from 'r'ubbish VCs : going further the matter of letters ?

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Un quartier de frange urbaine en milieu humide (xe-xve siècle) : les fouilles de la rue Fontgiève à Clermont-Ferrand (Puy-de-Dôme)

Cet ouvrage réunissant les contributions d’une quinzaine de chercheurs présente les principaux résultats d’une fouille archéologique préventive réalisée en 2012, à l’angle des rues Docteur Gautrez et Fontgiève, au nord-ouest du centre historique de Clermont-Ferrand. Les vestiges qui ont pu être étudiés illustrent l’évolution d’un quartier de périphérie urbaine sur le temps long, entre le Haut- Empire et la fin du Moyen Âge. Pour la période romaine, les constructions les plus marquantes correspondent à un moulin hydraulique aménagé sur la berge d’un chenal, à un bassin monumental circulaire, ou encore à un édifice bâti au-dessus d’un captage de source. L’humidité du milieu et la conservation d’une quantité importante de bois gorgés d’eau rendent ces découvertes d’autant plus exceptionnelles qu’elles permettent, d’une part, d’affiner les chronologies et, d’autre part, d’approcher des pans de l’architecture, de l’artisanat et de la vie quotidienne bien souvent inaccessibles. Au-delà, pour cette période, c’est le rythme d’occupation du site qui doit retenir l’attention, dans la mesure où il reflète concrètement les phases de développement et de repli de la capitale de cité, Augustonemetum. L’Antiquité tardive et le haut Moyen Âge sont pauvres en vestiges matériels, comme souvent dans ce type de contexte urbain, mais, pour autant, ces deux périodes ont pu être appréhendées par le biais des sources textuelles et par les données paléoenvironnementales. Celles-ci ont d’ailleurs été d’un apport crucial à la compréhension générale du site, toute période confondue, tant pour l’appréciation des structures que pour celle de l’environnement. La fin du Moyen Âge marque de son côté un nouvel essor de la ville, illustré par la constitution progressive, à partir de la fin du xiiie siècle, d’un quartier de faubourg. Si les vestiges de cette période sont moins spectaculaires que ceux du Haut-Empire, ils n’en demeurent pas moins exceptionnels puisqu’ils offrent une vision, là aussi rarement documentée par l’archéologie, de l’évolution d’une occupation médiévale urbaine extra muros. Cette monographie, constituant le premier ouvrage de synthèse archéologique sur un quartier ancien de Clermont-Ferrand, s’inscrit dans la liste des nombreuses fouilles préventives réalisées en centre-ville depuis 2010. Elle marque, espérons-le, le point de départ d’une série de volumes qui contribuera à restituer l’histoire du chef-lieu arverne

Multi-scale datasets for monitoring Mediterranean oak forests from optical remote sensing during the SENTHYMED/MEDOAK experiment in the north of Montpellier (France)

International audienceMediterranean forests represent critical areas that are increasingly affected by the frequency of droughts and fires, anthropic activities and land use changes. Optical remote sensing data give access to several essential biodiversity variables, such as species traits (related to vegetation biophysical and biochemical composition), which can help to better understand the structure and functioning of these forests. However, their reliability highly depends on the scale of observation and the spectral configuration of the sensor. Thus, the objective of the SENTHYMED/MEDOAK experiment is to provide datasets from leaf to canopy scale in synchronization with remote sensing acquisitions obtained from multi-platform sensors having different spectral characteristics and spatial resolutions. Seven monthly data collections were performed between April and October 2021 (with a complementary one in June 2023) over two forests in the north of Montpellier, France, comprised of two oak endemic species with different phenological dynamics (evergreen: Quercus ilex and deciduous: Quercus pubescens) and a variability of canopy cover fractions (from dense to open canopy). These collections were coincident with satellite multispectral Sentinel-2 data and one with airborne hyperspectral AVIRIS-Next Generation data. In addition, satellite hyperspectral PRISMA and DESIS were also available for some dates. All these airborne and satellite data are provided from free online download websites. Eight datasets are presented in this paper from thirteen studied forest plots: (1) overstory and understory inventory, (2) 687 canopy plant area index from Li-COR plant canopy analyzers, (3) 1475 in situ spectral reflectances (oak canopy, trunk, grass, limestone, etc.) from ASD spectroradiometers, (4) 92 soil moistures and temperatures from IMKO and Campbell probes, (5) 747 leaf-clip optical data from SPAD and DUALEX sensors, (6) 2594 in-lab leaf directional-hemispherical reflectances and transmittances from ASD spectroradiometer coupled with an integrating sphere, (7) 747 in-lab measured leaf water and dry matter content, and additional leaf traits by inversion of the PROSPECT model and (8) UAV-borne LiDAR 3-D point clouds. These datasets can be useful for multi-scale and multi-temporal calibration/validation of high level satellite vegetation products such as species traits, for current and future imaging spectroscopic missions, and by fusing or comparing both multispectral and hyperspectral data. Other targeted applications can be forest 3-D modelling, biodiversity assessment, fire risk prevention and globally vegetation monitoring