Dynamique des éléments minéraux dans la sève xylémique d'épicéas de 30 ans

Le flux hydrique circulant dans les troncs d'un perchis d'épicéa des Vosges a été directement mesuré par des sondes de fluxmètre radial au cours d'une saison de végétation. Parallèlement, les sèves xylémiques de branches et de troncs ont été extraites sur des échantillons prélevés à intervalle régulier, respectivement par pression sur les parties feuillées et par déplacement par une colonne d'eau. Ces sèves ont été analysées par ICP pour les principaux éléments minéraux (Ca, K, P, Mg, Si, Mn). Les flux d'éléments minéraux calculés par produit des flux hydriques par les concentrations ont été comparés au prélèvement en éléments minéraux, préalablement évalué par une étude classique de minéralomasse. La variation saisonnière des concentrations dans la sève de la base du tronc diffère suivant les éléments. Les teneurs en Ca, Mg, K et P sont maximales lors du débourrement (K) ou en début de croissance des pousses puis se stabilisent durant l'été pour raugmenter en fin de saison. Outre un pic lors de l'allongement des pousses, Si semble s'élever parallèlement à la progression de la sécheresse édaphique estivale. La variation saisonnière des concentrations de la sève de branche est moins marquée, mais similaire à celle observée pour les troncs. Les teneurs en éléments minéraux de la sève du haut du tronc et du houppier sont toujours très supérieures à celles de la base du tronc. Les flux de Ca et Mg dans la sève de la base du tronc sont d'importance comparable à ceux du prélèvement brut du peuplement estimé à partir de l'étude des biomasses. Ceux de P et K lui sont très supérieurs à la base du tronc. Ces résultats sont interprétés comme traduisant l'existence d'un pool mobile d'éléments minéraux dans l'arbre, particulièrement important dans les parties métaboliquement très actives comme le houppier et les racines.Mineral element dynamics in the xylem sap of 30-year old spruce. Xylem sap fluxes and mineral contents in branches and boles of a 30-yr-old spruce stand were monitored during the 1990 growing period. On average, four trees were felled fortnightly. Three portions of the bottom of each felled trunk and four branches per tree from the third and fourth whorl were sampled before sunrise. Sap extractions were carried out immediatly after felling. Bole xylem sap was displaced by the pressure of a 1-metre high column of water applied to the end of each portion sampled, whereas twig sap was extracted by applying slight air pressure (+1 MPa over potential) in a Schollander bomb. Xylem sap rise was recorded continuously in four trees with a heating probe device (fig 2). Sap water fluxes were multiplied by sap mineral contents in order to calculate the flux of mineral elements rising annually in the xylem tissues. This flux was compared to the mineral content and annual uptake of the stand, previously evaluated by a biomass and mineral content inventory (Le Goaster et al, 1991; Dambrine et al, 1991). Sap element content was found to have a systematically higher concentration in the crown compared to the bottom of the trunk (fig 3). Seasonal variations in the mineral concentrations of bole xylem sap depend on the element: Ca, Mg, K and P peak during or after bud break, then decrease and remain low and steady during summer with a final increase at the end of the growing period. Si seems to increase with drought (fig 4). Seasonal variations of branch xylem sap contents show a less pronounced pattern (fig 6). Concentrations of Ca, P and K peak at the beginning of shoot or needle growth, after the peak of bole sap concentrations. Mg content does not change greatly whereas Si increases in parallel with the drought. The ratio between branch and bole sap concentrations (fig 6) is highest for Ca and Mg, which are available to the trees in very limited amounts. Annual fluxes of Ca and Mg in bole xylem sap were found to be lower than (Ca) or of the same order of magnitude (Mg) as the gross uptake of the stand, assessed by a biomass and mineral content inventory (table IV). This discrepency between the Ca flux in sap and gross Ca uptake could be partly due to uncertainties in gross uptake evaluation. Conversely, fluxes of K and P in bole xylem sap are far higher than the gross uptake. Fluxes of all elements in branch xylem sap are higher (Ca, Si) or far higher (Mg, P, K) than the gross uptake. For P and K, fluxes are of the same order of magnitude as the total content of the stands crown (table V). These results strongly suggest the existence of a mobile pool of mineral elements, cycling continuously in the tree, and particularly in the crown. This is in agreement with previous investigations showing that root and branch sap concentrations were higher than those from the bole (table VI)

Which recovery from acid rains in the Vosges Mountains after 30 years ?

International audienceSome 30 years ago, surveys of the surface water quality in the Vosges Moutains revealed a strong headstream water acidification, with high concentrations in dissolved aluminium, and the observed consequences on the aquatic biodiversity (Probst et al., 1990). In the early 2000s some of these headstreams (Grand Clos, Ventron, Rouge Rupt, Cellet) were included in a set of 16 headstreams which constitute the Vosges Acidification Observatory of the Zone Atelier du Bassin de la Moselle. Their surface water has been sampled monthly and analyzed in terms of major anions and cations, as well as dissolved aluminium and, since 2012, in terms of dissolved organic matter.In spring 2021, a new survey of the headstreams investigated in the 1990s was organized. Nineteen streams could be sampled. All the streams running on granite bedrock had a higher pH than in 1990. Most of them showed lower aluminium concentrations. The improvement for headstreams running on sandstone seems more limited and new data need to be collected. The presentation will include data collected in summer 2021 as well as long term data for the headstreams included in the Vosges Acidification Observatory to check seasonal and yearly effects

The origin of closed depressions in Northeastern France: a new assessment

International audienceOver 10,000 closed depressions (CDs) are found in the silty plains of Northeastern France. These small wetlands support the growth of rare plant species. Although their origins, which could be anthropogenic or geologic due to salt/gypsum lens dissolution, have been debated for 150 years, they have not yet been the focus of an integrated study. In 39 geological borings along a 15-km² strip, no salt/gypsum lenses and more than 260 CDs were recorded using LiDAR. All of the investigated CDs have a bathtub form with a flat bottom. Complete excavations clearly showed a cut contact between the sediment and the horizontal marl substratum at the bottom, and a cut at the edges of the upper marl layers. Radiocarbon dating of sediment bottoms showed that sedimentation began between the second Iron Age and the Roman period. The frequencies of pollen and Sporormiella-type depict an open landscape with grassland, pasture and cropland. These convergent findings challenge the hypothesis that CDs formed naturally and suggest that they area instead anthropogenic. Because no soil deposits were found around the CDs, digging may have been intended to marl the surrounding acidic silty soils. The high density and small size of CDs will allow the detailed reconstruction of landscape and biodiversity modifications in the region for the two last millennia

Molecular recognition-induced liquid crystals from complementary diaminopyridine and flavin dyads

Complementary diaminopyridine-flavin dyads that provide liquid crystalline systems have been synthesised. The mesophases possess well-defined molecular architectures, thermal stability and wide ranges. Furthermore, chiral centres have been introduced to explore the role chirality plays on the end morphology. The introduction of stereo-centres into these systems offers effective control over mesophase morphology and generates well-defined columnar liquid crystals

Magnesium Isotope Variations to Trace Liming Input to Terrestrial Ecosystems: A Case Study in the Vosges Mountains

International audienceLiming with Ca and Mg carbonates is commonly used to reduce soil and stream acidity and to improve vegetation growth and nutrition in forests. Ten years ago, dolomite lime was experimentally applied to a forest catchment on granite in the Vosges Mountains (northeast France), which is characterized by acid soils and drained by an acid stream. The average Mg isotope composition of the dolomite lime (-1.75%) was low compared with that of tree foliage (-0.70%), granite and deep soil layers (-0.40%), and stream water (-0.80%) in the control catchment. After liming, the exchangeable Mg concentrations in surface soil layers, which were initially very low, increased, and the Mg isotope composition decreased (up to -0.60%). The decrease was smaller in deeper layers but not in proportion to the increase in exchangeable Mg content, suggesting contributions from mineralization of organic matter and/or displacement of exchangeable Mg from surface layers. Before application, Mg concentration in beech and fir leaves was low, and that of 1-yr-old fir needles was lower than that in current needles. Internal Mg translocation within fir needles also resulted in a lower delta Mg-26 of older needles. Three years after dolomite application, the Mg isotope composition of plant leaves was lower than that in the control catchment; this decrease (up to -1.00%) was attributed to direct uptake of Mg from dissolving dolomite. Liming doubled the concentration of Mg in the stream, whereas the Mg isotope composition decreased correspondingly from -0.80 to -1.20%, indicating a fast transfer of dolomite Mg to the stream. Our findings indicate that monitoring of delta Mg-26 may be a promising tool to study the fate of dolomitic inputs in terrestrial and aquatic ecosystems

Long-term monitoring of of headwater streams water quality in the Vosges Mountains (North-Eastern France)

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Biomasse et minéralomasse d'une plantation d'épicéa commun (Picea abies Karst) de forte production dans les Vosges (France)

Les résultats présentés concernent l'évaluation de la biomasse et de la minéralomasse d'une plantation d'épicéa commun (Picea abies Karst) de première génération, de forte production et présentant un très bon état sanitaire. Les objectifs de ce travail étaient : - de fournir les données concernant l'immobilisation et le prélèvement d'éléments nutritifs de cette pessière adulte de forte productivité, en vue de l'établissement du bilan entrées-sorties, sur une base saisonnière, annuelle et pluriannuelle, et - d'apporter des résultats à l'étude plus générale du fonctionnement des pessières sur sol acide dans le domaine des relations production - immobilisation - prélèvement d'éléments nutritifs. Ces résultats sont indispensables à la modélisation du fonctionnement de ces écosystèmes. Une méthodologie classique par échantilonnage destructif de 15 arbres répartis sur tout le spectre des classes de circonférence à 1,30 m a été utilisée. Des tarifs de biomasse et de contenu en éléments nutritifs ont été construits pour chacun des compartiments du peuplement, puis appliqués à l'inventaire. Les résultats montrent que ce peuplement a une biomasse sur pied très importante de 360 t de matière sèche pour le seul tronc (soit 900 m3), et une biomasse totale ligneuse de 470 t (incluant les racines). Son prélèvement courant est de 70,2; 4,9; 23,6; 24,1 et 4,4 kg·ha-1.an-1 respectivement pour N, P, K Ca et Mg. La comparaison des immobilisations et des stocks d'éléments disponibles montre que la pérennité d'un tel écosystème, aux réserves disponibles limitées pour la végétation, réside dans le flux d'éléments issus de l'altération des réserves. La question importante du rôle de l'histoire de l'occupation du sol précédant la plantation d'épicéa, dans la croissance de cette première génération forestière est posée.Biomass and nutrient content of a highly productive spruce plantation (Picea abies Karst) in the Vosges (France). The results presented in this report concern the biomass and mineral content of a first rotation plantation of Norway spruce (Picea abies Karst). The stand is situated in the western part of the Vosges mountains in the communal forest of Gemaingoutte. The brown soil, derived from a granito-gneissic bedrock, is acidic and relatively poor in "basic cations". No indicators of forest decline were observed on this highly productive stand. The aims of this study were to: - collect data on immobilisation and uptake of nutrients in this highly productive spruce stand, which could be used in calculating input-output assessments on a seasonal, annual and pluriannual basis; - provide results for a more general study on the manner in which a spruce ecosystem functions according to site characteristics, which would be of use in modelling the spruce ecosystem. A classical destructive sampling procedure was used: 15 trees were felled, biomass, and mineral content tables were established for the main tree components; these tables were used to evaluate the stand contents. The biomass of the stand was very high: 360 t for the stems, ie a volume of 900 m3; 470 t for the total stand including the root systems (cf table IV). The current uptake of the trees for N, P, K, Ca and Mg was 70.2, 4.9, 23.6, 24.1 and 4.4 kg respectively per ha per annum. The comparison between the actual available soil reserves and the immobilisation of nutrients within trees showed that these reserves were not sufficient to indefinitely sustain growth and that consequently soil fertility is a dynamic process linked to ecosystem input, ie atmospheric input and weathering (cf table VI). The fine earth of the soil constitutes a fairly large reservoir of nutrients. The question is raised regarding land use prior to planting the stand and the effect of this use on soil quality and therefore on growth of this first generation plantation