The successional status of tropical rainforest tree species is associated with differences in leaf carbon isotope discrimination and functional traits

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Influence d'un dessèchement du sol sur la nutrition minérale de jeunes plants de résineux

National audienceTwo experiments have been realized in a greenhouse on young plants of firs (Abies alba, nordmanniana et numidica) and spruce (Picea abies). Different durations and intensities of soil drying have been carried out. That year, the shoot increase as well as the differences of growth and new leaf area between treatments were small for firs and important for spruce. At the end of the year, the more important had been the soil desiccation, the smaller were the different elements contents in the firs needles. On spruce, the results are about the same with sampling during the growth period. On the other hand, at the end of the year, it is not the case any longer, except for potassium ; on the contrary, the water deficit had then increased the needles contents in nitrogen, manganese and calcium. Analysis of foliar potassium content appears therefore (ours results agree with litterature for this element) as a good test for water deficit diagnosis on young plants of conifers ; this test remains available even after the stress has disappeared. Concurrently, soil fertilization with potassium could perhaps reduce the effects of soil dessication on these plants.Deux expériences ont été menées en serre sur de jeunes plants, respectivement de sapins ( Abies alba, nordmanniana et numidica) et d’épicéa (Picea abies). Différentes durées et intensités de dessèchement du sol ont été appliquées. Cette année-là, la croissance aérienne ainsi que les différences de croissance et de biomasse foliaire entre traitements ont été faibles chez les sapins, importantes chez l’épicéa. En fin d’année, les concentrations des différents éléments dans les aiguilles des sapins étaient d’autant plus faibles que la sécheresse avait été plus marquée. Chez l’épicéa, les prélèvements foliaires effectués en cours de saison de végétation fournissent un résultat à peu près identique au précédent. Par contre, en fin d’année cela n’est plus valable que pour le potassium ; au contraire, le déficit hydrique a alors augmenté les teneurs foliaires en azote, manganèse et calcium. Il en découle, et pour cet élément cela concorde avec les renseignements bibliographiques, que l’analyse du potassium apparaît comme un test robuste de diagnostic de déficit hydrique sur de jeunes résineux, qui reste valable même après disparition du stress. Parallèlement, une fertilisation en potassium pourrait peut-être corriger en partie les conséquences de la sécheresse

13^{13}C and 15^{15}N isotopic fractionation in trees, soils and fungi in a natural forest stand and a Norway spruce plantation

$^{15}$N and $^{13}$C natural abundances of foliage, branches, trunks, litter, soil, fungal sporophores, mycorrhizas and mycelium were determined in two forest stands, a natural forest and a Norway spruce plantation, to obtain some insights into the role of the functional diversity of saprotrophic and ectomycorrhizal fungi in carbon and nitrogen cycles. Almost all saprotrophic fungi sporophores were enriched in $^{13}$C relative to their substrate. In contrast, they exhibited no or very little shift of $\delta$$^{15}$N. Judging from the amount of C discrimination, ectomycorrhizal fungi seem to acquire carbon from their host or from dead organic matter. Some ectomycorrhizal species seem able to acquire nitrogen from dead organic matter and could be able to transfer it to their host without nitrogen fractionation, while others supply their host with $^{15}$N-depleted nitrogen. Moreover ectomycorrhizal species displayed a significant N fractionation during sporophore differentiation, while saprotrophic fungi did not.Fractionnement isotopique 13C et 15N dans les arbres, le sol et les champignons pour un peuplement de forêt naturelle et une plantation d'épicéas. Les abondances naturelles du $^{15}$N et du $^{13}$C de la masse foliaire, des branches, des troncs, de la litière, du sol, des carpophores, des mycorhizes et du mycélium, ont été déterminées dans deux peuplements forestiers, une forêt naturelle et une plantation d'épicéas, afin d'obtenir quelques précisions sur le rôle de la diversité fonctionnelle des champignons saprophytes et ectomycorhiziens dans le cycle du carbone et de l'azote. Presque tous les champignons saprophytes présentent un enrichissement en $^{13}$C relativement à leur substrat. Par contre, ils ne présentent pas ou ne présentent que très peu de modifications du $\delta$$^{15}$N. En fonction de leur taux de discrimination du carbone, les champignons ectomycorhiziens semblent pouvoir acquérir du carbone à la fois à partir de leur hôte et de la matière organique morte. Quelques espèces semblent capables d'acquérir de l'azote organique du sol et de le transférer sans fractionnement à leur hôte alors que d'autres fournissent leur hôte en azote appauvri en $^{15}$N. De plus, les espèces ectomycorhiziennes présentent un fractionnement significatif de l'azote pendant la différenciation des carpophores, alors que les champignons saprophytes n'en présentent pas

Saprotrophic versus symbiotic strategy during truffle ascocarp development under holm oak. A response based on 13C and 15N natural abundance

The development of truffles in the soil is not well understood. It is not known if a direct transfer of carbohydrates takes place between the host tree and the developing ascocarps through ectomycorrhizal structures or whether sporophores become independent from their hosts after several weeks or months and are able to use dead host tissues or soil organic matter as carbon (C) and nitrogen (N) sources. To study saprophytic or symbiotic capacities of truffle ascocarps the natural abundance of 15N and 13C in foliage, wood, fine roots, mycorrhizae, fungal sporophores and soil were determined in a truffle orchard. The processes of carbon and nitrogen allocation remained unchanged during the entire period of ascocarp development of Tuber melanosporum. From 13C and 15N natural abundance measurements, T. melanosporum, T. brumale and T. rufum did not exhibit saprotophic strategy during ascocarp development, which is contradictory to common statements found in handbooks regarding truffle cultivation.Le développement des truffes dans le sol n’est pas encore bien compris. Les connaissances actuelles ne nous permettent pas de savoir s’il existe un transfert direct de sucres entre l’arbre hôte et les ascocarpes en développement via les structures ectomycorhiziennes, ou si les ascocarpes utilisent le carbone et l’azote directement issu de la matière organique du sol. Nous avons mesuré l’abondance naturelle du 15N et du 13C dans le sol, les feuilles, les mycorhizes, le bois et les carpophores d’une truffière naturelle à chêne vert afin de déterminer la stratégie de la nutrition carbonée des ascocarpes. Les processus d’allocation du carbone et de l’azote restent identiques pendant toute la phase de développement des ascocarpes de Tuber melanosporum. De ces mesures d’abondance naturelle du 15N et du 13C, il apparaît que T. melanosporum, T. brumale et T. rufum ne développent pas de stratégie saprophytique pendant le développement des ascocarpes, ce qui est en contradiction avec les idées habituellement véhiculées par les manuels de trufficulture

Evidence of hydraulic lift in a young beech and oak mixed forest usingO-18 soil water labelling

International audienceHydraulic lift (HL) by tree roots in a young, broad-leaved, mixed temperate European forest was investigated during the 2008 growing season by injecting O-18-enriched soil water at a depth of 75-90 cm under drought conditions experimentally imposed in a rain-exclusion system. Based on sap flow, leaf water potential, 2-D root distribution measurements, soil isotope profiles, and xylem water isotope composition, water acquisition and use by two tree species, beech (Fagus sylvatica) and oak (Quercus petraea) was compared. We showed that, unlike oak, beech experienced a marked decrease in sap flow and predawn leaf water potential with increasing soil drought. This behaviour was logical considering the shallower root system in beech than in oak. Six days after O-18-labelling, we observed isotopic enrichment in the shallower soil layers. Since the intermediate soil layers did not display any enrichment, our results clearly pointed to hydraulic lift by tree roots. The superficial enrichment that was observed in the vicinity of oak trunks and the increase in the isotopic signature of xylem sap in the oak trees but not in the beech trees confirmed the predominant role of oak in the hydraulic lift at our site. Even though facilitation for water acquisition among species was not observed here, our results suggest a potential positive contribution of species like oak toward maintaining species diversity in mixed forest ecosystems submitted to severe drought events

Depth of soil water uptake by tropical rainforest trees during dry periods: does tree dimension matter?

Though the root biomass of tropical rainforest trees is concentrated in the upper soil layers, soil water uptake by deep roots has been shown to contribute to tree transpiration. A precise evaluation of the relationship between tree dimensions and depth of water uptake would be useful in tree-based modelling approaches designed to anticipate the response of tropical rainforest ecosystems to future changes in environmental conditions. We used an innovative dual-isotope labelling approach (deuterium in surface soil and oxygen at 120-cm depth) coupled with a modelling approach to investigate the role of tree dimensions in soil water uptake in a tropical rainforest exposed to seasonal drought. We studied 65 trees of varying diameter and height and with a wide range of predawn leaf water potential (I-pd) values. We confirmed that about half of the studied trees relied on soil water below 100-cm depth during dry periods. I-pd was negatively correlated with depth of water extraction and can be taken as a rough proxy of this depth. Some trees showed considerable plasticity in their depth of water uptake, exhibiting an efficient adaptive strategy for water and nutrient resource acquisition. We did not find a strong relationship between tree dimensions and depth of water uptake. While tall trees preferentially extract water from layers below 100-cm depth, shorter trees show broad variations in mean depth of water uptake. This precludes the use of tree dimensions to parameterize functional models

Isotopic evidence in adult oak trees of a mixotrophic lifestyle during spring reactivation

In deciduous oaks, spring reactivation modifies the capability of ectomycorrhizal fungi (EMf to mobilise carbon from soil organic matter. Plant carbon budget is also unbalanced during spring reactivation: carbon demand is high to build new tissues and carbon may be provided by reserves only as new photoassimilates are not yet available. Using a stable isotope approach with C-13-labelled litter, we have estimated that after a six months experiment, less than 1% of carbon is derived from litter during spring reactivation and is channelled from the soil to the thick roots of Quercus petraea via ectomycorrhiza, even when leaves are photosynthetically active. Our results are promising for future detailed quantification of different carbon sources (i.e. reserves, photosynthesis and litter derived) in EMf oak trees. They also support the hypothesis that oak trees of Q petraea in north-eastern France are partially mixotrophic plants. (C) 2012 Elsevier Ltd. All rights reserved

Le mode d’acquisition du carbone pendant le développement des ascocarpes de Tuber melanosporum Vittad

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Carbon transfer from the host to Tuber melanosporum mycorrhizas and ascocarps followed using a 13C pulse-labeling technique

Truffles ascocarps need carbon to grow, but it is not known whether this carbon comes directly from the tree (heterotrophy) or from soil organic matter (saprotrophy). The objective of this work was to investigate the heterotrophic side of the ascocarp nutrition by assessing the allocation of carbon by the host to Tuber melanosporum mycorrhizas and ascocarps. In 2010, a single hazel tree selected for its high truffle (Tuber melanosporum) production and situated in the west part of the Vosges, France, was labeled with 13CO2. The transfer of 13C from the leaves to the fine roots and T. melanosporum mycorrhizas was very slow compared with the results found in the literature for herbaceous plants or other tree species. The fine roots primarily acted as a carbon conduit; they accumulated little 13C and transferred it slowly to the mycorrhizas. The mycorrhizas first formed a carbon sink and accumulated 13C prior to ascocarp development. Then, the mycorrhizas transferred 13C to the ascocarps to provide constitutive carbon (1.7 mg of 13C per day). The ascocarps accumulated host carbon until reaching complete maturity, 200 days after the first labeling and 150 days after the second labeling event. This role of the Tuber ascocarps as a carbon sink occurred several months after the end of carbon assimilation by the host and at low temperature. This finding suggests that carbon allocated to the ascocarps during winter was provided by reserve compounds stored in the wood and hydrolyzed during a period of frost. Almost all of the constitutive carbon allocated to the truffles (1% of the total carbon assimilated by the tree during the growing season) came from the host