Material-independent crack arrest statistics: Application to indentation experiments

An extensive experimental study of indentation and crack arrest statistics is presented for four different brittle materials (alumina, silicon carbide, silicon nitride, glass). Evidence is given that the crack length statistics can be described by a universal (i.e. material independent) distribution. The latter directly derives from results obtained when modeling crack propagation as a depinning phenomenon. Crack arrest (or effective toughness) statistics appears to be fully characterized by two parameters, namely, an asymptotic crack length (or macroscopic toughness) value and a power law size dependent width. The experimental knowledge of the crack arrest statistics at one given scale thus gives access to its knowledge at all scales

Short- and long-term influence of litter quality and quantity on simulated heterotrophic soil respiration in a lowland tropical forest

Heterotrophic soil respiration (SRH) alone can contribute up to 50% of total ecosystem respiration in tropical forests. Whereas the abiotic controls of SRH have been extensively studied, the influence of plant traits is less well characterised. We used field experiments and a modelling approach to test the relative influence of plant traits on SRH in lowland tropical forest in French Guiana. We measured leaf- and root litter traits for five common tree species and conducted a root decomposition experiment to evaluate the influence of root chemistry on decay rates. We measured SRH in trenched plots and used our field measurements to parameterize and test the Century model of soil C dynamics. Overall, the Century model performed well in simulating SRH, and species-specific root decomposition in Century corresponded well to decomposition rates measured in situ. Root litter characterized by low lignin-to-nitrogen ratios decomposed more rapidly than low-quality root litter during the first 6 months. Model runs over different time scales revealed that litter quality substantially influenced SRH on an annual time-scale by determining the rates of root- and leaf litter decomposition. However, litter mass had an overriding influence on SRH over the longer term in 20-year model runs. Synthesis Using simple plant trait data to parameterise the Century model, we were able to accurately simulate changes in SRH in a lowland tropical forest. Our results suggest that this approach could be used to predict changes in tropical soil C dynamics under global change scenarios by including data on changes in plant productivity and C inputs to the soil (for example litterfall and root turnover)

Analyse de l'efficience d'utilisation de l'eau des chênes pédonculé (Quercus robur L.) et sessile (Quercus petraea (Matt.) Liebl.) : relation avec les dépérissements des chênaies

The two sympatric European oak species Quercus robur and Q. petraea display distinct ecological requirements and drought sensitivities. Intrinsic water-use efficiency (WUE[index int]) was studied on the basis of both measurements of instantaneous gas exchange and carbon isotopic composition of plant tissue ([delta]13C). Retrospective analysis of [delta ]13C in wood cellulose of adult trees from the two species growing in mixed stands showed a time-stable interspecific difference. Gas exchange measurements performed on 2-year old seedlings grown in "common garden" confirmed the higher WUE[index int] of Q. petraea than Q. robur, mainly due to stomatal conductance variations between the two species. A genetic basis to these interspecific differences was suggested. The results provided new insights in the understanding of the ecology of these species. However, no clear relationships were observed between tree-ring growth or [delta]13C and sensitivity to drought-induced decline. The relationship between WUE[index int] and drought resistance is discussed.Les espèces sympatriques Quercus robur et Q. petraea occupent des niches écologiques distinctes et présentent des différences de sensibilité à la sécheresse. L'efficience intrinsèque d'utilisation de l'eau (WUE[indice int]) est étudiée sur la base de mesures instantanées d'échanges gazeux foliaires d'une part, et de mesures de composition isotopique du carbone ([delta]13C) dans les tissus végétaux d'autre part. L'analyse rétrospective de [delta]13C dans la cellulose du bois d'arbres adultes des deux espèces croissant en peuplements mélangés a montré une différence interspécifique stable au cours du temps. Conformément à ces observations, les mesures d'échanges gazeux effectuées sur des semis croissant en conditions contrôlées ont mis en évidence une WUE[indice int ]supérieure chez Q. petraea. Cette différence est essentiellement due aux variations interspécifiques de conductance stomatique. Par ailleurs, ni la croissance radiale, ni [delta]13C n'ont pu être reliés de façon univoque à la sensibilité au dépérissement induit par la sécheresse. La complexité de la relation entre WUE[indice int] et résistance à la sécheresse est analysée

Impact de la saison sèche sur les flux de CO2 en forêt tropicale humide guyanaise

National audienc

Contribution à l'étude de la variabilité spatiale des composantes du bilan de carbone d'un sol de forêt tropicale humide (Paracou, Guyane française)

L objectif principal de ce travail fut d analyser la variabilité spatiale de plusieurs composantes du bilan de C du sol en forêt tropicale humide. L étude menée en Guyane française a été essentiellement réalisée dans la forêt naturelle du site-atelier Guyaflux et dans une plantation d essences locales à proximité. Deux approches ont permis de répondre à cet objectif. La première visait à évaluer les effets des facteurs environnementaux (i.e. type de sol, humidité du sol ou structure forestière) sur la variabilité spatiale et interspécifique de plusieurs composantes. Parmi ces composantes les estimations de biomasse racinaire, production des racines fines (Ø < 2 mm) et respiration du sol (Rsol) ont mis en évidence une forte variabilité spatiale intra-parcelle. Au sein d une même parcelle, une relation empirique significative a été observée entre les données de surface terrière du peuplement et les mesures de Rsol. Conjointement, une étude comparative de la phénologie de croissance et de la décomposition des racines fines a permis d évaluer les variations entre cinq et huit espèces ligneuses, respectivement. La deuxième approche consistait à hiérarchiser les facteurs responsables de la variabilité spatiale de Rsol : une composante intégrative des processus impliqués dans le fonctionnement du sol. Rsol se décompose en une composante hétérotrophe (Rh) et autotrophe (Ra). Dans ce cadre deux modèles semi-mécanistes ont été paramétrés et testés grâce aux mesures de Rsol réalisées en continu sur le même site. Le modèle de bilan de C du sol CENTURY a simulé Rh, la respiration des micro-organismes décomposeurs de la matière organique. Le module McCree a simulé Ra, la respiration des racines vivantes et a été associé à CENTURY pour simuler Rsol totale. Une analyse de sensibilité réalisée sur ces prédictions de flux a montré que la qualité chimique (e.g. teneur en lignine) des litières ainsi que l humidité du sol affectent la variabilité spatiale des flux de CO2 provenant du sol. L utilisation de modèles mécanistes est ressortie très prometteuse dans ce travail de thèse, elle complète efficacement les analyses des expérimentations in situ en ajoutant une dimension particulière à chaque facteur d influence des processus biologiques extrêmement complexes en forêt tropicale.The aim of this study was to define the spatial variability of the C balance components of the soil in a tropical rain forest. This work was conducted in French Guiana in the Guyaflux experimental forest and in a close plantation of monospecific plots. Two approaches were used in this investigation. The first one aimed to analyze the effects of the environmental factors (i.e. soil texture, soil moisture or forest structure) on the spatial and interspecific variability in several components in situ. Among these components, the root biomass, the fine roots (Ø < 2 mm) production and the soil respiration (Rsol) measurements underlined a large spatial variability within plot. At this scale, there was a significant empirical relationship between the tree basal area and soil respiration. Interspecific variations in fine root elongation and decomposition rate have been jointly estimated for five and eight tropical tree species, respectively. The second approach classified the factors according to the magnitude of their effects on spatial variability of Rsol. Rsol is derived from both heterotrophic respiration by microorganisms (Rh) that decompose ground surface organic matter and autotrophic respiration by roots (Ra). Datasets from two years experiments of Rsol were used to evaluate the performance of semi-mechanistic model on the Guyaflux site. A soil organic matter model, CENTURY, simulating Rh component was coupled with the McCree model, which simulates the Ra of Rsol. The results of a parametric sensitivity analysis on the different predictions proved that the litter quality (e.g. lignin content) and the soil moisture were the two main factors responsible for the spatial variation of the CO2 flux on the soil. In this study, we showed that the use of simulation models linked to experimental field results is a promising approach to understand the impact of the biotic and abiotic factors on the processes deeply complex in tropical forest.NANCY1-Bib. numérique (543959902) / SudocSudocFranceF

Impact de la saison sèche sur les flux de CO2 en forêt tropicale humide guyanaise

National audienc

Soil fertilization transiently increases radial growth in sessile oaks but does not change their resilience to severe soil water deficit: Soil fertilization transiently increases radial growth in sessile oaks but does not change their resilience to severe soil water deficit

The future climate is projected to become increasingly problematic for European forest ecosystems, likely leading to the decline of many tree species. Forest management must adapt to cope with the negative effects of these changes. Despite the close interactions between nutrients and water in plant physiology, very few studies have investigated the link between soil fertility and tree response to soil water availability and vapour pressure deficit. The objective of this study was to test whether fertilization would modify forest tree resilience to soil water deficit. The potential impact of fertilization on trees during soil water deficit was assessed in a fertilization experiment carried out on sessile oaks. Three fertilization treatments (NCa, NPKCaMg and an unfertilized control) were applied to 40-year-old oaks that were cored almost 40 years later. The responses to drought in the fertilized and control trees were compared by analysing the radial growth chronologies following two complementary approaches. Firstly, the relationships between radial growth and inter-annual climate variations were investigated through dendroclimatic modelling including water deficit indices computed with the daily soil water balance model Biljou(C). Secondly, resistance, recovery and resilience to drought-induced crises were evaluated from tree-ring width and compared among treatments. In addition, the carbon isotope composition (delta C-13) of the tree rings was measured to evaluate potential differential gas-exchange processes between fertilized and control trees. Fertilization had an immediate, strong positive effect on oak growth which disappeared after nine years. NCa and NPKCaMg fertilizations had similar effects, suggesting that adding P, K and Mg did not increase radial growth. Whatever the treatment, ring width variations appeared to be identically constrained by climate, especially by the summer water deficits of the current and previous years. Tree resistance to soil water deficit was negatively affected by the severity of the drought event but did not significantly differ between control and fertilized trees. Tree resilience was not affected by fertilization and remained stable across crises. The tree-ring delta C-13 clearly increased during the severe 1976 water deficit but did not vary among treatments. We discuss the possible mechanisms involved in changes in growth and the lack of any significant effect of fertilization on resilience to drought and conclude that, under the relatively dry conditions and moderately poor soil fertility of our study site, fertilization could temporarily stimulate aboveground growth without inducing imbalances that would modify resilience and resistance to severe drought episodes

The long-term trend in leaf delta13C in two tree species from Northern Amazonia indicates an increase in water-use efficiency over the last two centuries

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