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Key message In order to record the seasonal changes in aboveground biomass production (trunk and branches) in a forest, changes in wood density must be taken into account. A 60-year-old beech forest displayed a large intra-annual variability in its aboveground woody biomass production efficiency. This variation followed a seasonal trend with a maximum during the summer while gross primary production was rather low.Context In the current context of land use and climate change, there is a need to precisely quantify the carbon (C) balance of forest ecosystems, and more specifically, of C allocation to tree compartments.Aims We quantified the seasonal changes in the aboveground biomass production (aBP) of a beech forest growing on two different soils: an alocrisol and a calci-brunisol. In addition, for the alocrisol ecosystem, we assessed the existence and degree of intra-annual variability in the ratio of wood aBP to gross primary production (GPP), i.e., the wood aBP efficiency.Methods The study site is a 60-year-old beech forest in northeastern France. An eddy covariance tower records continuously net ecosystem exchange. To investigate the temporal changes in aBP, mini-cores were drilled and diameter at breast height measurements were taken on a monthly basis from 45 trees for both stands studied over 2014.Results A clear difference in aBP was observed between the two soils with the alocrisol being more productive than the calcibrunisol. For the alocrisol, both woody aBP and GPP changed over the course of the year, reaching peak values during June (6 and 12.5 gC m(-2) day(-1), respectively). Wood applied bias photon-to-current efficiency aboveground Biomass Production Efficiency (aBPE) also showed important intra-annual variations, ranging from 0.09 in September to 0.58 in July. Wood density varied throughout the year, and not taking it into account would have led to an overestimation of aBP by as much as 20% in April and May.Conclusion Our study highlights the importance of taking wood density into account for intra-annual studies of aBP. Wood aBPE cannot be considered as constant as it fluctuated from 0.09 to 0.58 throughout the year for an annual value of 0.34. The potential error in wood aBPE stemming from not taking these changes into account amounts to 15%.

Whereas improved tree mineral nutrition is known to be one driver behind the higher growth in mixed vs pure stands, the effects of stand density and admixture of species with limited functional dissimilarity remain very poorly documented. Our study therefore addresses the impact of local neighbourhood, including density and species composition, on tree nutrition for two species with a limited contrast in traits. We investigated foliar nutrition in a young mixed plantation of beech ( Fagus sylvatica L.) and sycamore maple ( Acer pseudoplatanus L.), planted along a double gradient of density and species proportion. Target trees of both species were selected over the full diameter range along this double gradient, and neighbourhood (local density, species identity and diversity) was determined in a 3 m radius. Using standardized regression analysis, we investigated direct effects of tree size and neighbourhood, and indirect effects of neighbourhood through tree size. Mostly we observed positive tree size effects in both species, be it stronger in beech. For maple, direct positive species identity effects were observed for C, N, P, S, K, Mg, Mn, Zn and Al. For beech, we found indirect negative effects of stand density and diversity. These results were in general agreement with growth data, which suggests that nutrition is one of the driving factors behind complementarity in this stand. We conclude that at this stage the effect of tree size dominates over the neighbourhood effects, including density and diversity. However, the presence of beech reduced intraspecific competition in maple with a positive effect on the foliar nutrient content. As neighbourhood effects are expected to increase as the plantation matures, our findings highlight the potential of admixing tree species with limited dissimilarity in traits for practical use in forestry.

ey message Diachronic photogrammetric canopy height models can be used to quantify at a fine scale changes in dominant height and wood volume following storms. The regular renewal of aerial surveys makes this approach appealing for monitoring forest changes. Context The increasing availability of aerial photographs and the development of dense matching algorithms open up new possibilities to assess the effects of storm events on forest canopies. Aims The objective of this research is to assess the potential of diachronic canopy height models derived from photogrammetric point clouds (PCHM) to quantify changes in dominant height and wood volume of a broadleaved forest following a major storm. Methods PCHMs derived from aerial photographs acquired before and after a storm event were calibrated using 25 field plots to estimate dominant height and volume using various modeling approaches. The calibrated models were combined with a reference damage maps to estimate both the within-stand damage variability, and the amount of volume impacted. Results Dominant height was predicted with a root mean squared error (RMSE) of 4%, and volume with RMSEs ranging from 24 to 32% according to the type of model. The volume impacted by storm was in the range of 42-76%. Overall, the maps of dominant height changes provided more details about within-stand damage variability than conventional photointerpretation do. Conclusion The study suggests a promising potential for exploiting PCHM in pursuit of a rapid localization and quantification of wind-throw damages, given an adapted sampling design to calibrate models.

The timing of leaf phenophases greatly influences the functioning of trees. Phenological traits are thus considered major components of tree fitness, and are expected to be strongly selected under environmental or biotic pressures. To date, most phenological studies have been conducted at the population scale, with comparatively very few works at the scale of individuals. We take advantage of a unique phenological database, consisting of leaf unfolding (LU) and leaf senescence (LS) observations done at the individual scale for 5 years over 35 populations, representing >1200 dominant and co-dominant trees of three species (Quercus robur, Quercus petraea and Fagus sylvatica), to document the within-population variability of phenological traits in temperate deciduous forest trees. We show that individual phenological ranks mostly repeat from year to year among trees of a given population, though LU ranks tend to repeat more than LS ranks. Using simulated dynamics of soil water content, we suggest that inter-individual differences in the access to soil water may play a role in determining tree phenological ranks. The timing of leaf phenology is further correlated with individual growth in a given tree population. In Beech populations, early-leafers tend to grow more. On the other hand, Oak trees entering senescence later tend to grow more. The growth of Oak trees, being more prone to spring pathogenic attacks than Beech, may benefit less from a spring extension of leaf display.

Severe droughts strongly impact photosynthesis (GPP), and satellite imagery has yet to demonstrate its ability to detect drought effects. Especially changes in vegetation functioning when vegetation state remains unaltered (no browning or defoliation) pose a challenge to satellite-derived indicators. We evaluated the performance of different satellite indicators to detect strong drought effects on GPP in a beech forest in France (Hesse), where vegetation state remained largely unaffected while GPP decreased substantially. We compared the results with three additional sites: a Mediterranean holm oak forest (Puéchabon), a temperate beech forest (Hainich), and a semi-arid grassland (Bugacpuszta). In Hesse, a three-year reduction in GPP following drought was detected only by the Enhanced Vegetation Index (EVI). The Photochemical Reflectance Index (PRI) also detected this drought effect, but only after normalization for absorbed light. In Puéchabon normalized PRI outperformed the other indicators, while the short-term drought effect in Hainich was not detected by any tested indicator. In contrast, most indicators, but not PRI, captured the drought effects in Bugacpuszta. Hence, PRI improved detection of drought effects on GPP in forests and we propose that PRI normalized for absorbed light is considered in future algorithms to estimate GPP from space.

First Online: 07 June 2016; International audience; AbstractKey messageA daily 1-km Pan-European weather dataset can drive the BIOME-BGC model for the estimation of current and future beech gross primary production (GPP). Annual beech GPP is affected primarily by spring temperature and more irregularly by summer water stress.ContextThe spread of beech forests in Europe enhances the importance of modelling and monitoring their growth in view of ongoing climate changes.AimsThe current paper assesses the capability of a biogeochemical model to simulate beech gross primary production (GPP) using a Pan-European 1-km weather dataset.MethodsThe model BIOME-BGC is applied in four European forest ecosystems having different climatic conditions where the eddy covariance technique is used to measure water and carbon fluxes. The experiment is in three main steps. First, the accuracy of BIOME-BGC GPP simulations is assessed through comparison with flux observations. Second, the influence of two major meteorological drivers (spring minimum temperature and growing season dryness) on observed and simulated inter-annual GPP variations is analysed. Lastly, the impacts of two climate change scenarios on beech GPP are evaluated through statistical analyses of the ground data and model simulations.ResultsThe weather dataset can drive BIOME-BGC to simulate most of the beech GPP evolution in all four test areas. Both observed and simulated inter-annual GPP variations are mainly dependent on minimum temperature around the beginning of the growing season, while spring/summer dryness exerts a secondary role. BIOME-BGC can also reasonably predict the impacts of the examined climate change scenarios.ConclusionThe proposed modelling approach is capable of approximately reproducing spatial and temporal beech GPP variations and impacts of expected climate changes in the examined European sites.

International audience; With the evolution of ecosystem management and the actual climate change we are facing, there is a need to improve our knowledge of carbon (C) balance and more specifically of C allocation in the plants. In our study, we quantified the seasonal variation of gross primary production (GPP, obtained through eddy covariance measurements) and biomass production (BP, the C fixed into the biomass obtained thanks to inventory campaign) for a 60-year-old even-aged beech stand located in North East of France. We also assessed the seasonal evolution of the BP efficiency (BPE=BP/GPP; Vicca et al., 2012) and its potential determining factors for our site. For 2014, we found a net ecosystem exchange (NEE) of -549 gC m-2, corresponding to a C sequestration. This value breaks down between 1089 gC m-2 for the respiration of the ecosystem and -1639 gC m-2 for the GPP. On the same year, our stand built up 461.6 gC m-2 of tree biomass (leaves, trunk, branches, fine roots), leading to an annual BPE of 0.28, which is within the range of value found on other similar sites. There was a large temporal variation of C allocation to the different parts of the tree biomass during the growth season. Our results show that the growth first happened in the trunk and branches -with a peak value of 74.5 gC m-2 month-1 in May - whereas the fine roots biomass production started later (end of July) and reached a maximum at the end of the growth season (28.49 gC m-2 month-1 for September). The BPE varied also during the year from 0.13 in April to 0.31 in August, where the BP was the same than in July but the cumulated GPP was already decreasing. The seasonal variation may be mainly explained by climatic variations, whereas the shift between woody above-ground biomass and fine roots biomass could be explained by the phenology (linked to physiological mechanisms).

International audience; This paper analyses the variation in tree canopy conductance for water vapour (g$_{\rm c}$) in order to derive a general expression, including the effects of solar radiation ($R$), vapour pressure deficit ($D$), leaf area index ($LAI$) and extractable soil water. Canopy conductance was calculated from transpiration measured in 21 broadleaved and coniferous forest stands, under different climates: temperate, mountain, tropical and boreal. Common features in the dependence of g$_{\rm c}$ on climate and on soil water content were exhibited. When soil water was not limiting, g$_{\rm c}$ was shown to increase linearly with LAI in the range 0 to 6 m$^2$ m$^{-2}$ and reach a plateau value. Besides the positive effect of increasing $R$ and the negative effect of increasing $D$ on g$_{\rm c}$, it was surprisingly shown that a decrease in extractable soil water induced a similar reduction in g$_{\rm c}$ in various tree species, equally in coniferous and in broadleaved. Based on these findings, a general canopy conductance function is proposed.; Un modèle générique de conductance de couverts forestiers dépendant du climat, de la disponibilité en eau dans le sol et de l'indice foliaire. Ce travail réalise l'analyse des facteurs de variation de la conductance du couvert pour la vapeur d'eau (g$_{\rm c}$) avec l'objectif d'en donner une expression générale, prenant en compte les effets du rayonnement global ($R$), du déficit de saturation de l'air ($D$), de l'indice foliaire ($LAI$) et de la réserve hydrique extractible du sol. La conductance du couvert a été calculée à partir de la transpiration mesurée dans 21 peuplements forestiers feuillus et résineux, sous différents types climatiques : tempéré, montagnard, tropical et boréal. Ce travail a montré, pour ces divers peuplements, une dépendance similaire entre g$_{\rm c}$ et les facteurs climatiques, ainsi qu'avec la réserve hydrique extractible du sol ($REW$). En conditions hydriques non limitantes, on observe que g$_{\rm c}$ augmente linéairement avec le $LAI$ entre 0 et 6 m$^2$ m$^{-2}$, puis atteint un plateau. De façon surprenante, en dehors de l'effet positif sur g$_{\rm c}$ de l'augmentation de $R$, et l'effet négatif de celle de $D$, on montre que la diminution de $REW$ a des conséquences similaires sur g$_{\rm c}$ pour diverses espèces forestières, aussi bien feuillues que résineuses. A partir de ces observations, un modèle général de conductance de couvert est proposé ici.

Simulating the gross primary productivity (GPP) of terrestrial ecosystems has been a major challenge in quantifying the global carbon cycle. Many different LUE models have been developed recently, but our understanding of the relative merits of different models remains limited. Using carbon dioxide flux measurements from 155 eddy covariance sites, we here assess the performance of seven LUE models (CASA, CFix, CFlux, EC-LUE, MODIS, VPM and VPRM) and compared the major model algorithms. Comparison between simulated GPP and estimated GPP from flux measurements showed that model performance differed substantially among ecosystem types. In general, most models captured well the pattern and magnitude of GPP for deciduous broadleaf forests and mixed forests, but performed less well in evergreen broadleaf forests and shrublands. During cloudy and overcast days, six out of seven LUE models significantly underestimated GPP because the impacts of diffuse radiation on light use efficiency were ignored. Among the seven models, CFlux and EC-LUE exhibited the lowest root mean square error at 80% and 75% sites respectively. Moreover, these two models showed higher performance for simulating the interannual variability of GPP. Two pairwise comparisons revealed the seven models differed substantially due to the differences in the algorithms describing the environmental regulation on GPP, particularly water stress. This analysis highlights the need to improve LUE model algorithms by integrating impacts of diffuse radiation on LUE and reliable water stress equations., JRC.H.7-Climate Risk Management

International audience; Compared to conifers, broadleaf forests are less prone to soil and water acidification, because (1) they often have soils with larger exchangeable base cation pools, (2) the pollution scavenging capacity of deciduous trees is lower, and (3) they grow more slowly and are often less intensively managed. Since the 1980s, atmospheric deposition acidity has strongly decreased in forest ecosystems, so that the present acidification status of broadleaf forests should be improving. We used a 35 year-old beech plot in the Morvan Mountains (Burgundy, France) to question past and present acidification processes in broadleaf ecosystems. Soil exchangeable Mg, Ca and K pools measured from samples collected in 1974 and 2001 were compared and input-output budgets were computed over the 2003-2008 period. The objectives were (i) to assess Mg and Ca pool size changes over 1974-2008, (ii) to discuss the potential causes of these changes and, (iii) discuss the limits of conventional methods to study nutrient pool size changes (soil data comparison and nutrient budgets). Soil exchangeable Mg pools decreased during the 1974-2001 period while Ca and K pools remained constant, and very small. Soil solution monitoring and input-output budgets over the 2003-2008 period suggested an ongoing loss of exchangeable Ca and Mg, partly due to the desorption of sulphate from the soil which induced Mg, Ca and K depletion. Given the very low concentration in exchangeable base cations, and assuming no change in soil spatial variability, we computed that resampling soils at 10 years intervals may not unequivocally demonstrate a gain/loss of exchangeable base cations. Foliar Mg concentrations were continuously below the deficiency level, K and N concentrations decreased but Ca did not. From this, we discuss the validity/limits of the different approaches used, how the ecosystem can cope with such low levels of nutrients, and the fluxes and processes within the ecosystem that should be investigated in this perspective. (C) 2013 Elsevier B.V. All rights reserved.

National audience

Deux indicateurs de biodiversité associés à la diversité du peuplement arboré sont utilisés en France et en Europe, en l'absence de validation scientifique forte: (1) la richesse en essences ou en genres d'essences, comme un indicateur positif; et (2) l'abondance relative de l'essence dominante ("dominance") comme un indicateur négatif. Nous avons testé le caractère indicateur de ces indicateurs vis-à-vis de la biodiversité floristique du sous-bois, en les comparant à d'autres modèles écologiques, principalement associés à la composition et l'abondance des essences. Nous avons utilisé des modèles statistiques Bayésiens des données d'abondance et de richesse de groupes écologiques, définis sur la base du statut successionnel des espèces ou de leur tolérance à l'ombrage. Les distributions de probabilités utilisées dans les modèles sont nouvelles en écologie. Nous avons étudié la magnitude des effets avec des tests d'équivalence. 49 placettes adultes situées dans la région IFN de la Brie Francilienne ont été étudiées. Sur notre jeu de données, les indicateurs basés sur la richesse en genres ou la dominance étaient plus mauvais que les indicateurs basés sur l'abondance des genres d'essences. La magnitude des effets et l'identité du meilleur modèle indicateur variaient d'un groupe écologique à l'autre. Nos résultats montrent par ailleurs l'influence négative de la surface terrière des essences de demi-ombre sur le recouvrement de tous les groupes écologiques d'herbacées et de ligneux et sur la richesse des groupes non-forestiers et péri-forestiers de ligneux et d'herbacées. Comparé aux études du même genre, notre échantillonnage contrôlait fortement le type de station, éliminant ainsi partiellement la confusion potentielle entre les influences de la gestion et du type de station sur la biodiversité. / Two different biodiversity indicators based on tree species diversity are being used, in Europe and France respectively, without strong prior scientific validation: (1) tree species or genus richness as a positive indicator, and (2) relative abundance of the main species (dominance') as a negative indicator. We tested the relevance of these ecological models as indicators of understory vegetation biodiversity by comparing them to other ecological models, mainly related to tree species composition and abundance. We developed Bayesian statistical models for richness and abundance of ecological groups of understory vegetation species, classified according to successional status or shade tolerance. The count data probability distributions in the models were new to ecology. These models were fitted using data from 49 plots in mature lowland forests in the center of France (Bassin Parisien) with similar site conditions. We used equivalence and inequivalence tests to detect negligible and non-negligible effects. Tree genus richness and dominance resulted in models that were worse than ones based on the abundance of tree genus groups. Furthermore, the only significant results for dominance and tree genus richness were opposite to the ones implicitly assumed in the indicator system. However, the magnitude of the effects and which indicator provided the best statistical model varied among ecological groups of plants. Our results show the negative non-negligible effect of the basal area of undergrowth tree species on the cover of all ecological groups of herbaceous and woody species, and on the species richness of non-forest and peri-forest herbaceous and woody species. Compared to the literature, our sampling design strongly controlled forest and site type, thus removing to some degree the potential confusion between influences on biodiversity of management specific variables and other ecological variables. We discuss our results from both an ecological perspective and in terms of the value of these models as indicators of sustainable management. For example, the best-performing model was a multivariate model, which may be more difficult to explain to forest managers or policy-makers than an indicator simply based on tree genus richness.

Matériel et méthodes : nous donnons ici la synthèse de diverses données publiées et de données inédites provenant de nos propres collectes sur le terrain et de celles de nos collègues. L'emploi de pièges Malaise et de pièges jaunes englués démultiplie l'efficacité des méthodes classiques. Le tri, l'identification des nombreux échantillons et les descriptions d'espèces nouvelles pour la science s'étaleront encore sur plusieurs années et nous ne pouvons donner ici que les résultats d'un premier "écrémage". Certaines données publiées et diverses déterminations douteuses ou inabouties, nécessitant encore un contrôle ou un examen approfondi, ne seront citées que dans le prochain supplément à l'inventaire de Païolive.

The species structure of an ectomycorrhizal (ECM) community was assessed monthly for 15 months in the two horizons (A1 and A2) of an oak temperate forest in northeastern France. Ectomycorrhizal species were identified each month by internal transcribed spacer sequencing. Seventy-five fungal symbionts were identified. The community was dominated by Tomentellaceae, Russulaceae, Cortinariaceae, and Boletales. Four species are abundant in the study site: Lactarius quietus , Tomentella sublilacina , Cenococcum geophilum , and Russula sp1. The relative abundance of each species varied depending on the soil horizon and over time. Some species, such as L. quietus , were present in the A1 and A2 horizons. C. geophilum was located particularly in the A2 horizon, whereas T. sublilacina was more abundant in A1. Some species, such as Clavulina sp., were detected in winter, while T. sublilacina and L. quietus were present all year long. Our results support the hypothesis that a rapid turnover of species composition of the ECM community occurs over the course of a month. The spatial and temporal unequal distribution of ECM species could be explained by their ecological preferences, driven by such factors as root longevity, competition for resources, and resistance to environmental variability.

National audience

The angular distribution of diffuse photosynthetically active radiation (PAR) was characterised in the open and beneath deciduous and conifer forests in Quebec and British Columbia, Canada, under overcast and clear sky conditions, using a restricted field of view light sensor and hemispherical canopy photographs. The angular distribution of PAR was described by the relative light reading (RLR). In the open on overcast days, light was best characterized using the standard overcast sky distribution with the light intensity at the zenith set to four to five times greater than the light intensity at the horizon. RLR under forest stands was found to decrease with decreasing elevation angles under both overcast and clear sky conditions. Aspen (Populus tremuloides Michx.) and Jack pine (Pinus banksiana Lamb.) stands transmitted more light from a relatively wider angle around the zenith than the spruce (Picea glauca (Moench) Voss and Picea mariana (Mill.) BSP) stands, which transmitted light mainly from the zenith. RLR estimated with the hemispherical canopy photographs (RLRhc_corr) generally provided a comparable prediction of the effect of the canopy composition on the angular distribution of PAR., Distribution de la radiation photosynthétiquement active sous différentes conditions de ciel en milieu ouvert et sous peuplements forestiers de feuillus et de conifères au Québec et en Colombie-Britannique, Canada. La distribution angulaire de la radiation photosynthétiquement active (PAR) a été mesurée en milieu ouvert et sous peuplements de feuillus et de conifères situés au Québec et en Colombie-Britannique lors de journées nuageuses et ensoleillées à l'aide d'un senseur à angle restreint et de photographies hémisphériques. La distribution angulaire du PAR a été estimée par une lecture de lumière relative (LLR). En milieu ouvert lors de journées nuageuses, LLR déclinait du zénith vers l'horizon, avec des mesures au zénith de quatre à cinq fois plus élevées qu'à l'horizon. LLR mesurée sous couvert forestier déclinait du zénith vers l'horizon lors de journées nuageuses et ensoleillées. Les peuplements de peupliers (Populus tremuloides Michx.) et de pins gris (Pinus banksiana Lamb.) ont transmis relativement plus de PAR sous un angle plus large autour du zénith que les peuplements d'épinettes (Picea glauca (Moench) Voss et Picea mariana (Mill.) BSP) qui ont principalement transmis du PAR autour du zénith. La distribution angulaire du PAR estimée avec les LLR mesurées avec la photographie hémisphérique sous les différents couverts forestiers correspondait de façon générale aux résultats obtenus avec le senseur.

International audience

La gestion forestière vise des objectifs multiples: fonctions économiques, écologiques, sociales; maintien de la diversité, du rendement; pérennité des formations; gestion et conservation des ressources génétiques. Sur la base de connaissances et de modèles disponibles en matière de flux de gènes et de dynamique forestière, cette étude a permis: (1) l'élaboration d'un modèle démogénétique général prenant en compte simultanément les processus de reproduction (production et dispersion des graines et du pollen en lien avec la phénologie), de régénération ou recrutement, de croissance, compétition et mortalité; (2) l'implémentation informatique d'un tel modèle et l'intégration logicielle des relations disponibles pour cinq situations forestières diverses au sein d'une plate-forme de simulation qui mutualise des fonctionnalités génériques (création d'une bibliothèque "Genetics") tout en étant suffisamment modulaire, ouverte, et accessible aux chercheurs et aux utilisateurs; (3) la réalisation de simulations destinées, pour chaque situation étudiée, d'une part à identifier les éléments les plus sensibles du modèle démogénétique et les processus dont l'étude doit être approfondie et d'autre part à comparer divers modes de gestion forestière (sylviculture, exploitation forestière) pour ébaucher des indications quant à leurs effets sur les ressources génétiques.

The asexual ascomycete Cenococcum geophilum, known for its drought resistance, is a prominent fungus within the ecwmycorrhizal comunities in the beech forests in northeastern France. A high level of genetic diversity between-and within- C. geophilum populations was observed. This result suggests the occurrence of a high rate of mitotic or meiotic recombinations and an effect of stand features on population structure. We compared respiration activities of C. geophilum and Lacturius-type ectomycorrhias sampled from beech stands, during two growing seasons. When soil water potential is lower than -0.2 MPa, the respiration activity of C. geophilum ectomycorrhizas is signiticantly less altered than that of Laccarius-type ectornycorrhizas. Therefore, C. geophihim preserves the physiological integrity of beech root facing drought stress. ln vitro tests did not reveal any metabolic effects which could explain the drought resistance of C. geophilum. In addition, no links between C. geophilum genotype and in vitro physiological behaviour were detected., Parmi les centaines d'espèces du cortège ectomycorhizien de la hêtraie lorraine on trouve un ascomycète asexué, Cenococcum geophilum, qui présente des caractéristiques reconnues de résistance au stress hydrique. L'utilisation de techniques moléculaires a révélé une forte variabilité génétique au sein de cinq populations de C. geophilum. Cette variabilité, sous l'action de facteurs écogéographiques et de limitation des flux de gènes, conduit à une différenciation des .populations. Des mesures d'activité respiratoire d'ectomycorhizes prélevées in situ, pendant deux saisons de végétation, montrent que, lorsque le potentiel hydrique du sol est inférieur à -0,2 MPa, les ectomycorhizes de C. geophilum ont une activité physiologique supérieure à celle des ectomycorhizes du type Lactaire qui dominent numériquement. C. geophilum préserve donc mieux que le type Lactaire l'intégrité physiologique des racines en conditions hydriques limitantes. Des analyses in vitro n'ont pas mis en évidence de déterminisme métabolique à cette résistance. Elles n'ont pas non plus établi de lien entre résistance au stress hydrique et génotype de C. geophilum.

International audience; Five different heat storage terms were studied in a mixed deciduous forest. All terms should be taken into account for the calculation of the overall heat storage, because they all can be significant during certain weather conditions and hours. Heat storage in biomass is estimated using surface temperatures measured with an infrared radiometer, which seems to be a good method. The often neglected photosynthetic heat storage may not be omitted. On a seasonal basis soil heat storage seems to be the most important term. The overall heat storage shows a small tendency for releasing heat to the atmosphere during fall. Fluctuations in overall heat storage are a result of complex changes of several climatic parameters. Due to the high degree of coupling of the forest to the atmosphere, accurate measurements of overall heat storage for the determination of big leaf forest evapotranspiration are not of the utmost importance.; Détermination des termes du bilan d'énergie et calcul de l'évapotranspiration par un modèle "big-leaf " en peuplement feuillu mélangé. Cinq termes différents du bilan d'énergie ont été étudiés dans un peuplement feuillu mélangé. Etant donné que chacun de ces termes peut être significatif en fonction de l'heure et des conditions climatiques, ils doivent tous être pris en compte pour un calcul précis. L'énergie stockée dans la biomasse a été estimée à partir de la température de surface, mesurée à l'aide d'un radiomètre infrarouge. La méthode donne de bons résultats. Le stock d'énergie photosynthétique est non négligeable quoique souvent omis. Sur une base saisonnière, le stock d'énergie dans le sol est le terme le plus important. Lors de la chute des feuilles, on observe un flux positif d'énergie de l'écosystème vers l'atmosphère. Les variations du stock total d'énergie dans l'écosystème sont sous le contrôle d'un ensemble complexe de plusieurs paramètres climatiques. Cependant, étant donné le degré de couplage élevé entre la forêt et l'atmosphère, la mesure précise du stock total d'énergie dans l'écosystème n'est pas des plus importants pour le calcul de l'évapotranspiration par un modèle "big-leaf ".

National audience

L'objectif de cette étude était de quantifier les niveaux de contraintes hydriques subies par 41 peuplements du réseau RENECOFOR (15 hêtraies, 15 chênaies. 6 pessières et 5 sapinières) et d'évaluer leur rôle dans les variations inter-annuelles de croissance radiale des 1219 arbres correspondants. Les contraintes hydriques liées aux différents contextes pédo-climatiques des sites étudiés ont été quantifiées par modélisation mécaniste de bilan hydrique journalier (Granier et al. 1999) paramétré pour chaque placette (réserve utile, RU; indice foliaire, LAI). Un indice de stress caractérisant la durée et l'intensité du déficit hydrique, défini par rapport au seuil de régulation de la transpiration de O,4xRU, a été calculé pour chaque année et chaque peuplement (Bréda 1999). Les données journalières de 68 stations météorologiques du réseau Météo-France ont été utilisées comme variables climatiques d'entrée du modèle. Ces données ont permis de caractériser les conditions climatiques locales moyennes de croissance des arbres et de quantifier les niveaux annuels et mensuels de déficit hydrique. L'analyse des données climatiques a révélé une nette augmentation des températures depuis 40 à 50 ans variant de 1 à 2°C selon la station, le paramètre (Tmin, Tmax, Tmoy) et la période considérés. Les précipitations et la durée d'insolation ont moins variée et aucune tendance générale n'a été observée. II n'est pas apparu de liaisons directes simples entre le réchauffement et les variations de la durée d'insolation et le régime pluviométrique. Pour 7 des 41 peuplements, ces variations climatiques ont aggravé les niveaux de stress depuis 10 à 15 ans. La variabilité de la réserve utile, de l'indice foliaire et de la pluviométrie entre placettes interagissent de manière complexe sur le bilan hydrique des sols, mais expliquent en grande partie les différences de comportements observés entre sites. Pour les résineux bénéficiant de fortes pluies annuelles (> 1000 mm) et bien réparties pendant la saison, les contraintes en eau sont en général assez faibles et ne jouent pas un rôle déterminant dans les variations inter-annuelles de croissance des arbres. Pour ces peuplements, ce sont les températures d'hiver et de printemps qui semblent moduler une grande part des variations inter-annuelles. Pour la majorité des peuplements de feuillus, bénéficiant de régimes pluviométriques « moins favorables », le calcul du bilan hydrique s'est révélé être un outil performant pour détecter les contraintes hydriques affectant les peuplements et pour expliquer les variations de croissance radiale des arbres. Les analyses ont ainsi confirmé le rôle capital de l'alimentation hydrique du hêtre et du chêne dans la régulation de leur croissance. Cependant, le déficit de réserve est apparu plus déterminant pour le hêtre que pour le chêne (sensibilité moyennes des séries chronologiques plus fortes dans les hêtraies) aussi bien au niveau du nombre plus élevé de peuplements pour lesquels le déficit était une variable significativement corrélée à la croissance qu'au niveau du pourcentage de variance expliquée: 13 peuplements sur 15 avec des taux de variance expliquée de 12 à 43% pour le hêtre (moy=25%) et 10 peuplements sur 15 avec des taux de 8 à 32% pour le chêne (moy=19%). L'indice de déficit de réserve annuel (cumul de mai à octobre) est intervenu dans 7 cas. Pour 16 peuplements, c'est le déficit d'un mois où d'une combinaison de mois particuliers qui a expliqué le mieux les fluctuations de croissance. Pour ces sites, les périodes clés sont en relation étroite avec la capacité du sol à stocker l'eau. Pour les faibles réserves « 100 mm), ce sont les déficits du début de saison juin-juillet) qui interviennent sur la croissance annuelle alors qu'une réserve utile plus forte (> 150 mm) tamponne et/ou diffère l'arrivée du stress (relation entre croissance et déficit de août-septembre). Enfin, le calcul du bilan hydrique journalier s'est montré beaucoup plus puissant que les pluviométries mensuelles pour expliquer les variations de croissance radiale des peuplements.

International audience; Capillary solutions of two acidic forest topsoils were sampled for one year. On each of the two soils, there were three stands (Norway spruce, Douglas fir, Hardwood). Capillary solutions were extracted using the centrifugation method. Soil moisture under Hardwood stands was higher than under coniferous stands. Soil solutions under the coniferous tree species were more acidic and more concentrated in SO$_4^{-2}$-S and Na$^+$ than under Hardwoods, showing that coniferous tree species intercept atmospheric depositions more efficiently than hardwood tree species do. Soil solutions under hardwood stands were more concentrated in K$^+$, Alt, Si and Fe$^{3+}$ than those under coniferous stands. On the least desaturated site, soil solutions under the hardwood stand were less concentrated in NO$_3^-$-N and C than under the coniferous stands. The amount of rainfall significantly influenced results by diluting soil solutions. When the amount of rainfall was high, there were little difference between tree species.; Impact des essences forestières sur la composition des solutions du sol en conditions acides. Les solutions capillaires des horizons superficiels de deux sols forestiers acides ont été échantillonnées pendant une année. Sur chaque sol, trois peuplements étaient présents (Epicéa commun, Sapin Douglas, Feuillu). Les solutions capillaires ont été extraites des sols par centrifugation. L'humidité des sols sous feuillu est supérieure à celle sous résineux. Les solutions de sol sous résineux sont plus acides et plus concentrées en S-SO$_4^{-2}$ et en Na$^+$, ce qui témoigne d'une plus grande capacité de ces essences à capter les dépôts atmosphériques. Les solutions sous feuillus sont plus concentrées en K$^+$, Alt, Si et Fe$^{3+}$ que celles sous les résineux. Sur le site le moins désaturé, les solutions sous feuillus sont moins concentrées en N-NO$_3^-$ et C que celles sous les résineux. La quantité de précipitations influence notablement les résultats en diluant les solutions de sol. Lorsque la pluviosité est très importante, les différences entre essences sont peu marquées.