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13 Pág., Forest stand and environmental factors influence soil organic carbon (SOC) storage, but little is known about their relative impacts in different soil layers. Moreover, how environmental factors modulate the impact of stand factors, particularly species mixing, on SOC storage, is largely unexplored. In this study, conducted in 21 forest triplets (two monocultures of different species and their mixture on the same site) distributed in Europe, we tested the hypothesis that stand factors (functional identity and diversity) have stronger effects on topsoil (FF + 0–10 cm) C storage than environmental factors (climatic water availability, clay + silt content, oxalate-extractable Al–Alox) but that the opposite occurs in the subsoil (10–40 cm). We also tested the hypothesis that functional diversity improves SOC storage under high climatic water availability, clay + silt contents, and Alox. We characterized functional identity as the basal area proportion of broadleaved species (beech and/or oak), and functional diversity as the product of broadleaved and conifer (pine) proportions. The results show that functional identity was the main driver of topsoil C storage, while climatic water availability had the largest control on subsoil C storage. Functional diversity decreased topsoil C storage under increasing climatic water availability, but the opposite was observed in the subsoil. Functional diversity effects on topsoil C increased with increasing clay + silt content, while its effects on subsoil C were negative at increasing Alox content. This suggests that functional diversity effect on SOC storage changes along gradients in environmental factors and the direction of effects depends on soil depth., The European Union and the National Fund for Scientific Research (Fonds de la recherche scientifique) of Belgium supported this research as part the ERA-Net SUMFOREST project REFORM–Mixed species forest management. Lowering risk, increasing resilience (www.reform-mixing.eu). The lead author was further supported by Université catholique de Louvain for covid-19-related delays of his PhD project. Some of the triplets were selected, established and measured with funding from the EuMIXFOR FP1206 Cost Action project. The Polish State Forests Enterprise also supported one of the Polish co-authors (Grant No: OR.271.3.15.2017). The Orléans site, OPTMix was installed thanks to ONF (National Forest Service, France), belongs to research infrastructure ANAEE-F; it is also included in the SOERE TEMPO, ZAL (LTSER Zone Atelier Loire) and the GIS Coop network.

Heterogeneity of structure can increase mechanical stability, stress resistance and resilience, biodiversity and many other functions and services of forest stands. That is why many silvicultural measures aim at enhancing structural diversity. However, the effectiveness and potential of structuring may depend on the site conditions. Here, we revealed how the stand structure is determined by site quality and results from site-dependent partitioning of growth and mortality among the trees. We based our study on 90 mature, even-aged, fully stocked monocultures of Scots pine (Pinus sylvestris L.) sampled in 21 countries along a productivity gradient across Europe. A mini-simulation study further analyzed the site-dependency of the interplay between growth and mortality and the resulting stand structure. The overarching hypothesis was that the stand structure changes with site quality and results from the site-dependent asymmetry of competition and mortality. First, we show that Scots pine stands structure across Europe become more homogeneous with increasing site quality. The coefficient of variation and Gini coefficient of stem diameter and tree height continuously decreased, whereas Stand Density Index and stand basal area increased with site index. Second, we reveal a site-dependency of the growth distribution among the trees and the mortality. With increasing site index, the asymmetry of both competition and growth distribution increased and suggested, at first glance, an increase in stand heterogeneity. However, with increasing site index, mortality eliminates mainly small instead of all-sized trees, cancels the size variation and reduces the structural heterogeneity.Third, we modelled the site-dependent interplay between growth partitioning and mortality. By scenario runs for different site conditions, we can show how the site-dependent structure at the stand level emerges from the asymmetric competition and mortality at the tree level and how the interplay changes with increasing site quality across Europe. Our most interesting finding was that the growth partitioning became more asymmetric and structuring with increasing site quality, but that the mortality eliminated predominantly small trees, reduced their size variation and thus reversed the impact of site quality on the structure. Finally, the reverse effects of mode of growth partitioning and mortality on the stand structure resulted in the highest size variation on poor sites and decreased structural heterogeneity with increasing site quality. Since our results indicate where heterogeneous structures need silviculture interventions and where they emerge naturally, we conclude that these findings may improve system understanding and modelling and guide forest management aiming at structurally rich forests., This study was supported by the ERA-Net COFUND programme SUMFOREST, with the national funding organization Federal Ministry of Nutrition and Agriculture (REFORM, grant #2816ERA02S). The project has also received funding from the European Union’s Horizon 2020 research and innovation programme under grant agreement No952314 and under the Marie Skłodowska-Curie grant agreement No 778322, and the 2017 - 2018 Belmont Forum and BiodivERsA joint call for research proposals, under the BiodivScen ERA-Net COFUND programme, with the national funding organization Federal Ministry of Education and Research (grant #16LC1805B). The German Science Foundation funded the project “Structure and growth of mixed Scots pine/European beech stands compared with pure stands analyzed along an ecological gradient through Europe” (grant #PR 292/15-1). H.P. and E.U. were additionally supported by the Bavarian Ministry of Nutrition, Agriculture and Forestry through the projects “Pine-spruce mixed stands in Bavaria” (grant #7831-20339-2012) and “Maintenance of the long term experimental plots in Bavaria (W07, #7831-26625-2017). M.d.R. and R.R.-P. were supported by the projects CLU-2019-01 - iuFOR Institute Unit of Excellence” of the University of Valladolid and VA183P20–SMART, funded by the Junta de Castilla and Leon (Spain) and co-financed by the European Union (ERDF “Europe drives our growth”). J.C. was supported by the National Agency of Agricultural Research (Project No. QK21020307). K.B. was supported by the Polish Government MNiSW 2018–2021 Matching Fund No. 117/H2020/2018. M.M. was supported by the Estonian Research Council grant (PRG1586) and Estonian University of Life Sciences projects number P180024MIME and P200029MIME. R.S. was supported by the Slovak Research and Development Agency project No. APVV- 19-0035.

International audience; Aim Climate change is known to be a driver of changes in forest plant communities and to modify disturbance regimes. We investigated whether forest gaps favoured vegetation adaptation to warmer climates by accelerating the shift of plant communities to a warmer-adapted composition independently of canopy closure linked to natural forest dynamics. Location Temperate and mountainous forests of France. Time period 2002-2018. Major taxa studied Vascular plants. Methods Using floristic surveys conducted in 2002 and 2018 in 139 permanent plots set up in unmanaged forest gaps created by windstorms in 1999, we assessed the plant communities' changes in context of global warming. We also compared gap communities to closed communities inventoried in undisturbed forest. A community temperature index (CTI) and a community light index (CLI) were computed for each floristic survey with species temperature optima and Ellenberg light indicator values, respectively. Results CTI increased significantly in the gaps over the 16 years by 0.11 degrees C/decade on average. During the same period, the CLI decreased, indicating that community thermophilization was not the direct result of an increase in solar radiation linked to gap creation. Models showed that, after canopy recovery, thermophilization was stronger in mountains (+0.54 degrees C) than lowland sites (+0.12 degrees C) compared to undisturbed forests. The difference between former gaps and undisturbed forests resulted from the colonization of warm adapted species and the decline of cold adapted species in gaps. Main conclusions For a given canopy closure, our results show that thermophilization of understorey communities is higher in areas with previous disturbances than in undisturbed forests. By increasing the speed of thermophilization, the disturbance regime plays a key role in the adaptation of forest communities to climate warming. The effects of large-scale disturbances on forest plant composition should therefore not be overlooked, especially since disturbance regime tends to intensify with climate change.

International audience; Mechanical site preparation (MSP) is often performed prior to planting to improve the growth and survival of planted seedlings. In this study, we compared root development of 5-years-old Quercus petraea and Pinus nigra seedlings planted in plots that had been prepared with different methods, i.e. deep scarification, deep scarification combined with mounding-subsoiling, herbicide and a control without preparation. Seventy-two trees were excavated (36 per species) and their root system was measured by recording points in a three-dimensional space along their roots. The variation of the number of roots with depth and distance to root collar was assessed and analysed, as well as the root projection area. Our results showed that root development was better in the plots with mechanical preparation, for both Q. petraea and P. nigra, when compared to the control. Combining mounding to subsoiling made the roots extending deeper, especially for Q. petraea. A strong relationship was found between root projection area and root collar diameter, indicating the primary effect of lateral root spread on tree growth. The herbicide treatment induced the highest root growth, which raised questions about the potential negative effects of changes in soil properties caused by MSP methods.

Increasing species diversity is considered a promising strategy to mitigate the negative impacts of global change on forests. However, the interactions between regional climate conditions and species-mixing effects on climate-growth relationships and drought resistance remain poorly documented. In this study, we investigated the patterns of species-mixing effects over a large gradient of environmental conditions throughout Europe for European beech (Fagus sylvatica L.) and Scots pine (Pinus sylvestris L.), two species with contrasted ecological traits. We hypothesized that across large geographical scales, the difference of climate-growth relationships and drought resistance between pure and mixed stands would be dependent on regional climate. We used tree ring chronologies derived from 1143 beech and 1164 pine trees sampled in 30 study sites, each composed of one mixed stand of beech and pine and of the two corresponding pure stands located in similar site conditions. For each site and stand, we used Bootstrapped Correlation Coefficients (BCCs) on standardized chronologies and growth reduction during drought years on raw chronologies to analyze the difference in climate-tree growth relationships and resistance to drought between pure and mixed stands.We found consistent large-scale spatial patterns of climate-growth relationships. Those patterns were similar for both species. With the exception of the driest climates where pure and mixed beech stands tended to display differences in growth correlation with the main climatic drivers, the mixing effects on the BCCs were highly variable, resulting in the lack of a coherent response to mixing. No consistent species-mixing effect on drought resistance was found within and across climate zones. On average, mixing had no significant effect on drought resistance for neither species, yet it increased pine resistance in sites with higher climatic water balance in autumn. Also, beech and pine most often differed in the timing of their drought response within similar sites, irrespective of the regional climate, which might increase the temporal stability of growth in mixed compared to pure stands. Our results showed that the impact of species mixing on tree response to climate did not strongly differ between groups of sites with distinct climate characteristics and climate-growth relationships, indicating the interacting influences of species identity, stand characteristics, drought events characteristics as well as local site conditions., The networking in this study has been supported by the COST Action FP1206 EUMIXFOR. All authors thank the numerous persons who contributed to the fieldwork in the different sites across Europe. We acknowledge the E-OBS dataset from the EU-FP6 project ENSEMBLES (http://ensembles-eu.metoffice.com) and the data providers in the ECA&D project (http://www.ecad.eu).

14 Pág., While the impacts of forest management options on carbon (C) storage are well documented, the way they affect C distribution among ecosystem components remains poorly investigated. Yet, partitioning of total forest C stocks, particularly between aboveground woody biomass and the soil, greatly impacts the stability of C stocks against disturbances in forest ecosystems. This study assessed the impact of species composition and stand density on C storage in aboveground woody biomass (stem + branches), coarse roots, and soil, and their partitioning in pure and mixed forests in Europe. We used 21 triplets (5 beech-oak, 8 pine-beech, 8 pine-oak mixed stands, and their respective monocultures at the same sites) in seven European countries. We computed biomass C stocks from total stand inventories and species-specific allometric equations, and soil organic C data down to 40 cm depth. On average, the broadleaved species stored more C in aboveground woody biomass than soil, while C storage in pine was equally distributed between both components. Stand density had a strong effect on C storage in tree woody biomass but not in the soil. After controlling for stand basal area, the mixed stands had, on average, similar total C stocks (in aboveground woody biomass + coarse roots + soil) to the most performing monocultures. Although species composition and stand density affect total C stocks and its partitioning between aboveground woody biomass and soil, a large part of variability in soil C storage was unrelated to stand characteristics., The European Union and the National Fund for Scientific Research (Fonds de la recherche scientifique) of Belgium supported this research as part the ERA-Net SUMFOREST project REFORM–Mixed species forest management. Lowering risk, increasing resilience (www.reform-mixing.eu). The lead author was further supported by Université catholique de Louvain for covid-19 related delays of his PhD project. Some of the triplets were selected, established and measured with funding from the EuMIXFOR FP1206 Cost Action project. The Polish State Forests Enterprise also supported one of the Polish co-authors (Grant No: OR.271.3.15.2017). The Orléans site, OPTMix was installed thanks to ONF (National Forest Service, France), belongs to research infrastructure ANAEE-F; it is also included in the SOERE TEMPO, ZAL (LTSER Zone Atelier Loire) and the GIS Coop network.

International audience; Promoting mixed forests is often seen as a management option to increase the resilience of forests to droughts. However, mixing can also improve tree growth, and if faster-growing trees use more water, they may be exposed to higher risks of drought. Across Belgium, we tested whether increased tree growth in mixed sessile oak/common beech stands results in increased drought exposure of these forests in severe dry summers. We sampled wood cores from 222 trees in mixed and corresponding pure stands at eight different sites and measured ring widths and stable carbon isotope composition (delta C-13) in the wood formed during 2001 (wet summer as a re-ference year) and 2003 (severe dry summer). Pre-drought growth (PG), an index of tree growth in normal climatic conditions, was calculated as the mean ring width in the five years preceding the drought, and drought exposure was assessed as the difference in delta C-13 between the dry and the reference year. Growth of individual beech trees was faster in mixtures and both mixing and pre-drought tree growth were positively related to beech drought exposure. These two effects were not independent: the increased drought exposure in mixed stands was an effect of the faster growth. Analysis of delta C-13 values suggests that in reference conditions, beech trees in mixtures had a less conservative water use than trees in pure stands, but that they shifted to a functioning similar to that in pure stands under drought. By contrast, growth and drought exposure of oak trees did not differ between pure and mixed stands. Our results emphasize the different strategies of these two species regarding their response to mixing and drought. Beech maximizes its growth as a result of greater resource capture in mixed stands, but this increases the risk of suffering from severe droughts. Oak, on the other hand, exhibits lower variability in growth and in drought exposure across pure and mixed stands. We should stress that if mixing is used as a management strategy to mitigate the negative effects of climate change on temperate forests, forest managers should be aware that the performance benefits of mixed forests in non-drought years could in fact hinder the drought resistance of these forests in severe drought years.

Tree species-mixing has been suggested as one option to counteract the adverse effects of global change on tree mineral nutrition, yet the effect of mixing on nutrient availability remains poorly documented. We therefore analyzed the current foliar nutrient (N, P, K, Ca, Mg) quantities and ilr balances (isometric log transformed ratios between elements or groups of elements) for 261 European beech and 248 Scots pine trees from 15 sites, each consisting of one beech-pine mixed stand and the respective monocultures, across a gradient of environmental conditions in Europe. We hypothesized an overall positive effect of mixing on tree foliar nutrient content, and that this mixing effect would be stronger on nutrient-poor sites. Using linear mixed models and multivariate linear regression models, we first tested for the effects of species (beech/pine) and composition (pure/mixed) across all sites; we then investigated whether the species-mixing effect was related to site fertility. The nutrient composition of beech leaves and pine needles differed significantly for all ilr balances. For both species, significant mixing effects were detected for some nutrients and ilr balances; those effects, however, could not be consistently related to contrasted nutrient composition between species. For most nutrients and ilr balances, the mixing effect was influenced by the site nutritional status, but the pattern differed from expectation: absence or minor differences between monocultures and mixtures at the lower end of the chemical fertility gradient, and maximum differences in rich soils. The contrasting foliar nutrient composition of pine and beech trees and the site nutrient status only partly explained the mixing effects on tree mineral nutrition. Our results claim for a better understanding of nutrient-related mechanisms associated with complementarity and points towards the need to further expand the existing frameworks to account for the multivariate nature of tree nutrition., The main autor obtained a PhD grant from the ‘Fonds National de la Recherche Scientifique’ (FNRS-FRIA) and additional funding support from the Walloon forest service (Service Public de Wallonie—Département de la Nature et des Forêts) through the 5 year research programme “Accord cadre de recherches et de vulgarisation forestières”.

Mechanical site preparation methods that used tools mounted on lightweight excavators and that provided localised intensive preparation were tested in eight experimental sites across France where the vegetation was dominated either by (L.) Moench or (L.) Kuhn. Two lightweight tools (Deep Scarifier: DS; Deep Scarifier followed by Multifunction Subsoiler: DS+MS) were tested in pine ( L., var. (Loudon) Hyl. or Aiton) and oak ( (Matt.) Liebl. or L.) plantations. Regional methods commonly used locally (herbicide, disk harrow, mouldboard plow) and experimental methods (repeated herbicide application; untreated control) were used as references in the experiments. Neighbouring vegetation cover, seedling survival, height and basal diameter were assessed over three to five years after plantation. For pines growing in , seedling diameter after four years was 37% and 98% greater in DS and DS+MS, respectively, than in the untreated control. For pines growing in , it was 62% and 107% greater in the same treatments. For oak, diameter was only 4% and 15% greater in , and 13% and 25% greater in , in the same treatments. For pines, the survival rate after four years was 26% and 32% higher in and 64% and 70% higher in , in the same treatments. For oak, it was 3% and 29% higher in and 37% and 31% higher in . Herbicide, when applied for three or four years after planting, provided the best growth performances for pines growing in and and for oaks growing in . For these species and site combinations, DS+MS and DS treatments reduced the neighbouring vegetation cover for one to four years following site preparation.Molinia caeruleaPteridium aquilinumPinus sylvestrisPinus nigracorsicanaPinus pinasterQuercus petraeaQuercus roburM. caeruleaP. aquilinumM. caeruleaP. aquilinumM. caeruleaP. aquilinumM. caeruleaP. aquilinumM. caerulea P. aquilinumP. aquilinum

International audience; AbstractThere is a growing demand for moving towards sustainable agri-food systems which per nature covers a complex network of activities and domains; such systems will benefit from multi-criteria decision analysis (MCDA) methods. Although some reviews on MCDA in agri-food research have been published, none of them covered the whole value chain. In this article, a corpus of 954 articles published by INRA scientists from 2007 to 2017 was used to study the diversity and potentiality of MCDA techniques. For the first time, experts from more than 10 agri-food domains worked altogether to annotate the articles, carry out a multivariate analysis, and finally interpret the statistical results to identify the specificities of certain domains and the complementarities between domains and to suggest avenues for future agri-food research. One-third of the studies were based only on a list of indicators, even when their purpose was to choose, sort, or rank options. Regardless of the scientific discipline in the agri-food sector, MCDA studies rarely considered temporal dynamics, spatial scale changes, or stakeholder contributions. As the agri-food system becomes increasingly sustainable in the near future, the use of MCDA methods will accelerate. To become more effective, they will have to include ecosystem services, even outside the scope of ecological studies. Similarly, MCDA studies will need to include participatory science to involve stakeholders (i.e., public authorities, governmental agencies) and end-users (i.e., farmers, producers, industrials, consumers) in the construction of the multi-criteria evaluation but also in the resulting decisions.

International audience; Just as the aboveground tree organs represent the interface between trees and the atmosphere, roots act as the interface between trees and the soil. In this function, roots take-up water and nutrients, facilitate interactions with soil microflora, anchor trees, and also contribute to the gross primary production of forests. However, in comparison to aboveground plant organs, the biomass of roots is much more difficult to study. In this study, we analyzed 19 European datasets on above- and belowground biomass of juvenile trees of 14 species to identify generalizable estimators of root biomass based on tree sapling dimensions (e.g. height, diameter, aboveground biomass). Such estimations are essential growth and sequestration modelling. In addition, the intention was to study the effect of sapling dimension and light availability on biomass allocation to roots. All aboveground variables were significant predictors for root biomass. But, among aboveground predictors of root biomass plant height performed poorest. When comparing conifer and broadleaf species, the latter tended to have a higher root biomass at a given dimension. Also, with increasing size, the share of belowground biomass tended to increase for the sapling dimensions considered. In most species, there was a trend of increasing relative belowground biomass with increasing light availability. Finally, the height to diameter ratio (H/D) was negatively correlated to relative belowground biomass. This indicates that trees with a high H/D are not only more unstable owing to the unfavorable bending stress resistance, but also because they are comparatively less well anchored in the ground. Thus, single tree stability may be improved through increasing light availability to increase the share of belowground biomass.

International audience; With the rise of large herbivore populations in most northern hemisphere forests, browsing is becoming an increasingly important driver of forest regeneration dynamics. Among other processes affecting the regeneration, the concept of plant-herbivore feedback loops holds that browsed saplings are more subject to subsequent herbivory. This phenomenon is interpreted as a consequence of compensatory growth following browsing since fast growth is generally associated with higher digestibility and lower defense against herbivores. However, studies linking browsing-induced trait variations to subsequent attractiveness to herbivores are still lacking, especially in the forest context. In this study, we experimentally examine the existence of a feedback loop between oak (Quercus robur L.) and roe deer (Capreolus capreolus) and investigate its underlying morphological and chemical traits. We simulated single and repeated roe deer browsing on nursery-grown oak saplings and measured the changes in sapling height growth, lateral branching, leaf traits and winter shoot traits over two years. We conducted winter feeding trials with tame roe deer one year after the first treatment to test the effect of simulated browsing on sapling attractiveness. Simulated browsing reduced sapling height growth but had no effect on branching. Simulated browsing had no effect on leaf traits after half a year, but decreased the phenolic content and increased the fiber content of winter shoots the following winter. Contrary to our predictions, roe deer preferentially browsed control saplings over saplings previously browsed. After two years, repeated browsing promoted fast carbon acquisition leaf traits (high chlorophyll, high specific leaf area and low fiber content), reduced leaf phenolic content and increased leaf digestibility. We showed that a reduction in 1-year-old oak sapling height growth following browsing, combined with increased structural defense at the expense of chemical defense in winter shoots the following winter, was correlated with reduced browsing pressure, thereby challenging the feedback loop hypothesis. However, we also demonstrated that repeated browsing promoted fast carbon acquisition leaf traits in 2.5-year-old saplings, which tend to support the existence of a feedback loop on older and more intensively browsed saplings. As such, our study provides empirical evidence that morphological and physiological trait responses to browsing influence oak sapling attractiveness, but that the direction and magnitude of this effect depend on the ontogenic stage of the sapling and on the number of browsing events.

International audience; Oak reproduction is characterized by mast seeding with high inter-annual fluctuations in fruit production. Such resource pulses can greatly affect ecosystem functioning and may cause seed consumers to alter their mobility, demography, or diet. Consequences of mast seeding for seed consumers remain poorly understood as their long timescale makes them difficult to study. We investigated impacts of oak mast seeding on the feeding behavior of two sympatric European deer species: red deer (Cervus elaphus) and roe deer (Capreolus capreolus). We analyzed their rumen content over a 31-year period in tandem with 10 years of data on oak fructification (i.e. 8 years of field monitoring and two modelled years). Acorn production is strongly correlated with consumption by both deer species. In years of high fructification, acorns represent more than 50% and 35% of red and roe deer diet, respectively, confirming assumptions that deer favor acorns when these are available. Red deer eat more acorns than roe deer both between and within years. High acorn production in mast years appears to saturate the capacity of deer to consume acorns. As the proportion of acorns increase in their diet, red deer eat more grasses and less conifer browse. No dietary shift was found for roe deer. By inducing dietary shifts in consumers, oak mast seeding can have cascading effects on ecosystem processes, notably on the damages on conifers caused by red deer and the consequences for forest dynamics.

International audience; Mechanical site preparation (MSP) is widely used in forestry to improve plantation success. Although it is known to alter soil properties, its direct effects on soil structure have rarely been described. The cultivation profile is a visual soil evaluation (VSE) method developed in agricultural research to analyse the impacts of cultivation practices on soil structure. The objective of the study was to adapt the method to forest plantations in order to analyse the effects of MSP on soil quality. Cultivation profiles were performed in six experimental plantation sites located in Northern France. The method made it possible to compare the impacts on soil structure of three MSP methods. It provided a schematic representation of the soil structural quality and a quantitative estimation of the volume of soil favourable to seedling root growth. It also highlighted unexpected negative effects of some MSP methods on soil structure, such as the creation of small cavities, the presence of compacted soil volumes due to wheel tracks or smeared soil volumes due to tool pass, and the pseudogleisation of soil zones due to changes in water circulation in the soil. The relevance and limitations of VSE methods in the context of forest plantation as well as the expected future development of the methods are discussed.

Tree species mixing has been widely promoted as a promising silvicultural tool for reducing drought stress. However, so far only a limited number of species combinations have been studied in detail, revealing inconsistent results. In this study, we analysed the effect of mixing Scots pine and oak (pedunculate oak and sessile oak) trees on their drought response along a comprehensive ecological gradient across Europe. The objective was to improve our knowledge of general drought response patterns of two fundamental European tree species in mixed versus monospecific stands. We focused on three null hypotheses: () tree drought response does not differ between Scots pine and oak, () tree drought response of Scots pine and oak is not affected by stand composition (mixture versus monoculture) and () tree drought response of Scots pine and oak in mixtures and monocultures is not modified by tree size or site conditions. To test the hypotheses, we analysed increment cores of Scots pine and oak, sampled in mixed and monospecific stands, covering a wide range of site conditions. We investigated resistance (the ability to maintain growth levels during drought), recovery (the ability to restore a level of growth after drought) and resilience (the capacity to recover to pre-drought growth levels), involving site-specific drought events that occurred between 1976 and 2015. In monocultures, oak showed a higher resistance and resilience than Scots pine, while recovery was lower. Scots pine in mixed stands exhibited a higher resistance, but also a lower recovery compared with Scots pine in monocultures. Mixing increased the resistance and resilience of oak. Ecological factors such as tree size, site water supply and site fertility were found to have significant effects on the drought response. In the case of Scots pine, resistance was increased by tree size, while recovery was lowered. Resistance of oak increased with site water supply. The observed mixing effect on the tree drought response of Scots pine and oak was in some cases modified by the site conditions studied. Positive mixing effects in terms of resistance and resilience of oak increased with site water supply, while the opposite was found regarding recovery. In contrast, site fertility lessened the positive mixing effect on the resistance of Scots pine. We hypothesise that the observed positive mixing effects under drought mainly result from water- and/or light-related species interactions that improve resource availability and uptake according to temporal and spatial variations in environmental conditions. This work was supported by the European Union as part of the ERA-Net SUMFOREST project REFORM – Mixed species forest management. Lowering risk, increasing resilience (2816ERA02S, PCIN2017-026) and the Horizon 2020 research and innovation programme under the Marie Skłodowska-Curie grant agreement No 778322. All contributors thank their national funding institutions for supporting the establishment, mensuration and analysis of the studied triplets. The first author wants to thank the German Federal Ministry of Food and Agriculture (BMEL) for financial support through the Federal Office for Agriculture and Food (BLE) (grant number 2816ERA02S), as well as the Bayerische Staatsforsten (BaySF) and Landesbetrieb Forst Brandenburg for providing suitable research sites. Research on the Lithuanian triplets (LT 1, LT 2) was made possible by the national funding institution Research Council of Lithuania (LMTLT) (agreement number S-SUMFOREST-17-1). The French site FR 1 belongs to the OPTMix experimental site (https://optmix.irstea.fr), which is supported annually by Ecofor, Allenvi, and the French national research infrastructure ANAEE-F. A special thank is due to Peter Biber for supporting the statistical analysis.

Establishing mixed-species stands is frequently proposed as a strategy to adapt forests to the increasing risk of water scarcity, yet contrasted results have been reported regarding mixing effects on tree drought exposure. To investigate the drivers behind the spatial and temporal variation in water-related mixing effects, we analysed the δ13C variation in 22-year tree ring chronologies for beech and pine trees sampled from 17 pure and mixed pine–beech stands across a large gradient of environmental conditions throughout Europe. In the pure stands, average δ13C values were lower for beech (−27.9‰ to −22.2‰) than for pine (−26.0‰ to −21.1‰), irrespective of site conditions. Decreasing SPEI values (calculated over June to September) were associated with an increase in δ13C for both species, but their effect was influenced by stand basal area for pine and site water availability for beech. Mixing did not change the temporal constancy of δ13C nor the tree reaction to a drought event, for any of the species. While the mixing effect (Δ δ13C = δ13C pure stands − δ13C mixed stands) was on average positive for beech and non-significant for pine across the whole gradient, this effect strongly differed between sites. For both species, mixing was not significant at extremely dry sites and positive at dry sites; on moderately wet sites, mixing was positive for beech and negative for pine; at sites with permanent water supply, no general patterns emerge for any of the species. The pattern of mixing effect along the gradient of water availability was not linear but showed threshold points, highlighting the need to investigate such relation for other combinations of tree species., The networking in this study has been supported by COST Action FP1206 EUMIXFOR.

International audience; AbstractKey messageIn a mixed,Fagus sylvaticaL.-Acer pseudoplatanusL., young plantation, trees of both species absorbed water from superficial soil layers despite the presence of roots and water in deeper layers. Admixture proportion and tree density were weak predictors of water acquisition depth, as well as fine root vertical distribution, although it might be due to distinct periods of root and isotope investigations.ContextPromoting mixed forests and reducing stand densities have been proposed as effective ways to maintain the productivity of temperate planted forest stands in a changing climate.AimsThe objective of this study was to analyse how stand density and the degree of admixture of European beech and sycamore maple interactively influenced the water acquisition profile of individual trees.MethodsWe used a stable isotope (deuterium) approach to determine the profiles of soil water acquisition of both species in a 16-year-old plantation where trees had been planted along crossed gradients of tree density and species proportion. The profiles were then compared with the vertical distribution of fine root of these species in the plantation.ResultsAll the target trees mostly absorbed soil water from the first few centimetres of soil despite homogenous vertical water availability and the fact that a great part of the fine root biomass was located below 10 cm. Admixture proportion and tree density had negligible effects on soil water acquisition depth.ConclusionNo vertical differentiation of soil water acquisition between the two species was observed, suggesting that mixing these species does not promote reduction of belowground competition for resource acquisition. The vertical distribution of fine root may be a weak predictor of water acquisition depth.

Oak (Quercus petraea (Matt.) Liebl. and Quercus robur L. grouped), European beech (Fagus sylvatica L.) and hornbeam (Carpinus betulus L.) are three major species of western and central European forests. When conditions are suitable for the three species, silvicultural management often favours oak because of its greater economic interest. Forest managers know that beech and hornbeam are strong competitors for oak during the regeneration phase, but the conditions that influence the relative success of the regeneration of the three species growing in mixture are still poorly characterised. The natural regeneration of the three species 20 years after canopy openings was studied based on 108 study sites established in 2001 in French forests impacted by the windstorms Lothar and Martin in 1999. In spring and summer 2018 and 2019, all saplings over 0.1 m in height were counted, species were identified and diameter at breast height (DBH) was measured for all saplings over 1.30 m. The height of the saplings with the highest and median DBH in each plot was recorded for each species. Mixed models were used to analyse the combined effects of former stand type, distance from the edge of the gap and soil conditions on species presence, sapling abundance, DBH and height. The modelling approach highlighted the preponderant role of the initial seed rain on the presence and sapling density of the three species compared to interspecific competition or distance from the edge. The two latter factors had a secondary effect on sapling DBH and height. Beech and hornbeam saplings were more abundant than oak saplings regardless of soil conditions (on average, 3097, 3063 and 344 saplings ha−1, respectively), suggesting a strong competitive ability of these two species. Oak was present on 22% (43% for hornbeam and 68% for beech) of the studied plots, at a low density but with a height and DBH similar to that of beech or hornbeam. This result highlights the high dissemination capacity of beech and hornbeam, which prevents the establishment of a stand dominated by oak. When seeking to obtain oak-dominated stands in the lowlands of Europe, the abundance of beech and hornbeam can be a limiting factor that could lead to the disappearance of oak from large areas if no silvicultural operations are performed to promote it.

The recruitment of forest trees is driven by both bottom-up processes (the acquisition of resources) and top-down processes (herbivory). To initiate stand regeneration, foresters commonly reduce tree density to increase light levels for seedlings and enhance primary productivity. These changes in vegetation dynamics, however, could also influence effects of ungulate browsing, resulting in unintended consequences for forest management. Here, we assessed how effects of ungulate exclusion and canopy opening interacted to affect the regeneration of two oak species: Quercus robur and Quercus petraea. We monitored the growth and survival of oak seedlings for two to three growth seasons in paired fenced and unfenced plots under contrasting conditions of canopy openness (8% to 52%) at five sites in southern Sweden and three sites in northeastern France. We scored browsing in the unfenced plots by the four cervids occurring in these areas (Alces alces, Capreolus capreolus, Cervus elaphus and Dama dama). Fencing increased the growth of (mostly taller) seedlings occurring in Sweden and the survival of (mostly smaller) seedlings in France. Both effects increased as canopies became more open. Browsing reduced oak seedling growth in both countries, independently of canopy openness. Canopy openness increased browsing levels in Sweden. Cervid densities did not appear to modify how fencing affected oak seedling growth and survival. In both contrasting forest environments, creating gaps tended to enhance ungulate damage on young forest stands as browsing frequency increased. We conclude that net forest regeneration reflects a subtle equilibrium between enhancing resource availability, boosting seedling growth, and limiting herbivory, which curtails seedling growth and survival.

In most mixed-species forestry systems, regeneration is a critical step for individual tree survival and species coexistence. Species coexistence is driven by the fine-scale spatial patterns and competitive abilities of species. In naturally regenerated mixed oak (Quercus petraea and Quercus robur: QP and QR, respectively) stands, species coexistence was assessed by analysing the spatial distribution of both species and their growth responses to intra- and interspecific competition. Four sites located in North Eastern France with an established (1.4–2.8-m-high) mixed QP and QR regeneration were selected for the study. In a first step, individual seedling location was recorded along line transects to analyse the spatial distribution of the two species. Univariate pair correlation functions were used to test for spatial aggregation for each species and bivariate pair correlation functions were used to test for spatial associations between the two species. In a second step, seedlings were measured in small plots to analyse their response to local competition. Mixed linear models were used to estimate the combined effects of density, mixture, seedling size and seedling status on seedling growth. QP and QR were both highly aggregated in monospecific patches, and the two species were strongly segregated at a decametric scale. Transition zones between monospecific patches were reduced to a few meters. For both species, mixture had no effect on seedling growth. In all sites, the two species showed very similar diameter and height growth rates in response to density, mixture, seedling size and seedling status, and similar intra- and interspecific competitive abilities. Highly patchy patterns of QP and QR regeneration strongly reduce interactions between the two species and, in transition zones between monospecific patches where the two species are mixed, no species is expected to outcompete its companion species. For each species, long-term persistence at the stand scale is determined by intraspecific competition rather than by interspecific competition. At this developmental stage, the QP-QR mixture appears to be very stable and no specific silvicultural treatment is required to maintain the mixture.

Past failures of monocultures, caused by wind-throw or insect damages, and ongoing climate change currently strongly stimulate research into mixed-species stands. So far, the focus has mainly been on combinations of species with obvious complementary functional traits. However, for any generalization, a broad overview of the mixing reactions of functionally different tree species in different mixing proportions, patterns and under different site conditions is needed, including assemblages of species with rather similar demands on resources such as light. Here, we studied the growth of Scots pine and oak in mixed versus monospecific stands on 36 triplets located along a productivity gradient across Europe, reaching from Sweden to Spain and from France to Georgia. The set-up represents a wide variation in precipitation (456–1250 mm year−1), mean annual temperature (6.7–11.5 °C) and drought index by de Martonne (21–63 mm °C−1). Stand inventories and increment cores of trees stemming from 40- to 132-year-old, fully stocked stands on 0.04–0.94-ha-sized plots provided insight into how species mixing modifies stand growth and structure compared with neighbouring monospecific stands. On average, the standing stem volume was 436 and 360 m3 ha−1 in the monocultures of Scots pine and oak, respectively, and 418 m3 ha−1 in the mixed stands. The corresponding periodical annual volume increment amounted to 10.5 and 9.1 m3 ha−1 year−1 in the monocultures and 10.5 m3 ha−1 year−1 in the mixed stands. Scots pine showed a 10% larger quadratic mean diameter (p p p p p p p

Context Lowering stand density has been suggested to adapt forests to warmer and drier conditions. Whether common dendrometric rules used to guide growth models and support silviculture are still valid at these densities lower than usual needs be tested. This includes (1) estimating the stand growth-density relationship over wider density gradients (‘Langsaeter’s rule’), (2) testing the stability of site index, and (3) ‘Eichhorn’s rule’ (relating site index to stand growth) at lower densities. Additionally, environmental changes call for a more direct inclusion of environmental drivers in growth models. Material and methods We investigated the effects of relative stand density (RDI) on stand growth of even-aged stands of sessile oak. We also analyzed effects of climatic conditions to assess whether sessile oak is already at a critical stage that requires adapting forest management. We used a network of silvicultural trials designed to test the effects of maximum density gradients on stand dynamics along climatic gradients. Results As predicted by Langsaeter’s rule, gross stand growth increased with RDI then saturated for RDI > 0.4–0.5, mortality began after crown closure, and diameter increment decreased non-linearly with RDI. Dominant height growth first increased with RDI then saturated for RDI > 0.8. Positive effects of RDI on both stand and height growth caused Eichhorn’s rule to be stable over a large range of RDI (0.25–1). Effects of climate predictors were weak and only visible for radial growth indices. They however suggested antagonistic effects of water and temperature constraints in summer and autumn. Discussion Langsaeter’s and Eichhorn’s rules were found valid over a large range of RDI values. We suggest using them to guide growth models and management. However, assumptions behind the use of site index as a proxy for stand productivity were invalidated. Weak and opposed effect of temperature in summer and autumn makes the outcome of climate warming on sessile oak growth uncertain.

In understory trees, height–diameter allocation is subject several constraints: maximal height growth is limited, height growth is never null and, in terms of time dependency, diameter growth always precedes height growth. Understory trees experience a succession of canopy opening and closure events during their lifetime. Preferential allocation of their biomass to height or diameter growth is part of the acclimation process to their immediate environment. In this study, we investigated annual height and diameter increments in Fagus sylvatica understory trees submitted to canopy release. Annual height and diameter increments were obtained from retrospective stem analysis from the top of the tree to the stump on 39 understory trees. The relationship between height and diameter increments was investigated and temporal correlations among successive annual height and diameter increments were analyzed. An asymptotic relationship between annual height and diameter increment indicates that stem height growth was limited in understory trees. The intercept significantly differed from zero suggesting that height growth never stopped even when diameter growth was null. Following canopy release events, immediate diameter growth increase was observed while the height growth response was delayed, resulting in a time shift in the growth allocation strategy. Strong and asymmetric temporal correlations between annual height and diameter increments were observed: past annual diameter growth was positively correlated to present height growth. In understory trees, tree height is the main factor determining their potential growth since it determines their access to above-canopy light. However, the precedence of diameter growth over height growth suggests that tree growth is driven by diameter instead of height. This apparent discrepancy may be explained by the fact that, under closed canopy conditions, stem allometry reaches a functional threshold that forces tree to grow in diameter before growing in height.

International audience

13 Pág. Instituto de Ciencias Forestales (ICIFOR), Mixed forests are usually associated with higher aboveground carbon storage compared to the corresponding monocultures but information on the impact of tree species mixing on soil organic carbon (SOC) is still limited. Yet, maximizing SOC storage is crucial for ecosystem C sequestration and many other ecosystem services. This study used a triplet approach (ie. two-species mixed stand and respective pure stands at the same site) to assess the impact of tree species identity and mixing on SOC storage in eight pine-oak, eight pine-beech and five beech-oak triplets in Europe. We sampled the forest floor (FF) and 0–40 cm in the mineral soil per 10 cm interval. For each triplet type, we fitted basal area (BA) proportion of one component species (for species identity) and a BA-based plot-level True Shannon Diversity index (for species mixing) as explanatory variables for SOC stocks in linear mixed effects models, which included stone content and plot BA as covariates, and site as a random intercept. Considering the total soil depth (FF + 0–40 cm), species identity effect on SOC stocks was only significant for pine-beech and pine-oak triplets but explained more variability in SOC stocks than species mixing across triplet types. Species mixing effect was not significant for any triplet type in the total soil depth. While species identity consistently drove SOC storage in the topsoil layers across triplet types, species mixing explained more variability in SOC stocks in the deeper soil layers except for pine-oak triplets. The results showed that species identity is a stronger driver of SOC storage than species mixing. While tree species identity effect was strongly related to a conifers vs broadleaves signature, the drivers behind mixing effects remained elusive. The results suggest that targeted selection of tree species could better enhance SOC storage in European forests than a mere increase in species richness., The European Union and the National Fund for Scientific Research (Fonds de la recherche scientifique) of Belgium supported this research as part the ERA-Net SUMFOREST project REFORM – Mixed species forest management. Lowering risk, increasing resilience (www.reform-mixing.eu). Some triplets were selected, established and measured with funding from the EuMIXFOR FP1206 Cost Action project. The Polish State Forests Enterprise also supported one of the Polish co-authors (Grant No: OR.271.3.15.2017). We are grateful to Karine Henin for her key role in the laboratory analyses of the samples. We appreciate the technical assistance by Benjamin Willems for soil sampling across Europe. We would like to thank Thomas Pérot for the dendrometric measurements in the Orléans sites. Finally, we are debtful to the two anonymous reviewers who contributed to improving a previous version of this manuscript.

Premise of the study Thinning is a frequent disturbance in managed forests, especially to increase radial growth. Due to buckling and bending risk associated with height and mass growth, tree verticality is strongly constrained in slender trees growing in dense forests and poor light conditions. Tree verticality is controlled by uprighting movements implemented from local curvatures induced by wood maturation stresses and/or eccentric radial growth. This study presents the first attempt to compare the real uprighting movements in mature trees using a theoretical model of posture control. Methods Stem lean and curvature were measured by Terrestrial LiDAR Scanner (TLS) technology before and 6 years after thinning and compared to unthinned control poles. Measures for several tree and wood traits were pooled together to implement a widely used biomechanical model of tree posture control. Changes in observed stem lean were then compared with the model predictions, and discrepancies were reviewed. Key results Even under a highly constrained environment, most control poles were able to counterbalance gravitational curvature and avoid sagging. Thinning stimulated uprighting movements. The theoretical uprighting curvature rate increased just after thinning, then slowed after 2 years, likely due to the stem diameter increase. The biomechanical model overestimated the magnitude of uprighting. Conclusions Most suppressed beech poles maintain a constant lean angle, and uprighting movements occur after thinning, indicating that stem lean is plastic in response to light conditions. Acclimation of posture control to other changes in growth condition should be investigated, and lean angles should be measured in forest inventories as an indicator of future wood quality.

Research into mixed-forests has increased substantially in the last decades but the extent to which the new knowledge generated meets practitioners' concerns and is adequately transmitted to them is unknown. Here we provide the current state of knowledge and future research directions with regards to 10 questions about mixed-forest functioning and management identified and selected by a range of European forest managers during an extensive participatory process. The set of 10 questions were the highest ranked questions from an online prioritization exercise involving 168 managers from 22 different European countries. In general, the topics of major concern for forest managers coincided with the ones that are at the heart of most research projects. They covered important issues related to the management of mixed forests and the role of mixtures for the stability of forests faced with environmental changes and the provision of ecosystem services to society. Our analysis showed that the current scientific knowledge about these questions was rather variable and particularly low for those related to the management of mixed forests over time and the associated costs. We also found that whereas most research projects have sought to evaluate whether mixed forests are more stable or provide more goods and services than monocultures, there is still little information on the underlying mechanisms and trade-offs behind these effects. Similarly, we identified a lack of knowledge on the spatio-temporal scales at which the effects of mixtures on the resistance and adaptability to environmental changes are operating. Our analysis may help researchers to identify what knowledge needs to be better transferred and to better design future research initiatives meeting practitioner's concerns. This article is based upon work from COST Action FP1206, EuMIXFOR, supported by COST (European Cooperation in Science and Technology). The authors want to specially thank the large number of forest managers that were involved in the participatory process and kindly responded to the surveys. The first author also thanks the Spanish Ministry of Economy and Competitiveness for funding the project AGL2015-70425-R which partly supported his contribution to this study. MM acknowledges support by grant IUT21-4 of the Estonian Ministry of Education and Research and DS support by project III 43007 (2011–2017) from Ministry of Education, Science and Technological Development of Republic of Serbia.

International audience; The relationship between forest productivity and tree species diversity has been described in detail, but the underlying processes have yet to be identified. One important issue is to understand which processes are at the origin of observed aboveground overyielding in some mixed forests. We used a beech-maple plantation exhibiting aboveground overyielding to test whether belowground processes could explain this pattern. Soil cores were collected to determine fine root (FR) biomass and vertical distribution. Correlograms were used to detect spatial arrangement. Near-infrared reflectance spectroscopy was used to identify the tree species proportion in the FR samples and spatial root segregation. An isotopic approach was used to identify water acquisition patterns. The structure and the composition of the ectomycorrhizal fungal community were determined by high-throughput sequencing of DNA in the soil samples. We found no spatial pattern for FR biomass or for its vertical distribution along the gradients. No vertical root segregation was found, as FR density for both species decreased with depth in a similar way. The two species displayed similar vertical water acquisition profiles as well, mainly absorbing water from shallow soil layers; hence, niche differentiation for water acquisition was not highlighted here. Significant alterations in the fungal community compositions were detected in function of the percentage of maple in the vicinity of beech. Our findings do not support the commonly suggested drivers of aboveground overyielding in species-diverse forests and suggest that competition reduction or between-species facilitation of belowground resource acquisition may not explain the observed aboveground overyielding.

Objectif Determiner les parametres âge, sexe, nombre et phenotype des PSL associes au risque d’allo-immunisation dans une population de patients homogenes quant aux caracteristiques des produits transfuses. Methode Les parametres etudies ont ete collectes sur la base des declarations d’allo-immunisation anti-erythrocytaires declarees en IdF entre 2012 et 2017. Le nombre et le phenotype des PSL transfuses avant la declaration de l’allo-immunisation ont ete repris dans la base Receveur de l’EFS IDF. Pour les immunisations RH/K, seuls ont ete pris en compte les CGR pheno incompatibles dans ces systemes. Pour les immunisations dans les autres systemes, tous les CGR ont ete comptabilises, les phenotypes FY, JK, Ss n’etant pas connus pour ces CGR. Les patients ayant un anti-KEL1 et transfuses entre 2012 et 2017 ont ete extraits de cette base pour valider les resultats des anticorps declares. Resultats et Conclusion Malgre l’absence de declaration e-FIT dans plus de 50 % des cas et la difficulte d’exploiter des donnees heterogenes en terme de saisie, cette etude confirme le role du nombre d’expositions aux antigenes de groupes sanguins comme facteur de risque d’immunisation anti-erythrocytaire, avec des immunisations rapides pour le RH/K, et une apparition plus tardive mais neanmoins precoce (5 CGR pour 50 % des allo immunises) pour les autres systemes. Ces donnees montrent que le statut de haut et bas repondeur des patients peut etre rapidement connu, et le respect ou non du phenotype adapte en consequence chez les patients de plus de 70 ans. Enfin, le risque d’allo immunisation vis-a-vis des antigenes mineurs (FY, JK, MNS) par exposition aux produits plaquettaires est faible ( Tableau 1 , Tableau 2 , Tableau 3 ).

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.

Tree resilience to drought was higher in drier sites and lower for suppressed trees grown in higher density stands, highlighting the role of acclimation and selection in tree responses to drought. Ongoing climate change will drive more frequent drought events in the future, with potential impacts on tree community structure and functioning. Growth responses of tree communities may depend on their past water status and on competition pressure. We investigated the effects of site water status, population density, and tree social status on tree growth resistance and resilience following the severe drought of 1976 in even-aged stands of sessile oak (Quercus petraea). We used retrospective growth data collected in permanent plots experiencing contrasted climatic and stand density conditions. We used boosted regression trees to calibrate a tree growth model over 1960–1975, which was then used to provide a baseline of expected tree growth following 1976. Growth dynamics during and after 1976 was examined using the ratio between observed and expected growths over 1976–1983. Tree radial growth was on average 0.6 times its expected values in 1976 and was still 0.63 times its expected value in 1977. Despite experiencing higher summer soil water deficit in 1976, trees growing in drier sites exhibited remarkably faster growth recovery than those in moister sites. Suppressed trees grown in higher density stands recovered their normal growth rate slower than dominant trees. Forest growth is evidenced to be more vulnerable to drought in moister than in drier sites. Competitive pressures also alter tree capacity to recover from a severe drought, accelerating suppression of smaller trees in high-density stands. These results highlight the role of acclimation and selection processes in tree community responses to present and future climates.

Competition with neighboring trees of different species can affect crown size and shape. However, whether intra-specific differences in crown characteristics in mixed stands compared to pure stands are dependent on site conditions remains poorly understood. We used terrestrial laser scanning (TLS) to examine the differences in Fagus sylvatica crown characteristics at four sites, each of which contained pure stands of F. sylvatica and their mixture with Pinus sylvestris. These sites covered the area where the mixture occurs in Europe from south to north, representing a gradient of F. sylvatica productivity, defined as the mean increment of annual volume growth in pure F. sylvatica stands. Despite the large range in productivity, F. sylvatica trees in mixtures had larger crowns regardless of site conditions, with a higher proportion of their crown volume in the lower canopy compared to trees in pure stands. Larger crown volumes were related to higher live crown ratios and greater crown expansion, depending on the site. The magnitude of the mixing effect was variable among the crown characteristics evaluated, but overall our findings provide evidence that for a given species combination and density, the effect of mixture increased in the two most productive sites. TLS-derived novel crown metrics revealed that the mixing effect was affected by productivity, which was not captured by traditionally measured crown variables. © 2017 Elsevier B.V.

Key message The position of trees in the canopy impacts xylem structure and its inter-annual variation. After canopy release, the increase in the hydraulic conductivity of growth rings was driven by an increase in radial growth in large trees, and by both an increase in radial growth and changes in xylem structure in saplings. Context Forest canopies are frequently subjected to disturbances that allow understory trees to access the upper canopy. The effect of canopy release on xylem anatomy has been assessed in juvenile trees and saplings, while the potential acclimation of larger trees remains poorly documented. Aims We estimated the potential hydraulic conductivity of growth rings in large understory trees compared to overstory trees, and evaluated the responses to canopy release in large trees and in saplings. Methods We recorded radial growth, wood density, and vessel structure in beech trees according to their position within the canopy and their size. Xylem traits were followed during 6 years after canopy release for large trees, and during 2 years for saplings. Vessel diameter and frequency as well as ring area were used to compute the potential annual ring hydraulic conductivity. Results Large understory trees displayed lower radial growth increments and lower potential annual ring hydraulic conductivity than overstory trees. After canopy release, potential annual ring hydraulic conductivity increased in large trees, due exclusively to increased radial growth without any change in specific hydraulic conductivity. It increased in saplings due to both increased radial growth and increased specific conductivity. Conclusion Tree size impacted xylem structure and resulted in plasticity of the potential hydraulic conductivity of the annual tree ring following canopy release.

International audience; The objective of this study was to understand the diameter structure of three even-aged Atlas cedar stands and four others representing different development stages (thicket, sapling, pole stage and high forest stage), which occupy various surfaces in the Tikjda natural cedar forest (Djurdjura, Algeria). The modelling approach for the structures observed was to seek a fit with the theoretical Weibull distribution. Goodness-of-fit tests showed that this distribution was well chosen. In terms of shape, the structure was unbalanced because no silvicultural action was taken to gradually decrease tree numbers so that remaining stock diameter could increase with age. The comparison between the even-aged stand structures and those of stands at various stages of development showed that apart from a shift to the right of the modal class indicating tree growth, the slight decrease in numbers is mainly due to competition and natural selection since no silvicultural practises have been implemented in this cedar forest.; L’objectif de cette étude était d’appréhender la forme de la structure en diamètre de trois peuplements réguliers d’âge différent de Cèdre de l’Atlas ainsi que quatre autres représentant les stades de développement (fourré, gaulis, perchis et futaie adulte) occupant des surfaces variables dans la cédraie naturelle de Tikjda (Djurdjura, Algérie). La démarche de modélisation des structures obtenues consiste à chercher une adéquation avec la distribution théorique de Weibull. Les tests de qualité d’ajustement (goodness of fit) montrent la pertinence du choix de cette distribution. Du point de vue forme, les structures sont déséquilibrées du fait de l’absence d’opérations sylvicoles empêchant l’évolution vers une diminution progressive des effectifs et un gain de croissance en diamètre avec l’âge. En effet, la présentation groupée des structures aussi bien des plantations que des stades de développement montre qu’à part un glissement à droite de la classe modale indiquant un grossissement des arbres, la faible diminution des effectifs est due essentiellement aux phénomènes de concurrence et de sélection naturelle puisque aucune intervention sylvicole n’a été pratiquée dans ces peuplements.

Biomass equations are a helpful tool to estimate the tree and stand biomass production and standing stock. Such estimations are of great interest for science but also of great importance for global reports on the carbon cycle and the global climate system. Even though there are various collections and generic meta-analyses available with biomass equations for mature trees, reports on biomass equations for juvenile trees (seedlings and saplings) are mainly missing. Against the background of an increasing amount of reforestation and afforestation projects and forests in young successional stages, such equations are required. In this study we have collected data from various studies on the aboveground woody biomass of 19 common tree species growing in Europe. The aim of this paper was to calculate species-specific biomass equations for the aboveground woody biomass of single trees in dependence of root-collar-diameter (RCD), height (H) and the combination of the two (RCD2 H). Next to calculating species-specific biomass equations for the species available in the dataset, we also calculated generic biomass equations for all broadleaved species and all conifer species. The biomass equations should be a contribution to the pool of published biomass equations, whereas the novelty is here that the equations were exclusively derived for young trees. We thank the national research project “Ecosystem Services of Natural Forests at Forestry and Climate Policy (FKZ 3511 84 0200),” from the Federal Agency for Nature Conservation (BfN) of the Federal Ministry for the Environment Nature Conservation and Nuclear Safety (BMU) for funding this project.

Central nervous system anomalies in Pfeiffer syndrome (PS) due to mutations in the FGFR2 gene are poorly understood, even though PS is often associated with serious cognitive impairment. The aim of this study is to describe the neuropathological phenotype in PS. We present four severe fetal cases of sporadic PS with FGFR2 mutations who underwent termination followed by fetopathological and neuropathological examination. We studied the expression pattern of Fgfr2 in the mouse brain using radioactive fluorescence in situ hybridization. PS is associated with brain deformations due to the abnormal skull shape, but FGFR2 mutations also induce specific brain developmental anomalies: megalencephaly, midline disorders, amygdala, and hippocampus malformations, and ventricular wall alterations. The expression pattern of Fgfr2 in mice matches the distribution of malformations in humans. The brain anomalies in PS result from the combination of mechanical deformations and intrinsic developmental disorders due to FGFR2 hyperactivity. Several similarities are noted between these anomalies and the brain lesions observed in other syndromes due to mutations in FGF-receptor genes. The specific involvement of the hippocampus and the amygdala should encourage the precise cognitive screening of patients with mild forms of PS.

The area of forests planted with exotic tree species is increasing worldwide in order to fulfill various economic and environmental demands. Numerous species currently used in forest plantations are considered to be invasive in many parts of the world. Exotic plantation tree species are endowed with a series of life-history traits that are characteristic of invasive species: easy establishment, fast growth, high propagule pressure, and low or intermediate shade tolerance. In addition, plantation forestry presents many features that increase ecosystem invasibility, including a regime of frequent and intensive canopy disturbances, a dense network of roads and trails with continuous traffic, and wide plantation areas. Exotic trees planted for production purposes have strong direct positive economic impacts on the local and national economies of many countries, often leading to notable conflicts of interest when the species becomes invasive, as well as to negative impacts on the ecosystem. Studies and management programs mainly focus on Pinus and Acacia, which are used as model species. Complementary management strategies have been established to control these species at different stages of the invasion process. Knowledge gained is useful to predict problems in other regions of the world with the same species and to guide research or management actions on other problematic but less studied tree species.

By increasing resource availability, canopy opening enhances tree recruitment as well as the development of neighbouring vegetation. The proliferation of early successional and highly competitive vegetation may have dramatic consequences on seedling establishment. However, differences in competitive abilities have been shown among the plant growth forms commonly encountered in forests. We may thus expect that vegetation management leading to control of different plant growth forms would have different consequences on tree seedling growth and development. To test this hypothesis, we analysed the effects of an intensity gradient of four vegetation control treatments (untreated, coppice control, coppice and non-tree plant control, and coppice, non-tree and pioneer (tree) plant control) on plant species richness and natural tree regeneration in three post-storm sites. Higher plant species richness and a better natural tree regeneration were observed in the more intensive treatments that significantly improved the balance of the relative abundance of tree species. Suppressing the more competitive vegetation, mainly Rubus fruticosus and graminoids, led to the recruitment and growth of tree species sensitive to competition and good tree species diversity. Practical recommendations in terms of vegetation control relative to tree regeneration and plant species richness are given.

International audience; This study assessed the species composition and the development of regeneration plots in gaps created by a windstorm in a mixed-species broadleaved stand. The stand was a former coppice-with-standards characterized by a high broadleaved tree species diversity. Thirteen years after gap creation, all gaps were fully stocked and the regeneration was almost exclusively dominated by Fagus sylvatica and Acer pseudoplatanus seedlings, two species characterized by a high shade tolerance in their early stages. All other species (Quercus sp., Fraxinus excelsior , Carpinus betulus , Acer campestre , Acer platanoides , Sorbus torminalis , Tilia sp.) were either absent from the regeneration or completely over-topped by the two dominant species. These features were ascribed to a regime of periodic natural or anthropic disturbance of intermediate intensity, where small gaps are regularly created in the canopy. This regime results in a succession of short, open and closed canopy episodes that eventually promote shade-tolerant species. During this regime, the shade-tolerant species are able to build a strong advance regeneration that is ready to outgrow the other species when gaps are created. If the management objective is to maintain species diversity during the regeneration process, the development of this advance regeneration will have to be strictly controlled.

This study aimed at evaluating alternative methods to ensure regeneration success in temperate low-mountain forest stands by (1) estimating the effects of seed availability, competition from the adult stand and from neighbouring vegetation and interaction with the litter layer on seedling density, and by (2) comparing the effects of various silvicultural methods on regeneration success. The experiment was conducted in a monospecific beech (Fagus sylvatica) stand and a mixed silver fir-beech (Abies alba, Fagus sylvatica) stand with contrasted vegetation communities, in north-east France. Different methods of soil preparation, i.e. chemical (herbicide), mechanical (surface hoeing and deep scarification using light-weight machines) and biological (cover crops after surface hoeing) methods, were applied along a canopy opening gradient. After soil preparation (in 2009), vegetation colonisation and tree seedling density were monitored once a year from 2010 to 2013. Results were similar for the two sites. Seedling density the first year indicated a predominant effect of seed availability and soil scarification over potential competitive effects of adult stand and neighbouring vegetation. Despite continuous vegetation colonisation after soil preparation, seedling density remained stable over the 4 years of the experiment. For each of the 4 years, seedling density increased with canopy cover. Seedling density was higher after mechanical soil preparation than after herbicide application. Cover crops (following surface hoeing) appeared as the best method, ensuring both the lowest vegetation colonisation and the highest seedling density.

Even-aged forest stands are competitive communities where competition for light gives advantages to tall individuals, thereby inducing a race for height. These same individuals must however balance this competitive advantage with height-related mechanical and hydraulic risks. These phenomena may induce variations in height-diameter growth relationships, with primary depen-dences on stand density and tree social status as proxies for competition pressure and access to light, and on availability of local environmental resources, including water. We aimed to investigate the effects of stand density, tree social status and water stress on the individual height-circumference growth allocation (Delta h-Delta c), in even-aged stands of Quercus petraea Liebl. (sessile oak). Within-stand Delta c was used as surrogate for tree social status. We used an original long-term experimental plot network, set up in the species production area in France, and designed to explore stand dynamics on a maximum density gradient. Growth allocation was modelled statistically by relating the shape of the Delta h-Delta c relationship to stand density, stand age and water deficit. The shape of the Delta h-Delta c relationship shifted from linear with a moderate slope in open-grown stands to concave saturating with an initial steep slope in closed stands. Maximum height growth was found to follow a typical mono-modal response to stand age. In open-grown stands, increasing summer soil water deficit was found to decrease height growth relative to radial growth, suggesting hydraulic constraints on height growth. A similar pattern was found in closed stands, the magnitude of the effect however lowering from suppressed to dominant trees. We highlight the high phenotypic plasticity of growth in sessile oak trees that further adapt their allocation scheme to their environment. Stand density and tree social status were major drivers of growth allocation variations, while water stress had a detrimental effect on height in the Delta h-Delta c allocation.