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Forest models are instrumental for understanding and projecting the impact of climate change on forests. A considerable number of forest models have been developed in the last decades. However, few systematic and comprehensive model comparisons have been performed in Europe that combine an evaluation of modelled carbon and water fluxes and forest structure. We evaluate 13 widely used, state-of-the-art, stand-scale forest models against field measurements of forest structure and eddy-covariance data of carbon and water fluxes over multiple decades across an environmental gradient at nine typical European forest stands. We test the models' performance in three dimensions: accuracy of local predictions (agreement of modelled and observed annual data), realism of environmental responses (agreement of modelled and observed responses of daily gross primary productivity to temperature, radiation and vapour pressure deficit) and general applicability (proportion of European tree species covered). We find that multiple models are available that excel according to our three dimensions of model performance. For the accuracy of local predictions, variables related to forest structure have lower random and systematic errors than annual carbon and water flux variables. Moreover, the multi-model ensemble mean provided overall more realistic daily productivity responses to environmental drivers across all sites than any single individual model. The general applicability of the models is high, as almost all models are currently able to cover Europe's common tree species. We show that forest models complement each other in their response to environmental drivers and that there are several cases in which individual models outperform the model ensemble. Our framework provides a first step to capturing essential differences between forest models that go beyond the most commonly used accuracy of predictions. Overall, this study provides a point of reference for future model work aimed at predicting climate impacts and supporting climate mitigation and adaptation measures in forests., Global Change Biology, 28 (23), ISSN:1354-1013, ISSN:1365-2486

Ecology and forestry sciences are using an increasing amount of data to address a wide variety of technical and research questions at the local, continental and global scales. However, one type of data remains rare: fine-grain descriptions of large landscapes. Yet, this type of data could help address the scaling issues in ecology and could prove useful for testing forest management strategies and accurately predicting the dynamics of ecosystem services. Here we present three datasets describing three large European landscapes in France, Poland and Slovenia down to the tree level. Tree diameter, height and species data were generated combining field data, vegetation maps and airborne laser scanning (ALS) data. Together, these landscapes cover more than 100~000~ha and consist of more than 42 million trees of 51 different species. Alongside the data, we provide here a simple method to produce high-resolution descriptions of large landscapes using increasingly available data: inventory and ALS data. We evaluated the overall reliability of our workflow by comparing the stands dominant heights measured by ALS to those calculated from the trees we generated. Overall, the landscapes we generated are in good agreement with the landscapes they aim to reproduce.

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.

A growing body of research suggests mixed-species stands are generally more productive than pure stands as well as less sensitive to disturbances. However, these effects of mixture depend on species assemblages and environmental conditions. Here, we present the Salem simulator, a tool that can help forest managers assess the potential benefit of shifting from pure to mixed stands from a productivity perspective. Salem predicts the dynamics of pure and mixed even-aged stands and makes it possible to simulate management operations. Its purpose is to be a decision support tool for forest managers and stakeholders as well as for policy makers. It is also designed to conduct virtual experiments and help answer research questions. In Salem, we parameterised the growth in pure stand of 12 common tree species of Europe and we assessed the effect of mixture on species growth for 24 species pairs (made up of the 12 species mentioned above). Thus, Salem makes it possible to compare the productivity of 36 different pure and mixed stands depending on environmental conditions and user-defined management strategies. Salem is essentially based on the analysis of National Forest Inventory data. A major outcome of this analysis is that we found species mixture most often increases species growth, in particular at the poorest sites. Independently from the simulator, foresters and researchers can also consider using the species-specific models that constitute Salem: the growth models including or excluding mixture effect, the bark models, the diameter distribution models, the circumference-height relationship models, as well as the volume equations for the 12 parameterised species. Salem runs on Windows, Linux, or Mac. Its user-friendly graphical user interface makes it easy to use for non-modellers. Finally, it is distributed under a LGPL license and is therefore free and open source.

Key message We compared two methods for detailed individual tree measurements: single image photogrammetry (SIP), a simplified, low-cost method, and the state-of-the-art terrestrial laser scanning (TLS). Our results provide evidence that SIP can be successfully applied to obtain accurate tree architectural traits in mature forests. Context Tree crown variables are necessary in forest modelling; however, they are time consuming to measure directly, and they are measured in many different ways. We compare two methods to obtain crown variables: laser-based and image-based. TLS is an advanced technology for three-dimensional data acquisition; SIP is a simplified, low-cost method. Aims To elucidate differences between the methods, and validate SIP accuracy and usefulness for forest research, we investigated if (1) SIP and TLS measurements are in agreement in terms of the most widely used tree characteristics; (2) differences between the SIP traits and their TLS counterparts are constant throughout tree density and species composition; (3) tree architectural traits obtained with SIP explain differences in laser-based crown projection area (CPA), under different forest densities and stand compositions; and (4) CPA modelled with SIP variables is more accurate than CPA obtained with stem diameter-based allometric models. We also examined the correspondence between local tree densities extracted from images and from field measurements. Methods We compared TLS and SIP in a temperate pure sessile oak and mixed with Scots pine stands, in the Orléans Forest, France. Standard major axis regression was used to establish relations between laser-based and image-based tree height and diameter at breast height. Four SIP-derived traits were compared between the levels of stand density and species composition with a t test, in terms of deviations and biases to their TLS counterparts. We created a set of linear and linear mixed models (LMMs) of CPATLS, with SIP variables. Both laser-based and image-based stem diameters were used to estimate CPA with the published allometric equations; the results were then compared with the best predictive LMM, in terms of similarity with CPATLS measurement. Local tree density extracted from images was compared with field measurements in terms of basic statistics and correlation. Results Tree height and diameter at breast height were reliably represented by SIP (Pearson correlation coefficients r = 0.92 and 0.97, respectively). SIP measurements were affected by the stand composition factor; tree height attained higher mean absolute deviation (1.09 m) in mixed stands, compared to TLS, than in pure stands (0.66 m); crown width was more negatively biased in mixed stands (− 0.79 m), than in pure stands (− 0.05 m); and diameter at breast height and crown asymmetry were found unaffected. Crown width and mean branch angle were key SIP explanatory variables to predict CPATLS. The model was approximately 2-fold more accurate than the CPA allometric estimations with both laser-based and image-based stem diameters. SIP-derived local tree density was similar to the field-measured density in terms of mean and standard deviation (9.6 (3.5) and 9.4 (3.6) trees per plot, respectively); the correlation between both density measures was significantly positive (r = 0.76). Conclusion SIP-derived variables, such as crown width, mean branch angle, branch thickness, and crown asymmetry, were useful to explain tree architectural differences under different densities and stand compositions and may be implemented in many forest research applications. SIP may also provide a coarse measure of local competition, in terms of number of neighbouring trees. Our study provides the first test in mature forest stands, for SIP compared with TLS.

The growth and yield of mixed-species stands has become an important topic of research since there are certain advantages of this type of forest as regards functions and services. However, the concepts and methods used to characterize mixed stands need to be understood, as well as harmonized and standardized. In this review we have compiled a set of measures, indices, and methods at stand level to characterize the structure, dynamics, and productivity of mixed stands, and we discuss the pros and cons of their application in growth and yield studies. Parameters for the characterization of mixed stand structure such as stand density, species composition, horizontal (intermingling) and vertical tree distribution pattern, tree size distribution, and age composition are described, detailing the potential as well as the constraints of these parameters for understanding resource capture, use, and efficiency in mixed stands. Furthermore, a set of stand-level parameters was evaluated to characterize the dynamics of mixed stands, e.g. height growth and space partitioning, self- and alien-thinning, and growth partitioning among trees. The deviations and changes in the behaviour of the analysed parameters in comparison with pure stand conditions due to inter-specific interactions are of particular interest. As regards stand productivity, we reviewed site productivity indices, the growth–density relationship in mixed stands as well as methods to compare productivity in mixed versus monospecific stands. Finally, we discuss the main problems associated with the methodology such as up-scaling from tree to stand level as well as the relevance of standardized measures and methods for improving forest growth and yield research in mixed stands. The main challenges are also outlined, especially the need for qualitatively sound data. © 2015, Springer-Verlag Berlin Heidelberg.

The IPCC previsions for the upcoming decades include an increase in frequency and intensity of drought events in several regions worldwide, including Northern Europe. Drought significantly affects forest ecosystems through decreased productivity, increased vulnerability to biotic disturbances and increased subsequent mortality. How forest ecosystems maintain resistance and resilience to drought events are important questions. Our study aimed to assess whether species mixture or an individual tree size within a stand alters a given tree’s resilience and resistance to drought. A retrospective study of tree-ring widths allowed us to calculate resistance, resilience and recovery indices for five recent drought events: 1976, the 1990–1992 period, 2003, 2006 and 2010. These drought events were selected based on the SPEI (Standardized Precipitation Evapotranspiration Index) drought index. Our study sample consisted of 108 individual sessile oak ( Quercus petraea (Matt.)) or Scots pine ( Pinus sylvestris L.) trees sampled in 2012 and 2013 (Orleans forest, central France) in pure and mixed stands, divided into three diameter classes corresponding to three sizes: large, medium and small trees. Scots pine performed better than sessile oak during the 1990–1992 and 2010 droughts while the contrary was observed for the 2003 and 2006 droughts. They performed equally in 1976. We suggest that the differing sensitivity of the two species to spring and summer drought explained this result. In our study, stand composition had no effect on resilience or resistance for either species. The size effect in oaks was unclear as small oaks displayed either a better performance or a worse performance than large oaks. Small pines displayed better resistance and resilience than pines of a larger size. This work stressed the importance of taking into account stand composition and trees size as well as soil and climatic conditions for each drought events to achieve a better understanding of the diversity of responses to climatic variations among forest ecosystems.

L'étude du fonctionnement des peuplements mélangés n'est pas près de s'épuiser. En ce qui concerne l'effet du mélange sur la productivité, les travaux ont surtout porté sur la croissance radiale ou en surface terrière. Mais la productivité dépend aussi de l'influence du mélange sur la croissance en hauteur, plus difficile à appréhender : les deux effets sont-ils de même ampleur ? Vont-ils dans le même sens ? Quels sont les mécanismes ? Méthode d'étude et premières réponses.

International audience; AbstractKey messageStatic site indices determined from stands’ top height are derived from different forest inventory sources with height and age information and thus enable comparisons and modeling of a species’ productivity encompassing large environmental gradients.ContextEstimating forest site productivity under changing climate requires models that cover a wide range of site conditions. To exploit different inventory sources, we need harmonized measures and procedures for the productive potential. Static site indices (SI) appear to be a good choice.AimsWe propose a method to derive static site indices for different inventory designs and apply it to six tree species of the German and French National Forest Inventory (NFI). For Norway spruce and European beech, the climate dependency of SI is modeled in order to estimate trends in productivity due to climate change.MethodsHeight and age measures are determined from the top diameters of a species at a given site. The SI is determined for a reference age of 100 years.ResultsThe top height proves as a stable height measure that can be derived harmoniously from German and French NFI. The boundaries of the age-height frame are well described by the Chapman-Richards function. For spruce and beech, generalized additive models of the SI against simple climate variables lead to stable and plausible model behavior.ConclusionThe introduced methodology permits a harmonized quantification of forest site productivity by static site indices. Predicting productivity in dependence on climate illustrates the benefits of combined datasets.

Assessing how species productivity in mixtures is influenced by species shade tolerance and phylogeny would be helpful to improve our general understanding of the relationship between tree species diversity and productivity in forests. We investigated the effects of differences in shade tolerance and phylogenetic distances between pairs of species on the productivity of Quercus petraea growing in 18 mixtures in lowland temperate forests. We calculated the mixture effect as the difference in productivity of Q. petraea in mixed vs. pure stands. Our analyses were based on data from seven annual campaigns of the French National Forest Inventory covering 1,573 plots. The mixture effect on Q. petraea productivity increased when the shade tolerance of the companion species decreased. Compared to its productivity in pure stands, Q. petraea productivity in mixed coniferous stands varied from -14.6% up to +39.6% as the shade tolerance of the companion species inversely varied from highest to lowest. With broadleaved companion species, the mixture effect varied from -10% up to +13.9% with decreasing shade tolerance. We found no effect of phylogenetic distance between Q. petraea and the companion species on the mixture effect. Synthesis: Our results confirm that shade tolerance is an important driver of the diversity effect on productivity at species level in temperate forests and that phylogenetic distance is not a relevant proxy for species functional dissimilarity. This article is protected by copyright. All rights reserved.

Background Concern about climate change has motivated France to reduce its reliance on fossil fuel by setting targets for increased biomass-based renewable energy production. This study quantifies the carbon costs and benefits for the French forestry sector in meeting these targets. A forest growth and harvest simulator was developed for French forests using recent forest inventory data, and the wood-use chain was reconstructed from national wood product statistics. We then projected wood production, bioenergy production, and carbon balance for three realistic intensification scenarios and a business-as-usual scenario. These intensification scenarios targeted either overstocked, harvest-delayed or currently actively managed stands. Results All three intensification strategies produced 11.6–12.4 million tonnes of oil equivalent per year of wood-based energy by 2026, which corresponds to the target assigned to French wood-energy to meet the EU 2020 renewable energy target. Sustaining this level past 2026 will be challenging, let alone further increasing it. Although energy production targets can be reached, the management intensification required will degrade the near-term carbon balance of the forestry sector, compared to continuing present-day management. Even for the best-performing intensification strategy, i.e., reducing the harvest diameter of actively managed stands, the carbon benefits would only become apparent after 2040. The carbon balance of a strategy putting abandoned forests back into production would only break even by 2055; the carbon balance from increasing thinning in managed but untended stands would not break even within the studied time periods, i.e. 2015–2045 and 2046–2100. Owing to the temporal dynamics in the components of the carbon balance, i.e., the biomass stock in the forest, the carbon stock in wood products, and substitution benefits, the merit order of the examined strategies varies over time. Conclusions No single solution was found to improve the carbon balance of the forestry sector by 2040 in a way that also met energy targets. We therefore searched for the intensification scenario that produces energy at the lowest carbon cost. Reducing rotation time of actively managed stands is slightly more efficient than targeting harvest-delayed stands, but in both cases, each unit of energy produced has a carbon cost that only turns into a benefit between 2060 and 2080. Electronic supplementary material The online version of this article (10.1186/s13021-018-0113-5) contains supplementary material, which is available to authorized users.

An increasing amount of research is focusing on comparing productivity in monospecific versus mixed stands, although it is difficult to reach a general consensus as mixing effects differ both in sign (over-yielding or under-yielding) and magnitude depending on species composition as well as on site and stand conditions. While long-term experimental plots provide the best option for disentangling the mixing effects, these datasets are not available for all the existing mixtures nor do they cover large gradients of site factors. The objective of this study was to evaluate the effects and uncertainties of tree species mixing on the productivity of Scots pine–European beech stands along the gradient of site conditions in Europe, using models developed from National and Regional Forest Inventory data. We found a positive effect of pine on beech basal area growth, which was slightly greater for the more humid sites. In contrast, beech negatively affected pine basal area growth, although the effects switched to positive in the more humid sites. However, the uncertainty analysis revealed that the effect on pine at mid- and more humid sites was not-significant. Our results agree with studies developed from a European transect of temporal triplets in the same pine–beech mixtures, confirming the suitability of these datasets and methodology for evaluating mixing effects at large scale., This research was partially funded by the COST Action FP1206 through the Short Term Scientific Mission COST-STSM-ECOST-STSM-FP1206-091115-068270. This article is based on work from COST Action FP1206 (EuMIXFOR), supported by COST (European Cooperation in Science and Technology). The authors would like to thank the Spanish Ministry of Economy and Competitiveness for funding the research project AGL2014-51964-C2-2-R, which supported part of this study, and to the project CARE4C (# GA 778322). The authors are also grateful to Bayerische Staatsforsten, BaySF for providing the forest inventory data for Bavaria/Germany and to all those who make the National Forest Inventory possible in the different countries, and to those providing the data used for this study. We also thank the German Science Foundation, DFG, for funding the project PR 292/15-1 “Structure and Dynamics of mixed-species stands of Scots pine and European beech in comparison with mono-specific stands”.

Substituting renewable forms of energy for fossil fuels is a real challenge in developed countries and fuel wood will undoubtedly have an important role to play in forested areas. However, fuel wood harvesting may deplete nutrients in forest ecosystems and adversely affect site fertility in the long term. We compared the cost of compensatory fertilising and liming operations between whole-tree harvesting and traditional stem-only harvesting. We worked in beech (Fagus sylvatica L.) stands in eastern France and two site fertility classes (high and low). We used a growth model to calculate the nitrogen, calcium, potassium, magnesium and phosphorus contents of fine woody debris (FWD, diameter < 7 cm) and compared the benefits of selling this fine wood for energy to the cost of compensatory fertilising and liming. We showed that compensating and liming are more expensive than the profits derived from selling fine fuel wood, except when fuel wood prices rise considerably. This was mainly due to the high proportion of bark in FWD compared with other tree compartments. Indeed, bark is especially rich in nutrients such as nitrogen and phosphorus, which were the most costly nutrients to replace. Our study highlights the importance of considering the environmental costs of fuel wood production at the stand scale. Whenever fertilisation or liming is necessary, we recommend compensating at least for calcium and magnesium, two nutrients that account for a very small part of the costs. In any case, fertilisation should only be used if long-term site fertility has been compromised.

The effects of mixed stands on biodiversity are increasingly being studied since they are supposed to offer higher habitat heterogeneity. Nevertheless, for tree-associated diversity, including epiphytes and terricolous species near tree trunks, few studies exist, and still fewer compare mixed stands with each corresponding pure tree species stand. We evaluated and quantified the influence of forest composition on tree-associated bryophyte diversity (species richness, abundance, composition) in mixed and pure oak-pine stands in a French lowland forest. The main explanatory variables for bryophyte diversity at tree-level were the identity of the phorophyte tree species and the mixture type (pure versus mixed). At the plot level, the main explanatory variable was the stand type (pure oak, pure pine and mixed). We also explored the role of other variables including the chemistry of the bryophyte substrates (soil, bark) and water supply (stemflow, throughfall), as well as stand abundance variables (basal area, interfering plant cover). We analyzed data with Generalized Linear Models under Bayesian statistics, to take into account the spatial autocorrelation between plots and any under- or over-dispersion of our data. At the tree-level, bryophyte richness and abundance were higher on oak than on pine. Pine bryophyte richness was higher in mixed compared to pure stands, whereas for oak, mixed stands did not enhance bryophyte richness. At the plot level, mixed stands hosted bryophyte communities of similar richness to those in pure oak stands, whereas pure pine stands were clearly poorer. Our exploratory models suggested strong effects of water supply chemistry (stemflow and throughfall pH or conductivity) and basal area; the latter had a strong quadratic effect on epiphytic richness at the plot level. In terms of composition, three species were more likely to be found on pine phorophytes, seven species clearly occurred more frequently on oaks. Some species were more likely to be associated to pine in mixed than in pure stands, and one species was found more often under pines in pure stands. Therefore, bryophyte diversity at the landscape level should benefit from the simultaneous presence of the three stand composition types – pure oak, pure pine and mixed stands.

Questions: How to evaluate the mixture effect on basal area increment in two-species forest stands? Is a mixed Norway spruce–silver fir stand more productive than pure adjacent stands of either species? How to develop generic modelling approaches to assess mixture effects in forest stands? Location: In addition to a case study on Norway spruce–silver fir stands in French mountain forests, the generic approach used goes beyond local applications. Methods: We took advantage of National Forest Inventory data to develop a unique stand basal-area-increment model for pure and mixed stands of Norway spruce and silver fir that responds to ecological site conditions. The database was made up of 284 pure Norway spruce stands, 196 pure silver fir stands, and 323 mixed stands of these species. Results: Pure silver fir basal area increment is strongly influenced by spring climatic conditions, whereas pure Norway spruce is more influenced by soil conditions. The mixture of these species has a positive effect on silver fir, which decreases as the proportion of fir increases. In contrast, the mixture has no noticeable effect on Norway spruce. Conclusion: We developed a stand basal-area-increment model evidencing an advantage of the mixture on silver fir basal area increment, but not on Norway spruce. The mathematical formulation of the model developed is generic and can be used in all two-species mixture situations. It also makes it possible to compare different mixture situations with each other.

Article résumant et vulgarisant les résultats principaux de la thèse de Maude Toïgo. D'une part, nous exposons l'effet des conditions environnementales sur l'influence du mélange sur la productivité des forêts à partir des données de l'Inventaire Forestier de l'IGN. D'autre part, nous exposons l'effet du mélange sur la largeur et la densité des cernes en interaction avec le déficit en eau dans les peuplements mélangés chêne sessile - pin sylvestre de la forêt d'Orléans.

National audience

Cette communication expose des méthodes, basées sur la modélisation, pour estimer les stocks de carbone dans l'écosystème et les produits forestiers, et la façon dont ces stocks répondent à quelques variations de sylviculture. Une application à la distinction entre forêts anciennes et récentes est proposée.

Malgré l'importance et l'intérêt des peuplements mélangés, leur fonctionnement reste largement méconnu car la tâche est immense, ne serait-ce qu'en raison de la multiplicité des assemblages d'espèces. Ici les auteurs s'intéressent à la croissance : ils récapitulent les types de dispositifs qui permettent l'étude des peuplements mélangés et montrent l'intérêt des données de l'inventaire national pour caractériser, grâce à une méthode originale de modélisation, l'effetdu mélange sur la croissance des espèces concernées. De là naissent des hypothèses de mécanismes, que l'expérimentation permet d'approfondir.

International audience; The main aim of this paper is to illustrate, using a large set of indicators, the consequences of management choices imposed under five socio-economic forward-looking scenarios within a given case study area during the next 60 years. This 101,000 ha case study area is centred around the village of Pontenx-les-Forges in south west France. This paper details this INTEGRAL project case study area and some forest management schemes implemented by forest owners under various scenarios. Using a landscape simulator handling two forest growth models, the evolution of all forest stands in the landscape is simulated using 9 indicators over a 60-year period: growing stock, harvested volume, carbon sequestered in trees, mean tree volume, commercial value of growing stock, susceptibility to wind and fire and biodiversity indexes. One of the main results of this study was to show up the magnitude of effects on growing stock and harvested volumes: according to the scenarios, annual harvesting could vary by 50% as from 2030. As a consequence, other indicators are impacted, such as biodiversity, wind and fire susceptibility. All these changes are observed despite the fact that the dominant species remain the same and that forest owners are considered not very prone to changing their behaviour. In conclusion, options for future improvement are discussed and the opportunities offered by high definition data acquisition from remote sensing are considered.; L'objectif de cette étude est d'illustrer à l'aide d'indicateurs les conséquences de choix de gestion imposés par cinq scénarios socioéconomiques prospectifs appliqués à une large zone forestière pour les 60 prochaines années. Le cas d'étude choisi est la zone centrée sur la commune de Pontenx-les-Forges dans le sud-ouest de la France et couvrant 101000 hectares. Cet article présente une description de la zone d'étude et des itinéraires sylvicoles mis en ½uvre par les propriétaires forestiers selon des scénarios. À l'aide d'un simulateur pilotant deux modèles de croissance, l'évolution de la zone d'étude à l'échelle de chaque parcelle est synthétisée par 9 indicateurs sur une période de 60 ans : le volume sur pied, le carbone sur pied, le volume total exploité, la valeur commerciale sur pied, le volume de l'arbre moyen, la vulnérabilité au vent et au feu, et des indices de biodiversité. Un des principaux résultats de cette étude est de montrer l'amplitude des changements pour la production et le volume sur pied : selon les scénarios les récoltes annuelles peuvent varier de 50 % dès 2030. Par conséquent, d'autres indicateurs sont impactés comme la biodiversité, la vulnérabilité au vent ou au feu. Pourtant, l'espèce dominante est maintenue et le comportement partiellement conservateur des types de propriétaires est pris en compte. En conclusion, des améliorations pour de futures simulations sont envisagées ; dans ce but, des synergies avec la télédétection sont nécessaires pour la collecte des données d'initialisation sur de larges territoires, ce qui permettra d'améliorer la précision des résultats.

Summary There is a rising interest in the role of species diversity in ecosystem functioning and services, including productivity. Yet, how the diversity–productivity relationship depends on species identity and abiotic conditions remains a challenging issue. We analysed mixture effects on species productivity along site productivity gradients, calculated from a set of abiotic factors, in two biogeographic contexts (highlands and lowlands). We compared the productivity of 5 two-species mixtures (i.e. 10 cases of mixed species) with that of monocultures of the same species. Five main European tree species were considered: sessile oak (Quercus petraea Liebl.), Scots pine (Pinus sylvestris L.), European beech (Fagus sylvatica L.), silver fir (Abies alba Mill.) and Norway spruce (Picea abies (L.) H. Karst). Our data set was compiled from the 2006 to 2010 French National Forest Inventory data base and covers 2361 plots including pure and mixed stands. Overall productivity of mixtures in highlands, that is European beech–Norway spruce, European beech–silver fir and to a lesser extent, silver fir–Norway spruce, was found to be higher than expected from the productivity of corresponding monospecific stands. Overyielding was mainly due to European beech for the first two mixtures and to silver fir for the third one. No effect of mixture was found for sessile oak–Scots pine and sessile oak–European beech stands in lowlands. Overyielding of sessile oak mixed with Scots pine was not strong enough to significantly increase overall stand productivity. Overyielding of European beech was balanced by an underyielding of sessile oak. The mixture effect changed along site productivity gradients for six cases out of the 10 studied, with a stronger and positive effect on sites with low productivity. The magnitude of this change along site productivity gradients varied up to 89% depending on the tree species. Synthesis. The nature of species interaction in mixtures with regard to productivity changes with species assemblage and abiotic conditions. Overyielding is strongest when species grow in highlands on less productive sites. A negative link between mixture effect and site productivity was found, in line with the stress-gradient hypothesis.

Background and aims The structure of a forest stand, i.e. the distribution of tree size features, has strong effects on its functioning. The management of the structure is therefore an important tool in mitigating the impact of predicted changes in climate on forests, especially with respect to drought. Here, a new functional-structural model is presented and is used to assess the effects of management on forest functioning at a national scale. Methods The stand process-based model (PBM) CASTANEA was coupled to a stand structure module (SSM) based on empirical tree-to-tree competition rules. The calibration of the SSM was based on a thorough analysis of intersite and interannual variability of competition asymmetry. The coupled CASTANEA-SSM model was evaluated across France using forest inventory data, and used to compare the effect of contrasted silvicultural practices on simulated stand carbon fluxes and growth. Key results The asymmetry of competition varied consistently with stand productivity at both spatial and temporal scales. The modelling of the competition rules enabled efficient prediction of changes in stand structure within the CASTANEA PBM. The coupled model predicted an increase in net primary productivity (NPP) with management intensity, resulting in higher growth. This positive effect of management was found to vary at a national scale across France: the highest increases in NPP were attained in forests facing moderate to high water stress; however, the absolute effect of management on simulated stand growth remained moderate to low because stand thinning involved changes in carbon allocation at the tree scale. Conclusions This modelling approach helps to identify the areas where management efforts should be concentrated in order to mitigate near-future drought impact on national forest productivity. Around a quarter of the French temperate oak and beech forests are currently in zones of high vulnerability, where management could thus mitigate the influence of climate change on forest yield.

Pourquoi aborder les volumes en commençant par le volume total de l'arbre, peu familier et difficile à appréhender ? Parce qu'il est total, justement : indispensable pour assurer la cohérence des estimations par compartiment (tige, branches, à telle ou telle découpe).Et parce qu'on en a besoin pour les nouveaux enjeux, ceux du carbone par exemple. La méthode est complexe mais le résultat est là : un modèle général, paramétré pour 28 essences avec aussi une formule simplifiée pour des estimations approchées rapides.

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National audience

Growth compensations following a disturbance have been found in different species communities through experimentation, but there are few results obtained in natural conditions, in particular for forest ecosystems. The objective of this study was to determine whether there was growth compensation in a mixed oak–pine forest following a biotic disturbance: an outbreak of pine sawfly (Diprion pini) that caused massive defoliation of pines in Europe in the early 1980s. The data were collected in mixed oak–pine stands located in the plains of north-central France. We measured the ring widths of 223 oaks and 271 pines in nine mixed stands over a period ranging from 1972 to 2005. We established a model which incorporated climatic effects in order to predict the ring width under undisturbed conditions and to quantify the response of each species to the disturbance. We found that the growth of both species varied synchronously with a positive covariation outside of the disturbance. During the disturbance, the growth of both species covaried negatively especially in the plots where pine had been the most severely affected. For the year following the peak of the defoliations, the reduction in growth for pine was strong and ranged from −27% to −92% depending on the plot. In addition, the more significant the reduction in growth for pine, the more significant the increase in growth for oak. We found that a 100% reduction in pine growth was accompanied by a 61% increase in oak growth for the three years following the most severe defoliation. These results demonstrate that compensation between the two tree species following the insect outbreak did occur. We suggest that growth compensations would especially occur in the case of severe biotic disturbances but probably not in the case of climatic fluctuations.

Mélanger des espèces d'arbre au sein d'un peuplement forestier est une stratégie qui peut présenter plusieurs avantages du point de vue de la production de bois. Elle peut avoir un effet positif sur la productivité et permettrait de limiter les conséquences liées à la fragilisation ou à la disparition d'une espèce devenue inadaptée à une nouvelle situation climatique. Néanmoins, le lien entre mélange et productivité est complexe et dépend d'un ensemble de facteurs qui varient dans l'espace et le temps. L'utilisation des données de l'Inventaire forestier national et les expérimentations à long terme sont deux approches complémentaires pour améliorer les connaissances sur le fonctionnement des écosystèmes forestiers mélangés et être en mesure de proposer des modes de gestion adaptés aux différents types d'assemblage d'espèces. / Mixing tree species is a strategy that may have several advantages in terms of wood productivity. This strategy may have a positive effect on productivity and could limit the consequences of the weakening or disappearance of a species because of climate change. Nevertheless, the link between productivity and mixture is a hard topic and depends on a combination of factors that vary in space and time. Using data from the National Forest Inventory and long-term experiments are two complementary approaches to improve our knowledge on the functioning of mixed forests and to be able to define management strategies adapted to the different kinds of species assemblages and for different climatic conditions.

Cette etude presente un exemple de modeles de biomasse empiriques etablis pour le Hetre et repondant a des caracteristiques d’exhaustivite (prise en compte des branches et des fines decoupes), de performance (evaluation la plus precise possible) et de robustesse (face aux changements de productivite et aux interventions sylvicoles). Ce niveau de precision a pu etre atteint en ajoutant l’âge des peuplements aux variables explicatives traditionnelles des modeles de biomasse (diametre a 1,30 m et hauteur totale de l’arbre), et en adoptant une forme d’equation allometrique permettant de relier les parametres du modele aux phenomenes biologiques regissant la croissance des arbres. Les modeles ont ete appliques a une chronosequence situee en Bretagne. Les resultats obtenus ont ete compares avec ceux des tarifs de Bouchon (1982) actuellement utilises par les gestionnaires forestiers pour les estimations des stocks de biomasse. Il apparait que les outils utilises actuellement dans la pratique sous-estiment fortement les stocks de houppier qui representent pourtant l’essentiel de la biomasse potentiellement utilisable pour satisfaire les besoins en bois energie. Pour l’estimation des stocks de carbone a l’echelle nationale, l’emploi d’equations de biomasse permet d’augmenter la precision des estimations de 10 % par rapport aux calculs successifs d’un volume puis d’une masse.

A base for volume and biomass data of the aerial woody compartments of individual trees was constituted under the ANR EMERGE (French acronym for ‘designing models for a robust, all-purpose estimation of wood energy’) project by seven bodies involved in forest research and management in France (ONF, INRA, Cemagref, IFN, FCBA, CNPF and CIRAD) so as to come up with a robust model of woody resources in forests. These measurements, taken for various logging diameters, sometimes down to the slenderest branches, have been performed since the beginning of the 20th century forming a unique data base in France. The data cover a broad range of age groups, sizes, species, stand structures and environmental factors (soil, climate). Of particular interest is the distribution of the available data in relation to French resources as estimated in the National Forest Inventory so as to use sampling campaigns to collect missing information under the EMERGE project. Another significant feature is the potential for designing robust, all-purpose models of volumes and biomass for various logging diameters depending on species, stage of development, geographic gradients, variations in silvicultural practises and possible effect of date of measurement.

Cet article presente une evaluation des quantites de bois qui pourraient etre exploitables dans les forets francaises, les peupleraies et les haies pendant la periode 2007 a 2020. Dans un premier temps, les quantites disponibles brutes de bois, c’est-a-dire avant deduction des differentes consommations actuelles, ont ete evaluees en appliquant des scenarios sylvicoles de reference a l’ensemble des placettes de l’Inventaire forestier national, en fonction de leurs caracteristiques (essence, structure, fertilite, âge, volume a l’hectare observe). Les consommations actuelles ont ete retranchees a ces quantites. Elles ont ete estimees grâce a l’enquete annuelle de branche « exploitation forestiere » pour l’industrie, et a une estimation de la consommation de bois de feu par les menages. Il en ressort une disponibilite supplementaire de plus de 28 millions de metres cube de bois par an pour la bioenergie ou pour la trituration, et de pres de 15 millions en bois d’oeuvre, majoritairement en feuillus.

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Species choice is potentially an important management decision for increasing carbon stocks in forest ecosystems. The substitution of a slow-growing hardwood species (Quercus petraea) by a fast-growing conifer plantation (Pinus nigra subsp. laricio) was studied in central France. Simulations of carbon stocks in tree biomass were conducted using stand growth models Fagacees for sessile oak and PNL for Corsican pine. The changes in soil carbon were assessed using the Century model and data from two European soil monitoring networks: 16 km � 16 km grid and RENECOFOR. Carbon in wood products was assessed with life cycle analysis and lifespan of final products. However, only carbon stocks and their variation were accounted for: effects of energy-consuming materials or fossil fuel substitution are excluded from the analysis. To compare the growth of these two types of forest stands, an important part of the study was to assess the productivity of both species at the same site, using National Forest Inventory data. Simulations showed that (i) this species substitution would lead to an additional carbon storage of 1.6 tC/ha/year with the conifer plantation during its first rotation (64 years) (ii) at steady state, the time- averaged carbon stock of the conifer plantation over the whole rotation would be 42 tC/ha to 47 tC/ha lower than that of the hardwood even-aged forest, depending on applied silvicultural scenarios. The time-averaged carbon stocks including vegetation, soil and wood products are 221 tC/ha for sessile oak, and around 175 tC/ha for Corsican pine. The amount of carbon stored in wood products is low compared to the total carbon stock (5% for sessile oak, 8-8.5% for Corsican pine), mainly due to important losses during early wood processing, and to the short lifespan of wood products compared to the rotation length of forest stands.

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National audience; Results are presented from an ongoing research study of the productivity of forest trees in France. The results provided here are from studies on the height growth of beech [Fagus sylvatica] in northern France, and focus on the growth patterns that have been delineated (over the period 1870 to 2000), and the contrasts that have been found between the NE and NW regions. The study method adopted, and the growth model that has been derived, are described in detail. Different scenarios were simulated for the development of mature beech by the year 2100 in order to clarify the management options that are available; these consider age of exploitation, aspect and storm risk. The findings are discussed and conclusions drawn.

National audience

La capitalisation sur pied et la substitution d'essences sont deux stratégies sylvicoles analysées dans cette thèse, chacune pressentie pour être favorable au stockage du carbone, enjeu majeur de ce XXIème siècle pour la lutte contre le réchauffement climatique. La première correspond à un ralentissement des récoltes, qui devrait générer un puits de carbone additionnel par l'augmentation des stocks moyens de biomasse sur pied. La substitution d'essences à croissance lente comme le Chêne sessile ou le Hêtre par des essences résineuses à croissance rapide comme le Pin laricio ou le Douglas pourrait entraîner une absorption plus rapide du CO2. L'impact sur le carbone de ces deux stratégies est d'autant plus intéressant à étudier que les gestionnaires forestiers les pratiquent déjà. La construction d'une chaîne de modèles prenant en compte l'ensemble des compartiments dans lesquels le carbone est impliqué, à savoir la biomasse forestière, le sol, mais aussi les produits forestiers issus de l'exploitation, permet d'effectuer des simulations donnant des résultats quantitatifs sur ces pratiques. Nous montrons qu'une capitalisation sur pied engendrerait un puits de carbone additionnel dépendant de l'état initial des peuplements, de la fertilité et de la nouvelle sylviculture pratiquée. Il pourrait s'élever jusqu'à 142 tC/ha après obtention du régime permanent dans le cas le plus favorable. La substitution de feuillus à croissance lente par des résineux à croissance rapide serait également un puits de carbone important (jusqu'à 2 tC/ha/an) pendant plusieurs décennies, mais s'avérerait d'un stock de carbone moyen inférieur sur le long terme.

L'article relate une etude des changements de productivite intervenus dans les hetraies regulieres du Nord-Est de la France entre 1900 et 2000. L'etude est basee sur la comparaison retrospective de la croissance de deux generations, structurees en 14 couples de peuplements associant un jeune et un vieux, separes par 75 ans en moyenne et situes a proximite l'un de l'autre, dans la meme foret sur la meme station. Dans chaque peuplement, trois arbres dominants ont ete abattus, dont la croissance en hauteur a ete reconstituee par analyse de tige et la croissance radiale mesuree a 1,30 m. La jeune generation a connu une croissance nettement superieure, la hauteur dominante a 65 ans surclassant de plus de 4 m celle des vieux peuplements adjacents. Une modelisation originale des donnees a permis d'estimer la courbe de tendance de l'accroissement au cours du XXe siecle. Il apparait que l'essentiel de l'acceleration s'est produit entre 1950 et 1980. Integree dans le simulateur de croissance de peuplements forestiers Fagacees, cette tendance a ete utilisee pour comparer deux regimes d'eclaircies, appliques a deux generations nees en 1850 et 1950. Les simulations montrent une tres forte reduction de l'âge d'exploitabilite, pour un meme diametre dominant-objectif de 60 cm : sur station equivalente, la vieille generation conduite en eclaircies faibles est mure a 150 ans, contre 90 ans pour la jeune ayant subi des eclaircies fortes.

International audience; The maximum size-density relationship (MSDR) reflects the boundary site occupancy and the self-thinning line for a given species, being a useful tool in forestry. Studies focusing on the MSDR often do not cover the whole distribution of the studied species, which results in different boundaries for a given species in different regions. A common MSDR is lacking for the increasingly demanded large-scale studies. However, this information is important where silvicultural responses must be prioritized among monospecific stands or where comparisons among maximum and relative stand densities between and within species are required. For the purposes of this study, we used data from 9911 sample plots located in Scots pine and European beech monospecific stands. Both of these species are of considerable importance and widely distributed throughout Europe. The data came from National or Regional Forest Inventories of five European countries (Austria, Germany, France, Spain and Poland) and therefore were distributed across a wide range of climatic conditions. The main aim of this study was to determine whether the MSDR of these species depends on environmental variables and to develop a MSDR model for each species that explain this variability along a climate gradient. The resulting models showed that both parameters of species boundary lines were climate-dependent, but that the pattern of variation differed between species. Hence, the higher the humidity, the steeper the MSDR (more negative exponent) and the higher the intercept for beech, while in the case of pine, the higher the humidity, the straighter the MSDR and the lower the intercept. According to these models, the stand density indices, for a reference diameter of 25 cm, varied with the humidity in a different way for each species. Consequently, the ratio between the two species increases with humidity, although it also depends on stand diameter. These results are in accordance with the yield level theory and could contribute to the development of more precise silvicultural guidelines and growth models based on the self-thinning line. Moreover, they are of particular importance in the discussion of growth and the effects of mixing on mixed species stands.