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Abstract. We present repeated tree measurement data from 63 permanent plots in mountain forests in France. Plot elevations range from 800 (lower limit of the montane belt) to 1942 m above sea level (subalpine belt). Forests mainly consist of pure or mixed stands dominated by European beech (Fagus sylvatica), Silver fir (Abies alba), and Norway spruce (Picea abies), in association with various broadleaved species at low elevation and with Arolla pine (Pinus cembra) at high elevation. The plot network includes 23 plots in stands that have not been managed for the last 40 years (at least) and 40 plots in plots managed according to an uneven-aged system with single-tree or small-group selection cutting. Plot sizes range from 0.2 to 1.9 ha. Plots were installed from 1994 to 2004 and remeasured two to five times during the 1994-2015 period. During the first census (installation), living trees more than 7.5 cm in dbh were identified, their diameter at breast height (dbh) was measured and their social status (strata) noted. Trees were spatially located, either with x, y, and z coordinates (40 plots) or within 0.25-ha square subplots (23 plots). In addition, in a subset of plots (58 plots), tree heights and tree crown dimensions were measured on a subset of trees and dead standing trees and stumps were included in the census. Remeasurements after installation include live tree diameters (including recruited trees), tree status (living, damaged, dead, stump), and for a subset of trees, height. At the time of establishment of the plots, plot densities range from 181 to 1328 stems/ha and plot basal areas range from 13.6 to 81.3 m 2 /ha.

Climate change affects forest ecosystem processes and related services due to increasing temperature and increasing extreme drought event frequency. This effect can be direct through the alteration of the physiological responses of trees, but also indirect, by modifying interactions between trees and thus changing communities' composition. Such changes might affect species richness with high impacts on ecosystem functioning, especially productivity.Regarding management issues, mixed stands are usually considered a good option to maintain forest cover and ecosystem services under climate change. However, the possibility to maintain these mixed stands with management actions with positive effects on forest functioning under climate change remains uncertain and deserves further investigations. Relying on a simulation-based study with a forest gap model, we thus addressed the following questions: (1) Are monospecific stands vulnerable to climate change? (2) Would mixed stands significantly mitigate climate change effects on forest productivity and wood production under climate change? (3) Would conversion to mixed stand management affect significantly forest productivity and wood production under climate change compare to monospecific management?With a 150 years simulation approach, we quantified potential climate change effect (using RCP 8.5) compared to present climate and managements effect in the French Alps, focusing on five tree species. The gap-model we used included a management module, which allowed testing six silvicultural scenarios on different stands, with various composition, structure or environmental conditions, under climate change.These simulations showed that monospecific stands currently growing in stressful conditions would be too vulnerable to climate change to be maintained. Managing mixed stands or 3 conversion from pure to mixed stands would make it possible to maintain higher productivity in the long-term than monospecific stands, even under severe climate change. This pattern depends to species and sites considered. Our results will feed into discussion on forest management in the context of climate change.

Climate change affects forest ecosystem processes and related services due to increasing temperature and increasing extreme drought event frequency. This effect can be direct through the alteration of the physiological responses of trees, but also indirect, by modifying interactions between trees and thus changing communities' composition. Such changes might affect species richness with high impacts on ecosystem functioning, especially productivity. Regarding management issues, mixed stands are usually considered a good option to maintain forest cover and ecosystem services under climate change. However, the possibility to maintain these mixed stands with management actions with positive effects on forest functioning under climate change remains uncertain and deserves further investigations. Relying on a simulation-based study with a forest gap model, we thus addressed the following questions: (1) Are monospecific stands vulnerable to climate change? (2) Would mixed stands significantly mitigate climate change effects on forest productivity and wood production under climate change? (3) Would conversion to mixed stand management affect significantly forest productivity and wood production under climate change compare to monospecific management? With a 150 years simulation approach, we quantified potential climate change effect (using RCP 8.5) compared to present climate and managements effect in the French Alps, focusing on five tree species. The gap-model we used included a management module, which allowed testing six silvicultural scenarios on different stands, with various composition, structure or environmental conditions, under climate change.These simulations showed that monospecific stands currently growing in stressful conditions would be too vulnerable to climate change to be maintained. Managing mixed stands or 3 conversion from pure to mixed stands would make it possible to maintain higher productivity in the long-term than monospecific stands, even under severe climate change. This pattern depends to species and sites considered. Our results will feed into discussion on forest management in the context of climate change.

NDLR :L ap remière version de cet article est parue dans Innovations agronomiques,5 6, 2016. Nous remercions la rédaction de Innovations Agronomiques de nous avoir autorisés àe np ublier une deuxième version dans la Revue forestièref rançaise.L 'objet de cet article était de rendre compte de la démarche d'appropriation des travaux de recherche par les gestionnaires et notamment par le département Recherche, Développement et Innovation de l'ONF.L es auteurs de ce département tiennent àr emercier les laboratoires associés àc es travaux et qui sont cités tout le long du texte :I NRA, Irstea, FCBA, IRD, CIRAD...Tandis que la surface forestièref rançaise aa ugmenté de près de six millions d'hectares en un siècle (près de 60 %), la récolte aq uasiment doublé (IGN, 2013). Cette progression explique la position forte de la forêt française au rang européen, cependant la balance commerciale du bois et des produits dérivés (papiers, meubles) reste la seconde déficitairea près celle des produits pétroliers. Le décalage entrel 'offree tl ad emande nationale de bois tient àl af ois àu nm anque chronique de bois résineux de structure( en quantité mais aussi en qualité), qui oblige àu ne importation encorei mportante de bois du Nord ou de l'Est, mais aussi àu ne valorisation insuffisante de nos feuillus, malgré de nombreux travaux visant àp lus les utiliser,p ar exemple en construction (FCBA, 2011).Pourtant nous ne récoltons actuellement que 50 %d el ap roduction biologique, même si la forêt française est globalement jeune (beaucoup d'accrues dans le siècle dernier). La situation est moins accentuée en forêt publique et surtout domaniale (70 %d el ap roduction récoltée), où le stock de bois sur pied est maintenant stabilisé (Gamblin et Michon, 2011). Les régions identifiées de faible récolte sont plutôt en zone méditerranéenne pour des raisons de très faible productivité, ou en montagne cette fois pour des problèmes d'exploitabilité.Parallèlement, le matériau bois est porteur de beaucoup d'espoir dans sa capacité àr emplacer des matériaux fossiles et la pétrochimie :c onstruction, fibre, énergie, chimie verte. Sa fabrication naturelle en forêt et son utilisation par la société sont gages d'atténuation du changement climatique, par séquestration mais surtout par substitution, cette substitution agissant de manière cumulativee td éfinitive( Deleuzee tM icheneau, 2015).Face àc et espoir,l es évolutions climatiques àm oyen terme sont au contraires ource de fortes inquiétudes, avec des disparités selon les contextes pédoclimatiques. Les temps forestiers étant longs comme pour la futaie de Chêne, dont la durée de production atteint 190 ans, les décisions doivent êtrep rises en incluant un futur très incertain.