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Accord par mail pour la mise en ligne du pdf éditeur; National audience; Les mélanges d’essences et la baisse de la densité des arbres dans les peuplements sont des pratiques envisagées comme solution face au changement global. Elles peuvent aider à préserver les forêts et les services qu’elles rendent comme la production de bois, le stockage de carbone et la préservation de la biodiversité. Le dispositif OPTMix, unique en France, étudie les effets du climat, de la gestion sylvicole (mélange et densité) et de la présence des grands herbivores, et leurs interactions sur le fonctionnement des forêts de plaine. Réparti sur quarante hectares, il comporte trente-trois placettes de chêne sessile (Quercus petraea) et de pin sylvestre (Pinus sylvestris) en peuplements monospécifiques ou en mélange, fortement instrumentées pour mesurer le microclimat, la croissance des arbres, la dynamique du sous-bois dont la régénération du peuplement, la gestion des ressources (eau, lumière, nutriments), et la biodiversité (flore vasculaire, bryophytes, lichens, carabes, microfaune du sol). Les premiers résultats montrent que l’intérêt du mélange n’est pas systématique pour les différents services étudiés, et aussi que l’identité des essences détermine à la fois l’intensité et le sens des effets observés. Les dernières années de fortes sécheresses montrent également que l’intérêt des mélanges reste limité pour faire face aux changements climatiques annoncés. De plus, pour la biodiversité, il est recommandé de conserver une mosaïque de type de peuplements à l’échelle du massif, avec des purs de différentes essences et des mélanges plutôt que de généraliser les peuplements mélangés.

International audience

International audience; Les gestionnaires forestiers doivent relever plusieurs défis : adapter les forêts au changement climatique, répondre à la demande croissante de bois (augmentation des prélèvements de bois, et notamment de bois énergie), maintenir la biodiversité et la qualité des sols, et assurer le renouvellement des peuplements. Le site expérimental de terrain OPTMix (https://optmix.irstea.fr) permet d'étudier l'impact de ces changements sur le fonctionnement des écosystèmes forestiers et de tester différents scénarios de sylviculture. Trois facteurs sont testés et contrôlés: -composition du peuplement (chêne pur, pin pur, mélange de pin et de chêne) et -densité de peuplement (nombre d'arbres / ha) combinée avec -présence d'ongulés sauvages (chevreuils, sangliers, chevreuils) L'objectif est d'étudier les effets de ces facteurs et leur combinaison sur le fonctionnement de l'écosystème, notamment la productivité des arbres, l'utilisation et l'allocation des ressources (y compris l'eau et les nutriments), la dynamique de la biodiversité et de la végétation du sous-étage, y compris la régénération.

National audience; Présentation de la construction et du couplage de deux types de modèles de croissances. L'un sur des données transversales (donnees IFN), l'autre sur des données longitudinales (carottage). Application aux chênes sessiles et pin sylvestres. Utilisation du modèle couplé pour la projection à l'horizon 2100 selon les scenarios du GIEC RCP 2.6, 4.5 et 8.5.

International audience; Water shortage in relation with climate change is endured in many regions and is expected to increase in the next decades. To cope with this increasing pressure, the use of a mixture of tree species is nowadays promoted in commercial forestry instead of the standard monoculture widely practiced. The first objective of establishing mixed forest stands is to maintain productivity under the insurance hypothesis; at least one species of the mixture would resist and be resilient to different biotic and abiotic pressures. A postulate often claimed is that mixed forest stands would also help to save water, under the main hypothesis of niche complementarity; the competition for water would be lesser when two or more different species uptake it at different times, or at different locations. However the literature on the effects of mixing species on the forest ecosystem water balance showed contrasted and sometimes contradictory results. Where and under which conditions the complementary effect occurs remain to be specified. This question is addressed in the OPTMix project (Oak Pine Tree Mixture, https://optmix.irstea.fr) designed to test the effect of mixing oak (Quercus petraea) and pine (Pinus sylvestris) on different functions (wood production, biodiversity, resource availability among which, water) of the ecosystem. Thirty three plots divided in three replications were established in the national forest of Orléans, section of Lorris-Les-Bordes, and where mixed stands were compared to even-aged monospecific stands of either oak or pine. Two stand densities were managed, a relatively high one corresponding to traditional silviculture and a low one, assumed to reduce water consumption by trees at the ecosystem level. In each plot, 3 subplots were set up and designed to measure soil water content at 3 depths (-20, -40 and -60 cm from soil surface) as well as the temporary water table depth. Water resource has been measured every 2 hours and recorded every 12 h in dataloggers since the year 2014. In parallel tree water fluxes and water potentials were measured on critical periods of water deficit. Data recording and analyses are still in progress. So that preliminary results should be interpreted with caution. There are as follows. Mixing species had overall little influence on the different measured water variables. This probably results from the shallow soils of the Orleans' forest where both species share a reduced soil volume so that roots take off water in the same soil layers. However oak was a little less constrained by drought when mixed with pine, whereas pine was slightly affected by the presence of oak. These results are in accordance with species functional traits, oak regulates little water spending, whereas pine is much more conservative, i.e. its transpiration is reduced during drought. Consequently a little more water is available for oak when neighbour tree is a pine rather than an oak, decreasing thus competition. Consequences at the ecosystem scale will be discussed as far as the water preservation is concerned.

International audience; In contrast to N soil availability or carbon (C) reserves, the role of internal N reserves in plant interactions has been under-researched (Millard and Grelet, 2010). N resource can be highly deprived by grasses at the expense of tree seedlings. Accordingly increasing internal N pool of tree seedling could be a functional advantage in a competitive relationship for N resource. However, the N-loading of tree seedlings in nurseries before plantation within a forest community to increase their survival and early growth is still controversial. We hypothesized that a larger N-loaded pool in oak seedlings (Quercus petraea) during year n-1 might favour growth and function during year n through higher N remobilization, and thus lessen the impact of competition from Deschampsia cespitosa (tussock grass). In 2015, we cultivated two sets of one-year oak seedlings in 20 L pots with two nitrogen supply levels: no supply (N-unloaded) and 87 kg ha-1 (N-loaded), without competition and without water limitation. A greenhouse experiment was then undertaken in 2016 with the same two oak batches, but with 0, 1, 2 or 3 tillers of D.cespitosain batches containing either N-loaded or N-unloaded oak seedlings. Half the pots were subjected to drought for one month (August 2016). Soil water availability was the primary factor controlling plant growth and photosynthetic activity in oak, both with and without competition. Under well-watered conditions, a greater internal N pool was correlated with higher oak photosynthesis. A high internal N pool was also beneficial to oak seedlings under low competitive pressure from D. cespitosa. In addition, when grass density increased, intraspecific competition outweighed interspecific competition and attenuated the negative effect of D. cespitosaon oak seedlings. A carry-over effect of N-loading thus had a positive effect on growth and function in oak seedlings when no water stress occurred under low competition by D. cespitosa: this effect may derive from more N storage in perennial tissues, resulting in more N mobilized during vegetative growth. Our results underline the importance of the internal N pool in plant competition and are evidence that a higher potential regenerative success rate of oak seedlings is related to abiotic conditions, competitive pressure and internal N pool. Competitor density influenced habitat filtering (abiotic and biotic), inducing variations in biotic interactions and plant functioning.

L'objectif général du projet SATAN est d'évaluer les synergies et antagonismes entre les stratégies d'adaptation des forêts sur les stratégies d'atténuation à l'échelle d'un grand territoire forestier, la Forêt d'Orléans. Pour atteindre cet objectif, le projet est divisé en quatre tâches. La première est une tâche d'acquisition de connaissances sur l'adaptation des forêts au changement climatique. Dans cette tâche, nous étudions notamment l'influence de la densité et de la composition du peuplement sur le lien entre la consommation en eau et la croissance arbres. La deuxième tâche utilise ces connaissances, et remobilise des données acquises pour élaborer un modèle de croissance simple et robuste, prenant en compte l'influence du climat. Ce modèle doit être adapté aux conditions du site d'étude. Parallèlement, la troisième tâche définit les scénarios climatiques et sylvicoles adaptés à la région. Dans cette tâche figure également une analyse de la filière locale servant à bien identifier les usages des bois produits dans la région. Enfin, la quatrième tâche, non encore démarrée, utilisera les résultats des tâches 2 et 3 pour réaliser des simulations de synergie entre adaptation et atténuation à l'horizon 2100.

International audience; Several forest management strategies can be used to cope with climatic changes. Reducing stand density and mixing tree species are two of them. Several studies investigated these strategies separately, but we do not know what the outcome would be if we used them conjointly. The main objective of our study was to quantify tree species interactions effects on radial growth during a water stress period and to determine whether these effects changed with different levels of competition (different levels of stand density). This study was based on a long term triplet experiment (OPTMix) made of pure and mixed stands of mature Quercus petraea and Pinus sylvestris in Orleans’ forest (Central France). In this experiment, there are three repetitions of two densities (low and medium) in each composition (pure oak, mixed stands, pure pine). We followed radial growth of trees through 270 manual dendrometers spread over 21 plots. In each plot, the microclimate was monitored, and soil moisture sensors were set up at several soil depths in order to follow the drought. We found that the mixture had no effect on oak growth, but a small negative effect on pines when there was a water stress during the summer. Investigations are still in progress to determine if this result depends on stand density. Reducing stand density and the mixing of tree species are promising strategies, but understanding their combining effects still require further research.

International audience; Several forest management strategies can be used to cope with climatic changes. Reducing stand density and mixing tree species are two of them. Several studies investigated these strategies separately, but we do not know what the outcome would be if we used them conjointly. The main objective of our study was to quantify tree species interactions effects on radial growth during a water stress period and to determine whether these effects changed with different levels of competition (different levels of stand density). This study was based on a long term triplet experiment (OPTMix) made of pure and mixed stands of mature Quercus petraea and Pinus sylvestris in Orleans’ forest (Central France). In this experiment, there are three repetitions of two densities (low and medium) in each composition (pure oak, mixed stands, pure pine). We followed radial growth of trees through 270 manual dendrometers spread over 21 plots. In each plot, the microclimate was monitored, and soil moisture sensors were set up at several soil depths in order to follow the drought. We found that the mixture had no effect on oak growth, but a small negative effect on pines when there was a water stress during the summer. Investigations are still in progress to determine if this result depends on stand density. Reducing stand density and the mixing of tree species are promising strategies, but understanding their combining effects still require further research.

Ecological theories on species occurrence and interactions between them deeply rely on resources availability. Grasses are known for their ability to preempt belowground resources, especially nitrogen to the detriment of tree seedlings. Objective of the study was to determine how combination in contrasted soil inorganic nitrogen x light availability influence plants in terms of growth and resource acquisition in mixed stand of oak seedling (Quercus petraea) and Deschampsia cespitosa during early phase of mixture growth. We designed a greenhouse experiment testing the Deschampsia effect on oak, grown in pots, alone or as a mixture under two levels of crossed resources, nitrogen (fertilized, N+, or unfertilized pots, N-), and light (shaded, L-, or well-lit pots, L+). Our results showed oak endured competition from grass in all resource combinations except in N-/L-. Light environment increased intensity and importance of competition on oak. Unexpectedly Deschampsia was facilitated by oak in N+ pot. A higher root biomass, length, SRL, and lower root diameter confer to Deschampsia better prospection and capitation capacities, allowing it to take 96.6% of a source of 15N labeled inorganic nitrogen up. No root plasticity concordant with the foraging theory was observed. Why was Deschampsia facilitated by oak when nitrogen was not limiting is an intriguing question. One answer, still needing research, is that a better oak growth, and so production of more exudates, fosters Deschampsia growth. As a result, Deschampsia ability to harvest resources is reinforced. Oak seedling responds through driving the few resources it can collect to storage organs, following a conservative strategy.

International audience; Availability of aerial and soil resources in oak forests is likely to change in the near future: wood harvest will increase to fulfill biomass demand and atmospheric N deposition is still high. Accordingly, oak seedlings will benefit from a larger availability of both incoming irradiance and soil inorganic N (Ni) amounts. Such changes may also favor graminoids, jeopardizing oak regeneration. Methods Along an in situ light gradient, 60 1-year old oak seedlings were planted. Effects of M. caerulea and N fertilization were tested by hand-weeding and applying 90 kg inorganic N.ha-1. Seedling growth as well as tracking of 15N fate was monitored and seedlings were harvested in autumn 2015 after one growing season. Results Tree growth increases along light gradient. Neither presence of M. caerulea nor N fertilization has any effect on oak growth. Under high irradiance, 15N was preferentially allocated to leaves instead of stem. Eventually, only growth and N allocation were larger in fine roots when no N fertilization was applied or in mixture. Discussion Growth seedlings was positively responsive to light but not to changes in soil inorganic N resources due to presence of M. caerulea and/or N fertilization. Better growth might result from an optimized N allocation toward leaves. Fine roots biomass positively responded to N soil limitation, supporting foraging strategy. Nevertheless, lack of N resource due to biotic competition would promote graminoids uprooting in forest management practice to ensure a high success of regeneration.

International audience; Ecological theories on species occurrence and interactions between them deeply rely on resources availability. Grasses are known for their ability to preempt belowground resources, especially nitrogen to the detriment of tree seedlings. Objective of the study was to determine how combination in contrasted soil inorganic nitrogen x light availability influence plants in terms of growth and resource acquisition in mixed stand of oak seedling (Quercus petraea) and Deschampsia cespitosa during early phase of mixture growth. We designed a greenhouse experiment testing the Deschampsia effect on oak, grown in pots, alone or as a mixture under two levels of crossed resources, nitrogen (fertilized, N+, or unfertilized pots, N-), and light (shaded, L-, or well-lit pots, L+). Our results showed oak endured competition from grass in all resource combinations except in N-/L-. Light environment increased intensity and importance of competition on oak. Unexpectedly Deschampsia was facilitated by oak in N+ pot. A higher root biomass, length, SRL, and lower root diameter confer to Deschampsia better prospection and capitation capacities, allowing it to take 96.6% of a source of 15N labeled inorganic nitrogen up. No root plasticity concordant with the foraging theory was observed. Why was Deschampsia facilitated by oak when nitrogen was not limiting is an intriguing question. One answer, still needing research, is that a better oak growth, and so production of more exudates, fosters Deschampsia growth. As a result, Deschampsia ability to harvest resources is reinforced. Oak seedling responds through driving the few resources it can collect to storage organs, following a conservative strategy.

Rapport de la tranche 2016 de la convention ONF/Irstea Nogent 2016-2018 sur l'étude des peuplements mélangés. La convention comprends trois axes principaux : la modélisation des mélanges à large échelle à partir des données d'inventaire, les études à partir du dispositif OPTMix, une prospective sur la gestion des chênaies irrégulière et des chênaies en contexte hydromorphe.

Rapport de la convention 2015 portant sur la croissance et la production des forêts mélangées. Le rapport comprend 2 parties principales. La première sur l'étude des mélanges à large échelle à partir des données d'inventaire, la deuxième sur le dispositif OPTMix

National audience; Présentation d'un stage de M2 sur les effets du mélange sur la croissance en hauteur du pin sylvestre, en lien avec le climat.

National audience; The objective is to study the cross effects of: - stand composition (pure oak, pure pine, mixed pine-oak) and - stand density (number of trees/ha) combined with - presence of wild ungulates (roe deer, wild boar, red deer), on the ecosystem functioning such as tree productivity, resource use and allocation (including water and nutrients), biodiversity and understory vegetation dynamics including regeneration.

National audience; Analyse de l'effet du mélange sur la croissance pour cinq couples d'essences à partir des données de l'inventaire forestier. Analyse de l'influence des facteurs abiotiques sur cette relation. L'effet diminue avec le gradient de productivité.

National audience; Une étude rétrospective en forêt mélangée sur l’attaque massive d’un pathogène spécialiste, le lophyre du pin, montre que cette perturbation a profité aux chênes. Un très bon cas d’école qui illustre comment le mélange d’espèces peut limiter les pertes de croissance lors d’une perturbation affectant l’écosystème.

Ce rapport de la dernière tranche de la convention ONF/Irstea Nogent comprend les parties suivantes : - Modélisation des peuplements mélangés Chêne - autres, dont l'avancement de la thèse de Maude Toïgo - Un article RdVT sur la modélisation des mélanges Sapin-Epicea-Hêtre à partir des données IFN, et l'évolution du module Capsis Melies vers Deesses - Les études locales sur le mélange Chêne Pin en forêt d'Orléans, notamment un rapport sur le bilan du suivi en eau du sol en lien avec la croissance intra-annuelle.

National audience; La forêt subit des pressions croissantes relatives au changement climatique, à l’évolution de la gestion forestière (augmentation des prélèvements de bois, conduite du peuplement) et à l’accroissement des populations d’ongulés sauvages. L'unité Écosystèmes Forestiers d’Irstea étudie l'impact de ces changements sur le fonctionnement et la dynamique des écosystèmes forestiers et teste différents scénarios grâce à un réseau de placettes instrumentées en forêt domaniale d'Orléans. Le poster présente l'expérimentation et les travaux qui y seront menés par l'Unité EFNO.