Managing tree species diversity and ecosystem functions through coexistence mechanisms

International audienceAbstractKey messageA better transfer to managers of studies examining the functional role of tree species diversity would be achieved by explicitly addressing two missing links: the effect of management interventions on coexistence mechanisms and the relationships between coexistence mechanisms and ecosystem functions.ContextPlant species diversity has been shown to promote a wide array of ecosystem functions and ecosystem services. However, scientific results concerning relationships between species diversity or species mixing and ecosystem functions have not been well transferred to management practices so far. Part of the problem lies in the difficulty of assessing whether interesting species mixtures can persist over the long term and how management influences ecosystem functions.AimsWe argue that a better transfer of knowledge to managers would be achieved by addressing two missing links: (i) the effect of management interventions on coexistence mechanisms and (ii) the relationships between coexistence mechanisms and ecosystem functions.MethodsTo do so, we first provide a brief overview of the recent scientific results on relations between tree diversity (or two-species mixing) and ecosystem functions, focusing on studies dealing with productivity and stability in forests. We further introduce the key question of whether mixed stands are transient or permanent. We then briefly present key elements of modern coexistence theory and illustrate them with three examples in forest ecosystems. We finish by discussing how management interventions in forests can affect coexistence mechanisms and by addressing some methodological perspectives.ResultsWe provide examples of management actions (e.g. gap-based silviculture, preferentialselection of the most frequent species, preferential selection of the most competitive species, plantingweakly competitive species) that may increase the strength of coexistence mechanisms.ConclusionAnalysing long-term management impacts on species coexistence and ecosystem functions with a combination of long-term monitoring of large permanent plots and mechanistic dynamic model simulations will be useful to develop relevant practices favouring mixed forests in the long term

Tree-related microhabitats (TreMs) as key elements for forest biodiversity

Tree-related microhabitats (TreMs) are specific above-ground morphological singularities borne by standing living or dead trees. They are regularly observed and are crucial issues for forest management in such a way that standards are currently negotiated (e.g. PEFC, FSC, N2000). TreMs depend on tree characteristics. The largest trees play a pivotal role in TreM supply and only the largest trees bear all the TreM types. Living and dead trees are complementary in their supply of TreM types. For instance, cracks and fungus sporocarps are rather borne by snags than by living trees. TreMs distribution patterns are very different in old-growth forests or in managed stands. As «ephemeral resource patches », TreMs host a wide diversity of taxa and play a wide range of pivotal biological roles. Certain Trems, like dendrothelms, host poor but very specific species assemblages. Other TreMs are composite habitats and host several communities. TReMs participate in a complex functional habitat network in species life cycles. How TReMs contribute to local biodiversity depends both on forest type and taxon conservation status. Positive relationships between TReMs density and local species richness are sometimes thresholded. Practical considerations for forest management integrating TreMs include the search for quantitative thresholds. Set-aside areas more than 20 ha in area are needed in mixed forest to conserve TreMs diversity. After a drastic harvesting, the recovery of TreMs needs decades and TreM-associated taxa may have a delayed response. A hierarchical typology is now available as a reference for TreM recording in temperate and Mediterranean European forests. The modelling of TreM ontogenesis is at work, as well as simulation models evaluating long-term effects of management scenarii

Modeling the dynamics of microhabitats

The Integrate+ Conference took place in the Steigerwald Centre near Ebrach, Bavaria from 26 – 28 October 2016 and gathered nearly 80 participants with various professional backgrounds. Those included silviculture trainers, forest ecologists, conservation biologists, forest scientists and representatives from forest administrations and forest managers. The conference invited the participants to discuss and exchange on the project results, its extensive network activities and most importantly reflect on viable options on how to continue what has been initiated by Integrate+ beyond 2016

Comparaison entre l'interception de la lumière et l'endroit où se trouvent les bouquets pour prédire la croissance des arbres

International audienceFor the past few decades, empirical and process-based tree growth models have been developed concurrently, although few comparisons have been made of growth predictions obtained using both approaches. One main difference is the explicit quantification of foliage biomass as a key variable of process-based models. The aim of this work was to test if this difference has a significant impact on models behavior, especially when they are used to simulate new silviculture practices like intensive thinning.In the first part of the study, we developed a method to evaluate leaf area and light interception of the mean tree of an even-aged stand from the data of a yield table. Two cases were distinguished : in closed stand, leaf area was limited by a maximum Leaf Area Index, whereas in open stands, leaf area was limited by the height of the crown base. For the data set we used, a yield table for Norway spruce in the French Northern Alps, these situations complemented well, the first one adapting to young, dense stands and the second one to old, clearer ones. Light interception was then calculated using an interpolation between the situation of a closed stand (Beer-Lambert law) and an isolated tree (light interception proportional to leaf area).In the second part, we used this approach to build a growth model in which competition was described by the ratio of the light intercepted by a mean tree of the stand to the light intercepted by a tree of equivalent size in free growth.Pendant ces dernières décennies, les modèles de croissance des arbres empiriques et basés sur les processus ont été développés de manière concurrentielle, bien que quelques comparaisons de prédiction de croissance aient été effectuées en utilisant les deux approches. Une des principales différences est la quantification explicite de la biomasse des feuillages comme une variable clé des modèles basés sur les processus. Le but de ce travail était de tester si cette différence a un impact significatif sur le comportement des modèles, spécifiquement quand ils sont utilisés pour simuler de nouvelles pratiques de sylviculture telles que l'élagage intensif. Dans la première partie de l'étude, nous avons développé une méthode pour évaluer les zones des feuilles et l'interception de la lumière de l'arbre principal de bouquets d'âge équitable. Deux cas se distinguaient : en bouquet fermé, les zones de feuillages étaient limitées par un Index maximum de Zone de Feuillage, tandis qu'en bouquet ouvert, la zone de feuillage était limitée par la hauteur du pied principal. Pour l'ensemble des données que nous avons utilisé et qui provenait d'épicéas Norway dans les Alpes françaises du nord, ces situations se complémentaient bien, la première étant adaptée aux bouquets jeunes et denses et le second aux vieux et plus parsemés. L'interception de la lumière était alors calculée en utilisant une interpolation entre la situation d'un bouquet fermé (Beer Lambert bas) et un arbre isolé (interception de la lumière proportionnelle à la zone de feuillage). Dans la seconde partie, nous avons utilisé cette approche pour construire un modèle de croissance dans lequel la compétition était décrit par le ratio de la lumière interceptée par un arbre moyen du bouquet sur la lumière interceptée par un arbre de taille équivalente en croissance libre

Comparaison entre l'interception de la lumière et l'endroit où se trouvent les bouquets pour prédire la croissance des arbres

International audienceFor the past few decades, empirical and process-based tree growth models have been developed concurrently, although few comparisons have been made of growth predictions obtained using both approaches. One main difference is the explicit quantification of foliage biomass as a key variable of process-based models. The aim of this work was to test if this difference has a significant impact on models behavior, especially when they are used to simulate new silviculture practices like intensive thinning.In the first part of the study, we developed a method to evaluate leaf area and light interception of the mean tree of an even-aged stand from the data of a yield table. Two cases were distinguished : in closed stand, leaf area was limited by a maximum Leaf Area Index, whereas in open stands, leaf area was limited by the height of the crown base. For the data set we used, a yield table for Norway spruce in the French Northern Alps, these situations complemented well, the first one adapting to young, dense stands and the second one to old, clearer ones. Light interception was then calculated using an interpolation between the situation of a closed stand (Beer-Lambert law) and an isolated tree (light interception proportional to leaf area).In the second part, we used this approach to build a growth model in which competition was described by the ratio of the light intercepted by a mean tree of the stand to the light intercepted by a tree of equivalent size in free growth.Pendant ces dernières décennies, les modèles de croissance des arbres empiriques et basés sur les processus ont été développés de manière concurrentielle, bien que quelques comparaisons de prédiction de croissance aient été effectuées en utilisant les deux approches. Une des principales différences est la quantification explicite de la biomasse des feuillages comme une variable clé des modèles basés sur les processus. Le but de ce travail était de tester si cette différence a un impact significatif sur le comportement des modèles, spécifiquement quand ils sont utilisés pour simuler de nouvelles pratiques de sylviculture telles que l'élagage intensif. Dans la première partie de l'étude, nous avons développé une méthode pour évaluer les zones des feuilles et l'interception de la lumière de l'arbre principal de bouquets d'âge équitable. Deux cas se distinguaient : en bouquet fermé, les zones de feuillages étaient limitées par un Index maximum de Zone de Feuillage, tandis qu'en bouquet ouvert, la zone de feuillage était limitée par la hauteur du pied principal. Pour l'ensemble des données que nous avons utilisé et qui provenait d'épicéas Norway dans les Alpes françaises du nord, ces situations se complémentaient bien, la première étant adaptée aux bouquets jeunes et denses et le second aux vieux et plus parsemés. L'interception de la lumière était alors calculée en utilisant une interpolation entre la situation d'un bouquet fermé (Beer Lambert bas) et un arbre isolé (interception de la lumière proportionnelle à la zone de feuillage). Dans la seconde partie, nous avons utilisé cette approche pour construire un modèle de croissance dans lequel la compétition était décrit par le ratio de la lumière interceptée par un arbre moyen du bouquet sur la lumière interceptée par un arbre de taille équivalente en croissance libre

Exemples d'expériences de simulations. Liens avec l'expérimentation in situ

National audienceDans cette présentation, nous donnons des exemples d'expérimentations virtuelles effectuées avec la modèle de dynamique forestière Samsara2 (modèle individu centré spatialement explicite) dans le cas des futaies irrégulières de montagne. Nous présentons également un exemple d'utilisation du modèle et des résultats de ces expérimentations virtuelles pour la mise en place d'une expérimentation in situ rattachée au GIS-Coop

Effets des interactions biotiques sur la régénération des forêts le long de gradients climatiques

Comprendre comment les variations du climat pourraient influencer le recrutement des arbres en forêt est un défi car la germination, la croissance et la survie des semis sont particulièrement sensibles aux facteurs biotiques et abiotiques. Plusieurs modèles conceptuels généraux, (la stress gradient hypothesis SGH et latitudinal herbivory theory LHT,) suggèrent que la nature et l'intensité des interactions biotiques varient le long de gradients environnementaux. Ces modèles constituent une base théorique utile pour déterminer le rôle des interactions biotiques dans la réponse des communautés végétales aux variations du climat. Cependant, les études portant sur la SGH se sont limitées aux interactions directes entre plantes et ont négligé les interactions complexes. Les théories développées pour des niveaux trophiques supérieurs (comme la LHT) sont quant-à elles sujettes à débat et ne considèrent qu'un nombre limité d'interactions. Les interactions avec les micro-organismes du sol ont par exemple été négligées. Dans cette thèse, j'ai analysé comment les interactions directes et indirectes entre les semis, les arbres adultes, la végétation herbacée, les insectes herbivores et les pathogènes du sol varient en nature et en intensité le long de gradients climatiques. Pour explorer les interactions entre plantes, j'ai effectué une expérimentation s'appuyant sur la variabilité spatio-temporelle des conditions climatiques dans les Alpes. Les résultats montrent que les effets directs de compétition de la canopée et de la végétation herbacée augmentent avec la température. Cependant, pour les sites les plus chauds, la facilitation indirecte pourrait limiter la compétition directe des herbacées. Pour les interactions avec insectes herbivores,nous avons montré que le long de gradients d'altitude et de latitude, l'herbivorie est minimale là où les stress thermiques et hydriques est fort. Enfin, pour les interactions avec les micro-organismes du sol, nous avons mesuré en chambre de culture la survie de plantules de hêtre dans des sols, stérilisés ou non, collectés le long d'un gradient d'altitude. Les résultats montrent que les effets négatifs des micro-organismes sont plus faibles quand les communautés microbiennes proviennent de sols d'altitudes. Cette thèse suggère que les effets directs négatifs des plantes, des insectes herbivores et des pathogènes du sol sur les semis dominent dans les environnements chauds et productifs et s'atténuent dans des conditions plus froides. Les interactions indirectes semblent varier inversement et pourraient ainsi tamponner l'effet des interactions directes.Understand how climate change could influence forest tree recruitment is a challenge because germination, seedling growth and seedling survival are especially sensitive to biotic and abiotic factors. Several conceptual models (as the Stress Gradient Hypothesis , SGH, and the Latitudinal Herbivory Theory , LHT) propose that the nature and intensity of biotic interactions vary along environmental gradients. These models constitute a useful theoretical basis to determine the role played by biotic interactions in the response of vegetation communities to climate variations. However, SGH studies have nevertheless been limited until now to direct interactions between plants and have neglected complex interactions. Theories including higher trophic levels, as the LGH, are still debated and have focused on a limited number of interactions. For example, interactions with soil biota have been neglected. In this thesis, we have analysed how direct and indirect interactions between seedlings, adult trees, ground vegetation, herbivorous insects, and soil pathogens vary in nature and intensity along climate gradients. To study interactions between plants, we established an experimentation using the spatio-temporal climatic variability in the French Alps. The results show that the direct effects of competition from adult tree canopy and ground vegetation diminish at higher altitudes. However, for warmer sites, indirect facilitation by canopy could limit direct competition by ground vegetation. To study the interactions between seedlings and herbivorous insects, we have analysed the altitudinal and latitudinal variation of herbivory on young trees across Europe. Results show that herbivory is the lowest where temperature and water stresses are strong. Finally, to study interactions with soil microorganisms, we have measured in a greenhouse the survival of European beech seedlings on soils either sterilized or not, collected along an altitudinal gradient. The results show that the negative effect of microorganisms is lower when microbe communities come from higher altitudes. The results suggest that the negative direct effects of plants, herbivorous insects and soil pathogens on seedlings are dominant in warm and productive environments and diminish in cold environments. Indirect interactions seem to vary inversely and could buffer the effects of direct interactions.SAVOIE-SCD - Bib.électronique (730659901) / SudocGRENOBLE1/INP-Bib.électronique (384210012) / SudocGRENOBLE2/3-Bib.électronique (384219901) / SudocSudocFranceF

Intra-specific variability and the competition-colonisation trade-off: coexistence, abundance and stability patterns

International audienceIntra-specific variability often produces an overlap between species distributions of individual performances which can influence competition relations and community dynamics. We analysed a two-species competitioncolonisation model of vegetation with intra-specific variability in juvenile growth. On each patch colonised by both species, the winner was the juvenile with higher individual growth. Intra-specific variability disproportionately favoured the more fecund species because the tail of its distribution represented more ndividuals. In some cases, this process could even lead to a reversal of competition hierarchy and exclusion of the species with higher mean juvenile performance. In the space of species 2 mean growth and fecundity traits, the combinations of traits allowing coexistence with species 1 appeared close to an ideal trade-off curve. Along this curve, species 2 and species 1 coexisted at similar abundance. The balance of relative abundances diminished with the distance of species 2 from this curve. For a given level of relative species performances, coexistence stability increased continuously as species differentiation increased. In contrast to classical models that exhibit abrupt changes of equilibrium community properties when species traits vary, our model displayed continuous changes of these properties in relation to the balance of life traits within and among species. Intra-specific variability allows flexible patterns of community dynamics and could explain discrepancies between observations and classical theories