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Redesigning agrosystems with more ecological regulations can help feed a growing population, preserve soils for future productivity and reduce environmental impacts. However, guidelines for redesigning agrosystems from natural systems are limited. Reviewing the last knowledge of ecosystem functioning, we outlined four ecological systems synchronizing the supply of soluble nutrients by soil biota to fluctuating plant nutrient demand. This synchrony limits deficiencies and excesses of soluble nutrient, which usually penalize both production and regulating services of agrosystems such as nutrient retention and soil carbon storage. We detail how ecological systems promoting synchrony can be installed in agrosystems to improve their sustainability and reduce the use of mineral fertilizers.

International audience

International audience; New adaptation and mitigation strategies are needed to manage temperate forests in the context of climate change with more frequent droughts. Thinning intensification has been proposed as an adaptation strategy to improve forest resistance to drought. Partial cutting of forest stands by thinning operations mimics the natural disturbance by treefall events and can affects tree leaf litter traits by altering interactions among forest plants. The importance of plant litter traits and decomposability for C and nutrient cycling processes has been largely emphasized. However, the role of biotic interactions as drivers of intraspecific variability in litter traits remains surprisingly little studied. In this study, we used a large-scale, multi-site network of long-term tree removal experiments manipulating the abundance of a foundation tree species, i.e. Quercus petraea, to assess how plant interactions control intraspecific variation in oak leaf litter traits and decomposability. We studied 19 plots across eight experimental sites covering a large gradient of oak abundance, stand age and local abiotic context. Oak leaf litter quality strongly declined with tree removal in early forest successional stage. Litter became poorer in nutrients such as N and Mg and richer in secondary metabolites such as lignin and condensed tannins. Variance partitioning indicated that oak abundance explained as much variation in oak leaf litter traits as oak age and twice as much as soil inherent fertility. Confirmatory path analysis revealed that the decline of oak leaf litter quality induced by tree removal was most likely driven by a shift in understory plant species composition. This response pattern could reflect the plasticity of oak leaf litter traits to the shortage of nutrient supply related to the development of understory plants competitors with higher nutrient capture and retention ability. Our data also give consistent but weaker support that the decrease in oak leaf litter quality with tree removal could be driven by alleviated competition for light among canopy trees and subsequent enhanced crown exposure to light. This plasticity in oak leaf litter traits had important consequences for ecosystem functioning. The decline of oak leaf litter quality in turn slowed its decomposition and induced a decline of the forest floor turnover. This slower forest floor dynamics largely mitigated forest floor C loss associated to reduced litterfall caused by tree removal, resulting in a weak net forest floor C loss. Interestingly, the decline of oak leaf litter quality also induced a shift of litter N loss from N release to N immobilization. Overall, our study provides evidence that biotic factors such as plant interactions are major drivers of plasticity in leaf litter traits and decomposability. This finding contributes to the emerging view that phenotypic plasticity is fundamentally related to biotic interactions for sessile organisms, especially for long-lived and large plant species such as trees. Further, we demonstrated that this plasticity shaped by plant interactions can be a major driver of ecosystem processes. Taking this source of functional diversity into account could help us to better understand how changing biodiversity affects ecosystem functioning in the context of global change.

International audience; Plant-soil interactions are increasingly recognized to play a major role in terrestrial ecosystems functioning. However, few studies to date have focused on slow dynamic ecosystems such as forests. As they are vertically stratified by multiple vegetation strata, tree removal could alter forest plant community through tree canopy opening. Very little is known about cascading effect on soil biodiversity. We conducted a large-scale, multi-site assessment of collembolan assemblage response to long-term experimental manipulation of tree canopy opening in oak temperate forests. A total of 33 experimental plots were studied covering a large gradient of tree canopy basal area, stand age and local abiotic contexts. Collembolan abundance strongly declined with tree canopy opening in early forest successional stage and this was mediated by a shift in understory plant community composition. Collembolan functional groups had contrasting response patterns, which were mediated by different ecological factors. Epedaphic abundance and species richness increased with tree canopy opening in relation with the increase in understory plant species richness. In contrast, euedaphic abundance and species richness declined in early forest successional stage in relation with changes in understory plant community composition and species richness, as well as microclimatic conditions. Overall, our study provides experimental evidence that forest plant community can be a strong driver of collembolan assemblages. It also emphasizes the role of trees as foundation species of forest ecosystems that can shape soil biodiversity through their regulation of understory plant community and ecosystem abiotic conditions.

Apport de l’approche par traits fonctionnels pour comprendre les liens plantes-faune du sol dans les écosystèmes forestiers. Vèmes Journées de travail TEBIS

National audience; Apport de l’approche par traits fonctionnels pour comprendre les liens plantes-faune du sol dans les écosystèmes forestiers

International audience; Global warming induces new constraints on forest ecosystems and requires forest management adaptation. The reduction of stand density is currently debated in France as a potential tool to face increasing climate change-induced tree mortality risk due to summer drought by improving forest resistance to water stress. Our study aimed to assess the potential impact of this forest management adaptation on soil biodiversity, i.e. the detrital food web, and ecosystem functioning, i.e. litter and soil organic matter decomposition processes. We took advantage of a large-scale, multi-site experimental network of long-term forestry trials experimentally manipulating oak stand density through thinning operations in contrasted stand age and local abiotic context. Our results provide evidence that reducing stand density will have substantial impacts on the detrital food web structure, with cascading effects on soil functioning. While microbial biomass had little response, the effect of stand density on microbial-feeding nematodes was much more contrasted. Microarthropods such as collembolan and oribatid mites as well as diplopoda and anecic earthworms were depleted by stand density reduction. In contrast, endogeic earthworms were beneficially affected. Litter quality and decomposability decreased with stand density reduction while standard litter decomposition increased. Therefore, in situ litter decomposition was lower and carbon sequestration higher at intermediate stand density. Exploratory analysis using causal diagrams, i.e. path analyses, highlighted that those changes were mainly related to understory vegetation, microclimate and soil pH conditions alterations Overall, our study emphasizes that managing stand density of oak forests at intermediate level appears as the best way to optimize the trade-off between improving forest resistance to drought and preserving soil biodiversity and ecosystem functioning.

[Notes_IRSTEA]Document présent dans "Popular summaries", Session 1-3, p. 10-12 ; http://www.treesandstars.com/vmc8/ [Departement_IRSTEA]Territoires [TR1_IRSTEA]SEDYVIN; International audience; For most European temperate forests, recurrent droughts during seasonal growth are now recorded. To cope with these ongoing climate changes one attempt considered by forest managers is to reduce wood standing volume to globally decrease water consumption. Low wood standing volumes, and in particular a reduced tree leaf area index (LAI), globally lead to an increase of resources, light and water, in the understorey, as well as a modification of the microclimate. Previous studies reported that these modifications could benefit to the understorey flora, and in particular to some monopolistic genus such as Molinia, Rubus, Pteridium, Calluna,… What are the consequences of such modifications in resources, microclimate and flora composition induced by stand density management on biodiversity is the main question the project IMPREBIO attempted to resolve. The diversity in plants, including bryophytes, insects, soil macro- and meso-fauna, and gastropods, in interaction with game herbivory, was recorded in the field. It was then related to resources and microclimate generated by contrasted thinning regimes leading to different stand densities in pure even-aged oak (Quercus petraea and Q. robur) stands in Northern France. Two networks were used; the national cooperative for data on forest tree and stand growth modelling, and the LERFOB network on oak growth. In both networks contrasted thinning regimes were applied to oak stands with target levels of relative density index (RDI, following Reinecke) ranging from 0, totally open woodland, to 1, no thinning at all. A total of 100 plots distributed mostly in the northern mid-part of France were sampled, aged from 20 to 170 year-old. Light, air and soil temperatures, soil water content, and soil fertility were measured. A Structural Equation Modelling (SEM) approach was used to analyse the functional links between the oak stand main characteristics (RDI, stand basal area, age) and the species richness (alpha) of the different considered guilds. Overall no unequivocal trend was recorded between biodiversity considered as a whole and the contrasted stand densities. Conversely results depended strongly on the considered guilds and also on site features, highlighting the importance of the local soil and climate characteristics as primary environmental filters acting on understorey biodiversity. Stand age also influenced significantly the results, leading to complex interactions between the different ecosystem’s components. Many relationships between species richness and stand density were not linear, whereas a humped-model was often recorded, with the highest effects not recorded at both ends of the RDI gradient. The over-development of monopolistic plants in some sites at low RDI (low tree LAI) had a negative domino effect on other guilds such as other plants and insects that needs to be considered and further analysed. The understorey woody species such as Carpinus betulus also have a fundamental role on the microclimate and thus, in many relationships with species richness. As a consequence, low stand densities are not always beneficial to the diversity of every guild, whereas the different functional types must be considered in a given guild.

International audience; Global warming imposes new constraints on forest ecosystems and induces forest management adaptation. Consequently, the French forest policy currently promotes a reduction of stand density to face increasing summer droughts risk and provide higher supply of firewood. However, few studies have yet assessed the potential impact of this forest management intensification on soil biodiversity. We conducted a regional-scale assessment of earthworm assemblages response to forest tree density experimental manipulation. Paired plots treated using silvicultural-thinning regimes of contrasting intensity and located thoughout nine French forests encompassing a wide gradient of stand age and pedoclimatic context were studied (33 plots). Though tree density manipulation had not effect on the overall earthworm abundance and diversity, a shift in the community composition was observed. The different responses depending on earthworm species considered were related to their ecological traits. While litter-dwelling, i.e. epigeic, earthworms were unaffected by tree density manipulation, soil-dwelling species had constrated responses. Hence, endogeic earthworms abundance and diversity showed a strong increase with tree density reduction whereas anecic earthworms abundance tend to decrease. This may highlights a shift in litter supply location, with higher proportion of belowground litter, i.e. dead roots, with the reduction of tree canopy due to higher understory vegetation cover. Earthworm assemblages response to forest tree density was consistent over the stand age gradient. This change of earthworm community composition may have important consequences on ecosystem processes such as litter decomposition, forest floor and soil carbon dynamic, soil structuration and water regime and the subsequent supply of forest goods and services.

International audience; Plant-soil interactions are increasingly recognized to play a major role for terrestrial ecosystems functioning. However, few studies to date have focused on long-term dynamic ecosystems such as forests. Indeed, forest ecosystems are vertically stratified by multiple vegetation strata and the canopy cover manipulation through contrasted stand density management strategies could alter the overall architecture of forest plant community. Very little is known about the cascading effect on soil biodiversity and ecosystem function such as litter decomposition. The aim of this study was to assess the response of a major group of soil fauna, namely collembola, to oak stand density experimental manipulation. A total of 33 stands distributed over a large geographic area were studied covering a wide gradient of stand density, i.e. stand basal area from 2.5 to 43.7 m2.ha-1, stand age, i.e. 18 to 171-year-old, and pedoclimatic context conditions. Our results shown that the response of collembolan assemblages to stand density was highly dependent on stand age. Indeed, collembolan abundance and diversity tend to decrease with stand density reduction in young stands while no response was observed in old stands. Further, a substantial shift of collembolan species composition was observed within young stands and this was strongly associated with contrasted responses among functional groups. Hemiedaphic and euedaphic species abundance and diversity were strongly depleted by stand density reduction whereas epedaphic abundance was unaffected. In contrast, epedaphic diversity tended to be enhanced by stand density reduction. Patterns of collembolan assemblages response to stand density was consistent among pedoclimatic contexts, i.e. mull vs moder. Exploratory analysis using causal diagrams, i.e. path analyses, highlights that those changes were mainly related to understorey vegetation, microclimatic and soil pH condition alterations. These changes of collembolan assemblages could have important functional implications, especially on microbial respiration process through their grazing activities.

National audience; De nouvelles contraintes exercées par le réchauffement climatique font actuellement pressions sur les écosystèmes forestiers et vont imposer une adaptation de leur gestion sylvicole pour préserver leur santé. Ainsi le plan forestier national incite maintenant les gestionnaires forestiers à appliquer un régime d’éclaircie plus intensif afin de réduire la densité des peuplements forestiers. Ceci devrait permettre d’atténuer le risque de stress hydrique accru par des sécheresses estivales qui s’annoncent plus fréquentes à l’avenir. Cependant, peu d'études ont encore évalué l'impact potentiel de cette dynamisation de la gestion forestière sur la biodiversité du sol. Les collemboles représentent une communauté emblématique des sols forestiers. En effet, on les trouve en forte abondance au sein de l’épisolum humifère et ils jouent un rôle essentiel dans la régulation des populations fongiques décomposeurs de litière. De plus, les collemboles ont des sensibilités contrastées aux changements de conditions de milieux provoqués par les pratiques de gestion suivant les espèces. Ils sont ainsi de bons indicateurs biologiques de la qualité des sols forestiers. L’objectif de notre étude a été d’évaluer à échelle nationale la réponse des assemblages de collemboles à la manipulation de la densité en arbres en futaie régulière de chêne sessile.

International audience; In the framework of the Imprebio project, our study showed that long-term forest tree density manipulation could alter soil biota and the soil food web structure. This could have important implications for litter decomposition and could subsequently alter soil process and ecosystem functioning, i.e. carbon and nutrients cycling. We also showed that the soil biota reponse to forest tree density depend on forest successional stage and that the stand age effect was predominant compared to the tree density effect.

International audience; In the framework of the Imprebio project, our study showed that the diversity and community structure of most of the major groups of soil detritivores fauna do not respond to forest tree density manipulation over our large-scale assessment. This suggests that forest stand density management is likely to have limited impact on soil biodiversity on temperate young oak stand.

Impact des pratiques culturales de l’agriculture conventionnelle, l’agriculture biologique et l’agriculture de conservation sur les microorganismes et les nématodes du sol. 11. Journées d'Etude des Sols (JES)