Sylviculture intensive en région boréale : impact de la mixité des essences sur le processus de décomposition des litières et le stockage de carbone.

Depuis quelques années, la sylviculture intensive prend de l’ampleur au Canada afin de rapprocher la source de fibres des usines, d’accroître la productivité des plantations, et de diminuer la pression de coupe sur les forêts naturelles. Toutefois, un débat sur le type d’aménagement optimal des plantations oppose l’aménagement mono- et plurispécifique. Malgré des effets antagonistes possibles sur la productivité des arbres, il semblerait que les plantations mixtes procureraient des avantages au niveau des propriétés du sol, de la stabilité environnementale, mais aussi du maintien de la biodiversité et de la valeur esthétique. Les espèces végétales et leur mélange sont susceptibles d’influencer le processus de décomposition selon deux mécanismes qui opèrent à différentes échelles : l’effet ressource par la production de litière possédant des caractéristiques physiques et chimiques propres qui constituent la ressource nutritive pour les décomposeurs, et l’effet habitat qui correspond à l’effet à plus long terme de l’arbre sur son environnement en influençant les microconditions climatiques et édaphiques, et donc les communautés d’organismes présents dans le sol. Le mélange de diverses espèces engendrerait à la fois une plus grande diversité d’habitats et de ressources favorisant une plus grande diversité et abondance des organismes décomposeurs, ce qui accélérerait le processus de décomposition et la remise à disposition des nutriments dans le sol. Il est important de mieux comprendre le fonctionnement de ces écosystèmes pour en effectuer une bonne gestion et pour optimiser les services écosystémiques que ces plantations fournissent. Ainsi cette thèse visait à mieux comprendre l’influence de la mixité de deux essences forestières, à savoir l’épinette blanche et le peuplier hybride, en comparaison à des plantations pures sur le processus de décomposition des litières en séparant l’effet ressource de l’effet habitat. Cette étude a été menée sur des plantations intensivement aménagées qui étaient largement colonisées par des herbacées. Ainsi une autre partie de cette thèse consistait à déterminer si la présence d’herbacées était bénéfique au processus de décomposition des litières et à la libération des éléments nutritifs dans les différentes plantations mono- ou pluri-spécifiques. Finalement, dans l’optique de relier ces données avec le fonctionnement global de l’écosystème, le stockage de carbone de la phytomasse aérienne et dans le sol (excepté les racines) a été étudié dans les deux types de plantations. La qualité des ressources et l’effet du mélange des arbres et des litières sur les décomposeurs ont été étudiés dans une expérimentation de décomposition in-situ avec des litter bags de chaque type de litière (peuplier, épinette, herbacées, ainsi que leur mélange) dans chaque type de plantations. Cette étude a été appuyée par une expérimentation ex-situ utilisant une espèce cible d’organisme du sol, Folsomia candida (collembola). Un intérêt particulier a été porté sur le rôle et l’impact des métabolites secondaires contenus dans les espèces végétales concernées sur le processus de décomposition mais aussi sur leurs interactions avec les organismes décomposeurs. xx Concernant l’effet habitat, la plantation monospécifique de peuplier semblait être défavorable à la colonisation de la litière par la mésofaune, probablement dû à l’effet négatif des lixiviats de feuilles de peuplier mais également à la faible accumulation des litières. À l’inverse, la plantation pure d’épinette favorisait la décomposition de sa propre litière par la sélection d’organismes plus spécialisés. Les résultats de cette étude ne montraient pas d’amélioration du processus de décomposition avec le mélange du peuplier et de l’épinette (effet habitat) ou de leurs litières (effet ressource). En revanche, le mélange de ces deux espèces en plantation a tamponné les effets contrastés du peuplier et de l’épinette sur les organismes et le taux de décomposition observés dans les plantations monospécifiques. Le stockage de carbone et la productivité du peuplier étaient supérieurs dans les plantations mixtes par rapport aux plantations monospécifiques. Finalement, la litière d’herbacées semblait être bénéfique pour l’abondance d’organismes décomposeurs et favorisait la libération d’azote des litières d’arbres. Cet aspect pourrait contrebalancer l’effet négatif de la présence d’herbacées qui entrent en compétition avec les arbres pour les ressources. Ces résultats nous indiquent qu’après 10 ans, les plantations mixtes optimiseraient la productivité et le stockage de carbone, double avantage généralement recherché dans les systèmes sylvicoles

Resistance of subarctic soil fungal and invertebrate communities to disruption of below-ground carbon supply

The supply of recent photosynthate from plants to soils is thought to be a critical mechanism regulating the activity and diversity of soil biota. In the Arctic, large-scale vegetation transitions are underway in response to warming, and there is an urgent need to understand how these changes affect soil biodiversity and function. We investigated how abundance and diversity of soil fungi and invertebrates responded to a reduction in fresh below-ground photosynthate supply in treeline birch and willow, achieved using stem girdling. We hypothesised that birch forest would support greater abundance of ectomycorrhizal (ECM) fungal species and fauna than willow shrubs, and that girdling would result in a rapid switch from ECM fungi to saprotrophs as canopy supply of C was cut, with a concomitant decline in soil fauna. Birch forest had greater fungal and faunal abundance with a large contribution of root-associated ascomycetes (ericoid mycorrhizal fungi and root endophytes) compared to willow shrub plots, which had a higher proportion of saprotrophs and, contrary to our expectations, ECM fungi. Broad-scale soil fungal and faunal functional group composition was not significantly changed by girdling, even in the third year of treatment. Within the ECM community, there were some changes, with genera that are believed to be particularly C-demanding declining in girdled plots. However, it was notable how most ECM fungi remained present after 3 years of isolation of the below-ground compartment from contemporary photosynthate supply. Synthesis. In a treeline/tundra ecosystem, distinct soil communities existed in contrasting vegetation patches within the landscape, but the structure of these communities was resistant to canopy disturbance and concomitant reduction of autotrophic C inputs

Intensive grassland management disrupts below-ground multi-trophic resource transfer in response to drought

Modification of soil food webs by land management may alter the response of ecosystem processes to climate extremes, but empirical support is limited and the mechanisms involved remain unclear. Here we quantify how grassland management modifies the transfer of recent photosynthates and soil nitrogen through plants and soil food webs during a post-drought period in a controlled field experiment, using in situ 13C and 15N pulse-labelling in intensively and extensively managed fields. We show that intensive management decrease plant carbon (C) capture and its transfer through components of food webs and soil respiration compared to extensive management. We observe a legacy effect of drought on C transfer pathways mainly in intensively managed grasslands, by increasing plant C assimilation and 13C released as soil CO2 efflux but decreasing its transfer to roots, bacteria and Collembola. Our work provides insight into the interactive effects of grassland management and drought on C transfer pathways, and highlights that capture and rapid transfer of photosynthates through multi-trophic networks are key for maintaining grassland resistance to drought

Land management shapes drought responses of dominant soil microbial taxa across grasslands

Soil microbial communities are dominated by a relatively small number of taxa that may play outsized roles in ecosystem functioning, yet little is known about their capacities to resist and recover from climate extremes such as drought, or how environmental context mediates those responses. Here, we imposed an in situ experimental drought across 30 diverse UK grassland sites with contrasting management intensities and found that: (1) the majority of dominant bacterial (85%) and fungal (89%) taxa exhibit resistant or opportunistic drought strategies, possibly contributing to their ubiquity and dominance across sites; and (2) intensive grassland management decreases the proportion of drought-sensitive and non-resilient dominant bacteria-likely via alleviation of nutrient limitation and pH-related stress under fertilisation and liming-but has the opposite impact on dominant fungi. Our results suggest a potential mechanism by which intensive management promotes bacteria over fungi under drought with implications for soil functioning

Drought decreases incorporation of recent plant photosynthate into soil food webs regardless of their trophic complexity

Theory suggests that more complex food webs promote stability and can buffer the effects of perturbations, such as drought, on soil organisms and ecosystem functions. Here, we tested experimentally how soil food web trophic complexity modulates the response to drought of soil functions related to carbon cycling and the capture and transfer below‐ground of recent photosynthate by plants. We constructed experimental systems comprising soil communities with one, two or three trophic levels (microorganisms, detritivores and predators) and subjected them to drought. We investigated how food web trophic complexity in interaction with drought influenced litter decomposition, soil CO2 efflux, mycorrhizal colonization, fungal production, microbial communities and soil fauna biomass. Plants were pulse‐labelled after the drought with 13C‐CO2 to quantify the capture of recent photosynthate and its transfer below‐ground. Overall, our results show that drought and soil food web trophic complexity do not interact to affect soil functions and microbial community composition, but act independently, with an overall stronger effect of drought. After drought, the net uptake of 13C by plants was reduced and its retention in plant biomass was greater, leading to a strong decrease in carbon transfer below‐ground. Although food web trophic complexity influenced the biomass of Collembola and fungal hyphal length, 13C enrichment and the net transfer of carbon from plant shoots to microbes and soil CO2 efflux were not affected significantly by varying the number of trophic groups. Our results indicate that drought has a strong effect on above‐ground–below‐ground linkages by reducing the flow of recent photosynthate. Our results emphasize the sensitivity of the critical pathway of recent photosynthate transfer from plants to soil organisms to a drought perturbation, and show that these effects may not be mitigated by the trophic complexity of soil communities, at least at the level manipulated in this experiment.info:eu-repo/semantics/publishedVersio

Combatting global grassland degradation

Grasslands are under severe threat from ongoing degradation, undermining their capacity to support biodiversity, ecosystem services and human well-being. Yet, grasslands are largely ignored in sustainable development agendas. In this Perspective, we examine the current state of global grasslands and explore the extent and dominant drivers of their degradation. Socio-ecological solutions are needed to combat degradation and promote restoration. Important strategies include: increasing recognition of grasslands in global policy; developing standardized indicators of degradation; using scientific innovation for effective restoration at regional and landscape scales; and enhancing knowledge transfer and data sharing on restoration experiences. Stakeholder needs can be balanced through standardized assessment and shared understanding of the potential ecosystem service trade-offs in degraded and restored grasslands. The integration of these actions into sustainability policy will aid in halting degradation and enhancing restoration success, and protect the socio-economic, cultural and ecological benefits that grasslands provide

Resistance of subarctic soil fungal and invertebrate communities to disruption of below‐ground carbon supply

The supply of recent photosynthate from plants to soils is thought to be a critical mechanism regulating the activity and diversity of soil biota. In the Arctic, large-scale vegetation transitions are underway in response to warming, and there is an urgent need to understand how these changes affect soil biodiversity and function. We investigated how abundance and diversity of soil fungi and invertebrates responded to a reduction in fresh below-ground photosynthate supply in treeline birch and willow, achieved using stem girdling. We hypothesised that birch forest would support greater abundance of ectomycorrhizal (ECM) fungal species and fauna than willow shrubs, and that girdling would result in a rapid switch from ECM fungi to saprotrophs as canopy supply of C was cut, with a concomitant decline in soil fauna. Birch forest had greater fungal and faunal abundance with a large contribution of root-associated ascomycetes (ericoid mycorrhizal fungi and root endophytes) compared to willow shrub plots, which had a higher proportion of saprotrophs and, contrary to our expectations, ECM fungi. Broad-scale soil fungal and faunal functional group composition was not significantly changed by girdling, even in the third year of treatment. Within the ECM community, there were some changes, with genera that are believed to be particularly C-demanding declining in girdled plots. However, it was notable how most ECM fungi remained present after 3 years of isolation of the below-ground compartment from contemporary photosynthate supply. Synthesis. In a treeline/tundra ecosystem, distinct soil communities existed in contrasting vegetation patches within the landscape, but the structure of these communities was resistant to canopy disturbance and concomitant reduction of autotrophic C inputs

Globally invariant metabolism but density-diversity mismatch in springtails.

Soil life supports the functioning and biodiversity of terrestrial ecosystems. Springtails (Collembola) are among the most abundant soil arthropods regulating soil fertility and flow of energy through above- and belowground food webs. However, the global distribution of springtail diversity and density, and how these relate to energy fluxes remains unknown. Here, using a global dataset representing 2470 sites, we estimate the total soil springtail biomass at 27.5 megatons carbon, which is threefold higher than wild terrestrial vertebrates, and record peak densities up to 2 million individuals per square meter in the tundra. Despite a 20-fold biomass difference between the tundra and the tropics, springtail energy use (community metabolism) remains similar across the latitudinal gradient, owing to the changes in temperature with latitude. Neither springtail density nor community metabolism is predicted by local species richness, which is high in the tropics, but comparably high in some temperate forests and even tundra. Changes in springtail activity may emerge from latitudinal gradients in temperature, predation and resource limitation in soil communities. Contrasting relationships of biomass, diversity and activity of springtail communities with temperature suggest that climate warming will alter fundamental soil biodiversity metrics in different directions, potentially restructuring terrestrial food webs and affecting soil functioning

Global fine-resolution data on springtail abundance and community structure

Springtails (Collembola) inhabit soils from the Arctic to the Antarctic and comprise an estimated ~32% of all terrestrial arthropods on Earth. Here, we present a global, spatially-explicit database on springtail communities that includes 249,912 occurrences from 44,999 samples and 2,990 sites. These data are mainly raw sample-level records at the species level collected predominantly from private archives of the authors that were quality-controlled and taxonomically-standardised. Despite covering all continents, most of the sample-level data come from the European continent (82.5% of all samples) and represent four habitats: woodlands (57.4%), grasslands (14.0%), agrosystems (13.7%) and scrublands (9.0%). We included sampling by soil layers, and across seasons and years, representing temporal and spatial within-site variation in springtail communities. We also provided data use and sharing guidelines and R code to facilitate the use of the database by other researchers. This data paper describes a static version of the database at the publication date, but the database will be further expanded to include underrepresented regions and linked with trait data.</p

Global fine-resolution data on springtail abundance and community structure

CODE AVAILABILITY : Programming R code is openly available together with the database from Figshare.SUPPLEMENTARY MATERIAL 1 : Template for data collectionSUPPLEMENTARY MATERIAL 2 : Data Descriptor WorksheetSpringtails (Collembola) inhabit soils from the Arctic to the Antarctic and comprise an estimated ~32% of all terrestrial arthropods on Earth. Here, we present a global, spatially-explicit database on springtail communities that includes 249,912 occurrences from 44,999 samples and 2,990 sites. These data are mainly raw sample-level records at the species level collected predominantly from private archives of the authors that were quality-controlled and taxonomically-standardised. Despite covering all continents, most of the sample-level data come from the European continent (82.5% of all samples) and represent four habitats: woodlands (57.4%), grasslands (14.0%), agrosystems (13.7%) and scrublands (9.0%). We included sampling by soil layers, and across seasons and years, representing temporal and spatial within-site variation in springtail communities. We also provided data use and sharing guidelines and R code to facilitate the use of the database by other researchers. This data paper describes a static version of the database at the publication date, but the database will be further expanded to include underrepresented regions and linked with trait data.Open Access funding enabled and organized by Projekt DEAL.http://www.nature.com/sdatahj2024Plant Production and Soil ScienceSDG-15:Life on lan