Matthieu Chauvat

Successional changes of Collembola and soil microbiota during forest rotation

Dynamic approaches to forest ecosystems are surprisingly rare. Here we report about successional changes in collembolan community structure and microbial performances during forest rotation. The study was carried out in a chronosequence of four spruce forest stands (5-, 25-, 45-, and 95 years old; Tharandter forest, Germany). CO2 release significantly increased after clear-cutting and the amount of C stored in the organic layer subsequently declined. The early phase of forest rotation was characterized by a very active decomposer microflora, stimulation of both fungi and bacteria as well as by a high abundance of surface-oriented Collembola. In addition, collembolan species turnover was accelerated. While the biomass of fungi further increased at intermediate stages of forest rotation, the metabolic activity of the microflora was low, the functional diversity of bacteria declined and the collembolan community became impoverished. Euedaphic species dominated during this stage of forest development. These changes can be explained by both reduction in microhabitat diversity and depletion of food sources associated with an accumulation of recalcitrant soil organic matter. Results of the General Regression Model procedure indicate a shift from specific associations between collembolan functional groups and microbiota at the early stage of forest rotation to a more diffuse pattern at intermediate stages. Though the hypothesis that Collembola are relatively responsive to changes in environmental conditions is confirmed, consistently high community similarity suggests a remarkable persistence of some components of microarthropod assemblages. Our study provides evidence for substantial ecosystem-level implications of changes in the soil food web during forest rotation. Moreover, correlations between bacterial parameters and Collembola point to the overarching impact of differences in the composition of the microbial community on microarthropods.

Oribatid mite diversity and community dynamics in a spruce chronosequence

Abstract A comparative study on oribatid mite communities was carried out at four sites forming a chronosequence of spruce forest stands (stand age: 5, 25, 45 and 95-years; Tharandter Wald, Germany). A total of 70 species was found. The moderate decline of oribatid species richness at the 5-years-old stand together with a strong deviation from the log-series model indicate that the community had not recovered from initial disturbance after 5 years. Close community similarity between the 5 and the 95-years-old stands nevertheless suggests a delayed response of the oribatid fauna to strong changes in environmental conditions. Species richness significantly increased from 5 to 25-years-old stand. This was accompanied by a strong increase of oribatid abundance that was largely due to the increase of only two species: Oppiella nova and Tectocepheus velatus . Particularly strong deviation from the log-series model at this site suggests that community disturbance becomes most intense at intermediate stages of forest development. Further analyses revealed an alteration of the feeding structure as well as a stimulation of species that either prefer or are at least able to colonize deeper layers of the soil profile in the 25-years-old stand. This indicates marked changes in both nutritional conditions and microclimate in the uppermost horizons 25 years after clear-cutting. A very high C mineralization rate measured in the 5-years-old stand confirms the important contribution of decomposition processes to net ecosystem production in young stands. However, no strong correlation between oribatid mite community variance and environmental conditions could be established by means of a canonical correspondence analysis. It is concluded that oribatid mite communities are a fairly conservative element of the decomposer fauna in temperate spruce forests and might thus be an important component in the decomposer systems buffering mechanisms against strong environmental change. Our data indicate a multifaceted reaction of the oribatid community to long lasting changes in site conditions rather than a straightforward response to certain environmental factors.

Colonization of heavy metal-polluted soils by Collembola: preliminary experiments in compartmented boxes

Two-week laboratory experiments were carried out in plastic boxes separated in two connected compartments filled with a neutral soil (mull humus) at pH 7.7 and an acid soil (moder humus) at pH 4.3, containing their original faunas. Migration of Collembola from one compartment to the other was allowed through a perforated wall. The mull was contaminated with three concentrations of lead (as lead acetate) at 50, 6000 and 60,000 ppm. When combined with moder in the adjacent compartment, six times more individuals and five times more species were observed in the mull at the highest concentration applied, compared to mull combined with itself. Parisotoma notabilis proved to be highly sensitive to lead, and shifted to the moder compartment even at the lowest concentration. Densities of other mull species such as Pseudosinella alba were affected by medium to high concentrations of lead but these species did not move to the moder soil despite their high motility. Acidophilic species living in moder only, such as Willemia anophthalma, Proisotoma minima and Xenylla tullbergi, colonized contaminated mull treatments with densities increasing with lead concentrations but the result of this process was erratic. Folsomia manolachei was present in both humus forms but was much more abundant in the moder. This species colonized mull at medium to high lead concentrations, where it restored totally or partly its original abundance in the uncontaminated mull. These results suggested differences between mull and moder populations of Folsomia manolachei.

Forest management adaptation to climate change: a Cornelian dilemma between drought resistance and soil macro-detritivore functional diversity

Global warming induces new constraints on forest ecosystems and requires forest management adaptation. The reduction in stand density is currently debated as a potential tool to face increasing summer drought risk by improving forest resistance to climate change-induced tree mortality. However, few studies have yet assessed the impacts of this management change on soil biodiversity. We conducted a large-scale, multi-site assessment of the response of soil macro-detritivore assemblages and soil functioning to experimental manipulations of stand density. A total of 33 stands were studied covering a wide gradient of stand density, that is stand basal area from 25 to 437m(2)ha(-1), stand age, that is 18-171years old, and local abiotic context. We observed contrasting responses as a function of both taxonomic and functional groupings. Exploratory analysis using causal diagrams, that is path analysis, highlights that these changes were mainly related to alterations in understorey vegetation, microclimatic and soil pH conditions. The response of soil macro-detritivore assemblages to stand density manipulation was consistent over the gradient of stand ages. Among the litter-dwelling macro-detritivores, millipede abundance and diversity decreased with stand density reduction, while woodlice and epigeic earthworms were unaffected. Further, a shift in soil-dwelling earthworm community composition was observed in mull stands. Endogeic earthworm abundance showed a sharp increase with stand density reduction, which translated into an increase in soil respiration. In contrast, anecic earthworm abundance decreased and was strongly associated with a decline of the rate of forest floor turnover.Synthesis and applications. Our study provides strong evidence that reductions of stand density will have substantial impacts on soil macro-detritivore assemblages and cascading effects on soil functioning, particularly in mull stands. Managing stand density of oak forests at an intermediate level, that is 25m(2)ha(-1), appears to be best to optimize the trade-off between improving forest resistance to climate change and ensuring the conservation of functional diversity to preserve forest ecosystem functioning and stability. Our study provides strong evidence that reductions of stand density will have substantial impacts on soil macro-detritivore assemblages and cascading effects on soil functioning, particularly in mull stands. Managing stand density of oak forests at an intermediate level, that is 25m(2)ha(-1), appears to be best to optimize the trade-off between improving forest resistance to climate change and ensuring the conservation of functional diversity to preserve forest ecosystem functioning and stability.

Shifts and linkages of functional diversity between above‐ and below‐ground compartments along a flooding gradient

Summary Trait-based approaches have the potential to reveal general and predictive relationships between organisms and ecosystem functioning. However, the mechanisms underlying the functional structure of communities are still unclear. Within terrestrial ecosystems, several studies have shown that many ecological processes are controlled by the interacting above- and below-ground compartments. However, few studies have used traits to reveal the functional relationships between plants and soil fauna. Mostly, research combining plants and soil fauna solely used the traits of one assemblage in predictive studies. Above-ground (plants) and below-ground (Collembola) compartments were sampled over a flooding gradient in northern France along the Seine River. First, we measured the effect of flooding on functional and taxonomic assembly within both communities. We then considered the linkages between plant and Collembolan species richness, community traits and assessed whether traits of both compartments converged at high flooding intensity (abiotic filtering) and diverged when this constraint is released (biotic filtering). Species richness of both taxa followed the same bell-shaped pattern along the gradient, while a similar significant pattern of functional richness was only observed for plants. Further analyses revealed a progressive shift from trait convergence to divergence for plants, but not for Collembola, as constraints intensity decreased. Instead, our results highlighted that Collembola traits were mainly linked to the variations in plant traits. This leads, within Collembola assemblages, to convergence of a subset of perception and habitat-related traits for which the relationship with plant traits was assessed. Synthesis. Using a trait-based approach, our study highlighted that functional relationships occur between above- and below-ground compartments. We underlined that functional composition of plant communities plays a key role in structuring Collembola assemblages in addition to the role of abiotic variables. Our study clearly shows that functional diversity provides a new approach to link the above- and below-ground compartments and might, therefore, be further considered when studying ecological processes at the interface between both compartments.