Estelle Langlois

The relationships between soil seed bank, aboveground vegetation and disturbances in an eutrophic alluvial wetland of Western France

Summary We studied the species composition of vegetation and seed bank in an experiment with grassland and oldfield plots in an eutrophic alluvial wetland (called “Marais de Redon”) of Western France. In this wetland, artificial disturbances (mowing) and natural disturbances (cattle, roebucks, boars, voles…) are very frequent. In order to mime these natural disturbances, experimental disturbances were generated in March 1996 after the end of the winter flooding and the seedlings counted three months later. The seed bank, the undisturbed vegetation and seedlings emerging in disturbed quadrats were sampled. Detrended Correspondence Analysis (DCA) of the undisturbed quadrats, disturbed quadrats and seed bank samples showed significant differences of species composition. Similarity between seed bank and undisturbed aboveground vegetation was low and not very different between grassland and oldfield. Very few seedlings emerged in undisturbed vegetation both in grassland and oldfield, which indicates the importance of gaps for seed bank expression. The great majority of seedlings emerging after experimental disturbances were mainly recruited from the soil seed bank. This result contrasts with other studies where the seed bank contributed very little to the seedling flora and vegetative regrowth clearly predominated recolonisation after disturbances. In the seed bank, few species lost after succession from grassland to oldfield vegetation were still present as seeds in the soil, but in most cases, species lost were not recorded in the seed bank. The two hypotheses about changes in the seed bank during secondary succession, predicting decrease in species richness or species diversity and seed density were only confirmed for seed density parameter.

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.

Tree species richness induces strong intraspecific variability of beech (Fagus sylvatica) leaf traits and alleviates edaphic stress

Manipulating stand composition is an important management tool that foresters can use to affect the nature of forests and ecosystem processes. In mixed stands, interspecific interactions among trees can cause changes in tree performances. Nevertheless, these interactions are context dependent (cf. stress-gradient hypothesis, SGH). We thus investigated how intraspecific functional changes in leaf trait (19 traits) of European beech (Fagus sylvatica) were influenced by stand composition. We compared pure beech stands with four mixed stands containing from one to three additional tree species along a gradient of edaphic stress (gradient of soil water-holding capacity and rooting depth). First, we demonstrated that stand composition induced strong intraspecific leaf trait variation in beech for LDMC, LMA, phenolic compounds, leaf pH and magnesium concentration, suggesting higher nutrient acquisition by more diverse stands. Nevertheless, these results were modulated by edaphic stress. Mixed stands only conferred an advantage in relatively stressed sites (luvisol and leptosol). Besides, the addition of oak to beech stands had unexpected negative effects in sites with less severe stress (cambisol) as indicated by the null or positive LogRR of LMA, LDMC and phenolics. This study found that stand composition is an important though often-overlooked driver of intraspecific variability in leaf quality, and potentially reflects changes in beech tree physiology and productivity. Our results also suggest that positive interactions prevail in sites with stressful conditions. Such validation of the SGH is rare in natural or managed mature forests. Lastly, we strongly recommend that forest managers consider stand composition and abiotic factors when implementing forest growth models to improve their yield predictions.