Estelle Forey

Does disturbance drive the collapse of biotic interactions at the severe end of a diversity–biomass gradient?

It has been recently proposed that the decrease in diversity towards the severe end of the humped-back diversity–biomass model of Grime was driven by a collapse of facilitation due to extreme conditions of either stress or physical disturbance. In order to test the hypothesis that disturbance is the primary direct factor driving the collapse of interactions occurring along environmental severity gradients, we conducted a removal experiment in the highly stressed French coastal dunes along a gradient of disturbance due to sand burial. Four dune species were used as targets and transplanted with and without neighbours in four communities along the gradient. The experiment was conducted twice, a dry and an average year. Results of the experiment showed that during the dry year the effect of the environment was prominent and only one species was facilitated for survival in the least disturbed community. During the average year, interactions for growth were important only in the same community, with positive or negative responses depending on the natural position of the target species within the coastal dune gradient. In accordance with our hypothesis, most interactions for both survival and growth were observed in the least disturbed community exhibiting the highest diversity. There were no interactions in the most disturbed community with the lowest diversity.

Mapping local and global variability in plant trait distributions

Significance Currently, Earth system models (ESMs) represent variation in plant life through the presence of a small set of plant functional types (PFTs), each of which accounts for hundreds or thousands of species across thousands of vegetated grid cells on land. By expanding plant traits from a single mean value per PFT to a full distribution per PFT that varies among grid cells, the trait variation present in nature is restored and may be propagated to estimates of ecosystem processes. Indeed, critical ecosystem processes tend to depend on the full trait distribution, which therefore needs to be represented accurately. These maps reintroduce substantial local variation and will allow for a more accurate representation of the land surface in ESMs. Our ability to understand and predict the response of ecosystems to a changing environment depends on quantifying vegetation functional diversity. However, representing this diversity at the global scale is challenging. Typically, in Earth system models, characterization of plant diversity has been limited to grouping related species into plant functional types (PFTs), with all trait variation in a PFT collapsed into a single mean value that is applied globally. Using the largest global plant trait database and state of the art Bayesian modeling, we created fine-grained global maps of plant trait distributions that can be applied to Earth system models. Focusing on a set of plant traits closely coupled to photosynthesis and foliar respiration—specific leaf area (SLA) and dry mass-based concentrations of leaf nitrogen (Nm) and phosphorus (Pm), we characterize how traits vary within and among over 50,000 ∼50×50-km cells across the entire vegetated land surface. We do this in several ways—without defining the PFT of each grid cell and using 4 or 14 PFTs; each model’s predictions are evaluated against out-of-sample data. This endeavor advances prior trait mapping by generating global maps that preserve variability across scales by using modern Bayesian spatial statistical modeling in combination with a database over three times larger than that in previous analyses. Our maps reveal that the most diverse grid cells possess trait variability close to the range of global PFT means.

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.

Phylogenetic patterns and phenotypic profiles of the species of plants and mammals farmed for food

The origins of agriculture were key events in human history, during which people came to depend for their food on small numbers of animal and plant species. However, the biological traits determining which species were domesticated for food provision, and which were not, are unclear. Here, we investigate the phylogenetic distribution of livestock and crops, and compare their phenotypic traits with those of wild species. Our results indicate that phylogenetic clustering is modest for crop species but more intense for livestock. Domesticated species explore a reduced portion of the phenotypic space occupied by their wild counterparts and have particular traits in common. For example, herbaceous crops are globally characterized by traits including high leaf nitrogen concentration and tall canopies, which make them fast-growing species and proficient competitors. Livestock species are relatively large mammals with low basal metabolic rates, which indicate moderate to slow life histories. Our study therefore reveals ecological differences in domestication potential between plants and mammals. Domesticated plants belong to clades with traits that are advantageous in intensively managed high-resource habitats, whereas domesticated mammals are from clades adapted to moderately productive environments. Combining comparative phylogenetic methods with ecologically relevant traits has proven useful to unravel the causes and consequences of domestication.Phylogenetic distribution and phenotypic traits of livestock and crops reveal that domesticated species explore a reduced portion of the phenotypic space occupied by their wild counterparts and have particular traits in common.