Aurélie Thébault

PLANT TRAITS IN A STATE AND TRANSITION FRAMEWORK AS MARKERS OF ECOSYSTEM RESPONSE TO LAND-USE CHANGE

Understanding and forecasting changes in plant communities, ecosystem properties, and their associated services requires a mechanistic link between community shifts and modifications in ecosystem properties. In this study, we test the hypothesis that plant traits can provide such a link. Using subalpine grasslands in the central French Alps as a case study, we investigate the response of plant traits to changes in soil resource availability and disturbance regimes associated with changing grassland management as well as the effects of changes in plant traits on measured ecosystem properties. We found that fertilization leads to greater specific leaf area and leaf nitrogen content which leads to greater productivity and faster litter decomposition, and that grazing leads to higher leaf toughness and leaf dry matter content which leads to lower productivity and slower decomposition compared to mowing. A state and transition model was used as a flexible conceptual tool for integrating data on community composition, plant traits, and ecosystem properties in the context of management-mediated successional dynamics in subalpine grasslands. Focusing on the biology driving the transition between grassland states, we incorporated plant traits into the formulation of a state and transition model and demonstrated how they could be used to provide a mechanistic link between community shifts and ecosystem properties under complex management regimes with strong land-use legacies.

Leaf traits capture the effects of land use changes and climate on litter decomposability of grasslands across Europe

Land use and climate changes induce shifts in plant functional diversity and community structure, thereby modifying ecosystem processes. This is particularly true for litter decomposition, an essential process in the biogeochemical cycles of carbon and nutrients. In this study, we asked whether changes in functional traits of living leaves in response to changes in land use and climate were related to rates of litter potential decomposition, hereafter denoted litter decomposability, across a range of 10 contrasting sites. To disentangle the different control factors on litter decomposition, we conducted a microcosm experiment to determine the decomposability under standard conditions of litters collected in herbaceous communities from Europe and Israel. We tested how environmental factors (disturbance and climate) affected functional traits of living leaves and how these traits then modified litter quality and subsequent litter decomposability. Litter decomposability appeared proximately linked to initial litter quality, with particularly clear negative correlations with lignin-dependent indices (litter lignin concentr tion, lignin:nitrogen ratio, and fiber component). Litter quality was directly related to community-weighted mean traits. Lignin-dependent indices of litter quality were positively correlated with community-weighted mean leaf dry matter content (LDMC), and negatively correlated with community-weighted mean leaf nitrogen concentration (LNC). Consequently, litter decomposability was correlated negatively with community-weighted mean LDMC, and positively with community-weighted mean LNC. Environmental factors (disturbance and climate) influenced community-weighted mean traits. Plant communities experiencing less frequent or less intense disturbance exhibited higher community-weighted mean LDMC, and therefore higher litter lignin content and slower litter decomposability. LDMC therefore appears as a powerful marker of both changes in land use and of the pace of nutrient cycling across 10 contrasting sites.

Arbuscular mycorrhizal fungi reduce the differences in competitiveness between dominant and subordinate plant species

In grassland communities, plants can be classified as dominants or subordinates according to their relative abundances, but the factors controlling such distributions remain unclear. Here, we test whether the presence of the arbuscular mycorrhizal (AM) fungus Glomus intraradices affects the competitiveness of two dominant (Taraxacum officinale and Agrostis capillaris) and two subordinate species (Prunella vulgaris and Achillea millefolium). Plants were grown in pots in the presence or absence of the fungus, in monoculture and in mixtures of both species groups with two and four species. In the absence of G. intraradices, dominants were clearly more competitive than subordinates. In inoculated pots, the fungus acted towards the parasitic end of the mutualism–parasitism continuum and had an overall negative effect on the growth of the plant species. However, the negative effects of the AM fungus were more pronounced on dominant species reducing the differences in competitiveness between dominant and subordinate species. The effects of G. intraradices varied with species composition highlighting the importance of plant community to mediate the effects of AM fungi. Dominant species were negatively affected from the AM fungus in mixtures, while subordinates grew identically with and without the fungus. Therefore, our findings predict that the plant dominance hierarchy may flatten out when dominant species are more reduced than subordinate species in an unfavourable AM fungal relationship (parasitism).

Polyploidy and invasion success: trait trade-offs in native and introduced cytotypes of two Asteraceae species

Invasion success is favoured by the introduction of pre-adapted genotypes. In addition, novel pressures in the introduced range may lead to phenotypic changes related to fitness or competitive ability of introduced plants. Polyploidy appears to be over-represented in invasive plants, but differences between cytotypes in growth strategies including trade-offs among plant traits have received little attention so far in the context of biological invasions. We grew Centaurea stoebe L. and Senecio inaequidens D.C. in a greenhouse experiment to test for differences in fitness (shoot biomass, reproductive output) and competitive ability (vegetative size, specific leaf area, leaf dry matter content, root–shoot ratio) between diploid and polyploid cytotypes as well as between native and introduced plants. For both species, diploid and tetraploid genotypes occur in the native range, whereas only tetraploids are present in the introduced range. In the native range of both species, diploid and tetraploid genotypes had different growth strategies. Tetraploid genotypes of C. stoebe and S. inaequidens had, respectively, higher specific leaf area and stem height than diploid ones. Thus, for both species, native tetraploids appeared more competitive than native diploids, which could explain, at least partially, the invasion success of the pre-adapted tetraploid genotypes. The comparison of native and introduced tetraploid genotypes revealed differences in traits linked to competitive ability, which could be linked to novel selection in the new environment. In S. inaequidens, we found evidence for a competition-colonisation trade-off, whereas persistence of C. stoebe in the new range seemed to be linked to a competition-defence trade-off.

Response of herbaceous vegetation functional diversity to land use change across five sites in Europe and Israel

Only a few studies have examined responses of grassland functional diversity to management and major environmental gradients, in order to address the question of whether grassland use can promote functional divergence. For five grassland sites in Israel, Portugal, the Czech Republic, Mediterranean France, and the French Alps, where traditional grassland management is being abandoned, we quantified community-weighted means (CWM) and functional divergence (FDvg) for the three Leaf-Height-Seed (LHS) traits, individually and in combination. Responses of CWM and FDvg to land use were analyzed by mixed linear models with aridity, phosphorus, fertility, and the fractions of grasses and annuals as covariates. Responses of community-weighted traits to land use were consistent with current knowledge. More intense management favored plants with more rapid resource acquisition (high Specific Leaf Area, or SLA), whereas abandonment or less intense grassland management increased the dominance by tall plants with more c...

Species-specific effects of polyploidisation and plant traits of Centaurea maculosa and Senecio inaequidens on rhizosphere microorganisms.

Invasive plant species represent a threat to terrestrial ecosystems, but their effects on the soil biota and the mechanisms involved are not yet well understood. Many invasive species have undergone polyploidisation, leading to the coexistence of various cytotypes in the native range, whereas, in most cases, only one cytotype is present in the introduced range. Since genetic variation within a species can modify soil rhizosphere communities, we studied the effects of different cytotypes and ranges (native diploid, native tetraploid and introduced tetraploid) of Centaurea maculosa and Senecio inaequidens on microbial biomass carbon, rhizosphere total DNA content and bacterial communities of a standard soil in relation to plant functional traits. There was no overall significant difference in microbial biomass between cytotypes. The variation of rhizosphere total DNA content and bacterial community structure according to cytotype was species specific. The rhizosphere DNA content of S. inaequidens decreased with polyploidisation in the native range but did not vary for C. maculosa. In contrast, the bacterial community structure of C. maculosa was affected by polyploidisation and its diversity increased, whereas there was no significant change for S. inaequidens. Traits of S. inaequidens were correlated to the rhizosphere biota. Bacterial diversity and total DNA content were positively correlated with resource allocation to belowground growth and late flowering, whereas microbial biomass carbon was negatively correlated to investment in reproduction. There were no correlations between traits of the cytotypes of C. maculosa and corresponding rhizosphere soil biota. This study shows that polyploidisation may affect rhizosphere bacterial community composition, but that effects vary among plant species. Such changes may contribute to the success of invasive polyploid genotypes in the introduced range.

Complex interactions between spatial pattern of resident species and invasiveness of newly arriving species affect invasibility.

Understanding the factors that affect establishment success of new species in established communities requires the study of both the ability of new species to establish and community resistance. Spatial pattern of species within a community can affect plant performance by changing the outcome of inter-specific competition, and consequently community invasibility. We studied the effects of spatial pattern of resident plant communities on fitness of genotypes from the native and introduced ranges of two worldwide invasive species, Centaurea stoebe and Senecio inaequidens, during their establishment stage. We experimentally established artificial plant mixtures with 4 or 8 resident species in intra-specifically aggregated or random spatial patterns, and added seedlings of genotypes from the native and introduced ranges of the two target species. Early growth of both S. inaequidens and C. stoebe was higher in aggregated than randomly assembled mixtures. However, a species-specific interaction between invasiveness and invasibility highlighted more complex patterns. Genotypes from native and introduced ranges of S. inaequidens showed the same responses to spatial pattern. By contrast, genotypes from the introduced range of C. stoebe did not respond to spatial pattern whereas native ones did. Based on phenotypic plasticity, we argue that the two target species adopted different strategies to deal with the spatial pattern of the resident plant community. We show that effects of spatial pattern of the resident community on the fitness of establishing species may depend on the diversity of the recipient community. Our results highlight the need to consider the interaction between invasiveness and invasibility in order to increase our understanding of invasion success.

Assessing long-term land-use legacies in subalpine grasslands by using a plant trait-based generic modelling framework

Background: In European mountains, where semi-natural grasslands have high cultural and nature conservation value, land-use legacies from past ploughing are associated with distinct floristic compositions and ecosystem properties. It remains unknown if these differences are stable or if post-arable grasslands are only transient states that eventually follow a predictable successional pathway. Aims: We use a generic vegetation dynamics model based on plant traits to test these alternative hypotheses in subalpine grasslands of the central French Alps. Methods: We classified dominant graminoids into trait-based Plant Functional Types (PFTs) and simulated their long-term response to fertility, annual mowing and past ploughing. Results: Simulations demonstrated that a temporary dispersal limitation of long-lived tussock grasses could have allowed species-rich hay meadows to develop as alternative communities. We also show that when they are regularly mown, meadows are resistant to colonisation by long-lived tussock grasses. Conclusions: Our results thus support traditional management based on regular mowing, but also highlight the key role of landscape history through its effects on dispersal. Our study also demonstrates the usefulness of easy-to-measure plant traits and a widely applicable generic modelling framework for testing hypotheses on the long-term interactions of multiple drivers of vegetation dynamics.