Résistance à l’invasion des communautés végétales établies après perturbation. Rôle de la dynamique de colonisation et des effets de priorité

Invasive plant species cause serious environmental and sanitary issues and their control is today a major challenge. Disturbances involving vegetation removal and an increase in resource availability offer particularly favorable conditions for invasive plant colonization. Establishing a plant cover rapidly sequestering resources could be a relevant strategy to limit invasion. However, little is known about the characteristics enabling newly established communities to exert strong invasion resistance, especially in the early growth stages. In this thesis, I focused on two potential determinants of invasion resistance of herbaceous plant communities in the early growth stages after a major disturbance, which are (1) the concept of limiting similarity, stating that the coexistence of species sharing the same ecological niche is limited by competitive exclusion, and (2) priority effects, which occur when the establishment of a species affects the performance or survival of later arriving species. The application of limiting similarity to control invasive plants appears complex, ineffective and unsuitable for the most common situations. In contrast, integrating priority effects into invasive plant management strategies seems more promising. One strategy consists in restoring a plant cover exerting strong negative priority effects, decreasing the success of subsequent invasive plant establishment. In two greenhouse experiments, I explored the role of priority effects in early invasion resistance. In a first experiment, I manipulated species composition, sowing density and the elapsed time between community sowing and invasion by Ambrosia artemisiifolia, Bothriochloa barbinodis and Cortaderia selloana. A higher invasion resistance was observed when communities produced a high aboveground biomass, which was associated with the presence of productive species. Delaying invasive species arrival also decreased invasion success, but only if it allowed a sufficient increase in biomass production. A second experiment investigated how the identity of the first native colonizer (one of two grasses: Dactylis glomerata and Lolium perenne, or one of two legumes: Onobrychis viciifolia and Trifolium repens) and the timing of species establishment (synchronous vs. sequential sowing) influenced the structuration of the recipient community and its resistance to invasion by A. artemisiifolia. Small differences in assembly history of the recipient community substantially affected community structure, biomass production, soil nutrient content, as well as early invasion resistance. Sequential sowing generally decreased invasion resistance compared with a synchronous sowing. Early colonizers generated priority effects of variable strength most likely via belowground competition, which affected A. artemisiifolia’s invasion success. A prior establishment of the N-fixing legume T. repens particularly boosted A. artemisiifolia’s performance. In conclusions, this thesis work highlights the inadequacy of revegetation strategies based on limiting similarity and reveals promising perspectives of manipulating assembly history and priority effects for designing invasion resistant communities. Assembly history significantly influenced early invasion success by inducing differences in biomass production and resource preemption by the recipient community. Priority effects of newly established communities and associated invasion resistance could be enhanced by (1) giving as much time advance as possible to the recipient community over invasives, (2) introducing species displaying an ability to rapidly produce biomass and preempt soil resources, or (3) avoiding sequential sowing especially when early colonizers are nitrogen-fixing, productive species.Les plantes invasives posent d’importants problèmes environnementaux et de santé publique, et leur contrôle est aujourd’hui un défi majeur. Elles rencontrent des conditions particulièrement favorables après des perturbations conduisant à une suppression du couvert végétal et une remobilisation des ressources disponibles. La mise en place d’un couvert végétal séquestrant rapidement ces ressources parait alors une réponse probante pour réduire l’invasion. Néanmoins, les caractéristiques des communautés nécessaires pour exercer une résistance efficace, notamment dans les premières phases d’installation, sont encore peu connues. Dans cette thèse, je me suis intéressée à deux mécanismes qui pourraient influencer la résistance à l’invasion des communautés végétales herbacées lors des premiers stades d’installation après une perturbation majeure, que sont (1) la ‘limiting similarity’, impliquant que la coexistence d’espèces partageant la même niche écologique est limitée par l’exclusion compétitive, et (2) les effets de priorité, qui surviennent lorsque l’installation d’une espèce affecte la performance ou la survie d’une espèce arrivant par la suite. L’examen de la littérature confirme que l’application de la ‘limiting similarity’ pour lutter contre les plantes invasives est complexe et n’a, jusqu’à aujourd’hui, fait preuve d’efficacité. Elle apparait inadaptée aux situations les plus communes. Intégrer les effets de priorité aux méthodes de contrôle des plantes invasives après une perturbation semble d’avantage prometteur. Une des stratégies consiste en la mise en place d’un couvert végétal exerçant de forts effets de priorité négatifs, diminuant le succès d’installation des plantes invasives. Deux expérimentations en serre ont été réalisées à cet effet. Elles visent à jouer sur les effets de priorité de la communauté native receveuse composée d’espèces classiquement utilisées en revégétalisation, afin d’en comprendre l’implication dans la résistance à l’invasion. Dans une première expérimentation, le temps d’avance de la communauté receveuse sur l’arrivée de trois espèces invasives (i.e. Ambrosia artemisiifolia, Bothriochloa barbinodis et Cortaderia selloana), la composition en espèces et la densité des semis ont été manipulés. Une meilleure résistance à l’invasion a été observée lorsque les communautés produisent une forte biomasse aérienne, cette dernière étant associée à la présence d’espèces productives. Retarder l’arrivée des espèces invasives a également réduit le succès d’invasion, mais ceci uniquement lorsque la production de biomasse était suffisamment importante. Une seconde expérimentation a porté sur l’influence de l’identité de la première espèce installée (deux poacées : Dactylis glomerata ou Lolium perenne et deux fabacées : Onobrychis viciifolia ou Trifolium repens) dans la communauté receveuse ainsi que l’ordre de semis des espèces (semis simultané de la communauté ou séquentiel) sur la structuration de la communauté et les conséquences sur sa résistance à l’invasion par A. artemisiifolia. Des différences minimes dans la dynamique de colonisation de la communauté receveuse a substantiellement affecté sa structure, sa production de biomasse, la concentration du sol en nutriments, ainsi que sa résistance précoce à l’invasion. Le semis séquentiel a généralement diminué la résistance à l’invasion par rapport au semis simultané de l’ensemble de la communauté. Les espèces installées en premier ont généré des effets de priorité d’intensité variable, vraisemblablement par le biais de la compétition racinaire, impactant le succès d’invasion par A. artemisiifolia. L’introduction précoce de la fabacée fixatrice d’azote T. repens a particulièrement stimulé la performance de A. artemisiifolia. En conclusion, tandis que l’application de la ‘limiting similarity’ se révèle être incompatible avec la conception de communautés résistantes à l’invasion précoce, manipuler la dynamique de colonisation et les effets de priorité semble d’avantage prometteur. La dynamique de colonisation a considérablement influencé le succès d'invasion en induisant, chez la communauté receveuse, des différences de production de biomasse et de préemption des ressources. Les effets de priorité des communautés récemment établies et la résistance à l'invasion associée pourraient être améliorés en (1) maximisant le temps d’avance à la communauté receveuse par rapport aux espèces invasives, (2) introduisant des espèces capables de produire rapidement de la biomasse et de préempter les ressources du sol, et (3) évitant le semis séquentiel, en particulier lorsque les premières espèces installées sont des espèces productives fixatrices d'azot

Soil compaction enhances the impact of microwave heating on seedling emergence

International audienc

Phenological patterns of herbaceous Mediterranean plant communities in spring: is there a difference between native and formerly-cultivated grasslands?

Background and aims – Plant phenology, defined as the timing of recurring life events like leaf flushing, flowering, or fruiting, is highly sensitive to environmental factors such as photoperiod, temperature, and moisture. Phenological synchrony between interacting species – such as plants and their pollinators – is of major importance to the structure and functioning of ecosystems. Plant phenology might also be affected by changes in edaphic conditions. However, whether former agricultural activities may shift phenological patterns of plant communities remains poorly understood. In this study, we evaluated the impact of past agricultural practices on herbaceous plant community phenology in the protected Mediterranean xeric grassland of La Crau (France). Material and methods – We compared (1) species composition, and (2) phenological patterns of annuals, perennials, Bromus rubens (annual), and Lobularia maritima (perennial), in formerly-cultivated plots – abandoned for 30 years – and intact native grassland plots (steppe), both subjected to itinerant sheep grazing. Key results and conclusion – Our results suggest that former agricultural activities can affect species composition of Mediterranean xeric grassland communities with differences visible after 30 years of abandonment, but only altered phenological patterns slightly. We suggest that climatic factors and sheep grazing acted as strong habitat filters constraining community assembly at the phenological level

Altering native community assembly history influences the performance of an annual invader

International audienc

Advancing biological invasion hypothesis testing using functional diversity indices

International audiencePioneering investigations on the effects of introduced populations on community structure, ecosystem functioning and services have focused on the effects of invaders on taxonomic diversity. However, taxonomic-based diversity metrics overlook the heterogeneity of species roles within and among communities. As the homogenizing effects of biological invasions on community and ecosystem processes can be subtle, they may require the use of functional diversity indices to be properly evidenced. Starting from the listing of the main used functional diversity indices, alongside with the presentation of their strengths and limitations, we further focus on studies pertaining to the effects of invasive species on native communities and recipient ecosystems obtained using functional diversity indices. By doing so, we reveal that functional diversity of the recipient community may strongly vary at the onset of the invasion process, while it stabilizes at intermediate and high levels of invasion. As functional changes occurring during the lag phase of an invasion have been poorly investigated, we show that it is still unknown if there are consistent changes in functional diversity metrics that could indicate the end of the lag phase. Thus, we highly recommend to provide information on the invasion stage under consideration when computing functional diversity metrics. For the existing literature, it is also surprising that very few studies explored the functional difference between organisms from the recipient communities and invaders of the same trophic levels, or assessed the effects of non-native organism establishment into a non-analogue versus an analogue community. By providing valuable tools for obtaining in-depth diagnostics of community structure and functioning, functional diversity indices valuable tools for on-time implementation of restoration plans and improved conservation strategies. To conclude, our work provides a first synthetic guide for their use in hypothesis testing in invasion biolog

Key issues in North-western Mediterranean dry grassland restoration

International audienceDry grasslands of the North‐western Mediterranean Basin are semi‐natural species‐rich ecosystems, composed of many annual species and some structuring perennial species. As these grasslands have been used as rangelands for centuries, human management (grazing; fire regimes) is one of their main ecological and evolutionary drivers, along with the Mediterranean climate. Despite EU conservation policy, damage to such grasslands still occurs and efficient ecological restoration is needed. This paper reviews restoration constraints, effective techniques, and research perspectives for restoring them in the light of their ecological characteristics. Major restoration constraints specifically include i) a poor contribution of the internal (seed bank) and external (seed rain) species pools, and ii) low establishment. The latter may be the result of the low seed quantity or quality of perennial species, stochastic or extended droughts, high soil nutrient content that may lead to competition (e.g. between weed and target species), inappropriate grazing management or fire regimes leading to shrub encroachment, or the death of target species seedlings in early stages of restoration. Successful restoration techniques include seeding, transplanting, hay transfer and timely soil transfer, all of which have to be adapted to regional plant phenology and to local Mediterranean climate. They must be combined with appropriate grazing or fire management. Their success may depend on years with particular rainfall amounts or patterns or on particular target plant species. We discuss research perspectives contributing to a better understanding of seed dispersal, seedling recruitment and soil processes and suggesting the benefits of using ecosystem engineer species

Native plant community recovery after Carpobrotus (iceplant) removal on an island – results of a 10‐year project

International audienceSpecies of the genus Carpobrotus, or iceplant, are succulent mat‐forming perennial herbs often introduced for soil stabilization. They are common in coastal environments and may thus threaten island biodiversity. While their effects are well known on soils, plant communities and associated fauna, the effects of Carpobrotus control on vegetation recovery is poorly documented. The aim of the paper is to describe plant community trajectories after Carpobrotus removal