Écologie et impact d’une espèce invasive émergente en France : l’Ambroisie à épis lisses (Ambrosia psilostachya DC.).

L’Ambroisie à épis lisses (Ambrosia psilostachya DC.) est une Astéracée vivace originaire d’Amérique du Nord et introduite en France à la fin du XIXe siècle. Des observations de terrain font état d’une expansion récente et la formation de populations clonales assez denses pourrait présenter une menace pour la biodiversité dans certains habitats méditerranéens sensibles ou dégradés. Dans ce contexte, cette étude vise à identifier les habitats colonisés et les conditions écologiques favorables à son succès et à mesurer quels sont les impacts sur les communautés végétales des habitats semi-naturels envahis. Ambrosia psilostachya colonise principalement des milieux rudéraux (61 %), cultivés (14 %) et des habitats semi-naturels (pelouses sableuses, dunes) dégradés (12 %). Cette ambroisie vivace, si elle peut occuper les mêmes habitats qu’Ambrosia artemisiifolia (plante annuelle), sera présente dans les stations les plus stables de ces habitats. Ambrosia psilostachya présente de fortes abondances dans les végétations vivaces de début de succession (friches, bords de routes) mais est moins présente dans les habitats pionniers régulièrement perturbés ou stressants, suggérant que son optimum se situe à des niveaux de perturbations intermédiaires. La compétition interspécifique pourrait également agir comme le suggère son plus faible succès lorsqu’elle occupe des communautés composées d’espèces fonctionnellement proches, supposées utiliser les ressources de manière similaire. Aucun impact significatif sur la structure des communautés végétales occupées par A. psilostachya n’a été mis en évidence suggérant des conséquences limitées sur les communautés végétales. Toutefois, son développement dans les prairies à chiendents (Elytrigia spp.) serait à surveiller et une gestion intégrée des pâturages avec de fortes densités de plantes est à mettre en place. Ces premières données écologiques récoltées pourront permettre d’établir une meilleure cartographie du risque envahissant d’A. psilostachya en France.Western ragweed (Ambrosia psilostachya DC.) is a perennial member of the Asteraceae native to North America that was introduced into France at the end of the 19th Century. Field observations have provided evidence of a recent expansion and of the emergence of dense clonal populations potentially threatening the biodiversity of certain Mediterranean sensitive or degraded habitats. This study therefore aimed to identify the habitats colonized and the ecological conditions favouring successful colonization by this species, and to determine its impact on the plant communities of the semi-natural habitats invaded. Ambrosia psilostachya mostly colonizes ruderal (61 %) and cultivated (14 %) environments and degraded semi-natural (sandy grassland, dunes) environments (12 %). Western ragweed occupies the same habitats as the annual species Ambrosia artemisiifolia, but is found at the most stable sites in these habitats. Ambrosia psilostachya is highly abundant in the perennial vegetation at the start of succession (wasteland, roadsides), but is less frequent in more stressful, regularly disturbed pioneer habitats, suggesting a preference for sites with an intermediate level of disturbance. Interspecific competition may also have an effect, as suggested by its lower levels of success in communities consisting of functionally similar species thought to use the available resources in a similar manner. We found no significant impact on the structure of the plant communities occupied by A. psilostachya. However, the development of this plant in Elytrigia grasslands should be monitored and integrated pasture management, including a high plant density, should be implemented. The preliminary ecological data collected in this study should make it possible to establish a more accurate map of the risk of A. psilostachya invasion in France

Biological Flora of the British Isles: Ambrosia artemisiifolia

1. This account presents information on all aspects of the biology of Ambrosia artemisiifolia L. (Common ragweed) that are relevant to understanding its ecology. The main topics are presented within the standard framework of the Biological Flora of the British Isles: distribution, habitat, communities, responses to biotic factors, responses to environment, structure and physiology, phenology, floral and seed characters, herbivores and disease, and history, conservation, impacts and management. 2. Ambrosia artemisiifolia is a monoecious, wind-pollinated, annual herb native to North America whose height varies from 10 cm to 2.5 m, according to environmental conditions. It has erect, branched stems and pinnately lobed leaves. Spike-like racemes of male capitula composed of staminate (male) florets terminate the stems, while cyme-like clusters of pistillate (female) florets are arranged in groups in the axils of main and lateral stem leaves. 3. Seeds require prolonged chilling to break dormancy. Following seedling emergence in spring, the rate of vegetative growth depends on temperature, but development occurs over a wide thermal range. In temperate European climates, male and female flowers are produced from summer to early autumn (July to October). 4. Ambrosia artemisiifolia is sensitive to freezing. Late spring frosts kill seedlings and the first autumn frosts terminate the growing season. It has a preference for dry soils of intermediate to rich nutrient level. 5. Ambrosia artemisiifolia was introduced into Europe with seed imports from North America in the 19th century. Since World War II, it has become widespread in temperate regions of Europe and is now abundant in open, disturbed habitats as a ruderal and agricultural weed. 6. Recently, the North American ragweed leaf beetle (Ophraella communa) has been detected in southern Switzerland and northern Italy. This species appears to have the capacity to substantially reduce growth and seed production of A. artemisiifolia. 7. In heavily infested regions of Europe, A. artemisiifolia causes substantial crop-yield losses and its copious, highly allergenic pollen creates considerable public health problems. There is a consensus among models that climate change will allow its northward and uphill spread in Europe

Estimating economic benefits of biological control of Ambrosia artemisiifolia by Ophraella communa in southeastern France

The North American common ragweed, Ambrosia artemisiifolia, which produces highly allergenic pollen, is invasive in different parts of the world, including Europe. In 2013, common ragweed in northern Italy was found attacked by another accidentally introduced species, the North American leaf beetle Ophraella communa, which is used for biological control of common ragweed in China. Since the establishment of O. communa, ragweed pollen concentrations in northern Italy have significantly dropped. Here we set out to estimate the potential economic benefits of establishment of O. communa in the Rhône-Alpes region in south-eastern France, where detailed data on the economic impact of common ragweed are available. Extrapolating from the change in airborne ragweed pollen concentrations in the Milan area, we estimated that establishment of O. communa in the Rhône-Alpes region will reduce the number of days with ragweed pollen concentrations at which sensitive people express symptoms by 50% and the medical costs due to common ragweed by 5.2–6.8 M € annually. Our findings suggest that investments of public funds are justified to conduct a complete assessment of the potential risks and benefits associated with the accidental establishment of O. communa in Europe

Explaining variability in the production of seed and allergenic pollen by invasive Ambrosia artemisiifolia across Europe

To better manage invasive populations, it is vital to understand the environmental drivers underlying spatial variation in demographic performance of invasive individuals and populations. The invasive common ragweed, Ambrosia artemisiifolia, has severe adverse effects on agriculture and human health, due to its vast production of seeds and allergenic pollen. Here, we identify the scale and nature of environmental factors driving individual performance of A. artemisiifolia, and assess their relative importance. We studied 39 populations across the European continent, covering different climatic and habitat conditions. We found that plant size is the most important determinant in variation of per-capita seed and pollen production. Using plant volume as a measure of individual performance, we found that the local environment (i.e. the site) is far more influential for plant volume (explaining 25% of all spatial variation) than geographic position (regional level; 8%) or the neighbouring vegetation (at the plot level; 4%). An overall model including environmental factors at all scales performed better (27%), including the weather (bigger plants in warm and wet conditions), soil type (smaller plants on soils with more sand), and highlighting the negative effects of altitude, neighbouring vegetation and bare soil. Pollen and seed densities varied more than 200-fold between sites, with highest estimates in Croatia, Romania and Hungary. Pollen densities were highest on arable fields, while highest seed densities were found along infrastructure, both significantly higher than on ruderal sites. We discuss implications of these findings for the spatial scale of management interventions against A. artemisiifolia

Uncovering the genomic basis of an extraordinary plant invasion

Invasive species are a key driver of the global biodiversity crisis, but the drivers of invasiveness, including the role of pathogens, remain debated. We investigated the genomic basis of invasiveness in Ambrosia artemisiifolia (common ragweed), introduced to Europe in the late 19th century, by resequencing 655 ragweed genomes, including 308 herbarium specimens collected up to 190 years ago. In invasive European populations, we found selection signatures in defense genes and lower prevalence of disease-inducing plant pathogens. Together with temporal changes in population structure associated with introgression from closely related Ambrosia species, escape from specific microbial enemies likely favored the plant's remarkable success as an invasive species.Peer reviewe