Importance of seeds in the process of common ragweed invasion
Résumé
Ambrosia artemisiifolia L. (common ragweed) was introduced into Europe at the end of the 1900s and is now present in several European countries. This annual invasive plant produces seeds that are highly polymorphic. Common ragweed can produce only a few thousand highly viable seeds. Many studies have focused on the seed stage. Greater seedling emergence for the seeds placed near the soil surface could explain the success of this species in open habitats, where the probability of deeper burial is low. Emergence percentage was found to decrease as burial depth increased from 2 to 8 cm, and no germination nor seedling emergence was observed for the seeds buried from 10-cm depth. The huge plasticity in seed weight may help common ragweed to cope with a wide range of conditions and to establish in very different disturbed habitats. Control of seed production of ragweed appears to be a necessary step in its sustainable management. Despite a late seed production in the season, which should facilitate the management practices, nothing seems to be able to stop the spread of this species. Moreover, because of global warming, the reduction in the number and in the duration of days with frost may favour the expansion of common ragweed by increasing the period of seed production.