The historical spread of Ambrosia artemisiifolia L. in France from herbarium records

Aim The problems in public health and field management in France caused by Ambrosia artemisiifolia L. require a better knowledge of the introduction and naturalization of this species in both the past and present. Location France. Methods The pattern of spread of A. artemisiifolia was investigated through the study of herbarium specimens. More than 1200 specimens were found in 58 herbaria and virtual herbaria in France and in bordering countries. The spread was analysed by mapping the localities for each 30-year period since 1863. Specific indications as 'new plant' were used to determine the timing of the introduction of the species into a new area. Results It seems that the spread of A. artemisiifolia is not linked to its presence in botanical gardens. The study of specimen labels indicates that this species has been introduced in France in seed crops at various independent geographical points and at various times since its introduction in natural habitats. Commercial trade and American troops have contributed to its spread. Main conclusions The spread of the species in area and in time over France showed no clear front: new localities separated by large distances were colonized simultaneously. Cumulative numbers of localities show a continuous increase during the twentieth century. Herbarium specimens can be used to follow the spread of A. artemisiifolia.