On the identity of two Mexican species of Ageratina (Eupatorieae, Asteraceae): A. grandifolia and A. rivalis

Background: Ageratina, with 167 species, constitutes the most diverse genus of the Mexican Asteraceae and the sixth most diverse of the vascular plants in Mexico. The taxonomy of the genus is complex, due to its number of species and the numerous inadequately delimited taxa or with intricate and confusing nomenclature. In addition, little known species have sometimes remained under the synonymy of another different species, as in the case of A. rivalis, which has been considered as a synonym of A. grandifolia. Question: Is it possible to recognize Ageratina rivalis as a distinct species of A. grandifolia by critically analyzing its circumscription, nomenclature and geographical distribution? Taxon: Ageratina grandifolia and A. rivalis. Study site: Mexico. Method: A thorough review of herbarium specimens, field material, descriptions and geographic distribution of the species was made to contrast them. Results: Several differences exist in the morphology and geographical distribution of Ageratina rivalis and A. grandifolia. Descriptions, pictures, distribution maps, synonymy and a key to distinguish them from similar species are provided. The presence of A. grandifolia in Mexico City is also documented. Lectotypes are designated for Eupatorium conspicuum, E. conspicuum var. pueblense, E. grandifolium, and E. rivale. Conclusions: Morphology and geography support the distinction between A. grandifolia and A. rivalis. However, further research is desirable to corroborate or refute this statement, and especially to clearly circumscribe several similar species that seem to be related. This work contributes to a better understanding of the taxonomy and biogeography of the genus Ageratina in Mexico.