The genus Alnus had three taxa in the old floral resources of Iran. Recently, fve new taxa were added to the flora based on morphological studies. Individual samples were collected from 25 sites throughout three provinces covering Hyrcanian forests of Iran and classifed into the eight mentioned taxa and then prepared for micro-morphological studies. Four groups of taxa including A. subcordata var. subcordata, A. subcordata var. villosa, A. orientalis, and A. glutinosa subsp. antitaurica had a specifc structure. Similar structure was also found between two new species of A. djavanshirii and A. dolichocarpa, which were distinguished from the other groups. Simple trichomes with variation in size and density were observed among the taxa. Alnus subcordata var. villosa and A. glutinosa subsp. antitaurica had the densest trichomes on dorsal and ventral leaf surfaces. The results of this study show the suitability of ultrastructure features as a micromorphological marker for solving the taxonomical ambiguities of the genus Alnus in Hyrcanian forests.
