This study evaluated and compared the morphological characteristics of the pollen grains in the different taxa of the genus Alnus in Hyrcanian forests, north of Iran. The pollen characteristics of five new taxa of the genus Alnus were studied and compared in these forests for the first time. The male catkins from 84 alder individuals of 25 populations were collected throughout the Hyrcanian forests and then recognized and grouped in the eight taxa. After extraction and washing with the standard method of the acetolysis procedure, pollen grains were photographed using light and electron microscopy imaging. Their morphological characteristics were measured afterwards. Alder pollen shape was oblate, with 3 to 6 pores: 1.2% triporate, 42.2% tetraporate, pantoporate 53%, 6/3% hexaporate. The aperture type also was annulate or annulus. About 10.8% of the grains were classified in small size (less than 25 μm) and the rest were in average size (25-50 μm). The large diameter of the investigated pollen varied from 20.7 to 39.3 micron (mean= 29.5 μm) and the small diameter from 19.8 to 36.7 micron (mean= 28.5 μm). About 36% of the pollen grains were concave, 37.2% mixed and 26.8% also convex based on wall shape. Cluster analysis based on the quantitative traits of pollen grains grouped altogether taxa in single cluster and none of taxa were categorized into separate cluster. Discriminante analysis based on six quantitative pollen traits showed high similarity and overlap among different alder taxa. Totally, it was found that eight alder taxa have shown little divergence in terms of pollen morphological characteristics, and consequently determining the limitation of these taxa could not easily practicable.
