Stand characteristics and understory plant diversity were investigated in low-drained man-made stands of Alnus subcordata C.A.Mey and Taxodium distichum (L) L.C. Rich. The trees were planted with distances of 3 × 3 m and 4 × 4 m in northern Iran. In these stands, herbaceous and woody species were counted in plots of 20 × 20 m. Then, indexes of richness, H/ diversity, J/ equitability and Jaccard similarity (JI), tree growths, cover crown percentage, and litter layer thickness of each stand were assessed following 17 years after planting. The results revealed that the greatest diameter at breast height (D.B.H) and stem height were observed in Alnus 4 × 4 m. By contrast, crown cover percentage and litter thicknesses were greater in Taxodium stands. Species richness, H/ diversity and J/ equitability indexes, Jaccard similarity (JI) of Alnus stands were greater than those of Taxodium stands. In reality, small and light canopy of Alnus is the main reason that the solar radiation can penetrate easily to forest ground and affect understory plant diversity. Alnus as a native tree species, due to greater growth attributes and higher diversity indices in their stands are proposed for plantations in such low-drained sites of northern Iran.