Sixty different species of birch trees (Betula spp.) grow around various habitats in the Northern Hemisphere. From a horticultural standpoint, birches provide ornamentation in their colorful or textured exfoliating barks, springtime drooping catkin flowers, yellow fall foliage. Birches also attain an upright, single-trunked habit with open, graceful branching. Birches happen to belong to their own plant family named Betulaceae.
Alder trees (Alnus spp.) are other members of the birch family, Betulaceae. Therefore, they share many similar features to birches. Unless you are quite familiar with both alders and birches, you may not readily distinguish the two since their leaves, flowers and growth habits are so similar. Gardeners especially admire alder trees for their particularly long and showy catkin flowers in spring, which later yield cone-like fruits that change from green to brown. These dry fruits persist on the branches well after the fall foliage drops. Japanese alder (Alnus japonica) and gray alder (Alnus incana) attain a mature form much like birch trees.
Several species of aspen (Populus spp.) mimic many of the same ornamental features of birch tree species. While aspens are faster growing that birches, they all have oval to triangular leaves that have the same visual texture on the branches. Aspens also display golden yellow fall foliage. Although aspens are in the willow family (Salicaceae), they display drooping catkin flowering in early spring before the leaves, just like birch trees. The quaking aspen (Populus tremuloides) has a ghostly gray smooth bark. Other examples of aspens with light-colored barks and upright, birch-like growth habits are bigtooth aspen (Populus grandidentata) and the European aspen (Populus tremula).
Poplar and aspen trees are closely related. In fact, both aspens and poplars share the same genus designation, Populus. Poplars look like aspens which in turn mock many characteristics of birch trees. Some examples of poplars that are birch- and aspen-like are the black poplar (Populus nigra), western balsam poplar (Populus trichocarpa), and gray poplar (Populus x canescens). These examples of poplar trees have upright habits and pale colored and fissured bark that looks somewhat like some birch tree species'.