The birch family (betulaceae) consists of approximately 100 species of trees and shrubs, all of which are found principally in the cooler regions of the northern hemisphere. Birch, alder, hornbeams, hophornbeams, and hazels, also known as filberts, are all members of the birch family and share certain characteristics.
In the northern hemisphere, there are 25 known species of alders, according to "The Sibley Guide to Trees," written by bird and tree expert David Sibley and published in 2009. Seven are native to North America---the smooth alder, red alder, white alder, Arizona alder, speckled alder, green alder, and seaside alder. Alders are small multi-trunked trees. Many grow up to 20 feet tall, while others only reach the height of a shrub. Depending on the age and species, their diameter can be anywhere from a few inches to two feet. They thrive in moist, recently disturbed landscapes---where fires, clear-cuts or glaciation may have occurred. You can recognize these trees by their small, woody cones, which are visible year-round. Alders are helpful in stabilizing and fertilizing areas such as clear-cuts, rock slides, and riverbanks. According to Sibley, "Alders fix nitrogen from the air in a symbiotic relationship with bacteria in its root system, which nourishes the tree." So when nitrogen-rich alder leaves fall, they speedily fertilize barren ground.
Another member of the birch family, the hornbeam is known for its very hard wood and smooth gray bark. Although there are 25 known species of hornbeams in the world, according to "The Sibley Guide to Trees," only one (the American hornbeam, otherwise known as Carpinus caroliniana) is native to North America. The American hornbeam, commonly known as the blue beech, water beech, or ironwood, often reaches about 30 feet tall, though some have been known to grow up to 69 feet. Other species of hornbeams occasionally cultivated include the Japanese hornbeam and the European hornbeam. Unlike all other species in the birch family, the male flowers of hornbeams emerge from small lateral buds in the spring, instead of remaining exposed all winter. The hornbeam still has alternate, deciduous leaves, and staminate and pistillate flowers are in different inflorescences on the same tree, as they are in all other members of the birch family.
There are 12 known species of hazel trees and shrubs in the world, according to "The Sibley Guide to Trees," two of which are native to North America. Beaked hazel (or beaked filbert) is only tree-like in stature in the western United States---from California up through Washington. As such, some call it the California hazel. American hazel, the other native species, grows no taller than 27 feet. Hazels grow in damp to dry soil near forest edges, rocky slopes, and stream-side or in the understory of some forests. Growers cultivate four exotic species of hazel in North America, including the Turkish hazel and giant hazel. Hazels have tiny red female flowers and thick yellow catkins, similar to the birch, that dangle about 2 inches long before leaves even arrive.