Birches are part of the Betulaceae family, related to alders and hornbeams.The genus is native to regions throughout the northern hemisphere, and contains about 35 species. Birches are distinguished by the peeling bark characteristic of some species, like the paper birch (Betula papyrifera), and by the distinctive male and female "catkins" that are present on every tree. Birches are popular ornamental trees, but at various times in various cultures, the wood, bark, sap and roots of the birch have also been used to make everything from saunas to soda.
Phillips and Rix in "The Botanical Garden" comment that birches are prone to natural hybridization when various species grow in close proximity to each other. This trait has helped the genus evolve over time. Birches sometimes produce pollen and egg cells that are diploid, containing twice the normal number of chromosomes. The union of these diploid cells results in tetraploid offspring with four times the normal number of chromosomes. These tetraploid offspring have the potential to germinate into plants with enough unique characteristics to be deemed new species.
In parts of northern Scandinavia, the mountain birch (Betula czerepanovii) hybridizes with another species, Betula nana. These hybrids show great variation in form and genetic traits. Scientists, including a team led by Dr. Friederike Wagner of Utrecht University in the Netherlands, believe that the hybridization process among these trees is driven by climate conditions, including the reduced sunlight that causes the region to have a relatively short growing season.
Yellow Birch Hybridization
In southeastern Michigan, yellow birch (Betula alleghaniensis) may sometimes hybridize with bog birch (Betula pumila). Most of the resulting trees have dark bark as opposed to the lighter bark of the yellow birch. However, scientists studying these birch populations concluded that some of the "hybrids" might actually be natural yellow birch variants, which also occur frequently.
Paper and Bog Birch Hybrids
In 1959, researchers from the University of Minnesota reported on hybrids of paper birch (Betula papyrifera) and bog birch (Betual pumila) in Minnesota and Wisconsin. The new species, identified in 1916, was christened Betula sandbergii. Most of the hybrids had the reddish brown, nonpeeling bark of the bog birch and grew to a height between that of the tall (up to 80 feet) paper birch and the shorter (up to 10 feet) bog birch.
In the New World, paper birch has reportedly hybridized with many other species including yellow birch (Betula alleghaniensis), sweet birch (Betula lenta) and river birch (Betula nigra). There are also a number of named hybrids that have resulted from crosses between paper birch and other small, shrubby birch species, varieties or hybrids. The many crosses are reflective of the fact that paper birch varieties are the most widely distributed birches in North America.