(water birch)
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Betula occidentalis is a hybrid species formed by mating events between resin birch (
B. neoalaskana) and dwarf birch (
B. nana). As such, its size and morphology are intermediate between these two species, and show considerable variation, sometimes with greater similarity to resin birch, sometimes more closely resembling dwarf birch. Most commonly,
B. occidentalis is a spreading shrub with many stems, reaching 3-4 m tall, but it can sometimes grow as a small tree or low shrub. The bark is pink-brown, not peeling, trunks not generally reaching diameters over 10 cm. Leaf shape is
ovate to round, or sometimes pointed, with a broad to narrow wedge-shaped base, short tip and coarsely serrated margins. Leaf size is intermediate between dwarf birch and paper birch, 2-5 cm long. Plants have separate male and female catkins that are grown on the same twig. Male flowers are two
stamens surrounded by a brown-green
calyx, female flowers are an ovary with
styles, protected by a scale-like brown
bract. Fruits are
nutlets with papery brown wings. This hybrid can be distinguished from its parent species most easily by leaf shape and size.
Phenology of hybrid birch has not been studied, and it undoubtedly varies depending on the parent species and site conditions. Betula occidentalis flowers precociously in early spring, and then undergoes budburst. New leaf buds and catkins are formed at the end of summer, which lay dormant through the winter. Seeds mature throughout the summer and are dispersed in late fall.
Like other birches, B. occidentalis is wind-pollinated and wind-dispersed. Plants have separate male and female catkins that are grown on the same twig. The level of hybridization between this taxon and its parent species is unknown. Fruits are cone like, with papery brown wings to aid in wind dissemination.
Betula occidentalis is a boreal North American species that occurs throughout most of Alaska where its parent species are found, from the southern edge of the Brooks Range to the southwestern region, across southcentral Alaska to Yukon Territory. These hybrids occur occasionally throughout the Park, with the highest frequency in the mid-elevations of the northeastern portion of the Park. This taxon is also found near the rivers of the northwestern corner, and has a few scattered localities south of the Alaska Range in the park.
Details are shown in the Plots & Charts found at right, depicting recent Denali data.
Like many other characters, the climatic tolerance of hybrid birch appears to be between that of its parent species, with its elevational occurrence peaking between the means for Alaska birch and dwarf birch. Hybrid birch has occurs in Denali at a wide range of elevations (157-1138 m), preferring medium to high elevations. It has the highest level of cover at 500-700 m (though less than 3%) with lesser but still measurable presence at 700-1100 m. This species is equally frequent on all aspects, but covers slightly more area on southern slopes. This species prefers moderate slopes, though it can be found from flat areas to inclines of 36 degrees.
Details are shown in the Plots & Charts found at right. For more on how to interpret these figures, visit Understanding Data Presented.
Somewhat poorly drained wet sites to well drained moist sites.
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