Green alder is a shrub in the birch family (Betulaceae) that is one of the most abundant shrub species in the park, sometimes forming dense thickets on slopes and in valleys from the lowlands into the subalpine zone, where it can dominate large areas. Leaves are shiny dark green on top, pale and shiny green below, blade ovate or elliptic, margins finely serrate, sometimes with course, wavy double serrations. Leaves are resin coated when young. The bark is smooth, with small horizontal pink openings (lenticels). Male catkins are long, narrow and pendant, with brown scales and yellow-green flowers comprising a reduced calyx and four stamens. Female catkins are ovate cones on long stems, with long brown scales overlapping the two-styled flowers. The fruits are winged nutlets, the wings wider than the body. The species is distinguishable from thinleaf alder (Alnus tenuifolia) by its conspicuously shiny leaves, long-peduncled female catkins (as compared to thinleaf alder), and the nutlet wings being broader than the body.
Like other alders, mountain alder flowers in early spring, prior to budburst. Fruits mature over the summer and fall in late autumn. Old female catkins persist through winter, while the male catkins are shed.
Alder is both wind-pollinated and dispersed. Trees have separate inflorescences (catkins) for male and female flowers. The fruits are winged nutlets, the wings wider than the body, the wings aiding in wind-dispersal. Shrubs also spread vegetatively from underground rhizomes or suckers, forming large thickets. Plants form a symbiosis with a nitrogen-fixing bacterium (Frankia) in their roots, allowing alder to colonize nutrient poor habitats.
The thinleaf and green alders have been used for a variety of medicinal applications. Tea brewed from inner bark was used by the Dena'ina to address stomach problems, tuberculosis, induce vomiting, and break fevers (Kari 1995). Alder species have root nodules which house symbiotic nitrogen-fixing bacteria. As such, they can live in nutrient poor areas, and are important ecological drivers. Economically, the wood is used to smoke salmon, and occasionally as firewood.
Green alder is an incompletely circumpolar species that occurs widely in northern areas of the globe, including the west coast of Greenland. In North America, this species occurs across the continent, reaching its southern limit in isolated sites in the mountains of North Carolina in the east and California in the west. The species is broadly distributed across Alaska, but is less common north of the Brooks Range. Its range reaches south through the Alaska Peninsula and Southeast Alaska. Alnus viridis is broadly distributed in Denali and is particularly abundant on the south side of the Alaska Range, and in the Kantishna Hills. There are two subspecies in Alaska, subsp. fruticosa and subsp. sinuata. Subspecies sinuata has a more Pacific coastal distribution, in the southern half of Alaska, and extending to northern California and western Wyoming. Alnus viridis subsp. fruticosa grows from southcentral Alaska to the Brooks Range.
Mountain alder has a strong correlation with increasing slope, reaching 9% cover on sites above 28 degrees. However, the average site is on a slope of 9 degrees, and it is common on most inclines. This species has a broad elevation range in Denali, growing from 122 to 1236 m, it covers the most area at 700-900 m, and its frequency peaks at 500-700 m. It is less common on north-facing slopes, compared to other aspects.