Thinleaf alder is a large shrub in the birch family (Betulaceae), often growing in dense thickets along lowland rivers and terraces. Thinleaf alder can grow up to 9 m tall, with much-branched and multiple stems. Leaves are thin and ovate, margins coarsely doubly serrate. The leaves are dull gray-green above, and paler beneath, the lower surface hairy or almost hairless. The bark is gray when trees are young, maturing to reddish-brown, marked with many horizontal lines (lenticels). Twigs are red and hairy when young, maturing to gray and hairless. Shrubs have separate male and female catkins, usually borne on the same branch. Female catkins are ovate and cone like, 0.5-2.5 cm long, with dark brown scales, and occurring in clusters of 3-9. Each catkin is made up of many female flowers: two to a scale, consisting of one ovary with two styles. Male catkins are long and pendant with yellow-green scales. Each male flower consists of four highly reduced sepals and four stamens, with one brown scale per three flowers. Peduncles (catkin stems) are shorter than the catkins. Fruits are nutlets without wings. Thinleaf alder is distinguished from green alder (Alnus viridis) by the doubly-serrate leaf margins, the dull, (not shiny as in green alder) leaves and by having peduncles shorter than the catkins (again in contrast to green alder.
Thinleaf alder flowers precociously, meaning the flowers open before budburst. Flowering occurs in May and early June. The fruits develop through the summer, and are dispersed in late autumn. Old female catkins persist through winter, while the male catkins are shed.
Alder is both wind-pollinated and dispersed. Shrubs have separate male and female catkins, usually borne on the same branch. The seeds are nutlets without wings. 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). The Tlingit and Yup'ik used alder to treat cuts and scrapes (Garibaldi 1999). Bark is used by the Yup'ik to dye animal skins (Jernigan et al. 2015). Alder has root nodules containing a nitrogen-fixing bacteria, Frankia, making these shrubs important ecologically for introducing a limiting nutrient, and able to occupy poor soils. Economically, the wood is used to smoke salmon, and occasionally as firewood.
Thinleaf alder is a North American species that occurs solely from the Rocky Mountain states to the west, reaching the southern border in New Mexico. In Alaska, thinleaf alder occurs in the southern Interior, Southwest, and Southcentral portions of the state. In Denali, thinleaf alder commonly grows on river flats in low elevation areas on both sides of the Alaska Range crest.
Thinleaf alder is almost exclusively found on areas without an incline in Denali, the average site at a slope of 0.9 degrees. Additionally, this species is restricted to lower elevations, with almost all individuals found beneath 400 m. It is far less common than mountain alder in Denali.
Thinleaf alder is an early successional species that grows in mineral soils. This species also has a symbiotic relationship with bacteria in root nodules to allow the fixation of nitrogen and allow it to grow in nutrient poor soils.