Alnus viridis is a deciduous, shrub or small tree that attains a height of 10 m with multiple stems. Its flexible stems are well adapted to frequent snow avalanches. Plants produce abundant seed that disperses during fall and winter. Germination from seed on disturbed habitats is the primary form of reproduction, through sprouting. Seedling establishment is best on moist mineral soil. Plants fix atmospheric nitrogen through a symbiotic relationship with bacteria (Uchytil 1989e).

Alnus viridis is a widespread species in the southern arctic, subarctic, and the mountainous regions of the Northern Hemisphere. Two of the three subspecies grow in northwestern California. Plants of ssp. fruticosa are local in the coastal plain in Del Norte Co.; ssp. sinuata is common in montane and subalpine habitats of the Klamath Mountains and adjacent North Coast. Another name commonly used for the later subspecies is A. sinuata.

NatureServe (2007a) recognizes the alliance from the Pacific Northwest, northern Rocky Mountains, and Cascades. Stands are common in drainages subject to heavy snow accumulations and snowmelt flooding, and frequent fluvial disturbance acts to discourage coniferous tree establishment. In California, they form narrow stringers along montane streams in the Klamath Mountains, usually adjacent to coniferous forests. A. viridis ssp. sinuata appear to be more shade tolerant than Alnus incana ssp. tenuifolia, regularly occurring in the shade of adjacent taller conifer stands along forested streams. Stands are not as extensive as those of A. incana.

Plot-based descriptions exist for Bridge Creek, Haypress Meadow, and Sugar Creek cRNAs (Cheng 2004) in the Klamath Mountains (M261A). The species occurs on South Fork Mountain in Northern California Coast Ranges (M261Ba) but not in the Yolla Bolly Mountains (Keeler-Wolf and Keeler-Wolf 1974). We need additional sampling of this alliance to verify its distribution and range of conditions in which it occurs in California.