Red alder is another iconic Northwest Coast lowland plant. It ranges from Southern Alaska in the North to Santa Cruz County in the South, mostly on the West slope of the Cascades, though there is a disjunct population in far Eastern Washington, Central Idaho and Western Montana. It is the largest species of Alder in North America (a recent champion tree, in Clatsop County, OR, was measured at 104′ height, 49′ spread, 245″ circumference).

Red alder is a short-lived, early-seral species; its seeds are both wind pollinated and wind dispersed, and its seedlings do best on bare mineral soil. It is difficult to propagate vegetatively. Young red alders grow very quickly, but rarely reach their hundredth year. Red alder is tolerant of saturated soils, but not of drought.

Alnus rubra is actinorhizal. It a forms a symbiotic relationship with an actinomycete filamentous bacterium in the genus Frankia, which has the ability to fix atmospheric nitrogen. The relationship begins with an intracellular infection of a root hair, which leads to cell division in the pericycle, eventually growing a multi-lobed root nodule. Both the roots and the leaf litter significantly increase the amount of available nitrogen in the soil. Douglas fir grows faster in conjunction with red alder than without it.

Red alder has also become the most important commercial hardwood in the PNW. Although the wood rots quickly out of doors, its workability makes it a favorite for furniture, cabinetry, and toymaking. Red alder is not a chosen browse of most ungulates (deer will preferentially eat dropped leaves in the fall, but avoid them on the tree), but the bark is an apparent delicacy for mountain beavers. Red alder trees are an important winter forage for pine siskins, chestnut backed chickadees, and kinglets. Sapsuckers are considered a pest of commercial stands; their feeding holes damage the wood, sometimes even girdling the tree, or weakening it to the point that the top blows out in a storm.