The Red Alder (Alnus rubra) is the tallest species of alder in the world and one of the most ecologically important trees in the Pacific Northwest. A pioneer species that rapidly colonizes disturbed sites, it plays a crucial role in forest succession by fixing atmospheric nitrogen and preparing the ground for conifer establishment. Its wood, once considered a weed tree by loggers, has gained recognition as a beautiful, workable hardwood with a warm, reddish-brown tone.
• The species epithet "rubra" means "red," referring to the reddish-orange color of the inner bark when exposed to air
• The largest species of alder in the world, reaching heights of 30+ meters
• One of the most important nitrogen-fixing trees in North American forests — enriches soil for subsequent tree generations
• A pioneer species that is among the first trees to colonize logged or burned areas in the Pacific Northwest
• Red alder can add up to 200 kg of nitrogen per hectare per year through root nodules
• The wood was historically considered a "weed tree" by timber companies but is now valued for furniture and cabinetry
• Found along the Pacific coast from Kodiak Island, Alaska (60°N) to the Santa Cruz Mountains of central California (37°N)
• Primarily a lowland species, occurring from sea level to approximately 1,000 meters
• Most abundant west of the Cascade Range crest in the moist, maritime-influenced climate zone
• Also found in the interior ranges of British Columbia and Washington where moisture is adequate
• Common along streams, rivers, floodplains, and in moist valleys
• First described scientifically by Karl Sigismund Kunth in 1825, based on earlier work by August von Bongard
• Indigenous peoples of the Pacific Northwest, including the Coast Salish, Haida, Tlingit, and Chinook, used Red Alder extensively for carving, dyeing, and medicine
• The species has been managed through natural regeneration for timber production since the mid-20th century
• Red Alder forests cover approximately 2 million hectares in the Pacific Northwest
• The tree plays a foundational role in the ecology of Pacific Northwest riparian forests
Size and habit:
• The largest alder species, typically growing 15 to 25 meters tall, occasionally reaching 30 to 35 meters
• Trunk is 30 to 80 cm in diameter, with smooth, thin, grayish-white to greenish-gray bark
• Bark often develops patches of white lichen, giving mature trunks a mottled appearance
• Crown is narrowly conical to oval, with ascending branches
• Relatively short-lived, typically surviving 40 to 80 years
Leaves:
• Simple, alternate, broadly ovate to elliptical, 7 to 15 cm long and 4 to 10 cm wide
• Dark green above, paler and slightly hairy below
• Margins are doubly serrate (toothed) with rolled edges
• Leaves have a distinctive, slightly crinkled texture
• Turn pale yellow before dropping in fall
Flowers:
• Male flowers are in long, pendulous catkins 5 to 15 cm long, appearing before leaves in late winter
• Female flowers are in small, upright, woody cones 1 to 2 cm long
• Wind-pollinated
Fruit:
• Small, woody, cone-like structures (strobili) 1.5 to 3 cm long
• Resemble tiny pine cones — an unusual feature for a broadleaf tree
• Each cone contains numerous small, flattened, winged seeds
• Cones persist on the tree through winter
• Seeds are wind-dispersed and produced in enormous quantities
Habitat:
• Moist lowland forests, riparian corridors, floodplains, and recently disturbed sites
• Requires adequate moisture — annual rainfall of 600 to 3,000 mm in its native range
• Grows in full sun to partial shade
• Tolerates a wide range of soil types including gravel, clay, and sandy soils
• Particularly well-adapted to stream banks and seasonally flooded areas
Ecological role:
• Nitrogen fixation through symbiotic Frankia bacteria in root nodules — adds 100 to 200 kg of nitrogen per hectare per year
• This nitrogen enrichment creates fertile conditions for subsequent conifer establishment (Douglas fir, Western Hemlock)
• Rapid early growth shades out competitive brush species, creating favorable conditions for conifer seedlings
• Leaves are high in nitrogen and decompose rapidly, forming nutrient-rich humus
• Riparian Red Alder stands provide critical shade and nutrient input to salmon-bearing streams
• Provides habitat for numerous bird species, including cavity-nesting birds
• Seeds are eaten by finches, siskins, and other small birds
• Beaver use Red Alder extensively for dam construction and food
• Plays a critical role in stream bank stabilization and flood mitigation
• Propagation from seed, which requires no pretreatment and germinates in 1 to 4 weeks
• Seeds are tiny — surface sow on moist, bare mineral soil in spring
• Natural regeneration is prolific on disturbed sites with exposed mineral soil
• Also propagated from bare-root seedlings for reforestation
• Extremely fast-growing — can reach 3 to 5 meters in the first 3 to 5 years
• Plant in full sun for best growth
• Requires adequate moisture — at least 600 mm annual rainfall
• Adaptable to a wide range of soils including poor, gravelly, and clay soils
• Tolerates seasonal flooding and wet soils
• Hardy in USDA zones 5 to 9
• Short-lived — typically 40 to 60 years in landscape settings
• Not suitable as a specimen shade tree due to relatively short lifespan
• Best used for reforestation, stream bank restoration, and ecological plantings
• Nitrogen-fixing ability makes it valuable for reclaiming degraded sites
• Often planted in mixed-species reforestation to improve soil conditions for conifers
• Prune only when necessary — pruning wounds can lead to decay
• Wood is increasingly valued as a fine hardwood — light reddish-brown to tan, with a fine, even texture and attractive figure
• Used for furniture, cabinetry, millwork, turning, carving, and decorative veneer
• Historically used for smoking salmon and other fish — alder-smoked salmon is a Pacific Northwest culinary tradition
• Wood is used for pulp and paper production
• Used for plywood core stock and pallet construction
• Indigenous peoples of the Pacific Northwest carved Red Alder into masks, bowls, spoons, and ceremonial objects
• The reddish inner bark was used as a dye for red and orange coloring of baskets and blankets
• Bark was used in traditional medicine for various ailments including skin conditions and respiratory complaints
• Catkins are edible and can be eaten as an emergency food
• Widely used in ecological restoration for stream bank stabilization, mine reclamation, and reforestation
• Nitrogen-fixing ability makes it one of the most important species for restoring degraded Pacific Northwest forests
• Used in agroforestry systems as a soil-improving companion tree
• The tree is being studied for bioenergy production due to rapid growth
豆知識
Red Alder is the fastest-growing tree in the Pacific Northwest and can fix up to 200 kg of nitrogen per hectare per year through special bacteria in its root nodules — essentially manufacturing its own fertilizer. Indigenous peoples of the Pacific Northwest discovered that the inner bark turns a brilliant reddish-orange when exposed to air, and used it for centuries to dye baskets and blankets a rich red color that became one of the most valuable trade commodities in the pre-contact Pacific Northwest.
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