Common Alder

Alnus glutinosa is native to virtually all of Europe, extending into western Asia and northern Africa.

• Found across Europe from Ireland and Portugal in the west to Russia and the Caucasus in the east
• Extends northward to Scandinavia (64°N) and southward to North Africa (Morocco, Tunisia)
• Also found in western Turkey, northern Iran, and the Caucasus region
• Occurs at elevations from sea level to approximately 1,200 meters (occasionally to 1,800 m in southern Europe)
• Has been present in the European landscape since the last Ice Age — pollen records show Alnus was among the first trees to colonize post-glacial landscapes
• First described scientifically by Carl Linnaeus in 1753
• The tree has been important to European cultures for millennia — Bronze Age lake dwellings in Switzerland and Germany were built on alder pilings
• In Celtic mythology, the alder was a sacred tree associated with the god Bran and was believed to have protective powers
• The wood was so valued for underwater construction that medieval Venice was built on millions of alder logs driven into the lagoon mud
• In Irish folklore, the alder was associated with fairies and was considered unlucky to cut down
• The species has been widely planted for land reclamation, erosion control, and riparian restoration

Alnus glutinosa is a medium-sized, deciduous tree with a conical to rounded crown and dark, glossy foliage.

Size and habit:
• Typically grows 10 to 20 meters tall, occasionally reaching 25 to 30 meters under optimal conditions
• Trunk is 20 to 60 cm in diameter, with smooth, dark grayish-brown bark that develops shallow fissures with age
• Crown is broadly conical, becoming more rounded with age
• Often multistemmed when growing in open positions

Leaves:
• Simple, alternate, round to broadly obovate, 5 to 12 cm long and 4 to 10 cm wide
• Dark green and glossy above, paler below — one of the most lustrous leaves of any European tree
• Leaf margins are doubly serrate (toothed) with a distinctive blunt or notched tip
• Young leaves and buds are slightly sticky (glutinous) — hence the species name
• Leaves remain green late into autumn, eventually turning pale yellow-brown before dropping

Flowers:
• Male flowers in long, pendulous, reddish-brown catkins 5 to 12 cm long, very conspicuous in late winter
• Female flowers in small, upright, green, ovoid cones 5 to 8 mm long
• One of the first trees to flower in late winter to early spring — catkins appear before leaves
• Wind-pollinated

Fruit:
• Small, woody, cone-like structures (strobili) 1.5 to 2.5 cm long — unusual for a broadleaf tree
• Green when immature, turning dark brown to black when mature
• Cones persist on the tree through winter after releasing small, flattened, winged seeds
• Superficially resemble tiny pine cones
• Seeds are produced in enormous quantities and dispersed by wind and water

Alnus glutinosa is a keystone species in European riparian and wetland ecosystems.

Habitat:
• Riparian forests, river banks, stream margins, lake shores, wet meadows, and damp woodlands
• Tolerates prolonged flooding and waterlogged, anaerobic soils that kill most other tree species
• Found in areas with annual rainfall of 500 to 2,500 mm
• Grows in full sun to partial shade
• Tolerates a range of soil types including clay, loam, and peat

Ecological role:
• Nitrogen fixation through symbiotic Frankia bacteria — adds 50 to 150 kg of nitrogen per hectare per year to poor soils
• This nitrogen enrichment prepares the ground for subsequent colonization by less tolerant tree species (willow, ash, oak)
• Roots stabilize stream banks and prevent erosion along waterways
• Leaf litter is nitrogen-rich and decomposes rapidly, enriching aquatic ecosystems
• Provides critical shade over streams, regulating water temperature for fish and aquatic organisms
• Seeds are eaten by siskins, goldfinches, and other finch species
• Supports over 90 species of insect, including many moths and butterflies specific to alder
• Alder carr (dense wetland woodland) is one of the most biodiverse habitats in Europe
• The tree is a pioneer species that colonizes bare river gravels and wet, disturbed sites
• Root nodules provide habitat for nitrogen-fixing bacteria and associated soil organisms

Listed as Least Concern by the IUCN, but facing significant threats in parts of its range.

• A major threat is Phytophthora alni, a water mold that causes alder dieback — first identified in the UK in 1993 and now widespread across Europe
• Alder dieback has killed tens of thousands of trees across Europe, particularly along polluted waterways
• Climate change may reduce habitat suitability in southern Europe
• Riparian habitat loss due to river canalization, drainage, and agricultural intensification has reduced alder populations
• In many European countries, alder is protected under forestry regulations
• Riparian restoration programs increasingly use Common Alder to restore degraded waterways
• The species benefits from protection of wetland and riparian habitats under EU legislation

Planting:
• Propagation from seed, which requires cold stratification for 2 to 4 weeks
• Seeds are tiny — surface sow on moist, bare soil in spring; germination occurs in 2 to 4 weeks
• Also propagated from bare-root seedlings for forestry and restoration planting
• Fast-growing, particularly in youth — 50 to 100 cm per year
• Plant in full sun to partial shade
• Requires moist to wet soil — ideal for stream banks, wet meadows, and pond margins
• Tolerates waterlogging, clay, and peat soils
• Hardy in USDA zones 3 to 7 — extremely cold-hardy
• Do not plant in dry, well-drained soils — the tree requires consistent moisture
• Minimal pruning required
• Relatively short-lived for a tree — 60 to 80 years typical, occasionally 100+
• Best used for ecological restoration, riparian planting, wetland gardens, and wildlife habitat
• Not ideal as a specimen shade tree due to relatively short lifespan
• Excellent choice for reclamation of degraded wetlands and mine sites
• Can be coppiced — regrows vigorously from cut stumps
• Often planted in mixed-species riparian buffers alongside willow and ash

Uses:
• Wood is uniquely resistant to rot when permanently submerged in water — the preferred foundation material for medieval and early modern European cities built on water
• Venice, Amsterdam, and many other cities are built on alder wood pilings that remain sound after centuries underwater
• Wood turns orange-red when freshly cut, then fades to pale tan — used for furniture, turning, and carving
• Used for making clogs, bowls, and decorative items in traditional European crafts
• Wood is used for smoking fish and meat, imparting a distinctive flavor
• Historically used for charcoal production, particularly for gunpowder manufacturing
• Bark is used in traditional medicine as an astringent, for wound healing, and for treating rheumatism
• Bark and twigs yield a yellow, reddish, or black dye depending on the mordant used
• Leaves were used in traditional folk medicine as a poultice for wounds
• Catkins are edible and can be eaten as an emergency food
• Widely planted for riparian restoration, stream bank stabilization, and wetland reclamation
• Nitrogen-fixing ability makes it valuable for reclaiming degraded and nutrient-poor sites
• Important in European agroforestry systems as a soil-improving species
• The tree features prominently in Celtic, Norse, and Irish mythology as a sacred tree of protection and courage
• Alder coppice woodlands were historically managed for fuelwood, poles, and charcoal for thousands of years