Grey Alder
Alnus incana
The grey alder is a medium-sized, fast-growing deciduous tree of the birch family, characterized by its smooth, silver-grey bark that remains largely unfissured even in old age. As a pioneer species, it colonizes moist sites, gravel flats, and riverbanks, contributing to soil improvement through a symbiotic relationship with nitrogen-fixing bacteria (Frankia). The leaves are ovate, doubly serrated, and typically feature a grey-pubescent underside.
Details
Habitat function
Riparian woodland, provides shelter and nesting sites for birds, root systems serve as hiding places for aquatic organisms.
Nutrient uptake
High nitrogen fixation through root nodule symbiosis; moderate nutrient requirements.
Food source for
Eurasian siskin, redpoll (seeds); various insect larvae (leaves).
Human use
Wood for hydraulic engineering, model making, and woodturning; reforestation of slag heaps and erosion control.
Ecology
Ecological role
Pioneer species for primary colonization of raw soils, soil nitrogen enrichment, erosion control on steep slopes and banks.
Natural predators
Alder leaf beetle (Agelastica alni), woolly alder aphid, various fungal diseases such as alder dieback (Phytophthora alni).
Competitor species
Competition from more shade-tolerant species like ash or beech in later successional stages.
Ecosystem service
Soil stabilization, flood protection through bank reinforcement, habitat for birds (e.g., siskin), nitrogen fixation.
Threats
River engineering, loss of floodplain areas, spread of alder dieback (Phytophthora).
Scientific profile
Profile
Protection & threats
Main threats
Loss of natural river dynamics due to engineering, groundwater lowering, hybridization with Alnus glutinosa, and the spread of Phytophthora alni.