Alder

Alnus glutinosa

1. Identification and origin

Alnus glutinosa (L.) Gaertn. is a tree species of the Betulaceae family, known as the common alder. It is native to Europe, North Africa and part of Western Asia, occurring naturally in wet areas, riverbanks, watercourses and waterlogged soils. It is a pioneer species, frequently associated with riparian ecosystems.

2. Economic and ecological importance

The alder is of high ecological importance owing to its capacity for symbiotic nitrogen fixation through Frankia spp., contributing to soil fertility. It is used in:

• stabilisation of banks and recovery of riparian areas
• production of light timber for carpentry, paper pulp and small constructions
• improvement of degraded soils
• support for biodiversity in wet ecosystems

Although it is not an agricultural crop, it is relevant in forest management, habitat conservation and erosion control.

3. Botanical characterisation

Medium-sized tree, with a rounded crown and rapid growth. The leaves are simple, rounded, with a truncate apex and serrated margins, presenting a sticky texture when young. The flowers are unisexual, grouped in catkins: the male ones pendulous and the female ones erect. The fruits are woody strobiles that release small winged seeds. Reproduction is by seed or vegetative regrowth.

4. Soil and climate requirements

It prefers temperate and humid climates, tolerating cold winters. It develops well in waterlogged, clayey or organic soils, being one of the few tree species adapted to prolonged water saturation. It tolerates a pH between 5.0–7.5. It is sensitive to prolonged drought and to highly compacted soils outside wet environments.

5. Main pests

• Defoliating beetles: Agelastica alni causes significant defoliation.
• Aphids (Aphididae): sap sucking and production of honeydew.
• Mites (Tetranychidae): silvering and reduction of the photosynthetic area.
• Lepidoptera: occasional caterpillars that cause partial defoliation.
• Opportunistic wood-boring insects: damage to weakened trees.

6. Main diseases

• Alder phytophthora: Phytophthora alni causes wilting, collar necrosis and sudden death, being the most serious disease of the species.
• Cankers and necrosis: Nectria spp. and other opportunistic fungi.
• Rust: Melampsoridium hiratsukanum causes leaf spots and pustules.
• Root rots: Armillaria spp. in poorly drained soils but with additional stress.
• Sooty mould: development of saprophytic fungi on insect honeydew.

7. General cultural and environmental management

Management of the alder includes:

• selection of healthy plants adapted to wet areas
• maintenance of riverbanks and watercourses with diversified riparian vegetation
• monitoring of symptoms of Phytophthora alni, removing infected trees
• avoiding hydrological changes that cause water stress
• promoting natural regeneration or planting in ecological restoration projects
• avoiding soil compaction and mechanical damage to the roots

The alder is essential in ecological corridors and in the stabilisation of wet ecosystems.

Bibliographic references

• EPPO Global Database – Alnus glutinosa.
• CABI – Invasive Species / Forestry Compendium – Alnus glutinosa.
• European Environment Agency – reports on riparian ecosystems.
• Gibbs, J. N. et al. (2003). Phytophthora alni and alder decline in Europe. Forestry.
• Mitchell, A. (1974). A Field Guide to the Trees of Britain and Northern Europe. Collins.
• Thomas, P. A. (2016). Biology, ecology and conservation of Alnus species. Botany Letters.