EN black alder – PT amieiro – ES aliso común – FR aulne glutineaux – DE Schwarz-Erle
Alnus glutinosa (L.) Gaertn. is a deciduous fast-growing and broadleaved tree. It is relatively small (it usually grows to 10-25 m), with rough and fissured bark and dark green leaves with an obovate shape. Alders are monoecious species (they develop male and female catkins) and produce woody fruits with seeds that can float and be dispersed by water.
Black alders require high water availability and tend to dominate riparian areas around perennial flow streams and wetlands being native in most of Europe and in punctual areas of N Africa.
Variations in the number of chromosome sets were found throughout this area, with diploid populations in most of the distributional range and tetraploid populations in the in the warmer edge of the species distribution (Iberian Peninsula, North Africa and Dinaric Alps). Considering this ploidy variation, the Alnus from the Iberain Peninsula and North Africa have been recently described as Alnus lusitanica Vít, Douda and Mandák recently (Vít et al. 2017).
Alder forests are a priority for conservation by the European Habitat Directive 92/43/CEE and they are considered a priority habitat (91E0* “Alluvial forests with Alnus glutinosa and Fraxinus excelsior”). In the Iberian Peninsula, black alders can be found with other tree species such as ash (Fraxinus spp.), birch (Betula spp.), willow (Salix spp.) or poplar (Populus spp.), and with several herbaceous species and ferns.
Alder forests are paramount to riparian areas as they provide important services in these ecosystems:
Nitrogen fixation
They form symbiosis with the actinomycete Frankia, facilitating the colonization of other species in nitrogen-poor soils.
Riverbank protection
They stabilize unstable surfaces, mitigating floods
Maintenance of the trophic chain
They are on the base of the trophic chain and the seeds are an important winter food for some birds such as siskins.
Water quality improvement
They can act as green filters that protect water bodies from nitrate contamination and prevent eutrophication.
Creation of habitats
They are pioneer species that can colonize disturbed sites and contribute to the creation of important habitats with high biodiversity.