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   Alnus glutinosa (tree)
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      Alnus glutinosa (Photo: USDA-NRCS PLANTS Database) - Click for full size   Alnus glutinosa (Photo: USDA-NRCS PLANTS Database) - Click for full size   Alnus glutinosa seeds (Photo: USDA-NRCS PLANTS Database) - Click for full size
    Taxonomic name: Alnus glutinosa (L.) Gaertn.
    Synonyms: Alnus alnus (L.) Britt., Alnus barbata C.A. Mey., Alnus rotundifolia Miller, Betula alnus glutinosa L., Betula glutinosa L. Lam.
    Common names: alder, black alder, common alder, European alder, European black alder, sticky alder, swartels (Afrikaans)
    Organism type: tree
    Alnus glutinosa is a member of the birch family (Betulaceae) and is native to Eurasia and the northern part of Africa. Members of the Alnus genus are commonly known as alders; A. glutinosa is known as black alder. It can grow up to 50 feet high. It is often cultivated for erosion control, to improve soil and as an ornamental. A. glutinosa is fast growing and can grow in a wide range of soils; this trait increases its invasiveness in susceptible environments. Association with species of the genus Frankia enable A. glutinosa to fix nitrogen (Frankia is a genus of nitrogen fixing filamentous bacteria that live in symbiosis with actinorhizal plants and form root nodules, similar to Rhizobia). A. glutinosa invades damp ecosystems near water sources, wetalnds and riparian zones.
    Alnus glutinosa is a member of the birch family (Betulaceae); members of the genus Alnus are commonly known as alders; A. glutinosa is commonly known as black alder. A. glutinosa is quick growing tree, with a relatively short life span - up to 160 years. Trunk may be single or multi-stemmed, with a smooth greyish-green bark that turns a speckled grayish-brown, with warty horizontal stripes and irregular, shallow fissures. Flowers appear before the emergence of leaves, in early spring; A. glutinosa is monoecious. Male flowers are catkins that are slender, reddish-brown and 2.5 - 3.8 cm in length, while females flowers are reddish-brown cone-like catkins around 4 mm, and cluster near branch tipes in groups of 2 - 5. Leaves, when young, are gummy, and develop into leathery, dark green, oval-to-orbicular shape, with serrated edges. Leaves are blunt-tipped and are 5 - 12.5cm x 5 - 10cm in dimension. Fruit form in autumn, initially as green, cone-like woody catkins around 2cm long. These ripen to brown and contain many, small, winged nutlets. The root system is nodulated and dense, and can be fairly shallow. (Cao 2009; Claessens et al. 2010; Funk 2005; McVean 1953; USDA Forest Service 2006).
    Occurs in:
    lakes, natural forests, planted forests, riparian zones, ruderal/disturbed, urban areas, wetlands
    Habitat description
    Alnus glutinosa is able to grow in a variety of habitats and in most soils. While tolerating dry soils, it prefers moist, damp conditions, especially near water sources. It commonly grows in riparian zones, wetlands, along ponds and lakes. It also grows in forests (including early successional forests, forest edges), forest wetlands and in urban areas. Association with species of the genus Frankia enable A. glutinosa to fix nitrogen (Frankia is a genus of nitrogen fixing filamentous bacteria that live in symbiosis with actinorhizal plants and form root nodules, similar to Rhizobia). Its ability to fix nitrogen means it can grow on poor soils with low nutrient status. A. glutinosa can also grow on both acidic and basic soils, with a pH range of 4.2 - 7.5. Mature trees are frost hardy (to -54 °C), but 130 frost-free days are required for successful reproduction and growth. It is not very tolerant of shade and prefers full sunlight. (Claessens et al. 2010; Funk 2005; McVean 1953; USDA Forest Service 2006).
    General impacts
    Alnus glutinosa can form dense monospecific stands fairly quickly (< 10 years) that exclude other species. In the wetlands, it creates a higher degree of shade that can inhibit the growth of some native species. The dense root network can increase the accumulation of sediments and alter water flow.

    A. glutinosa can alter soil nutrient status. A symbiosis exists between A. glutinosa and Frankia sp. (Frankia is a genus of nitrogen fixing filamentous bacteria that live in symbiosis with actinorhizal plants and form root nodules, similar to Rhizobia); allowing A. glutinosa to fix atmospheric nitrogen (Bond et al. 1954). The ability to provide its own source of nitrogen enables A. glutinosa to grow on a wide range of soils and increases its invasiveness. Nodulation can occur is most of these soil conditions, but occurs best at pH 5.5 – 7.2 (Griffiths & McCormick 1984). This nitrification process increases the nitrogen status of the soil, which is also contributed to by leaf fall and decomposition (Claessens et al. 2010). The availability of phosphorous in the soil is also increased (Giardina et al. 1995).

    A. glutinosa readily hybridises with other alder species, and known hybrids include A. glutinosa x A. rubra, A. cordata x A. glutinosa and A. glutinosa x A. orientalis (Funk 2005).

    While several diseases and pests associated A. glutinosa exist, one of the most notable is the fungus Phytophthora alni, which has been spreading through alder populations in Europe since the 1990's (Claessens et al. 2010; Funk 2005). Trees present on river banks and near water are the most susceptible, as the presence of free water is necessary fot the development and transport of P. alni (Chandelier et al., 2006; Gibbs et al. 1999). Symptoms of the disease include dead roots, dead bark, small yellow-ish leaves and tar-coloured spots on trunk bases, and over the following years, branches die and death of the tree can occur (Claessens et al. 2010).

    Alnus glutinosa has various environmental uses, including erosion control, soil improvement and as an ornamental. It is also used for building materials (wood) and as a traditional medicine. (USDA-ARS 2010). Alders have been recommended for afforestation of disturbed areas throughout much of the temperate world. Establishing European alder on mined sites apparently improves their suitability for earthworm habitat. (Funk 2005). A. glutinosa can be used as a nurse crop for high-value species that are sensitive to nitrogen input to increase nitrification of the soil, e.g. black walnut (Juglans nigra) in Illinois, U.S.A (Bohanek & Groninger 2005). The nurse crop (i.e.A. glutinosa) can also be thinned from plantations, and sold for commercial gain. It assists with water filtration and purification in waterlogged soils (Peterjohn and Correll, 1984; Pinay and Labroue, 1986; Schnitzler and Carbiener, 1993), and the root system helps to control floods and stabilize riverbanks (Piégay et al., 2003). It is also thought to be a suitable species in the restoration of functional alluvial ecosystems (Schäfer and Joosten, 2005), for example in transforming spruce and poplar plantations to more natural stands. Claessens et al. 2010. In the Netherlands, A. glutinosa protects river banks from erosion and also aids in maintaining water nutrient status and temperature (Claessens et al. 2010).

    In its native range, A. glutinosa is valuable for wildlife. It can contribute to biodiversity by providing habitats for a specific flora and fauna both on the tree itself and in the flooded root system (Dussart, 1999). In addition, both the leaves and the cones are a food source for animals. Seed-eating birds rely on the tree over the winter, as the cones open gradually over winter, releasing seed (Funk 2005). A. glutinosa also provides food for deer, rabbits and hares is recommended as shelter for pheasants (Alaska Natural Heritage Program 2005; Funk 2005).

    Geographical range
    Native range: Africa (Algeria, Morocco, Tunisia); Asia (Iran, Kazakhstan, Turkey); Europe (Albania, Austria, Belarus, Belgium, Bulgaria, (former) Czechoslovakia, Denmark, Finland, France, Germany, Greece, Hungary, Ireland, Italy, Netherlands, Norway, Poland, Portugal, Romania, Russian Federation, Spain, Sweden, Switzerland, Ukraine, United Kingdom, (former) Yugoslavia). (USDA-ARS 2010)
    Known introduced range: Australia, Canada, Chile, New Zealand, South Africs, United States. (Castro et al. 2005; GCW 2007; Henderson 2007; WDNR 2009).
    Introduction pathways to new locations
    Floating vegetation/debris:
    For ornamental purposes:
    Landscape/fauna "improvement": Erosion control
    Other: Garden escape/garden waste.

    Local dispersal methods
    Consumption/excretion: Dispersal by birds occurs sometimes, though more usually the seeds are split open during consumption (McVean 1953).
    For ornamental purposes (local):
    Forestry (local):
    Garden escape/garden waste:
    Horticulture (local):
    Other (local): Gravity
    Vegetative reproduction:
    Water currents: Fruit float and are efficiently dispersed by water currents (Alaska Natural Heritage Program 2005).
    Wind dispersed:
    Management information
    Alnus glutinosa can be successfully controlled by a combination of physical and chemical means. Trees are often felled and the stumps treated with herbicides. However, follow-up procedures are necessary as A. glutinosa is known to resprout. A. glutinosa is not affected by low rates of the pre-emergence herbicide pendimethalin but is damaged by higher concentrations (2.0 kg a.i. ha–1) and above), as well as by napropamide. Other effective herbicides include glyphosate (stump treatment) and triclopyr triethylamine (foliar application). (Champion et al. 2008; Funk 2005; Kelly & Southwood 2006a,b; Willoughby et al. 2007).
    While Alnus glutinosa can reproduce sexually and vegetatively, reproduction occurs predominately by seed (Alaska Natural Heritage Program 2005). Once alders mature - 3 - 30 years (depending on ecotype and environment) they produce prolific amounts of seed every three or four years, with an average of 240,000 per tree (Claessens et al. 2010). Cut stump are known to resprout, and fallen green branches have been observed to take root in soft, swampy wetland areas (McVean 1953). Additionally, A. glutinosa can be readily propagated by tissue culture (Tremblay et al. 1984, in Funk 2005). Root suckers are rare (Alaska Natural Heritage Program 2005).
    Lifecycle stages
    Alnus glutinosa have the potential to immediately germinate, however, stratification (six weeks) increases germination success. The optimum conditions for A. glutinosa seed germination occur at pH 4 and 25 °C in either light or dark. Once germinated, seedlings are susceptible to low soil oxygen status and drought. Maturity takes 3 - 30 years, depending on ecotype and environment. A. glutinosa is relatively short-lived, with a lifespan of up to 160 years. (Alaska Natural Heritage Program 2005; Claessens et al. 2010; McVean 1953)
    Compiled by: IUCN SSC Invasive Species Specialist Group (ISSG) with support from the Auckland Regional Council (ARC)
    Last Modified: Friday, 27 August 2010

ISSG Landcare Research NBII IUCN University of Auckland