The white alder (Alnus incana) is an ephemeral tree or shrub, belonging to the Betulaceae family, which generally reaches 8 to 10 m in height, but in some cases it can even exceed 20 m. It has sparse greenish gray foliage, with twisted branches and a slender, irregular stem characterized by white bark mottled with glossy light gray, which from smooth becomes more cracked and pink with time. Young branches die. The main root system enters into symbiosis with nitrogen-fixing actinobacteria (especially Frankiella alni) forming actinorrhizae, that is, lateral roots that look like nodules in which atmospheric nitrogen is fixed, transforming it into ammonium ion, which is then used by Ontane to synthesize amino acids. acids. and therefore proteins. The buds are obtuse in shape and pubescent. The leaves are deciduous and oval in shape, with a wedge or rounded base, a doubly toothed margin and an acute apex. They are smooth and dark green above and hairy and whitish below. 4 to 8 cm long and 3.5 to 5 cm wide, they are not sticky like the leaves of Alnus glutinosa, the black alder. Flowering occurs between February and April, and dormant flowers have been present on the plant since late the previous fall. The male flowers, characterized by conspicuous brown-violet bracts, are grouped in a number of 3-5 elements to form pendulous green catkins 4-7 cm long. The female flowers form hairy inflorescences in the form of a pine or cocoon, 0.5-1.5 cm long and grouped in groups of 3-5 forming a raceme. The fruit is an achene containing small oval seeds with side wings. The achenes have leathery wings and are grouped in woody ovoid pseudostrobilides, about 2 cm long and first green then black, grouped in turn to form groups of 2 to 4 members. The plant releases large amounts of allergenic pollen which, as in the case of other Betulaceae, can give rise to SOA, the oral allergy syndrome, in which patients allergic to pollen develop food allergies to fruits which contain molecules similar to those that trigger the reaction. to pollen. Foods that trigger SOA linked to Betulaceae include celery, parsley, carrots, fennel, apples, peaches, plums, cherries, apricots, pears, kiwis, hazelnuts, almond and peanut.
Climate and terrain
Its distribution in Europe reveals that the white alder has characteristics intermediate between the black alder and the green alder, with the exception of soil pH preference, the black alder being a lover of more basic soils than the other alders. In Italy it is present from 0 to 1300-1600 meters of altitude, in the north of the peninsula up to the Marches and in Tuscany and Sardinia, in the Apennines and the Alps. It is a hardy, heliophilous and pioneer species, which requires proximity to water even if it does not seek a real dive. It looks for more drained and stony soils than black alder, in some cases real alluvium on which it can form its own riparian forests, i.e. alders known as white alder characterized by less rich undergrowth. than in alders with black alder. alder. However, it can live in moist and even calcareous soils,
Plant and cultivation techniques.
Black alder propagates by seed immediately after harvest, by suckers in winter or by grafting in summer for cultivars. White alder seeds dried at 5-10% humidity can be stored for a long time in airtight containers placed at low temperature (0 to -5°C for storage up to 5 years, -15 to -18°C for longer storage times). Due to its characteristics, alder does not require fertilization, on the contrary, it can be used to improve soil fertility. Governed by the scrubland, it can consolidate the slopes of landslides. As an ornamental plant and in its most showy cultivars as the case may be, it does its best in winter associated with branching species such as willows and cornus, or during the growing season as a background for flowerbeds. mixed bands.
pests and diseases
Among the pests of white alder, the alder mite (Acalitus brevitarsus), the classic red fork mite (Cossus cossus) and the red mite (T. urticae) are remembered. In addition, white alder is prone to white disease caused by Microsphaera alni and blister enlargement caused by Taphrina sadebeckii.