Olsza

Olsza, also known by its scientific name Alnus, is a genus of deciduous trees and shrubs in the birch family, Betulaceae. There are about 30-35 species of olszas, found in temperate and boreal regions of the Northern Hemisphere, as well as in parts of South America. These trees are commonly found in moist soils, along riverbanks, lakeshores, and in wet woodlands.

Olszas are characterized by their serrated or toothed leaves, which are often ovate or elliptical in shape. They produce catkins, which are drooping or erect clusters of small flowers. The female catkins develop into small, woody cones that contain winged seeds. Many olsza species have a symbiotic relationship with the bacterium *Frankia alni*, which lives in nodules on their roots and fixes atmospheric nitrogen, enriching the soil.

Common species include the common olsza (Alnus glutinosa), native to Europe, North Africa, and parts of Asia, and the green olsza (Alnus viridis), which has a circumpolar distribution. Olszas are often used in riparian restoration projects due to their ability to stabilize soil and their nitrogen-fixing capabilities. Their wood, though not as strong as some other hardwoods, has been historically used for charcoal, turnery, and in situations where resistance to water is beneficial, such as for pilings and clogs.

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