Plant

Alder

Alnus (genus)

Also known as: Alnus

A genus of around 35 species of deciduous trees and shrubs in the birch family (Betulaceae) — distributed across the temperate Northern Hemisphere and into parts of the Andes. The species are unusual among non-legume trees for fixing atmospheric nitrogen via root-nodule symbiosis with *Frankia* actinobacteria — a parallel to the legume-rhizobium symbiosis. This makes alders pioneer species in disturbed and nutrient-poor wetlands. Iconic wetland-edge trees of European and North American streams and floodplains. The wood is the foundational material of the Venetian piling system on which the city has been built for 1,500 years.

Photo via Wikimedia Commons — see source for license.

Scientific

Alnus (family Betulaceae — same family as [[birch]] and [[hazelnut]]) contains ~35 species, most in temperate Northern Hemisphere wetlands and stream margins, with a few extending into the Andes (Alnus acuminata).

The species’ nitrogen-fixation is one of the most economically and ecologically significant non-legume fixation systems. Frankia, an actinobacterial genus, infects alder roots and produces nodules functionally similar to legume nodules — pulling atmospheric nitrogen into ammonia that the tree (and surrounding plants) can use. This is part of why alders are textbook pioneer species in disturbed wetlands: they bring their own nitrogen.

Cultural and historical — Venice

[[san-francisco|The City]] of Venice is built on millions of alder, oak, and larch pilings driven into the Venetian lagoon’s muddy bottom. The choice of alder for the bulk of the piling work reflects a specific property: alder wood underwater is nearly indestructible (the lack of oxygen prevents fungal decay) and the wood petrifies over time into a stone-like substance. The pilings under the Rialto Bridge, Basilica di San Marco, and many other historic Venetian structures are 500–1500-year-old alder, still functionally load-bearing.

Equivalent piling traditions used alder for foundations in Amsterdam, Bruges, and many other Northern European cities built over peat or wetland.

Ecological role

Alders are critical to riparian ecology. They line streams and rivers across temperate Europe, North America, and Asia; their nitrogen-fixation enriches surrounding soil; their roots stabilize banks against erosion; their leaf litter feeds aquatic invertebrates and ultimately the entire stream food web. Restoration of degraded riparian zones often involves alder replanting as a primary restoration species.

Sources

[[wikipedia|Wikipedia]] — Alder

A plant entry in the [[0mn1one|0mn1.one]] [[directory]].

What links here, and how

Inbound connections from across the wiki, grouped by lens and by relationship. These appear automatically — every entity page declares what it links to, and that data populates here on the targets.

Cultural

shares approach with

Hornbeam auto-linked from body mention
Larch auto-linked from body mention

General