Alder

The Alder — Alnus

General Biology

Alder is a deciduous tree in the birch family, Betulaceae. The common or black alder (Alnus glutinosa) is our native species, typically reaching 20–30 metres, with dark fissured bark and rounded, notched leaves that are characteristically sticky when young — hence glutinosa. It is remarkably long‑lived for a pioneer tree, surviving 150–200 years, and is uniquely adapted to life where few other trees thrive: wet ground, riverbanks, floodplains and beside lochs and burns. Its wood is water‑resistant and dense, turning a rich orange when first cut — a distinctive feature that fades to pale brown on exposure.

Nitrogen Fixation

Alder has an extraordinary partnership with a bacterium called Frankia alni that forms nodules on its roots. The tree provides the bacteria with sugars and shelter; in return, Frankia takes nitrogen gas from the air and converts it into ammonia, a form the tree can use for growth. This means alder can flourish on nutrient‑poor, wet, even peaty soil where other trees struggle — and crucially, it enriches the soil around it, making it possible for other trees and plants to follow. It is one of the few British trees with this ability, and it shapes entire ecosystems wherever it grows.

Global Distribution

There are around 35 species of alder found across the Northern Hemisphere — Europe, Asia and North America — with a few extending into the Andes and Central America. Common alder ranges from the British Isles eastwards to Siberia and the Caucasus, and south to the Mediterranean mountains. In the UK it is widespread, but most abundant in the wetter west and north, where it lines almost every watercourse in Highland Perthshire and the Cairngorms. It is a true hydrophilic — "water lover" — and you will rarely find it far from flowing water or damp ground.

 Diseases

The most serious threat today is Phytophthora × alni, a water‑borne pathogen that causes Alder Dieback. It invades through the roots and creates tarry, bleeding cankers on the trunk; the canopy thins, yellows, and the tree dies — often within just a few years. First identified in the UK in 1993, it has now been recorded across much of Scotland, including sites on the Tay and Tummel systems. The spores swim in water and are carried downstream, making riverbank trees particularly vulnerable. Other minor ailments include tar spot fungus on leaves and various leaf beetles, but these rarely threaten the tree's survival.

Organisms that depend on it

Alder supports at least 140 specialist insect species — more than many broadleaves. These include the striking alder leaf beetle, several sawflies, and numerous moth caterpillars. Its seeds are a vital winter food source for siskins, redpolls and goldfinches — they hang in woody cones that release tiny nutlets throughout the coldest months, when little else is available. Otters use its roots as sheltered holts; dippers build nests in the bank beneath its overhang; and salmon and trout spawn in the cool, shaded waters its branches protect from direct sun. It is truly a keystone species of the riparian ecosystem.

Symbiotic Relationships

Beyond nitrogen‑fixing Frankia, alder forms associations with mycorrhizal fungi that help it take up phosphorus and other minerals from wet, often nutrient‑poor soil. Some of these fungi are water‑tolerant — another adaptation that allows alder to thrive where other trees would drown. The leaf litter is also uniquely important: it decomposes more slowly than most, releasing nutrients steadily into streams and providing a stable food base for freshwater invertebrates — which in turn feed fish.

Alder and Beavers

Alder is one of the beaver's favourite trees (although it ranks behind  aspen, poplar, willow, and birch) — and this is far from one‑sided. Beavers coppice alder, cutting stems and allowing it to sprout vigorously from the base. Unlike most trees, alder responds to browsing by growing back thicker and bushier, rather than being killed. The dams beavers build create ponds that slow floodwaters, trap silt, and keep water levels stable — all of which benefits alder's preferred wetland habitat. In return, alder provides beavers with food, building material, and a tree that can sustain repeated cutting. It is one of the great mutually beneficial relationships in nature.

Seed Dispersal

Alder produces flowers in catkins very early in spring — before the leaves emerge — with male catkins long and dangling, female ones small and woody. Once pollinated, the female develops into a small, cone‑like structure that turns woody and brown by autumn, opening to release thousands of tiny, winged nutlets. These are light enough to be carried short distances by wind, but their real advantage is they float. Water is alder's main highway: seeds drop into burns and rivers, drift downstream, and are deposited on fresh mudbanks where they germinate in their thousands — perfectly timed to take advantage of receding spring floodwaters.

Mythology and Folklore

To our ancestors, the alder was a tree of dual nature — its pale wood turning vivid orange when exposed to air made it seem to "bleed", linking it to life and death, fire and water. In Irish myth it was one of the seven sacred trees; in the Celtic tree calendar, it marks the spring equinox. It was seen as the tree of protection — especially against drowning — and yet it was also the wood chosen for shields, because it was tough and would not shatter like oak or ash when struck. This dual character — strong yet yielding, growing in water yet burning well — made it mysterious and revered.

Human Uses

Because alder wood does not rot when submerged, it has been used for underwater piles, sluices, canal locks and bridge foundations since Roman times — much of Venice sits on alder piles. When dry it is light and easily worked, used for bowls, clogs, tool handles and wooden shoes. It burns steadily but not fiercely, making it excellent for smoking fish and cheese — a traditional use that continues today. Dyes from its bark and cones yield black, green and reddish hues, and it was once the primary source of charcoal for making gunpowder because it burns fast and evenly.

Evolution and Chemistry

Fossil evidence shows alder evolved in the temperate forests of the Northern Hemisphere during the Eocene, around 35–50 million years ago. As climates cooled and forests retreated, its ability to fix nitrogen and grow in wet ground allowed it to persist in refugia and recolonise rapidly at the end of the last Ice Age — it was one of the very first trees to return to Scotland after the ice melted. Chemically, its tissues are rich in tannins — especially in the bark and cones — which makes its leaves decompose slowly and resist breakdown in water. It also contains salicylates, related to aspirin — one reason deer and other browsers tend to avoid eating its foliage in summer, preferring the twigs in winter when concentrations are lower.