Alder

Alder - Alnus sp.

Alders belong to the scientific genus Alnus which live throughout the cooler regions of the Northern Hemisphere, southwards over the high mountains of central America and down to the Andes.

Depending on whose book you read, there are some 17 species and five of those are really shrubs or dwarf trees. Alders seldom grow very tall.

Alders possess special nodules on their roots which contain nitrogen fixing bacteria which improve the soil.

Most alders are pioneer species. They invade and thrive in gaps and clearings in forests, often on the poorest soils. Many tolerate a high water-table and periodic flooding.

Alder shows all the features of a successful pioneer survival strategy - it produces numerous small seeds, rapidly colonises bare open ground, has fast initial growth and a short life-span. They need good light, open ground and cannot tolerate shade and competition - in natural conditions they are overtaken by larger trees which out-shade them.

Five species of alder are commonly found in the UK - one native and four imported.

In British forestry, they are of little value as timber trees but can be used as "nurses" to protect and bring on more valuable trees or as soil improvers on sites with undeveloped soils like reclamation sites. Most alders coppice well.

Alnus glutinosa leaves, male amd female catkins, from Bilder ur Nordens Flora

The black alder - Alnus glutinosa - is native throughout the British Isles and much of Continental Europe. It is found widely but particularly on wetter soils, often at high elevations on infertile terrains although it does not thrive on acid-peat or dry-sandy substrates. It can form small woods or "carrs" on boggy land.

The grey alder - Alnus incana - is found across central Europe. Introduced to Britain in 1780, it resembles the native Black Alder in its lifestyle and site requirements.

Italian alder - Alnus cordata - from southern Italy and Corsica, was brought to Britain in 1820. Like all alders, it is a strong light demander and withstands exposure and pollution well, copes better with drier, calcareous soils than its relatives and is a good landscape tree.

Grey and Italian alders have been planted as windbreaks round orchards.

The red or Oregon alder - Alnus rubra - is native to large tracts of the Pacific Coast of north America, where it is closely associated with Sitka spruce. First introduced into Britain in the late 1800s, it has been tried as a timber tree with mixed results although it is a significant pulp wood in its native home.

The green alder - Alnus viridis - has a shrubby growth form and lives in arctic and alpine regions of Europe. It provides vital stability and fertility when planted to start reclaiming bare derelict land like China Clay spoil in Cornwall.

Over the last few years, many riverside alders in Britain have succumbed to a fungus from the Phytophthora group.

Similar to poplar, alder wood is one of the weakest hardwoods. However it is resistant to decay under water so was sometimes used for sluice gates and also for charcoal in making gunpowder.

Alders have catkins - the male and female ones open in early spring, before the leaves appear, so the wind can carry pollen from male to female flowers.

Each long, dangling male catkin is a complex structure loaded with about 120 individual flowers and 480 pollen-producing stamens. After the clouds of yellow pollen are shed, the male catkins wither and fall.

The plump, club-shaped female catkins have tufts of hair or stigma to trap the pollen drifting in the air.

After pollination, female alder catkins ripen into a woody "false-cone". In autumn, the scales open and release the seeds to be dispersed both wind and water in this riverside plant. The empty "false-cones" hang on the twigs for several years, and make alder easy to recognise.

Now cleared and drained for intensive agriculture, much fenland was originally dominated by alder carrs.

Further reading : Gibbs, J. N. & Lonsdale, D. (1998). Phytophthora disease of alder. Forestry Commission Information Note 6, 5pp.