Common Alder, Alnus glutinosa

A New Year’s Day walk by the river at Sweetbriar Marshes in Norwich. It is sunny and the winter light is bright and sharp. The trees are looking fine, especially the Alders, whose male catkins glow red-purple in the sunshine. The trees also bear last year’s woody, black female fruit cones, which will persist through the winter. These were proving popular with Siskins and Goldfinches, which were eating the small seeds. (Redpolls like them too but we didn’t see any yesterday.)

The Common Alder, Black Alder or European Alder, Alnus glutinosa, is a British native tree, a member of the Betulaceae (birch family). It can also be found across most of Europe and in Russia as far as Siberia and in Turkey, the Caucasus, Iran and in Morocco, Algeria and Tunisia in North Africa. It has been introduced to North America and is naturalised in the north-east United States and eastern Canada.

The Common Alder grows in damp soil and in Norfolk it is mainly a tree of the river valleys, occurring in carr woodland in the company of sallow and birch trees and shrubs such as guelder rose and dog rose. Carr woodland is an intermediate stage of vegetation, midway between swamp/fen and drier woodland. Carr is derived from the Old Norse kjarr, meaning a swamp. A good example can be found at Ranworth Broad, a Norfolk Wildlife Trust reserve, where visitors can walk through carr woodland from dry land to the edge of the broad along a boardwalk.

Alder trees are well adapted to swampy ground. Alder seeds are light and have air-filled membranes that allow them to float on water, so they can be carried to new sites by air and water. Alders have root nodules that contain filamentous nitrogen-fixing bacteria, Frankia. The fixed nitrogen benefits the tree and the carbon dioxide fixed by the tree benefits the bacteria.

Alders provide a habitat for many insects and fungi and, in wetter parts of the country, the bark supports lungworts such as Lobaria pulmonaria (read more). Otters sometimes build their holts amongst the roots (reference). The Common Alder can be grown in low maintenance and wildlife gardens and as a windbreak. Alder timber is very resistant to decay and has been used to make water pipes, troughs and piles under houses and bridges, for example in Venice (reference). Other uses included making charcoal and clogs.

Italian Alders on Belvoir Street, Norwich

On drier ground a related species, the Italian Alder, Alnus cordata, is often planted. This species is native to southern Italy, Sardinia and Corsica and can tolerate much drier soils than the Common Alder. The Italian Alder is planted as a screen or windbreak or on mine spoil heaps.

Italian Alder is a very tough street tree and there are a lot in Norwich. There is one in the Belvedere Centre garden in Norwich and several more on Belvoir Street, just outside the garden. Planted in the mid 1970s these are now tall, stately trees. George Ishmael, who worked for Norwich City Council in the 1970s, told me that many of Norwich’s Italian Alders were planted as nurse trees to protect slower growing trees such as oaks. The plan was to remover the nurse trees once the other trees had become established but this didn’t happen and only the Italian Alders grew to become substantial trees.

Italian Alders grow too tall for small gardens and their dense foliage, which usually stays on the tree well into December, restricts what can be grown on the ground below.

Like the Common Alder, the Italian Alder has attractive cone-like fruits and reddish male catkins. The leaves are glossier than those of the Common Alder. Both alders retain their leaves until late in the autumn and on sheltered streets in Norwich Italian Alders don’t lose their leaves until mid December. Like the Common Alder, the Italian Alder fixes nitrogen.

The Plants For A Future website lists more uses for Common and Italian Alders.

Both species of Alder as susceptible to to Phytophthora disease of alder, caused by Phytophthora alni, described in my next blog post.