Common Alder is fairly common in Exeter and is more often a shrub rather than a tree. There are many Common alder shrubs along the banks of the river Exe. There are also two small trees between the back of the old bus station and Sidwell street. They produce very unusual cone-like fruits that often stay on the tree throughout the winter. As a native tree species it is important we keep the ones we have and continue to plant more.
If you are lucky enough to come across a tree rather than the shrub form they are usually conical in shape. Common alder is actually quite short lived and can live to up to 170 years but is typically much less. It does best in moist areas so anywhere near streams and rivers, although it will still tolerate drier soils. Common alder has a very special ability - to fix Nitrogen in the soil. It has a symbiotic relationship with a friendly bacterium called Frankia alni. This friendly partnership allows the airborne Nitrogen that is trapped underground to be converted into soil based nitrates. Similar to Silver Birch - it is a pioneer species so will be the first to colonise open / barren land. Due to the huge benefits that Common alder provide to the ground - they should be planted more widely in industrial estates and other areas with general pollution. They are massively overlooked by councils across the UK as they are one of the few native tree species that can grow on extremely poor soils.
The leaves are usually racquet-shaped and leathery
Common alder loves water
The leaves are 3–9cm long and dark green in colour. They are racquet-shaped and leathery, with serrated edges. The leaf buds are purple or grey and form on long stems and can be quite sticky. The leaf tip is never pointed and is often indented. The bark is dark and fissured and is often covered in lichen. The twigs have a light brown spotted stem which turns red towards the top. The young twigs are sticky to touch.
During pollen release the male flowers are lovely shades of yellow, red and brown
The fruit is a woody cone like structure and the seeds are dispersed by wind and water
The flowers are on catkins which appear between February and April. Alder is monoecious, which means that both male and female flowers are found on the same tree but different parts. The male catkins are yellow and pendulous, measuring 2–6cm. The female catkins are green and oval-shaped and are grouped in numbers of three to eight on each stalk. Once pollinated by the wind, the female catkins gradually become woody and appear as tiny, cone-like fruits in winter. They open up to release seeds, which are dispersed by wind and water.
These unusual growths are caused by a gall that only effects the female flowers. It is called Alders tongue.
During the early winter the male flowers are easy to spot.
Another great thing about Common alder is the roots can help against flooding as they can soak up a lot of water as well as helping to stabilise the structure of riverbeds and other bodies of water. Interestingly the wood of Common alder does not rot under water. This allows all sorts of creatures to make homes within the roots such as otters and other water dwelling mammals. As it is often by water it also provides shelter for smaller fish. Aquatic insects such as water beetles, caddis flies and stoneflies feed off the leaves that fall into the water. Small birds such as Goldfinch eat the seeds and many insects and spider species are associated with Common alder.