Conical in shape, mature trees can reach a height of around 28m and live to approximately 60 years. The bark is dark and fissured and is often covered in lichen. Twigs have a light brown, spotted stem which turns red towards the top. Young twigs are sticky to touch. Alder (Alnus Glutinosa) is the most common tree in wetland forests, and it improves soil fertility through its ability to fix nitrogen from the air. Alder is found throughout Scotland, although its presence in Shetland is due to planting. Today it is rare in the Outer Hebrides and the far northwest of the mainland, but this is the result of deforestation, rather than a limit in the tree’s range. Alder occurs at elevations of up to 500 metres. It typically grows alongside streams and rivers, or on wet ground. As a result it is often found in narrow bands along watercourses, although in the right conditions it can form alder carr, which is a type of wet woodland.