This section is from the book "British Wild Flowers - In Their Natural Haunts Vol5-6", by A. R. Horwood. Also available from Amazon: A British Wild Flowers In Their Natural Haunts.
That the marsh vegetation is a more or less natural formation is shown by the usual occurrence of a certain tree association, which gives it a distinctive character. The tree flora of the moors is deficient in this country, and only the Pine and Birch are found at high altitudes in peaty soils. In a marsh formation the Alder is usually the dominant tree. The fen or fen carr is also made up of Weeping Willow and Sweet Gale forming a scrub, which merges into an Alder and Willow wood; Ash, the pedunculate Oak, the hoary and black Poplars, and the small-leaved Elm also occur frequently.
The shrubs forming the scrub layer are made up of the smaller Willows, Guelder Rose, Birch, Field Maple, Hawthorn, Sloe, Elder, Gooseberry, Currant, and Brambles and Roses, with such climbers as Honeysuckle and Ivy.
But trees may be absent, in which case marsh approaches the bog formation more closely.