British NVC community W4
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NVC community W4 is one of the woodland communities in the British National Vegetation Classification system, characterised by a canopy of downy birch over a field layer of purple moor-grass or other calcifugous herbs on a peaty substrate. It usually forms as a secondary woodland over partially drained bogs or wet heaths.
W4 Betula pubescens-Molinia caerulea woodland is widely distributed, but rarely extensive, throughout the lowlands and the upland fringes of Britain. It occurs on moist, peaty, rather acidic soils, particularly on or around peat bogs that are drying out, usually as a result of drainage, although it can also be found on acidic mineral soils in suitable locations. Its characteristic (constant) species are downy birch (occasionally replaced by silver birch), purple moor-grass and various types of Sphagnum moss.[1]
The habitat is typically a rather open woodland canopy, mainly of birch but often with some alder, willow and oak, with a typically sparse shrub layer that may contain some hawthorn and alder buckthorn. The field and ground layers are sometimes formed of either a lawn of Sphagnum or a taller sward of purple moor-grass or, on less moist soils, a dense cover of bracken.[2] In the less common W4b subcommunity, various rushes and sedges are abundant. There are no tall trees in a W4 wood, as downy birch grows only to about 20 m and it is not long-lived, so there is usually an abundance of dead and decaying trees, giving the habitat a "moribund" look.[3][1]
Fungi are often abundant, with birch polypore on the rotting tree stumps and tawny grisettes and various types of Russula on the boggy ground.[4]
The types of bog-moss that occur in the Sphagnum-dominated stands often reflect the conditions that were present before the tree cover. Sphagnum recurvum agg. (flat-topped bogmoss etc.), feathery bogmoss and red bogmoss suggest raised mires or schwingmoors, while blunt-leaved bogmoss and spiky bogmoss typically occur in places where there is some base-rich surface water. Fringed bogmoss is perhaps the most characteristic of W4 woodland, as it is shade tolerant and frequent here.[5][6]
Birch woodland (usually W4 but sometimes W10 or W16) is often considered problematic on heathland nature reserves, particularly as the decomposing leaf litter can suppress the ground flora.[4][7]
Subcommunities
There are three subcommunities:[1]
- W4a Dryopteris dilatata – Rubus fruticosus subcommunity is a type with only patches of Sphagnum, if any, and a higher quantity of typical woodland shrubs such as bramble and honeysuckle, similar in some ways to W10 oak woodland. Woodwalton Fen and Malham Tarn are classic sites for this subcommunity.
- W4b Juncus effusus subcommunity has a wetter ground layer, with more abundant purple moor-grass and wetland plants such as soft rush and smooth-stalked sedge. There are sometimes patches of Sphagnum palustre and, in some stands, abundant pennywort.[5]
- W4c Sphagnum spp. subcommunity is the more acid, oligotrophic variety, with abundant Sphagnum of various species (most characteristically, S. fimbriatum) combined with other bog plants such as cottongrass (Eriophorum spp.) and cranberry. Clarepool Moss is a locus classicus.