Woodland Habitat Audit - Appendix page 2

1. Oak-honeysuckle (hornbeam, sweet chestnut) woodland (NVC W10). This woodland type is found on soils that are moist and of mid range pH, and is defined more for the lack of species indicative of drier, wetter, more acid, or more basic conditions, than for any particular preferential species. It is probably the most widespread woodland community in London’s Sites of Metropolitan Importance, if one accepts that many of the unattributed hornbeam and some of the sycamore woodlands probably belong here, and almost certainly the commonest type in London. Typical trees are pedunculate oak and silver birch, while ash and maple are scarce. The best bluebell woods occur here, but moister soils have wood anemone instead, and creeping soft-grass, bracken and bramble are common. Most sweet chestnut woods belong here, as do many hazel coppices. The NVC describes five sub-communities, four of which appear to occur in London (W10e being the exception). We have identified this type provisionally in all of the Metropolitan sites except for some on the chalk in the south-east and on the fertile alluvium of the Colne Valley.
2. Ash-maple (sycamore) woodland (NVC W8). This woodland type is found on soils that are moist and base-rich. It has a very wide range of species and is one of the richest of London’s woodland communities. It is the second commonest type in the Sites of Metropolitan Importance. Indicative species include field maple and ash, but also sycamore, elm, buckthorn, guelder rose, dog’s mercury, wood sage, ramsons, lesser celandine and primrose. It shares pedunculate oak, birch, hawthorn, hazel, bramble, bluebell, wood anemone and ivy with oak-honeysuckle woods. Some of London’s hornbeam woodlands belong here. Seven sub-communities are described all but two of which (W8f & g) occur in London. We have provisionally identified this community in almost all of London’s Metropolitan sites. Most of the sycamore woodland in London should be classified here.
3. Birch-oak woodland (NVC W16a). This type is found on acid and nutrient-poor soils, usually as the result of succession on previously heathland or acid grassland sites, where the displaced habitat is often preferred. It is the third most abundant type in the Sites of Metropolitan Importance. The dominant trees are commonly pedunculate oak and silver birch, but sessile oak and downy birch may be dominant. Ash, hazel, sycamore, hawthorn and bluebells are uncommon and any bramble and honeysuckle sparse. Both Scots pine and sweet chestnut can occur in this type. It is species-poor and has few indicative species, but the presence of much heather, wavy hair-grass, purple moor-grass, rowan, gorse or bilberry distinguishes it from oak-honeysuckle woodland. Holly is prominent in the understorey of some stands. It shares bracken with oak-honeysuckle woodland. This woodland has few spring flowers, although lily-of-the-valley can occur. Two sub-communities are described, but just this one occurs in London.
4. Beech hangers (NVC W12). This type is the most abundant beech wood in the Sites of Metropolitan Importance, and is found mainly on the well-drained chalk in the south and east of London. It grows in similar places to ash-maple woodland, which it may replace through succession until beech becomes the sole dominant canopy tree. However, the beech tends to be on the steeper scarps and the ash-maple on the lower slopes. Few other trees occur, but there may be some ash, yew, silver birch, holly and whitebeam. The heavy shade and root competition from the beech restrict the diversity of ash-maple woodland species, all of which can occur in small quantities. Only dog’s mercury, sanicle, ivy, bramble, and wall lettuce occur in any abundance where the beech canopy is mature. Characteristic species are yew, wall lettuce and sanicle. Three sub-communities are described and all occur in London.
5. Yew woodland. (NVC W13). This type is rare (or absent) in London. It is found on dry and exposed chalk. It is a more extreme type than the beech hangers and, like them, few other species survive under the heavy shade.
6. Beech-bramble woodland. (NVC W14). This is the beech dominated equivalent of the oak-honeysuckle woodland and, like it, is distinguished from related woodlands more by what it lacks than by any characteristic species. This community probably follows oak-honeysuckle woodlands in succession on better-drained sites. It proved difficult to identify in the London data, and there appears to be little of it in the Sites of Metropolitan Importance. However, if it was grouped with beech hangers or acid beech into one survey parcel, it could easily be overlooked. Pedunculate oak and silver birch are the only other frequent trees and, where there is little browsing, holly commonly forms a sub-canopy beneath the beech. The other species of oak-honeysuckle woodland are largely excluded by the heavy shade and root competition of the dominant beech. Bramble carpets the ground in the older stands, with a little bracken and honeysuckle. There are no recognised sub-communities.
7. Acid beech woodland. (NVC W15). This is the beech dominated equivalent of the birch-oak woodland. It often has large amounts of the dominant species of the latter, pedunculate oak and silver birch, and of Scots pine. Also it often has much holly, some rowan and a scattering of bracken and wavy hair-grass. It might be better considered a sub-community of birch-oak woodland. Characteristic species include pill sedge, bilberry, heather, holly and more rarely common cow-wheat and wood sorrel. None of this community was positively identified within the Sites of Metropolitan Importance but, as with the previous community, it could have been subsumed into beech hangers, or in this case, into birch-oak woodland. Four sub-communities are described and all could occur in London.
8. Alder nettle woodland (NVC W6). This species-poor wet woodland type is probably the most widespread wet woodland community in London. It occurs mainly beside rivers and lakes where nutrient levels are maintained by periodic flooding. It is distinguished mainly by the abundance of nettle and scarcity or absence of species like common reed, lesser pond sedge, wild angelica, meadowsweet, purple loosestrife, common loosestrife, hemp agrimony, marsh marigold and common valerian. Usually it has a canopy of alder, but it may have crack willow, downy birch, osier willows or grey willow, and a little oak, ash or sycamore. The shrub layer may have grey willow, bramble, honeysuckle and elder. With the nettle may be cleavers, reed canary-grass, great willowherb, yellow iris, hedge bindweed and broad-buckler fern.
9. Swamp carr. (NVC W5). This species-rich wet woodland comes about through succession in reedbeds and sedge fens. In the young stages it has much grey willow as well as alder, but the alder predominates later. There may also be a few ash, oak, downy birch, alder buckthorn, guelder rose and hawthorn. Bramble is the only frequent shrub, but there may be some honeysuckle. Reed eventually becomes shaded out by the tree canopy but lesser pond-sedge can survive under the mature canopy. Other fen species in this community include nettle, meadowsweet, common valerian, common marsh-bedstraw and water mint. This community is very likely to occur in London, but was not confidently identified in the Metropolitan Sites data. Characteristic species include lesser pond-sedge, remote sedge, marsh thistle, opposite-leaved golden saxifrage, hemp agrimony, alder buckthorn, yellow iris, gypsywort, purple loosestrife and bittersweet. Three sub-communities are described and two could occur in London.
10. Fen carr (NVC W2). This community too can be the result of succession in a fen community, but it can also result from the cessation of mowing of a marsh. Its national distribution is concentrated in East Anglia and the West Midlands and it may not occur in London. It is distinguished from alder carr by the infrequency of reed and frequency of bulky sedges, downy birch, grey willow and of sphagnum species in one sub-community.
11. Flush alder wood (NVC W7). This alder dominated woodland typically occurs on slope flushes within oak woodland types in north-west Britain and the Weald. It may occur in London. It is distinguished from swamp and fen carrs by the scarcity of sedges and other fen species. It may have nettle on the ground, but is distinguished from the nettle woodland by having more yellow pimpernel, meadowsweet, lady fern, remote sedge, grasses and creeping buttercup.
12. Grey willow carr (NVC W1). This species-poor woodland is dominated very largely by grey willow and marsh bedstraw is usually found in its ground layer. It is found on mineral soils around lowland water bodies and probably occurs in London. There may be a few trees of other wetland species, or hawthorns and brambles, and grasses, bittersweet, ivy, water mint and soft rush below, but sedges and tall grasses are generally absent.
13. Birch-purple moor-grass (NVC W4). This is a simple community of wet acid soils, usually dominated by downy birch. It may have some alder, silver birch and a little oak, but no ash. The understorey may have willows, especially grey willow. Beneath this is largely purple moor-grass, usually with some sphagnum. Different subcommunities may have bramble, honeysuckle, broad buckler fern, Yorkshire fog, tufted hair-grass, creeping soft grass, soft rush and heather. Probably all three subcommunities occur in London in association with heathland and mire.
14. Hawthorn hedge & scrub (NVC W21). The majority of London’s scrub, woodland edge and hedgerows fall within this single community type, characterised by much hawthorn and bramble, some blackthorn and dog rose and, in London, by cherry plum and plum. Ivy and young trees of ash and sycamore are common. It is regarded as a successional stage to the oak, ash, sycamore, hornbeam and maple woodlands, except where management arrests succession (as in hedgerows). The chalk scrub subcommunity (NVC W21d) is found on dry, base rich soils, and may include much dogwood, wayfaring tree, roses, yew, elder, privet, black bryony and old-man’s beard, wood false-brome beneath, and trees from the base-rich woodland types. Several orchids can occur in this scrub type and it often occurs in a mosaic with chalk grasslands. A poorer subcommunity of these soils, wood false-brome scrub (NVC W21c) is more common in the north of Britain, but may describe chalk scrub in places where colonisation by a variety of species is difficult because of isolation from sources. This community often has wild strawberry and common dog-violet. The ash-elm subcommunity (NVC W21b) is found on heavier basic soils and is the scrub equivalent of ash-maple woodland, sharing with it many trees, shrubs and ground flora. It may also succeed the other subcommunities as the scrub canopy closes. Much of London’s suckering elm scrub falls here and other characteristic trees include field maple. On made ground and abandoned agriculture the elder-buddleia subcommunity (NVC W21a) is widespread in London and it covers such a range of composition that the NVC subcommunity probably should be further subdivided, it typically has much nettle and cleavers beneath and a good variety of other herbs, such as red dead-nettle, common chickweed, creeping thistle, lesser burdock, hogweed, hedge bindweed, false oat-grass, Yorkshire fog, couch, squirrel-tail fescue, hawkweed oxtongue, sterile brome and other wasteland species. On chalky rubble this community borders on pure buddleia scrub. On less extreme basic soils succession to ash-maple woodland occurs and on more mesotrophic soils succession is to oak-honeysuckle woodland. All the subcommunities may be found on London’s railsides.
15. Blackthorn scrub (NVC 22b). This scrub community is typically dominated by blackthorn, but otherwise parallels the hawthorn hedge and scrub community. The dense canopy of blackthorn makes this community generally poorer in species compared with the hawthorn community. It is widespread on London’s railsides.
16. Gorse scrub (NVC W23a). This scrub community is found on base-poor free-draining soils. Apart from common gorse, it may have broom and much bramble, young birch and oak. Very little grows on the ground below the gorse, but the community commonly occurs in a mosaic with grasslands having common bent and heath bedstraw. It occurs where the more acid woodlands have been felled, on woodland edges, or in succession to acid woodlands, but not in hedgerows. It is common where London’s railsides pass through appropriate soils. Where there are appreciable amounts of heather or dwarf gorse the community is included in the heathland audit.
17. Bramble-Yorkshire fog scrub (NVC W24). This scrub type has elements of mesotrophic (‘neutral’) grassland and occurs on moist soils where woodland has been cleared, on rides and woodland edges or where bramble has invaded neutral grassland. Other common grasses are cock’s-foot, red fescue and false oat-grass. There are often patches of nettle, hogweed, cow parsley, creeping thistle, spear thistle and rosebay willowherb. Underneath the bramble canopy are ivy and a scattering of woodland herbs. There are few or no other shrubs. The community is widespread on London’s railsides and as a successional stage following garden use or wasteland.
18. Bracken-Bramble scrub (NVC W25). This is the equivalent of bramble-Yorkshire fog scrub on base poor freely draining soils. As there, this community has few or no other shrubs and few other associates, and occurs where woodland has been cleared, on rides and woodland edges or where bramble and bracken have invaded base poor grassland. It too is found on abandoned sites and on railsides where the soil is suitable.