THE EFFECT OF URBANISATION ON ANCIENT WOODLANDS

Oliver Gilbert and David Bevan

Published in "British Wildlife" Volume 8, number 4, April 1997
© British Wildlife Publishing and the authors



A cultivar of Ivy growing in Ecclesall Wood, Sheffield



In Sheffield there are about 50 blocks of amenity woodland covering 650ha. These vary in size from 1ha to the extensive Ecclesall Wood occupying 117ha. Though widely affected by hardwood - planting, they are basically oak - birch - bluebell (Quercus - Betula - Hyacinthoides) woods which from the medieval period until World War One were managed as coppice-with-standards. Today they are managed as amenity woodlands by the local authority, having been acquired at various times within the last 100 years. Historical work by Melvyn Jones (1986a,b) has shown that all of these woods are ancient (in existence before 1600); the majority of these now lie within the continuously built-up area.

Figure 1 The number of introduced species in 100m lengths of the urban ecotone, in one section of Ecclesall Wood, Sheffield. Dots show the distribution of Highclere Holly.

Table 1 The most prevalent introduced ground-layer species in Ecclesall Wood listed in order of decreasing abundance.

Other cities, such as Birmingham, Hull and Manchester, are almost devoid of ancient woodland as a result of their land-use history and flat topography. London, however, still retains a remarkable range of such woods. A survey in 1984 found that Greater London contained about 2,500ha of ancient woodland, which represents 1.6% of the surface area (Spencer 1986). The extensive oak-hornbeam (Quercus Carpinus) woodlands around Ruislip are a fine example, covering 331ha and representing the largest area of ancient woodland in the capital. Other well-known ancient woods include the recently threatened Oxleas Wood complex in Greenwich and Ken Wood on Hampstead Heath, which is London's most central SSSI.
Close by, in the London Borough of Haringey around Highgate and Muswell Hill, lie three fragments of ancient oak-hornbeam woodland, Highgate, Queen's and Coldfall Woods, where some of the effects of urbanisation have been studied. As in Sheffield, these were once worked coppice-with-standards, are publicly owned and are now managed as amenity woodlands.

The urban ecotone

It is often assumed that advanced successional stages, such as woodland, are particularly resistant to invasion by introduced plants owing to the operation of factors such as competitive exclusion. However, these urban woods are being ingressed by foreign plants on a large scale. These plants are not uniformly distributed, but are concentrated around the fringes, where they form an urban ecotone. A fine example can be seen around Ecclesall Wood in the western suburbs of Sheffield. Expanding housing reached the outskirts of this wood in the early 1920s. In order to save it from speculative builders, who had already nibbled at the margins and bisected it with a road, it was purchased for the public and opened as a public open space in 1928. Since then, management has been low key and unplanned.
A walk along the wood margin in early June reveals a normal range of species such as Red Campion, Silene dioica, Hedge Woundwort, Stachys sylvatica, Bluebell, Hyacinthoidies non-scripta, and Dog's Mercury, Mercurialis perennis, but, in addition, growing in among the Brambles, Rubus fruticosus agg., Bracken, Pteridium aquilinium, and Creeping Soft-grass, Holcus mollis, are unexpected plants such as Montbretia, Crocosmia x crocosmiiflora, Garden Solomon's-seal, Polygonatum x hybridum, Welsh Poppy, Meconopsis cambrica, and Irish Ivy, Hedera 'Hibernica'. To investigate this urban ecotone in more detail, four types of wood margin were identified, backed onto by gardens, by quiet minor roads, by major roads, and by horse-grazed fields. A substantial length of each type of margin was divided into 100m sections and the number of introduced herbaceous and woody species present in each section assessed. It was rarely necessary to penetrate more than 40m into the wood before native vegetation became dominant.
The results from one section of the wood (Fig. 1) showed that, at a mean of ten species per 100m, invasion was most intense along residential edges, followed by margins bounded by minor roads (six spp/100m); then came major roads (two spp/100m), while there was no invasion of alien species associated with the field margins. To explain these findings and discover the introduced plants' origins and dispersal mechanisms, it is necessary to examine the composition of the assemblages more closely.
A total of 89 introduced species is present in Ecclesall Wood, which represents 39% of the flora. Within the northern section of the wood (c. 40% of the total area), which was studied in greatest detail, the most prevalent introduced ground-layer species, in order of decreasing abundance, are shown in Table 1. These 14 species vary in frequency from well over 25 clumps or patches (Spanish Bluebell, Welsh Poppy, Montbretia) to a minimum of six. Species that have efficient methods of vegetative spread, such as Lesser Periwinkle, Variegated Yellow Archangel and Pick-a-back-plant, have established dense patches up to 10m across. During the 65 years the wood has been under urban influences, many introduced species have gained only a tenuous foothold and are present in just two or three localities despite being shade-tolerant. These include Monk's-hood. Aconitum napellus, Columbine, Aquilegia vulgaris, Lily-of-the-valley, Convallaria majalis, Leopard's-bane, Doronicum pardalianches, Wood Spurge, Euphorbia amygdaloides ssp. robbiae, Lungwort, Pulmonaria officinalis, and Fringe-cups, Tellima grandiflora. Many of these less frequent species are expanding slowly by vegetative growth and building up their innoculum pressure of seed/propagules, so they will eventually become more widely established.
The majority of the introduced species are horticultural plants which can be found in gardens surrounding the wood. The linked questions that need to be answered are "How did they get from the gardens to the wood?" and "Why are they limited to the margins?" Their restriction to the marginal ecotone could be related to distance from seed source, disturbance, soil eutrophication or increased light. Their method of dispersal could involve the activity of animals such as birds, dogs or Grey Squirrels, Sciurus carolinensis (burying bulbs), wind or humans. The majority of evidence points to the last factor. A considerable amount ot tipping of garden refuse takes place, particularly over back-garden fences and beside minor roads, which is where the highest concentration of introduced species occurs. So the dispersal method of most species is as garden 'throwouts' of troublesome and over-exuberant species. Once they get established in the wood margin, these often highly competitive species spread by vegetative growth or more rarely by seed, e.g. Spanish Bluebell (including hybrids) and Welsh Poppy. Though the interior of the wood will not be suitable for all the species that currently make up the urban ecotone, the more shade-tolerant ones are gradually moving inwards like the closing of an iris diaphragm.
Nineteen of the introduced species in the urban ecotone are shrubs; all are still rare. Snowberry, Symphoricarpos albus, and Spiraea alba will eventually become conspicuous through their strongly suckering growth, while Tutsan, Hypericum androsaemum, currants, Ribes, and several cotoneasters may spread by seed.

Woodland interior

The interiors of ancient woods are more resistant to invasion, but certain woody plants are able to penetrate them. The best-known invader of acid woodland is Rhododendron, R. ponticum, and, in Burnham Beeches, the Yellow Azalea, R. luteum, is also spreading by seed. Elsewhere Shallon, Gaultheria shallon, is causing problems. In Ecclesall Wood, Sheffield, Rhododendron is less of a problem than the Highclere Holly Ilex x altaclerensis, which is widespread in surrounding gardens, from where it gets bird-sown into the wood. To investigate its distribution, all hollies were surveyed in a sample 25ha area of the wood. The results (Fig. 1) revealed that 77 of the 649 hollies present (11.9%) were Highclere Holly; their density was not obviously related to the closeness of gardens. By contrast, Norway Maple, Acer platanoides, present along one margin as a pavement tree, is regenerating densely in the urban ecotone, but is unable to penetrate more than 12m into the wood.
The ivys form a singular group. The Irish Ivy and the Persian Ivy, Hedera colchica, form dense undulating carpets on the wood floor, while other cultivars such as H. helix 'Sagittifolia' climb up the trees, providing unexpected vignettes.

Dotted Loosestrife and Norway Maple in the urban ecotone, Ecclesall Wood

Coppicing and the entry of introduced species

The entry of introduced species into recently cut coppices has been studied at Coldfall Wood in the London Borough of Haringey. This wood is a 14ha ancient oak-hornbeam coppice-with standards. Ring counts of the over-mature Hornbeam, Carpinus betulus, coppice poles indicate that they were last cut between 50 and 60 years ago. Together with the standard oaks they now form a dense canopy, preventing light from reaching the woodland floor. As a result, there is virtually no ground flora and the wood is a dark and rather forbidding place.


Left Coppice 1, Coldfall Wood, north London, with a dead hedge in the foreground; December 1990
Right Coppice 1, Coldfall Wood in August 1993

With a view to increasing the structural and species diversity of the wood, a series of coppice 'falls' has been cut since December 1990 (Bevan, 1992). Three of these are of similar size (0.4ha). The first lies close to the St Pancras and Islington Cemetery (Fig. 2), and the second adjacent to a housing estate; these two coppices are situated at the woodland edge. The third coppice lies near the centre of the wood. Each coppice was surrounded by a dead hedge, using brushwood from the felled Hornbeams.



Figure 2 Map showing location of the three coppice falls (1, 2, 3) in Coldfall Wood. Inset locates Coldfall (C) Highgate (H) and Queen's Woods (Q) in north London

Table 2 Introduced plants found in two or more coppice falls, Coldfall Wood

The arrival of new plants during the first three years in each of the three coppices has been carefully monitored. A total of 171 new arrivals (both native and introduced) was recorded, but they were not evenly distributed between the coppices. Coppice 1 recruited 83 species, Coppice 2 had 147, and Coppice 3 only 36. The high number of species in Coppice 2 is largely explained by its being a wetter site. However, its proximity to housing is also significant and explains the large numbers of mtroduced species found (Table 2). Many of the species recorded were new to the wood and some, such as Trailing St John's-wort, Hypericum humifusum, and Pale Sedge, Carex pallescens, were new borough records and were thought to have derived from seed dormant in the seed bank since the previous coppice had been cut, more than 50 years ago (Bevan, 1994). In 1996, more than 100 Wild Service Tree, Sorbus torminalis, seedlings were recorded from beneath a parent tree in the central coppice. No such seedlings leave been recorded from the uncoppiced part of the wood.
Overall, 40 of the total species (23%) were introductions. These also were unevenly distributed, with the central coppice supporting only six. Many of the early arrivals were ruderal species, such as Oxford Ragwort, Senecio squalidus, American Willowherb, Epilobium ciliatum, and Canadian Fleabane, Conyza canadensis, with wind-dispersed seeds, taking advantage of the recently disturbed ground and high light-levels. The presence of Guernsey Fleabane, Conyza sumatrensis, in all three coppices is of interest. This exotic South American species has been recorded from mainland Britain only since 1974 (Wurzell 1988). Its remarkably rapid spread in the London area since 1987 has been documented by Burton (1995). Such early colonists are unhkely to persist as the coppices mature; indeed, the majority had gone by the end of the third year. The more shade-tolerant introductions could become a permanent part of the woodland flora. They include a number of trees and shrubs such as the two garden blackberries, Rubus armeniacus and R. laciniatus, Himalayan Cotoneaster, Cotoneaster simonsii, Holm Oak, Quercus ilex, Swedish Whitebeam, Sorbus intermedia, and Tutsan. Most of these are bird-sown from local gardens and have been recorded from around the edge of the wood as part of the local urban ecotone; coppicing will possibly accelerate their spread. Neither Cherry Laurel, Prunus laurocerasus, nor Sycamore, Acer pseudoplatanus, both notoriously invasive in some situations, currently shows any sign of causing problems in Coldfall Wood.
The presence of a small colony of Cowslips, Primula veris, in Coppice 2 is likely to be the result of a deliberate and well-intentioned introduction of wild-flower seed. Similarly, the occurrence of Flax, Linum usitatissimum, Canary-grass, Phalaris canariensis, and Bread Wheat, Triticum aestivum, in Coppice 1 probably resulted from the scattering of bird seed. None of these is likely to persist, but their presence demonstrates another characteristic of urban woods: the deliberate introduction of plants by local people. The occasional appearance of Daffodil, Narcissus cv, along pathsides in Coldfall Wood is another such example. They seldom flower after their first spring, owing to lack of light, but can persist vegetatively for many years.



Hybrid Spanish Bluebell in Coldfall Wood, North London.

In Queen's Wood, Highgate, Himalayan Honeysuckle, Leycesteria formosa, is well established in another recently cut coppice and is present elsewhere in the wood. Tutsan, which was recorded from the wet coppice in Coldfall Wood (Coppice 2), is widespread in Queen's Wood along the banks of a stream and elsewhere. Both these ornamental plants are likely to have originated from local gardens as bird-sown introductions, although Tutsan was known from around Highgate as a native species up to the mid 18th century. As in Sheffield, Highclere Holly is widespread in both Queen's and Highgate Wood. The Spanish Bluebell hybrid, Hyacinthoides non-scripta x H. hispanica, is also frequent in the Haringey Woods and is now more commonly seen than either of its parents. A mature Indian Horse-chestnut, Aesculus indica, originally planted on the western boundary of Queen's Wood, has recently given rise to numerous seedlings which form a conspicuous component of this woodland's urban ecotone (Clement & Foster 1994).

Surfaced paths

Woodland footpaths in Sheffield are surfaced with alkaline furnace ash, which raises the pH of the surrounding soil from 4.0 to around 6.6. This has the effect of producing hnear bands of untypical vegetation. Closest to the path is a discontinuous belt of ruderal species such as Annual Meadow-grass, Poa annua, Greater Plantain, Plantago major, and Chickweed, Stellaria media, growing on soil of pH >6. These are flanked by the nutrient-demanding, tall herbs: Wood Avens, Geum urbanum, Nipplewort, Lapsana communis, Enchanter's nightshade, Circaea lutetiana, and the grass Giant Fescue, Festuca gigantea, which elsewhere in the Wood are limited to flushed sites such as stream sides (pH5-6). The belt of altered soil is 1-3m wide on each side of the path before the normal ground flora of Creeping Soft-grass returns (pH4-5). Over 70% of people using the wood are accompanied by a dog. These animals may play a part in distributing the seeds of those path-side plants which have hooked or sticky fruits.

Discussion

Ancient woodlands in cities can be seen to be playing their part in one of the great themes of urban ecology - the proliferation of introduced species. How should this phenomenon be regarded? Urban woods must not be managed as imitation countryside and sanitised by the removal of invading species. These old woods are adapting to their new surroundings in a most natural way, by selecting from the incoming propagules those species most suited to the prevailing conditions. A beautiful example of ecology in action. It is their destiny to evolve in harmony with human horticultural taste and fashion. Similar phenomena to those recorded here must be taking place in urban woods all over the country.

Acknowledgement

David Bevan wishes to thank Dr David Corcoran for helpful discussions, computing advice, and help with preparation of the London maps.

References

Bevan, D 1992 The Natural History of Haringey's Ancient Woodlands. Lond. Nat 71: 9-20
Bevan, D 1994 Coppicing Haringey's ancient woods. Urban Wildlife News 11 (2):8
Burton, RM 1995 The present distribution of Guernsey fleabane Conyza sumatrensis in the London Area. Lond. Nat 74: 169-170.
Clement, EJ, & Foster, MC 1994 Alien plants of the British Isles. BSBI London
Jones, M 1986a Ancient woods in the Sheffield area: the documentary evidence. Sorby Record 24: 7-18
Jones, M 1986b Sheffield's ancient woods. Sheffield Hallam University
Spencer, JW 1986 Inventory of ancient woodlands-Greater London (Provisional). Nature Conservancy Council, Peterborough
Wurzell, B 1988 Conyza sumatrensis (Retz.) E. Walker established in England. Watsonia 17: 145-148

Oliver Gilbert has just retired as Reader from the Department of Landscape, Sheffield University. His interests embrace all aspects of urban ecology and the British lichen flora.

David Bevan is the Conservation Officer of the London Borough of Haringey. He is a past president of the London Natural History Society and has a particular interest in urban woodland.

You can e-mail me at: conserving.bevan@virgin.net

Last maintained 10th Jan 1998


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