Home Forum Booklet Gallery Mendips Shetland Bute West Wilts Dry v Mortar Misc data Links

 
ECOLOGICAL REPORT - LIFELINES DRY STONE WALL SURVEY
for the MENDIP HILLS AONB Service 2008
Betts Ecology, Bank House, Martley, Worcester WR6 6PB
T +44 (0)1886 888445; F +44 (0)1886 888782; E nature@bettsecology.com  www.bettsecology.com

A SUMMARY by John Presland

The report

This report describes an ecological survey of 74 dry stone walls in the Mendip Hills Area of Outstanding Natural Beauty, where they are distinctive landscape features above 200 metres, but rare lower down. The AONB consists largely of an exposed, high, bare plateau of carboniferous limestone with intensively managed calcareous grassland and a small number of arable fields. Observations were recorded of the direction each wall faced (its aspect), its altitude, height, the total number of plant and lichen species on each and the numbers of species of vascular plants, bryophytes (mosses and liverworts) and lichens separately. Statistical devices were used to clarify relationships between the various kinds of data collected. The work reported is, as far as I know, the most comprehensive study of the vegetation of dry stone wall plants and lichens so far atttempted. The report is downloadable in full at http://www.mendiphillsaonb.org.uk/publications/up_175815_lifelines-post_submission.doc

Overall results

The dry stone walls ran mostly north/east to south/west or north/west to south/east. They ranged from tall and narrow recently restored walls through to short and wide derelict walls. Wide and low were more numerous than tall and narrow, and reduced height tended to be accompanied by increased width, suggesting that a large proportion of walls are in a state of disrepair or collapse, and that the fallen stone has been allowed to increase the width of what is remaining, rather than been removed.

No relationship was found between wall aspect or altitude and the numbers of total species, vascular plants, bryophytes and lichens. The lower the wall, the greater was the number of species recorded, and this applied also to vascular plants and bryophytes separately, but not to lichens. Increasing wall width was associated with a greater number of total species and vascular species, but not bryophytes or lichens.This relationship was thought to be due to enrichment by organic matter accumulating in the derelict and collapsed walls providing an environment suitable for a wider range of vascular plants. .

Overall, 149 species were recorded, though 57 (38%) of these were recorded only once or twice. There were 75 vascular plants (17 grasses, 7 ferns and 51 flowering plants), 51 bryophytes (23 acrocarpous mosses, 16 pleurocarpous mosses and 3 liverworts) and 23 lichens. Of these, 36 (24 %) species occurred on only one wall with another 21 (14 %) species on only two walls. The full list of species and their frequencies and abundance values are given in Appendices, while a map in the main text shows the number of observations at each site.

In studying the plant and lichen findings, it should be realised that no definition is given of a dry stone wall. It looks as though the the scope is very wide, including any plant or lichen on or near any structure which is or was a dry stone wall. There was “evidence of mortar” in some, so that the results could refer partly to mortared wall flora. It is not stated whether or not retaining walls without mortar or walls with a dry stone structures with concrete or a layer of mortared capping stone on top are included, so they might be, which would also affect the flora. Probably, this broad approach reflected a wish to emphasise biodiversity. However, comparison with other studies, which focus on dry stone walls as more distinct communities, needs to take it into account.

Further analysis and findings

The findings were also related to the frequency concepts from the National Vegetation Classifcation (Rodwell 1991 et seq), where species are called constants if they occur in 61% or more of samples of a community and frequent species if they occur in 41% or more but less than 61%. On these Mendip walls, the constants were the bryophyte Homalothecium sericeum and the lichen Verrucaria baldensis, while Schistidium apocarpum sensu lato was a frequent species. The less frequent category called occasional was represented by Bryum capillare var. capillare, Caloplaca flavescens, Hypnum cupressiforme, Neckera complanata, Rubus fruticosus, Tortula muralis var. muralis, Verrucaria nigrescens and Zygodon viridissimus. Leaf litter was scarce and bare rock a constant.

The authors further analyse the results to identify five plant communities, relating them to the National Vegetation Classification and to separate observations made in upper (wall top), lower and middle zones of the walls:

Group 1 - A pioneer community of crustose lichens covering large expanses of bare, inhospitable wall surface in a mainly open aspect. The substrate was predominantly limestone with occasional sandstone, a few walls contained mortar and one wall had been infilled with breccia. Constants were the lichens Caloplaca flavescens, Verrucaria baldensis and Verrucaria nigrescens and the moss Homalothecium sericeum. The moss Schistidium apocarpum sensu lato was frequent. Occasional species were the crustose lichens Aspicilia calcarea, Caloplaca teicholyta and Placynthium nigrum and the small acrocarpous mosses Grimmia pulvinata, Tortula muralis var. muralis and Zygodon viridissimus. There were 63 plant species (22 vascular; 25 bryophytes; 16 lichens) recorded in total, of which 14 were recorded only once. There was little difference between the numbers and composition of species recorded in each wall zone. It is claimed that the community described best matched the National Vegetation Classification OV42 Cymbalaria muralis community, wall crevice vegetation typical of sunny communities.The dominant vegetation within 1 m of wall was Arrhenatherum elatius, Cirsium arvense, Dactylis glomerata, Holcus lanatus, Lolium perenne and Urtica dioica.

Group 2 - A species poor community with abundant bryophytes and lichens and a few vascular plants developing on mostly dilapidated limestone dry stone walls in partial shade. The bryophytes Homalothecium sericeum and Schistidium apocarpum sensu lato and the crustose lichen Verrucaria baldensis were constants. Frequent species were Bryum capillare var. capillare, Neckera complanata and Verrucaria nigrescens and there were the occasional bryophytes Hypnum cupressiforme, Tortella tortuosa, Tortula muralis var. muralis and Zygodon viridissimus and Ivy (Hedera helix subsp. helix) and Bramble (Rubus fruticosus). The species composition was similar in the different zones. There was a total of 102 plant species (50 vascular; 34 bryophytes; 18 lichens), 17 recorded only once. Leaf litter was scarce and bare rock constant. The walls were mostly overshadowed by mature Sycamore, Hawthorn and Beech, but a few were exposed to the elements. Tall plants of waste places often shaded the lower zone. Dominant vegetation within 1 m of wall was Arrhenatherum elatius, Dactylis glomerata, Holcus lanatus, Lolium perenne, Pteridium aquilinum, Rubus fruticosus and Urtica dioica. The best match in the National Vegetation Classification was held to be OV27 Chamerion angustifolium community, a tall herb weed community that exploits open ground.

Group 3 - A community of moderate species richness with an extensive and diverse bryophyte cover dominated by pleurocarpous bryophytes with occasional vascular plants growing on moderately shaded, mostly limestone, dry stone walls in mostly stock-proof condition, one containing mortar. Lichen cover was scarce. The bryophyte Homalothecium sericeum was constant, while frequent bryophytes were Amblystegium serpens var. serpens, Brachythecium rutabulum, Eurhynchium praelongum, Hypnum cupressiforme, Neckera complanata and Thamnobryum alopecurum. Occasional bryophytes were Bryum capillare, Plagiomnium undulatum, Porella platyphylla, Schistidium apocarpum sensu lato, Tortella tortuosa and Zygodon viridissimus and occasional vascular species Geranium robertianum, Glechoma hederacea, Rubus fruticosus and Urtica dioica. There were 80 plant species (35 vascular; 35 bryophytes; 10 lichens) recorded, 19 only once. There was little difference between the different zones, with the middle zone not much in evidence. Leaf litter was scarce and bare rock constant. Dominant vegetation within 1 m of wall was Brachypodium sylvaticum, Crataegus monogyna, Mercurialis perennis, Rubus fruticosus and Urtica dioica. The best matched National Vegetation Classification community was held to be W8e Fraxinus excelsior - Acer campestre - Mercurialis perennis woodland: Geranium robertianum subcommunity, a woodland community with an extensive and diverse bryophyte cover.

Group 4 - A species poor community dominated by bramble scrub on neglected limestone dry stone walls with a moderate degree of shade. Rubus fruticosus was constant, Crataegus monogyna and Sambucus nigra frequent, and Homalothecium sericeum occasional. There were nine plant species in total (3 vascular; 5 bryophytes; 1 lichen). The only other species present, at low frequency, were Eurhynchium praelongum, Neckera complanata, Tortula muralis var. muralis, Verrucaria baldensis and Zygodon viridissimus. No bare rock or leaf litter was recorded. Dominant vegetation within 1 m of wall was Bromus hordeaceus, Cerastium fontanum, Chamerion angustifolium, Crataegus monogyna, Cynosurus cristatus, Festuca rubra, Galium aparine, Holcus lanatus, Lolium perenne, Rubus fruticosus, Sambucus nigra and Trifolium repens. This was said to match the National Vegetation Classification W21a Crataegus monogyna - Hedera helix scrub: Hedera helix - Urtica dioica subcommunity, a woody community that develops and establishes on many kinds of neglected ground.

Group 5 - A shrubby/woody plant community dominating derelict limestone walls in a shaded position and often with earth and humus. The constants were the bryophytes Hypnum cupressiforme and Mnium hornum. Frequent species were Cladonia macilenta, Isothecium myosuroides, Lonicera periclymenum, Oxalis acetosella, Polystichum setiferum and Rubus fruticosus. Occasional species were the bryophytes Eurhynchium praelongum and Orthotrichum diaphanum and the vascular species Deschampsia flexuosa, Digitalis purpurea, Holcus mollis, Hyacinthoides non-scripta, Polypodium interjectum, Pteridium aquilinum and Vaccinium myrtillus. Zones were hard to distinguish. Dominant vegetation within 1 m of wall was Corylus avellana, Holcus mollis, Hyacinthoides non-scripta, Oxalis acetosella, Pteridium aquilinum and Rubus fruticosus. The best National Vegetation Classification match was held to be W10 Quercus robur - Pteridium aquilinum - Rubus fruticosus woodland, a seminatural woodland community.

The different communities are related in the process of vegetational succession, with, for instance, Group 1 acting as a precursor of Group 3. Lichens and some bryophytes act as pioneer species on wall surfaces most exposed to the elements. They are well adapted to such wall habitat conditions as desiccation, instability, extreme temperatures and little or no nutrient which discourage the growth of vascular plants. . Eventually, humus from decaying lichens and bryophytes provides a rooting medium for these more demanding plants and some protection for them against drought. Overall, however, lichens and bryophytes remain the dominant members of the community.

Data are also provided on animal life, or evidence of its existence, noted during the survey and lists of animals likely to be found there culled from other sources. They specifically mention: use of the walls by medium and small sized mammals, some of whom can use the crevices for protection or homes; invertebrates in moss cushions and encrusting lichens or in the gaps between the loose stones and rock and within holes; and perches, shelter, foraging and feeding sites for birds.

Issues and conclusions

A number of associated issues are touched on in the report, either arising from the data, or from other sources or indicating the assumptions and beliefs of the authors. The data indicate that the dry stone walls of the Mendip Hills Area of Outstanding Natural Beauty constitute a distinctive habitat type for plant growth. They support fauna and flora of local, national and international importance. Many United Kingdom Biodiversity Action Plan species of conservation concern are likely to use the habitat for part or all of their life cycle. Numerous species pass through, across and along the walls. The most important walls for biodiversity and nature conservation are those intermediate between newly restored walls and derelict walls - both extreme habitats which provide unstable, disturbed and highly stressed conditions for the majority of species. Semi-derelict walls are likely to be more attractive to wildlife and provide more niches than a tightly-built or collapsed wall.

Comment and discussion based on other sources or opinion focus firstly on the importance of dry stone walls as potential links between species and habitats in the wider landscape and within a fragmented mosaic of important semi-natural habitats such as wildflower grassland, heathland and woodland. They believe that the most valuable dry stone walls for biodiversity and nature conservation are those within and adjacent to United Kingdom priority habitats. The dry stone walls cross many of the designated sites and have the potential to support wildlife. They may also have an important role to play in some species dispersal between the protected habitats. However, the habitat is under threat from ongoing vegetation succession, weathering, neglect and human impact, including destruction by insensitive works or lack of management. Its importance is insufficiently recognised. It has, for instance, received little attention in the United Kingdom Biodiversity Action Plans and very little reearch has been carried out to develop knowledge about its plant communities, most research having focused on mortared walls. Also, the National Vegetation Classification, the recognised standard for describing UK plant communities, makes little if any mention of dry stone wall plant communities, possibly because walls are man-made structures and there has been a lack of awareness of the nature conservation aspects of artificial urban habitats and the importance of these to local people for educational, recreational, cultural, health and spiritual reasons.

Recommendations

1. Repair dry stone walls rather than strip them down and re-build.
2. Carry out repairs in such a way as to preserve wildlife value, eg. stones should be replaced so that any bryophytes or lichens have a similar position and aspect to that on the original wall.
3. Undertake on-going maintenance, eg. remove woody growth like ivy, bramble and saplings.
4. Undertake an ecological survey to assess both the fauna and flora value of any wall before carrying out any major rebuilding or maintenance work.
5. Incorporate the appropriate management of all relevant species in the care of the dry stone walls.
6. Aim to establish buffer strips of at least 2 m of rough grassland along both sides of dry stone walls.
7. Carry out further research to consider the status of urban rock habitats.

Reference

Rodwell, J. S. (Ed) (1991 et seq.). British Plant Communities. Cambridge University Press,
Cambridge, UK.