|
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.
|
