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Is there a limestone dry stone walls community?
JOHN PRESLAND, 175c Ashley Lane, Winsley, Bradford-on-Avon,
Wiltshire BA15 2HR
Introduction
Earlier items on this site and articles elsewhere (Presland 2008a) have
described the dry stone wall plant and lichen community on limestone dry stone
walls in the West Wiltshire parish of Winsley and compared it with that of
mortared walls in the same area. The findings raised questions as to whether the
flora described there for dry stone walls represents a distinct type of plant
community specific to these walls, or whether it is better regarded as relating
to a community of walls in general, or even of a wider range of stone-based
environments. These issues are addressed below in a recently published article
(Presland 2008b) which is summarised below.
Comparing different walls
The first question to ask is how far the Winsley community is typical of dry
stone walls in general. This is difficult because of the scarcity of relevant
studies. Two studies of walls in general in Malmesbury, a North Wiltshire town
also in the Cotswold area (Wiltshire Trust for Nature Conservation 1986; Oliver
and Wall 2004), found all the flowering plants of Winsley dry stone walls,
except for Geranium pyrenaicum, all the ferns except for Polypodium
interjectum and some of the bryophytes and lichens, groups which did not
receive as much attention, so some of the others could have been overlooked. Dry
stone walls were not looked at specifically Likewise, Grose (1957) lists much
the same vascular plants on limestone walls in Wiltshire, but again does not
distinguish dry stone walls or the Cotswold area. One of the earlier of my
articles (Presland 2008a) compared the Winsley findings with those of Payne
(1989) in the Mendips, which provided further support for the notion of a
limestone dry stone wall community. However, we plainly need more information to
reach reliable conclusions on the typicality of Winsley dry stone walls.
The next question is whether the plants I recorded represented a community
distinct from that which grows on mortared wall. My 2008a article found the
floras were very different, and some support for this was reported from Payne’s
study. Again we need more data.
There is also the question of whether the findings apply to all dry stone walls
or only those made of limestone. The only study located of dry stone wall flora
on acid walls was in the Shetlands (Williams 1988). It also noted differences
between dry stone and mortared walls, but the lists of plants are completely
different from those recorded in Winsley. Furthermore, there was only one
species (Sedum rosea) which is characteristic of rocky substrates, all the
others being invaders from other habitats. Clearly it is not the community found
in Winsley or on dry stone walls studied elsewhere in the Cotswolds or in the
Mendips. It is likely that other studies of acid dry stone walls would produce
similar results, though there is a need to check this by surveys elsewhere.
To summarise the argument so far, it is a reasonable hypothesis that limestone
dry stone walls host a flora which is different from that of limestone mortared
walls and different form that of any kind of wall made from acid rocks.
Is there a distinctive plant community on limestone dry stone walls?
Even if the flora of dry stone walls is different from that of mortared walls,
and the flora of limestone dry stone walls different from that of their acid
counterparts, it does not follow that the limestone dry stone wall flora is
unique. It could still be regarded as part of a wider community type not
necessarily confined to walls, but including other rocky environments.
Unfortunately, this has been little investigated. The detailed classification of
British plants communities accomplished in recent years (Rodwell 2000) has not
identified an association for this environment. Is it possible, though, that it
could be encompassed by communities or subcommunities identified by Rodwell for
rocky environments? There are three possibilities - OV39b, OV41a and OV42, all
of which have walls and, at least by implication, mortar mentioned as among
their habitats, though there is no reference to dry stone walls. Each is
compared below with the Winsley dry stone wall flora, using Rodwells’s criteria
for identifying constant species of the community (occurring in 61% or more of
samples of that community) and associated common or frequent species (41% or
more samples). More detailed comparative information can be found in the fuller
article.
OV39b - This has Homalothecium sericeum and Sedum acre as
constants, as does the Winsley dry stone wall community. Porella platyphylla
is another constant and is frequent in Winsley. Saxifraga tridactylites
is frequent in both communities. This leaves twelve constants and four frequent
species of OV39b neither constant nor frequent on Winsley dry stone walls. Five
Winsley constants and one frequent species are neither constant nor frequent in
OV39b. Plainly, there is a small overlap in the defining species, but there are
marked differences between the two communities and no clear common element in
the habitat apart from calcareous rock. It is significant that Asplenium
trichomanes, constant in OV39b, is locally frequent on mortared walls in
Winsley but has not colonised the dry stone walls at all.
OV41a - This has no constant species, whereas Winsley dry stone walls have
seven. Of these seven, only Homalothecium sericeum and Tortula muralis
are frequent species in OV41a. None of the three Winsley frequent species are
either constant or frequent in OV41a. The two communities seem to have little in
common. The only common element in the habitats is rock, which apparently does
not even need to be calcareous in OV41a.
OV42 - The only constant is Cymbalaria muralis, which was not observed at
all on the Winsley dry stone walls. The two frequent species are
Homalothecium sericeum and Schistidium apocarpum, both constant in
Winsley. This leaves five Winsley constants and three Winsley frequent species
neither constant nor frequent in OV42. The differences between the two
communities are marked and the only common feature in the habitats is walls,
which, in OV42 do not have to be calcareous and do not have to be dry stone
walls - indeed, there is an implication that the OV42 walls are mortared.
Cymbalaria muralis is extremely common on mortared walls in Winsley, but has
not appeared at all on the dry stone walls.
The Winsley dry stone wall community, it seems, does not fit the descriptions of
any of these Rodwell communities. Some plants characteristic of those
communities have not colonised the dry stone walls even though available in the
area to do so, so can be regarded as not part of the Winsley dry stone wall
community. It looks, therefore, as though a new community type is needed to
accommodate the latter. The possibility that it could be a new subcommunity of
one of the existing OV communities cannot, however, be dismissed.
Implications
The data above point in the direction of the existence of a distinctive plant
community on limestone dry stone walls. This contrasts with Darlington’s (1981)
claim that it is difficult to envisage a wall as a plant environment without
taking into account what happens in the surrounding area and especially in the
immediate vicinity, and it might be better to regard walls as specialised
features of larger environments. However, he shows little awareness of dry stone
walls, and the flora of those in Winsley show hardly any relationship to the
flora of the surrounding area, apart from occasional invasion of Geranium
pyrenaicum, and appears to be an ecological entity in itself.
In view of the paucity of evidence, it can, at this point, be only a hypothesis
that there is an identifiable plant community which is characteristic of
limestone dry stone walls but not of other habitats. The studies referred to
here could be the first attempt to describe such a community in detail. On the
basis of the Winsley survey findings, Cotswold and Mendip limestone dry stone
walls could be seen as having a plant community consisting of crustose lichens
(including Caloplaca aurantia, Caloplaca citrina, Lecanora campestris,
and Aspicilia (Lecanora) calcarea), Homalothecium sericeum, Tortula
muralis, Grimmia pulvinata, Schistidium apocarpum, Bryum capillare, Geranium
lucidum, Saxifraga tridactylites, Sedum acre, and Porella platyphylla,
with occasional Geranium pyrenaicum, Orthotrichum anomalum, Rhynchostegium
confertum, Polypodium interjectum and Xanthoria parietina. Other
species might be regarded as occurring less commonly under particular conditions
- such as Encalypta vulgaris, Erophila verna, Sedum rupestre, Galerina
pumila, Ceterach officinarum, Phyllitis scolopendrium, Umbilicus rupestris,
and Cladonia pyxidata.
How far the pattern described here applies more widely and how the range of
plant communities of dry stone walls fit into the National Vegetation
Classification needs clarifying by a great deal more investigation.
References
DARLINGTON, A. (1981) Ecology of Walls. London: Heinemann Educational.
GROSE, D (1957) The Flora of Wiltshire. Wiltshire Archaeological and
Natural History Society, Devizes.
OLIVER, J. E. & WALL, J. (2004) Walls of Malmesbury. Unpublished lists of
plants recorded on walls in Malmesbury.
PAYNE, R. M. (1989) The flora of walls in the Chew Valley. Somerset
Archaeology and Natural History 133: 231-242
PRESLAND, J.L. (2008a) The flora of walls: Dry stone versus mortared.
BSBI News 108: 7-11.
PRESLAND, J. L. (2008b) Is there a limestone dry stone walls Community? BSBI
News 109: 9-12.
RODWELL, J. S. (ed.) (2000) British Plant Communities Volume 5: Maritime
Communities & Vegetation of Open Habitats. Cambridge: Cambridge University
Press.
WILLIAMS, L. (1988) Observations on the flora of wall habitats on Yell,
Shetland, in J. A. Fowler (Ed.) Ecological Studies in the Maritime Approaches
to the Shetland oil Terminal: Report of the Leicester Polytechnic to Shetland,
August 1986 and July 1987. Leicester Polytechnic, Leicester.
WILTSHIRE TRUST FOR NATURE CONSERVATION (1986) The Walls of Malmesbury: a
record of the work of the Biological Recording Group, 1986. Devizes:
Wiltshire Trust for Nature
POSTSCRIPT
Since the article was written, the report of Betts Ecology on dry stone walls in
the Mendips has come to my notice. Its findings are summarised
here. This study identifies different dry stone wall communities as examples of
a number of Rodwell communities. However, it appears to do this by envisaging
the dry stone wall flora not as an entity itself, but as part of several wider
communities encompassing the surrounding habitats. It is not, therefore,
necessarily at variance with the conclusions here. It is just not examining the
same communities. A detailed comparison of the Winsley and Betts studies could
be of considerable interest. I hope to undertake this when time and energy
allow.
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