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OBSERVATIONS ON THE FLORA OF WALL HABITATS ON
YELL, SHETLAND By Leslie Williams From ECOLOGICAL STUDIES
IN THE MARITIME APPROACHES TO THE SHETLAND OIL TERMINAL 1986-1987: Report of
the Leicester Polytechnic Expeditions to Shetland, August 1986 and July 1987. Edited
by J.A. Fowler. The David Attenborough Laboratories, School of Life Sciences, Leicester
Polytechnic, Scraptoft, Leicester LE7 9SU, June 1988 Introduction "The flora of
wall habitats has received little attention in the past and the bibliographies
in Spence (1979) and in Berry and Johnston (1980) do not list any studies from
Shetland. Elsewhere studies have included that of Segal (1969) on European
walls, Rishbeth (1948) on Cambridge walls, Woodell and Rossiter (1959) on
Durham walls and Kent (1961) on Middlesex walls. Factors affecting the flora of walls include
aspect, construction, shading, moisture content and the type of adjacent habitats
which have an important role in providing species to colonise the walls. It is
difficult to demonstrate how much influence each of these has and in some cases
difficult to actually measure the factors involved. Previous studies in Britain have therefore
been confined to simple observations of the flora of walls in an area. This
survey followed a similar line of enquiry. Methods Observations on the
flora of walls in the Graveland Peninsula, Yell, Shetland and some to the east
of Whale Firth were undertaken between 1-11 July 1986. A full vascular plant
list was made for each wall, while the presence of lichens, mosses and ferns
were noted. Records were made of the wall construction using four categories.
These were drystone walls built of individual stones and without mortar;
drystone walls with mortared copings (copings or coping stones are the
finishing set of protective stones on the top of a wall); mortared walls, some
being abandoned crofts where the roof had been removed; and retaining walls in which
drystone walls retained a bank of peat or soil on one side. The type of coping
was noted and details on the length, height, aspect, condition and any unusual
features of the wall, together with a sketch map of their location. Lengths were spaced as the method did not justify
the use of accurate measurements. Where possible, both sides of the wall were
examined. Results Forty-eight walls totaling
about 2,027 m in length were examined. Twenty-four walls (c. 1,437 m) were
drystone walls, two walls (c. 35 m) both in the vicinity of Windhouse (HU
489919) were drystone walls with mortared copings, 18 walls (ca 320 m) were
mortared; and there were four retaining walls (c. 235 m). Two unusual coping
styles were noted during the survey. At Gardins, West Sandwick (HU 447 882)
coping stones were piled onto the top of a conventional drystone wall,
something like a thick version of the Galloway style. Peat under-copings were
found in a drystone wall at Haa of Graveland (HU 462942) and these were
colonised by vascular plants. A summary of the results is given in Table 1.
Note that 'other lichens' are probably under-recorded. Poa pratensis/P.
subcaerulea were not keyed out further in this survey and are two members of a
group of closely related species, the third one being Poa angustifolia which is
the more likely of the three species to colonise walls in Britain (Darlington, 1981) but appears to be absent from Shetland.
P.subcaerulea is more common in Shetland (Berry and Johnston, 1980) and on Yell
(Thurlow and Fowler, 1986) than P. pratensis. Hubbard (1984) listed walls as
one of the habitats on which P. pratensis is found, though Darlington (1981)
states that it is rare on walls. The low
height of the specimens found in the Yell walls, seldom exceeding 30-40
cm and the sharply pointed glumes suggests that P. subcaerulea was present.
General observations
were:
a.
Vascular
plants were much more common on mortared than drystone tails where they were
only found on accumulations of peat or soil.
b.
On all
walls vascular plants were more frequent towards the top of the wall.
c.
Drystone
walls with mortared copings had a flora similar to walls which were mortared
throughout.
d.
The
flora of retaining walls was similar to that of the adjacent moor or habitat
that they retained.
e.
The lichen Ramalina sp. was seldom found below a height of 1
m above ground level. It was more frequent on the north and west sides of walls.
Discussion The richer flora of
mortared walls compared with that of drystone walls and the absence of vascular
plants from drystone walls except where soil or peat accumulate have been noted
elsewhere (Darlington 1981). At two locations where drystone walls merged into
mortared walls, a change that was not obvious externally, the junction was
marked by the appearance of grass growth on top of the mortared section. The
tendency of vascular plants to be found only towards the top of walls, also
appears to be universal; and in Yell the plants were usually rooted in
weathered mortar or in soil/peat where present. The phenomenon is largely
related to the preference of vascular plants for horizontal rather than
vertical surfaces and is directly related to the greater ability of horizontal surfaces
to retain soil and water. In particular, the type of coping on the wall top had
an important effect upon the wall flora by providing such horizontal surfaces
and helping to retain water within the rooting substrate. In this respect the
construction of the wall just below the coping was probably of more importance
than the coping stones themselves, for the lower layer is usually as flat as
possible in order to make a stable base for the coping stones. Kent (1961) found
that 65% of the species on Middlesex walls were plants of cultivated and waste
ground, waysides and parks, as expected in a built-up county. Other studies confirm the importance of
nearby habitats as a seed source for walls. On Yell the vascular plants found on
the walls were all present in the adjacent moorland; though some common
moorland species such as Heather (Calluna vulgaris) were absent from the wall
flora. Of the 20 most common Middlesex wall plants, many of which were also
common on walls elsewhere in mainland Britain, none were noted on the Yell
walls. The preference of
Ramalina sp. lichens for the north and west sides of walls is a general feature
of walls elsewhere. The north side usually receives less solar insolation than
the south side and hence temperature extremes are less and more humid
conditions prevail on the north sides. West sides often receive more
rainfall. The species present was probably
all Ramalina siliquosa and its absence from wall surfaces within about 1 m of
ground level may be explained by the observation in Dobson (1981) that it is
eaten by sheep in Shetland. The species is common on hard siliceous rocks in
maritime districts, such as the garnetiferous mica-plagioclasegneiss (Mykura
1976) from which almost all the Yell walls are built. A retaining wall at
Grimister (HU 467932) appeared to have been recently rebuilt and a good search
revealed only seven plants of Ramalina along the 145 m of west facing wall
which varies in height between 1, 3-1, 5 m. While the stone from which the wall
was rebuilt probably carried this species, future counts of this wall could
provide information of the colonisation of this species with time. The effect of the
wall on other flora was noted at Windhouse (HU 489919) where six very stunted
Sycamore (Acer pseudoplatanus) of planted origin on the south side of the wall,
had been protected from the wind to the height of the wall (1.6 m) but had been
unable to grow higher. At Uphouse (HU
468931) an Elder (Sambucus nigra) had grown on the south side of a wall,
actually using the face stones of the wall for support and protection. Starlings
(Sturnus vulgaris) were nesting in the Windhouse drystone walls. Other species seen
using walls were the Wren (Troglodytes troglodytes, Wheatear (Oenanthe
oenanthe), rabbits and sheep. No methods are yet available for dating walls
using biological indicators, as can be done with hedgerows in mainland Britain.
Apart from physical factors as summarized in Brooks (1977), lichens may prove to
be the most useful indicators, but lichen growth may depend upon many environmental
factors and the majority of drystone walls require rebuilding every few
centuries so this approach may be of limited use. Acknowledgements I would like to
thank Jenny Bott and Gill Wakeley for the identification of Ramalina siliquosa.
References Berry, RJ and
Johnston, JL (1980) The Natural History of Shetland, Collins. Brooks, A (1977)
Drystone Walling, British Trust for Conservation Volunteers. Darlington, A (1981)
Ecology of Walls, Heinemann. Dobson, F (1981)
Lichens, An Illustrated Guide, Richmond Publishing Co. Hubbard, CE (1984)
grasses, Penguin Books. Kent, DH (1961) The flora
of Middlesex walls, London Naturalist, 40: 29-43 Mykura, W (1976)
Orkney and Shetland, British Regional Geology, Natural Environment Research
Council, HMSO Rishbeth, J (1948)
The flora of Cambridge walls, J Ecology, 36: 136-48 Segal, S (1969)
Ecological Notes on Wall Vegetation, The Hague; Junk. Spence, D (1979)
Shetland's Living Landscape, A study in Island Plant Ecology, The Thule Press,
Shetland.
Table 1. Number and percentage of walls colonised by
Flora, Yell, Scotland
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