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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

 

 

                                              Wall Type

Species

Drystone
wall

 

24 walls,
1,437 m

Drystone wall with mortared
coping
2 walls,
35 m

Mortared
walls

 

18 walls,
320 m

Retaining wall

 

 

4 walls
235 m

All walls

 

 

48 walls,

2,027 m

 

No.

%

No.

%

No.

%

No.

%

No.

%

Lichen
Ramalina sp.

22

92

1

50

11

61

1

25

35

73

Other lichens

6

25

2

100

5

28

1

25

14

29

Moss

Musci

8

33

1

50

6

33

3

75

18

38

Fern

Pteridiphyta

1

4

 

 

 

 

 

 

1

2

Mouse-ear Chickweed
Cerastium fontanum

 

 

1

50

1

6

 

 

2

4

Chickweed
Stellaria media

 

 

 

 

1

6

1

25

2

4

Procumbent Pearlwort
Sagina procumbens

1

4

2

100

1

6

1

25

5

10

Rose-root
Sedum rosea

 

 

1

50

 

 

 

 

1

2

Sheeps Sorrel
Rumex acetosella

 

 

 

 

2

11

2

50

4

8

Sorrel
Rumex acetosa

3

13

 

 

2

11

2

50

7

15

Stinging Nettle
Urtica dioica

1

4

 

 

1

6

 

 

2

4

Heath Bedstraw
Galium palustre

2

8

 

 

1

6

 

 

3

6

Daisy
Bellis perennis

 

 

 

 

1

6

 

 

1

2

Soft Rush
Juncus effusus

 

 

 

 

 

 

1

25

1

2

Viviparous Fescue
Festuca vivipara

1

4

 

 

2

11

 

 

1

6

Red Fescue

Festuca rubra

2

8

1

50

3

17

 

 

6

13

Perennial Rye-grass

Lolium perenne

 

 

 

 

1

6

 

 

1

2

Poa sp

3

13

 

 

2

11

 

 

5

10

Poa trivalis

 

 

 

1

50

1

6

 

2

4

Poa  subcaerulea/pratensis

4

17

2

100

13

72

1

25

20

42

Cocksfoot

Dactylis glomerata

 

 

1

50

 

 

 

 

1

2

Wavy Hair-grass

Deschampsia flexuosa

2

8

 

 

 

 

 

 

2

4

Early Hair-grass

Aira praecox

7

29

 

 

13

72

2

50

22

46

Yorkshire Fog

Holcus lanatus

 

 

 

 

2

11

1

25

3

6

Sweet Vernal-grass

Anthoxanthum odoratum

 

 

 

 

1

6

1

25

2

4

Agrostis sp.

 

 

 

 

4

22

1

25

5

10

Creeping Bent

Agrostis stolonifera

 

 

 

 

1

6

 

 

1

2