COMPARING THE FLORAS OF LIMESTONE DRY STONE AND MORTARED WALLS IN WINSLEY, WEST WILTSHIRE
John Presland

Introduction

Another item by John Presland on this site (Dry stone wall flora in Winsley, West Wiltshire), describes a systematic study of the flora of limestone dry stone walls in a South Cotswold parish. A fuller account is available elsewhere (Presland 2008a). For comparison purposes, the flora of mortared limestone walls in the same parish was investigated. Full details of the investigation and its results is also provided elsewhere (Presland 2008b). It was not as systematic or comprehensive as the dry stone wall study, observations being reduced to those essential to make a useful comparison.

How do dry stone and mortared walls differ?

Dry stone walls are free standing and built upwards, without mortar, with stones in successive rows overlapping each other. Two horizontal structures like this are built with a gap between, leaning towards each other, and the centre is filled with stones of varying sizes to give a solid structure. At intervals long stones are, in most areas, laid wholly or partly across the wall to hold the two sides together. A line of coping stones is commonly laid on top, though this is uncommon in Winsley. Presland (2008a) provides more detail, illustrations and references, and there is also information on the home page of this site.

Mortared walls typically are built with more shaped and regularly rectangular stones held together by mortar. There are differences between mortared and dry stone walls which might result in differing floras. The mortar is a source of nutrients and water and provides anchorage for roots and rhizomes. The more regularly shaped stones of mortared walls fit closer together, also with effects on anchorage and availability of nutrients and water.

Key findings

Somewhat more that 40 species were found in the dry stone walls and approximately 50 species on the mortared walls. The fuller article (Presland 2008b) lists species present on only one of the wall types and species more common on one type than on the other.

In particular, the following plants were judged to be locally frequent on the mortared walls but were not noted at all in the dry stone wall survey:

Asplenium trichomanes (Maidenhair Spleenwort)
Campanula portenschlagiana (Adria Bellflower)
Cymbalaria muralis (Ivy-leaved Toadflax)
Lepraria lobificans (a lichen) - though it did occur on one non-retaining non-mortared wall in the mortared wall study area.
Parietaria judaica (Pellitory-of-the-wall)
Pseudofumaria lutea (Yellow Corydalis)

Plants common on the dry stone walls but absent from the mortared ones were:

Geranium pyrenaicum (Hedgerow Cranesbill)
Orthotrichum anomalum (Anomalous Bristle-moss)
Sedum acre (Biting Stonecrop)

Particularly striking was the observation that mosses were surprisingly rare, even Homalothecium sericeum, Tortula muralis, Grimmia apocarpa, Schistidium apocarpum, and Bryum capillare, all common on the tops of the dry stone walls. None of them occurred on more than three short stretches of these mortared walls.

The older mortared walls which almost alone hosted plants were almost certainly constructed of the same kind of stone as the dry stone walls studied, and could therefore be expected to have a similar flora. It is, therefore of interest to have found that some species occurred only on the dry stone walls, while others were on only the mortared walls. It appeared, also, that some species occurred at very different frequencies in the two environments. Perhaps a clinching comparison emerges from listing all the species which were judged to be at least locally frequent on mortared walls, which amounts to 11, and then identifying the 11 species which were recorded at the greatest number of sites on the dry stone walls. These are shown in the Table. There was no overlap at all, which is a strong argument for regarding the two communities as different.

Comparison of species on mortared and dry stone walls in Winsley

Can the findings be generalised?

The key question to ask here is whether these differences are typical of those which occur between the two types of wall or whether they are due to factors operating only locally.

The literature on the flora of walls contributes very little towards answering this question. A number of authors have described the flora and ecology of walls, both in general and in specific areas. General accounts have been provided by Segal (1969) and Darlington (1981), who both give detailed accounts of the ecology. However, hardly any of these sources make any distinction between mortared walls and dry stone walls and the latter may not be mentioned at all. A more recent publication (South Court Environmental Limited 1994) shows a similar neglect of dry stone walls.

Only two studies of relevance have been located. Both are reproduced in full on this site. One was carried out by Payne (1989) in the Mendips. Comparison of his findings with those in Winsley encourages the notion of distinct communities. The four species which Payne could most confidently identify as having a statistical preference for dry stone rather than other walls were Polypodium interjectum, Saxifraga tridactylites, Geranium lucidum, and Sedum acre. In the Winsley study, Polypodium interjectum and Sedum acre were found only on dry stone walls, while Saxifraga tridactylites and Geranium lucidum were common on dry stone walls and rare on the mortared. This represents a high level of agreement. He found Asplenium ceterach (now Ceterach officinalis), and Asplenium ruta-muraria to have a “marked relative aversion” to dry stone walls, which is also reflected in the Winsley findings. There is a puzzling discrepancy for Asplenium trichomanes, found by Payne to prefer dry stone walls but occurring only on mortared walls in Winsley. Payne did not include lichens and bryophytes in his study, so the comparison can only be limited. Williams (1988) also found differences between dry stone and mortared walls made of acid rocks on Shetland, but the flora was quite different anyway, so meaningful comparisons could not be made. It looks as though acid walls need to be looked at separately.

Implications

A case has been made for regarding dry stone and mortared limestone walls as distinct communities in at least two localities. How widely the distinction applies and which plants consistently feature in each list must await data on a range of walls in further places. The same applies to walls built with acid rocks. I am in contact with other people interested in this area, and would welcome hearing from anyone else who would consider carrying out surveys. A guide to conserving the flora of limestone dry stone walls is in print (Presland 2007), which describes the plants, vascular and non-vascular, likely to be found in such situations, and this may help others interested in carrying out the necessary further surveys in limestone areas - though not where the rocks are acid.

References

PAYNE, R. M. (1989) The flora of walls in the Chew Valley. Somerset Archaeology and Natural History 133: 231-242

PRESLAND, J. (2007) Conserving the Flora of Limestone Dry Stone Walls. Wiltshire Natural History Publications Trust, Salisbury.

PRESLAND, J. (2008a) Dry stone walls in Winsley. Wiltshire Botany 10: 23-28.

PRESLAND, J.L. (2008b) The flora of walls: dry stone versus mortared. BSBI News 108: 7-11.

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.