DEFINING AND CONSERVING LIMESTONE DRY STONE WALL COMMUNITIES

By John Presland

In a series of studies, all summarised on this site, I have proposed that limestone dry stone walls are occupied by a unique community of plants and lichens which had not previously been described. This proposal was based on studies of walls in the Wiltshire village of Winsley, which is in the extreme south of the Cotswolds. To confirm or refute it, data would be needed on limestone dry stone walls elsewhere. Two other studies have been located - the earlier one by Payne (1989 on this site) and the later one by Hill (2008, summarised on this site as the Lifelines Report, but not including her name). The former provides considerable support, but the latter has a methodology so different from mine that comparisons are difficult. However, I make the attempt below.

Full details of the structure of dry stone walls can be found elsewhere on this site, Briefly, they are walls constructed of two horizontal structures of overlapping stones, usually with interconnecting stone to keep them together and sometimes with capping stone bridging the top. The space between is filled with smaller stones. No mortar is used in the construction. In a booklet derived from Hill’s report Hardcastle and Nisbet (2008, summarised on this site) provide a similar description. Payne does not describe them. However, I think we can take it that all three studies concern basically similar structures, including walls where much of the original design has been lost with the passage of time but a wall structure is still recognisable.

Surveying the walls requires a definition of the plant community, and here we are immediately into difficulties. My own approach was to record only species clearly rooted on approximately the top two thirds of walls, since others could be regarded as belonging to the community adjacent to the wall rather than that of the wall itself. Thus, basically ground-rooted plants growing up through the wall, such as Cleavers (Galium aparine), were excluded. Ivy (Hedera helix) and Brambles (Rubus spp) were also excluded since they stem from roots in the ground, even though secondary roots may invade the wall. Furthermore, they suppress the more characteristically wall-growing plants. Payne doesn’t give information on this, but his records suggest that he included plants near the base of the wall, which would be likely to have some contact with ground moisture and nutrients. Hill, however, included a much wider variety of wall communities, encompassing more or less anything still identifiable as a wall, even if overgrown by the surrounding community. However, she does divide the whole into a number of subcommunities, more or less representing different stages in colonisation.

Firstly, let’s compare my findings with those of Payne. He recorded only vascular plants. He found the species with strongest preferences for dry stone walls were Polypodium interjectum (Intermediate Polypody), Saxifrage tridactylites (Rue-leaved Saxifrage), Geranium Iucidum (Shining Cranesbill) and Sedum acre (Biting Stonecrop), and all occurred on 45% or more of the 40 dry stone walls he studied. The last three occurred on 56% or more of the 27 Winsley dry stone wall sites, while Polypodium interjectum was found on 20%. Other species recorded in both surveys were Geranium robertianum (Herb Robert), Dactylis glomerata (Cocksfoot), Ceterach officinale (Rusty-back), Anthriscus sylvestris (Cow Parsley) and Lolium perenne (Perennial Rye-grass), though none of them are characteristically wall plants. A handful of further non-wall species occurred on one site but not the other. These included Hedera helix and Galium aparine, recorded in the Mendips, and present but not recorded in Winsley because they were interpreted as rooted in the ground. Some of the discrepancies could have been due to inclusion of the bottom third of the wall in the Mendips but not in Winsley. Others can be seen as occasional invaders that could crop up anywhere. The only significant discrepancies were the occurrence in the Mendips of Asplenium trichomanes (Maidenhair Spleenwort) and Cymbalaria muralis (Ivy-leaved Toadflax), neither of which were found at all on Winsley dry stone walls, despite being available to colonise from nearby mortared walls; and the identification of Asplenium ruta-muraria (Wall-rue) and Ceterach officinale as having an aversion to dry stone walls in the Mendips but occurring on a few such walls in Winsley. It could be, of course, that some apparent dry stone walls in both places contained concealed mortar. Overall, however there is a strong degree of agreement between the two studies.

Now, we turn to comparing Hill’s findings with mine. Hill described 5 communities on her 74 Mendip walls:

Group 1 - A pioneer community of crustose lichens covering large expanses of bare, inhospitable wall surface in a mainly open aspect. There were 63 species (22 vascular; 25 bryophytes; 16 lichens) recorded in total, of which 14 were recorded only once.

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. There was a total of 102 species (50 vascular; 34 bryophytes; 18 lichens), 17 recorded only once.

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 walls, one containing mortar. Lichen cover was scarce. There were 80 species (35 vascular; 35 bryophytes; 10 lichens) recorded, 19 only once.

Group 4 - A species poor community dominated by bramble scrub on neglected limestone dry stone walls with a moderate degree of shade. There were nine species in total (3 vascular; 5 bryophytes; 1 lichen).

Group 5 - A shrubby/woody plant community dominating derelict limestone walls in a shaded position and often with earth and humus. There were many bryophytes and a collection of woodland vascular plants.

Plainly, groups 1-3 were similar environments to those sampled in Winsley, and Group 4 is common in Winsley, though not included in the survey. If one looks at the seven most commonly found bryophytes in Winsley, all are present in all of Mendip Groups 1-3, suggesting that we were looking at the same community. A further range of bryophytes were found in the Mendips but not Winsley, though most of them have been identified in Winsley subsequently. Lichens were not included in the Winsley survey, but, again, most of those noted in the Mendips are now known at Winsley. There is one major flaw in this pattern of agreement - the vascular plants. Saxifrage tridactylites and Geranium lucidum, occurring on 56% and 84% of the 27 Winsley dry stone wall sites, were not recorded at all in the Mendip survey, while Sedum acre on 80% of Winsley sites was in less than 20% of Mendip sites for all three groups and overall. Polypodium interjectum was found in both studies at low frequency.

Turning to Payne and Hill, it is puzzling that, though both took place in the Mendips, the results were very different. Hill’s Groups 4 and 5 were not really recognisable in Payne, so he was presumably not counting them as dry stone wall communities. Even in the groups where one might expect correspondence there are very puzzling discrepancies. Payne found the species with strongest preferences for dry stone walls were Polypodium interjectum, Saxifraga tridactylites, Geranium lucidum and Sedum acre, and all occurred on 45% or more of the 40 dry stone walls he studied. Yet the middle two were totally absent from Hill’s data, and the other two were on less than 20% of all groups.

Plainly, there are differences between the three authors in their concepts of a dry stone wall community. Payne and Presland describe very similar communities, though Presland excludes plant growing near the base of the wall and plants rooted in the soil but growing through the soil, whereas Payne appears to include them. Hill’s thinking is much more inclusive. She identifies her Group 2 with Community OV27 in Rodwell’s National Vegetation Classification, which is a tall herb community, normally dominated by Chamerion angustifolium (Rosebay Willowherb). Group 3 is identified with W8e Fraxinus excelsior - Acer campestre - Mercurialis perennis (Ash - Field Maple - Dog’s Mercury) woodland: Geranium robertianum (Herb Robert) subcommunity, a woodland community with an extensive and diverse bryophyte cover. Group 4 she identifies as 21a Crataegus monogyna - Hedera helix (Hawthorn - Ivy) scrub: Hedera helix - Urtica dioica (Ivy - Nettle) subcommunity, a woody community that develops and establishes on many kinds of neglected ground. Group 5 she matches with W10 Quercus robur - Pteridium aquilinum - Rubus fruticosus (Peduncuate Oak - Bracken - Bramble) woodland, a seminatural woodland community. Plainly, none of these are specifically wall communities, though Groups 2 and 3 do show affinities with the Presland and Payne dry stone wall communities. Even her Group 1, again close to the Presland and Payne pattern, has discordant features, matched as it is with OV42 Cymbalaria muralis community, wall crevice vegetation typical of sunny communities. The dominant plant here is Cymbalaria muralis, which in Winsley is entirely a mortared wall plant, which doesn’t colonise dry stone walls at all despite being plentiful in the area. Its occurrence in Hill’s sample may be because a few Group 1 walls contained mortar.

What are the implications of these different kinds of thinking for conservation? Payne does not discuss the matter. Hill and Presland, despite the methodological differences and some strange discrepancies, do broadly agree that dry stone walls need to be protected so that their plant communities are not lost, and the detailed management strategies described are much the same. Hill, however, appears to see them less as communities in themselves, but rather as parts of more inclusive networks of a number of different plant communities. From this she claims that the most valuable dry stone walls are those within and adjacent to United Kingdom Priority Habitats. This does, perhaps, underemphasise the unique character of communities such as those described in Winsley, which are on their own in predominantly agricultural land. If they are lost, most of their species largely go with them, whereas this is not so with walls that bear, say, predominantly woodland flora.

Another key function of dry stone walls emphasised by Hill is that of linking channels between communities which allow species to spread geographically, rather than becoming isolated and dying out. However, Hill appears to see this as relating to communities other than the walls themselves. This is surprising, since she found no evidence that such species do travel along the walls. Indeed, many of them wouldn’t survive on the inhospitable wall surfaces in open country, so it seems unlikely to be a common phenomenon. If, however, the walls are seen as important in themselves, the channel concept becomes important. The characteristic wall plants must have spread along the walls, because, for the most part, there is nowhere else for them to come from. If the more remote walls are not protected, this function is at risk.

The differences here are of emphasis. Wherever dry stone walls harbour communities of native or long-established species, those communities need to be protected. Careful thought is needed, however, as to where the priorities lie.

April 2010