Flora of Dry Stone Walls

THE VASCULAR FLORA OF ACIDIC DRY STONE WALLS

John Presland

Introduction

Earlier publications (Presland 2007, 2008a. 2008b, 2008c) have summarised pre-existing information on the flora of limestone dry stone walls and proposed a description of this flora which claimed that it was unique. Existing evidence appeared to fit this claim, but more data is needed to test it more severely. No comparable account has been located for dry stone walls made of acidic rocks, which is understandable, since very little data are available. However, an attempt is made here to summarise what there is and make suggestions as to what it shows. It may be produced in several parts. Here, we cover the vascular plant component. Bryophytes and lichens present a harder challenge, and will be attempted subsequently. In order to find more detailed information about the interaction of bryophytes and lichens on the surfaces of limestone walls, it is necessary to process a lot of information resources and research by famous biologists and laboratory technicians, we entrusted this theoretical part of the study to the best writing services https://bestwritingservice.com/, because only qualified executors of scientific and educational tasks work here.

Species reported

Only four sources have been located, apart from earlier work by Farmer which is now included in a larger study reported for the first time in this article, and these are dealt with in turn below. None of them found a substantial number of species occurring.

Walsh (2008) studied a woodland wall and a moorland wall in Lancashire. The moorland wall bore no vascular plants at all, whereas the woodland wall had only Holcus lanatus (Yorkshire Fog), Cryptogramma crispa (Parsley Fern) and Cystopteris fragilis (Brittle Badder-fern).
Hannah (2007) summarising observations over a number of years on the Isle Bute where the stone of walls is “moderately acidic”), recorded virtually no vascular plant flora on the more exposed dry stone walls, though the lower courses sometimes carried similar vegetation to the adjacent wall-foot, eg Hypochaeris radicata (Cat’s-ear), Achillea millefolium (Yarrow), Agrostis capillaries (Common Bent), etc., while on seriously neglected walls brambles (Rubus fruticosus agg.) and ivy (Hedera helix) would sometimes get a hold.
Williams (1988) examined 24 dry stone walls in Shetland, the total length being 1,437 m, and also 24 walls of other types, total length 590m. The latter comprised two dry stone walls with mortared copings, 18 mortared walls and four retaining walls. On the clearly dry stone walls, he recorded an unidentified fern, Galium palustre (Heath Bedstraw), Festuca vivipara (Viviparous Fescue), Festuca rubra (Red Fescue), Poa sp, Poa subcaerulea (= P. humilis) (Speading Meadow-grass) or P. pratensis (Smooth Meadow-grass), Deschampsia flexuosa (Wavy Hair-grass), and Aira praecox (Early Hair-grass).
Ratcliffe (2002) reports that the dry stone walls of the Lake District have little else but mosses and lichens, but some support sparse growth of calcifuge ferns, including Cryptogramma crispa (Parsley Fern).

A survey in Argyll

Farmer recently recorded the flora of 10 dry stone walls in Argyll, where they are built of acidic rock. The locations and grid references of the walls were as follows:

A - Glen Lonan NM 961281
B Glen Lonan NM 935280
C - Glen Lonan NM 925282
D – Kilchrenan NN 013198
E - Glen Lonan NM 925282
F - Oban NM 841284
G - Inverawe NN 025312
H - Glen Salach NM 979382
I - Inveresragan NM 986356
J - Kerrera NM 806264
K - Achnacloich NM 965340

Only plants clearly rooted on the wall were recorded, and not those growing on the ground at the base or low down on the wall where water and nutrients from the soil were available to them. Notes were made on the shade and humidity of locations. The results are shown in Table 1, together with the typical habitats of each species as given by Stace.

Table 1. Vascular plants on 10 Argyll dry stone walls and typical habitats

1 = present, not shaded
s = in shade of trees
h = humid shade, i.e. particularly dense woodland shade and humid conditions resulting in lush wall growth with untypical species.
b = shaded for part of year by bracken growing next to wall
* = identity awaits confirmation

Species noted on dry stone walls	A	B	C	D	E	G	H	I	J	K	Typical habitats (from Stace)
Sedum anglicum (English Stonecrop)									1		Rocks, sand, shingle
Oxalis acetosella (Wood Sorrel)	s	s			s	s					Woods, hedgebanks, shady rocks, often on humus
Geranium robertianum (Herb Robert)										s	Woods, hedgerows, banks, scree, shingle
Viola riviniana (Common Dog-violet)		s									Woods, grassland
Potentilla erecta (Tormentil)		s			1						Grassland and dwarf shrubland on heaths, moors, bogs, mountains, roadsides and pastures, mostly on acid soils
Luzula pilosa (Hairy Woodrush)		s									Woods, hedgerows, moors
Sorbus aucuparia (Rowan)		s	1			h					Woods, moors, rocky places
Anthothanthum odoratum (Sweet Vernal-grass)		s									Grassy places
Galium saxatile (Heath Bedstraw)		s			s				b		Dry grassland, rocky places and open woods on acid soil
Digitalis purpurea (Foxglove)	1		1					b		s	Open places, especially woodland clearings, heaths and mountain-sides and waste ground, on acid soils
Polypodium vulgaris (Common Polypody)		s	1	1		s	1	b	1		Rocks, walls, tree trunks and banks, often on acid soils
Holcus lanatus (Yorkshire Fog)	1										Rough grassland, lawns, arable, rough and waste ground, open woods
Juncus effusus (Soft Rush)					1						Wetlands and damp woods
Betula sp (Birch)					s	h					Various locations on acid soils
Athyrium filix-femina (Lady Fern)					s						Damp woods, hedgebanks, mountain screes, marshes
Festuca filiformis (Fine-leaved Sheep’s-fescue)					1						Grassy places
Agrostis capillaris (Common Bent)		s	1		s	s		b			Grassy places, rough ground,usually on acid soils
Cirsium vulgare (Spear Thistle)					1						Grassland, waysides, cultivated, rough and waste ground
Dryopteris dilatata (Broad Buckler-fern)		s*									Woods, hedgebanks, ditches, heaths, mountains
Dryopteris fillix-mas (Male Fern)						1*					Woods, hedgebanks, ditches, mountains
Fraxinus excelsior (Ash)						s					Woods, scrub, hedgerows
Crepis paludosa (Marsh Hawks-beard)						s					Wet places in open woodland, grassland, fens
Vaccinium myrtillis (Bilberry)						h					Heaths, moors, woods

Is the flora unique?

Tables 1, 2, 3 and 4 list the principal typical habitats of the species noted on dry stone walls, as described by Stace (1991).

Table 2. Species recorded on dry stone walls in Lancashire and typical habitats

Species noted on dry stone walls	Typical habitats (from Stace)
Holcus lanatus	Rough grassland, lawns, arable, rough and waste ground, open woods
Cryptogramma crispa	Rocky places on acid soils on mountains
Cystopteris fragilis	Shady, basic rocks an walls

Table 3. Species recorded on dry stone walls in Bute and typical habitats

Species noted on dry stone walls	Typical habitats (from Stace)
Hypochaeris radicata	Grassy places
Achillea millefolium	Grassland, banks and waysides
Agrostis capillaris	Grassy places, rough ground,usually on acid soils
Rubus fruticosus agg.	Various
Hedera helix	Trees, banks, rocks and sprawling on the ground

Table 4. Species recorded on dry stone and other walls in Shetland and their typical habitats

Species noted on dry stone walls	No. of dry stone walls	No. of walls other than dry stone	Typical habitats (from Stace)
Sagina procumbens	1	4	Paths, lawns, ditchsides, short turf
Rumex acetosa	3	4	Grassy places
Urtica dioica	1	1	Many habitats
Galium palustre	2	1	Damp meadows, pond-sides, ditches, marshes, fens
Festuca vivipara	1	2	Grassy places, usually on rocky ground
Festuca rubra	2	4	Grassy places and rough ground
Poa sp	2	2
Poa subcaerulea (= humilis) or P. pratensis	4	16	Subcaerulea grassland, roadsides, old walls. Pratensis meadows, pastures waysides, rough and waste ground
Deschampsia flexuosa	2	0	Acid heaths, moors, open woods, drier parts of bogs
Aira praecox	7	15	Dry sandy, gravely or rocky ground, walls, heaths, dunes
Cryptogramma crispa			Rocky places on acid soils on mountains

Of the species noted, only Cystopteris fragilis, Polypodium vulgare and Aira praecox have walls included in their habitats. Several others list rocks, which might suggest some tendency to colonise walls. All can occur in habitats other than walls and rocks. It looks as though they do not constitute a specifically wall community, but have simply spread on to the walls from other habitats nearby. In support of this, it was found that 9 out of the 10 Shetland dry stone wall species occurred also on other types of wall, in most cases more frequently, despite the total number and length of such walls being much less. Thos on Yell were all present in the adjacent moorland; though some common moorland species such as Heather (Calluna vulgaris) were absent from the wall flora.

Where on the wall do the plants grow?

There are different ways of deciding whether a plant is growing on a wall or not. We can distinguish:

Rooted in the ground but growing through or over the wall
Rooted on the side of the wall near ground level
Rooted on the side of the wall well above ground level
Rooted on the top of the wall

Categories 1 and 2 may simply be regarded as extensions of the ground flora, whereas 3 and 4 are indisputably part of the wall flora, though they may be in the ground flora as well. Our own choice has been to consider only 1 and 2 as wall species. However, other studies may be more inclusive - or simply not specify the wall location - and this can make comparisons difficult.

The studies reported here show such differences. Farmer has excluded 1 and 2, and the virtual absence of vascular plants in Walsh’s study suggests he did the same. Williams does not specify locations, but does say that vascular plants were more frequent towards the top of the wall, because they prefer horizontal rather than vertical surfaces and is directly related to the greater ability of horizontal surfaces to retain soil and water. Hannah considered both, concluding that vascular plants were almost entirely on the lower courses, while the basically ground-rooted brambles and ivy would sometimes occur. In these cases, of course, soil and water would be provided from the ground.

Comparisons

A number of issues present themselves for consideration in the light of the data so far available. Is there a typical acidic dry stone wall community? How do acidic dry stone walls differ from walls of other types? How much acidic walls vary between themselves? We will deal with each of these in turn.

There is little support from these results for the notion that there is a typical assemblage of plants which grow on acidic dry stone walls. Firstly, each study produced a species lists with little in common with the others. Only three species were recorded in more than one of the studies - Holcus lanatus in Argyll and Lancashire; Cryptogramma crispa in Lancashire, Shetland and the Lake District; and Agrostis capillaris in Argyll and Bute. The possibility cannot, however, be dismissed that a wider sampling would find greater commonality.

Secondly, only three of the 39 species had walls described as a common habitat in Stace’s flora - Polypodium vulgare, Aira praecox and Cystopteris fragilis. Several others typically occur on rocks, which might suggest some tendency to colonise walls. The majority, however, have nothing in their habitat descriptions to indicate they belong to walls, and all the species can occur in habitats other than walls and rocks. It looks, therefore, as though they do not constitute a specifically wall community, but have simple spread on to the walls from other habitats nearby. In support of this, it was found that 9 out of the 10 Shetland dry stone wall species occurred also on other types of wall, in most cases more frequently, despite the total length of such walls being much less. Those on Yell were all present in the adjacent moorland.

There is, however evidence that acidic dry stone walls differ from walls of other types. Comparison with the findings for limestone dry stone walls in studies referred to at the beginning of the article gives quite clear results for vascular plants. The limestone walls bear a characteristic collection of species which spread specifically from one wall site to another, with little relationship to surrounding habitats. In contrast, the acid walls described here have hardly any distinctive vascular species at all, those that do occur being colonists from other habitats. Limestone mortared walls have also have a distinctive vascular flora, so are also different.

One would, also, expect comparisons between dry stone and mortared walls made from acidic rocks to show considerable differences, since the mortar is calcareous and should therefore allow a much wider variety of species. Retaining walls should also be much richer because of the moisture and nutrients coming through from the soil which is being retained. Within the walls he studied, Williams included two dry stone walls with mortared copings, 18 mortared walls and four retaining walls. Table 5 shows the species growing on each of these walls which were not found on the dry stone walls. The only species occurring on the dry stone wall but not on any other type were an unidentified fern and Deschampsia flexuosa. The others also occurred on mortared walls, walls with mortared caps or retaining walls in the area and were more frequent there, and these walls also carried 13 additional species. It is apparent that walls with any mortaring and retaining walls bore more species than dry stone walls and, where there were species in common, they were mostly more common on walls that were not dry stone. Farmer found that some species occurred exclusively on mortared walls in Argyll - Asplenium trichomanes, A. ruta-murari, A. adiantum-nigrum, Cymbalaria muralis and Erinus alpinus. Though these various findings do suggest a difference between the types of wall, it is only the mortared walls which have a significant number of distinguishing species.

Table 5. Shetland species occurring on different types of wall


Species	No. of dry stone walls with mortared coping (out of 2 walls, total length 35 m)	No. of mortared walls (out of 18 walls, total length 320 m)	No. of retaining walls (0ut of 4 walls, total length 235 m)
Cerastium fontanum (Mouse-ear Chickweed)	1	1
Stellaria media (Chickweed)		1	1
Sedum rosea (Rose-root)	1
Rumex acetosella (Sheeps Sorrel)		2	2
Bellis perennis (Daisy)		1
Juncus effusus (Soft Rush)			1
Lolium perenne (Perennial Rye-grass)		1
Poa trivalis (Rough Meadow-grass)			6
Dactylis glomerata (Cocksfoot)	1
Holcus lanatus (Yorkshire Fog )		2	1
Anthoxanthum odoratum (Sweet Vernal-grass)		1	1
Agrostis sp.		4	1
Agrostis stolonifera (Creeping Bent)		1

Acidic dry stone walls without mortar or retained soil can vary between themselves. Walsh compared two walls in different sites - sheltered in woodland and exposed on moorland. The woodland wall had 3 species of grass or fern, whereas the moorland wall had none at all. Farmer found that walls in the shade of trees become completely covered with a dense growth of moss into which leaves and other debris fall and decompose, and just about any shade-tolerant plant can grow on the resulting material - even the marsh/streamside plant Crepis paludosa was seen growing happily on top of a "dry" stone wall in a wood. This habitat is so different from the bare dry stone walls of open country in the same county, which often have no vascular plants at all. If they do have one species, it is likely to be Polypodium vulgare, and if they have two, the second is likely to be Sedum anglicum, both plants featuring rocks among their typical habitats. Digitalis purpurea is probably the third most likely. Sorbus aucuparia seedlings often occur in the top of the wall if there are trees within range (but not shading the wall). Other vascular species are very occasional. Most of these species on walls in open habitats were also found on walls in shady conditions, So the difference is more in the number of plants that can grow rather than particular species - 11 species were on the “open” walls and 17 on the “shaded”.

Human activities affect what grows on the walls, as suggested by Farmer’s finding of turfs which appeared to have been placed on the walls, allowing a wider variety of species to grow there.

Conclusions

For the locations and communities described in this article, it can be concluded that:

The vascular flora of the acid dry stone walls studied was made up of a small number of species, the total recorded for all locations combined being 39.
There is little support from these results for the notion that there is a typical assemblage of plants which grow on acidic dry stone walls. It looks, rather, as though the species which occur are not specifically wall plants, but have simply spread on to the walls from other habitats nearby.
These acid dry stone walls differed from limestone dry stone walls in not having a characteristic community.
Dry stone walls made from acidic rocks have a different flora from walls of the same rock which are mortared or retain soil, and only the mortared walls have a significant number of distinguishing species.
There were more species on parts of dry stone walls where moisture and nutrients are most available, ie, near the ground or on the flat top, particularly in woods, where debris can settle and evaporation of moisture is reduced by the shady conditions.
Human activities affect what grows on acid dry stone walls

The number of studies located was small, as were some of the samples. The results cannot, therefore, be generalised. They do, however, form a basis for comparison of such walls elsewhere, which might eventually allow such generalisations. At the moment, it is difficult to say precisely what should be surveyed for this purpose. We are tempted to argue that future surveys should disregard acidic walls in woodland or walls with significant amounts of soil on top, since their vascular flora is not in any way different from other habitats nearby. However, even unshaded dry stone walls, though bearing some species which are at least partly wall plants, still have a flora which strongly reflects non-wall habitats nearby. A current analysis of the bryophytes and lichens which also grow on the walls may throw more light on these issues, though indications so far are that it will support rather than disconfirm the tentative conclusions drawn here. The website dry-stone-wall-flora.co.uk will eventually provide information about it, along with any future findings on vascular plants.

Ideally, further studies should try to look more rigorously at the acidic-basic status of walls. Finding Cystopteris fragilis, for instance, suggest basic rock or mortar, and some of the bryophytes found were also more characteristic of basic habitats. The geology of some of the areas is very varied, particularly Shetland, and it may not always be clear what a particular wall is made from. There are distinctions also to be made as to what constitutes a wall flora, since in some cases, particularly in woods, the invasion of walls by plants from other habitats is so marked that the wall seems almost like a strip of the surrounding community itself. Such rigour will, however, be difficult to achieve, and information about what is found in areas wherever acidic dry stone walls are thought to be the norm could help in providing data on which to design more specific studies.

References

HANNAH, A. (2007) Some Notes on the Flora of Bute Walls dry-stone-wall-flora.co.uk

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.

PRESLAND, J. L. (2008c) Is there a limestone dry stone walls community? BSBI News 109: 9-12.

RATCLIFFE, D. (2002) Lakeland . HarperCollins New Naturalist, p. 167.

STACE, C. (1999) Field Flora of the British Isles. Cambridge University press, Cambridge.

WALSH, M. (2008) Two Drystone Walls in Lancashire: An Environmental Appraisal

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