THE FLORA OF WALLS IN THE CHEW VALLEY
R. M. PAYNE

Somerset Archaeology and Natural History: the proceedings of the Somerset Archaeological and Natural History Society for 1989, Vol. 133, pages 231-242

INTRODUCTION

During the two years 1988 and 1989, 305 walls in the Chew Valley were kept under observation, with the particular aim of assessing the relative frequency of different plant species on the various kinds of walls in the area. Only flowering plants and pteridophytes were studied. Stone bridges and some buildings were included in the survey but not quarries, cliff faces or any other semi-natural habitats. It seems that no comparable study has previously been made in the west of Britain, though Thompson (1925) recorded 104 species growing on pollarded willows and alders along our River Chew - a somewhat similar habitat - and West (1911) listed 78 species growing on a single wall near the sea in the extreme south west of Ireland.

The Chew Valley, in North Somerset and Avon, being the area drained by the small River Chew and its tributary streams, extends from above Chewton Mendip in the south to Keynsham in the north (where the Chew flows into the Avon), and from Marksbury and Emborough in the east to Compton Martin and Felton in the west, a total area of some 140km2. The altitudinal range is from 285m to 15m. The area is almost entirely rural with a few large villages but only one town, Keynsham. For this reason pollution is not an important factor in determining the composition of the flora: in any case the higher plants are much less affected by pollution than lichens and bryophytes. Large parts of the area have a high rainfall, the average on the Mendip Hills (where the Chew rises) being some 45 ins per annum, Chew Magna (in the centre of the valley) 38 ins, while down at Keynsham the total is probably about 32 ins.

Almost all the walls are of local stone and predominantly calcareous in nature - White Lias, Carboniferous Limestone, Dolomitic Conglomerate and Grey and Red Sandstone. Even where the stone is not calcareous, eg. Pennant Sandstone at Pensford, the mortared joints, where most plants grow, provide a localised calcareous environment. There are virtually no brick walls except for a very few railway bridges and in suburban roads in the southern districts of Keynsham. There are many miles of drystone walls, particularly at the higher altitudes.

The 305 walls studied were chosen with three criteria in mind: firstly, to cover the whole area (the initial aim being to select at least one wall in each 1km square): secondly, to include reasonable numbers of walls of particular kinds, eg dividing and retaining walls, river walls, churches, drystone walls etc - some of these categories may, of course, overlap; thirdly, to select, as far as was consistent with these two more important criteria, walls with a relatively rich flora, so as to obtain as large a sample as possible of plants colonising walls in the area. Needless to say, every wall selected had at least one plant species growing on it during some part of the survey period: as Kent (1961) noted in his paper on Middlesex walls, it was not uncommon to find on a later visit that a wall had been repointed or rebuilt to the detriment of the flora! Retaining walls with a substantial depth of soil on top were generally excluded from the survey, since plants growing on their tops could not reasonably be distinguished from the flora of the adjoining field or wood. For the same reason plants at the extreme base of a wall which appeared to be rooted in the ground were not recorded.

Except for churches (see below), the unit for the survey was either a single wall or, occasionally, a series of very short adjacent walls of identical construction and aspect (for example, a farmyard wall with a break for a gate in the centre). In the case of long drystone and other field walls a sample length of up to 50 metres was chosen.

As Darlington (1981) points out, the angle of inclination of a wall has an important effect on colonisation, in that the nearer to the horizontal the greater the range of plants. Partly for this reason the survey excluded the shelving river embankments along the lower Chew in Keynsham and also the sloping concrete and stone walls of the Chew Valley Reservoir. With the single exception of the aqueduct revetment in Harptree Combe, which is marginally out of the vertical, all the walls surveyed had vertical sides.

The nomenclature of plants used in this paper is that of Clapham, Tutin and Moore (1987), and to save space the names of authors are omitted.

LIST OF PLANTS

A total of 291 species of flowering plants and ferns were seen on the Chew Valley walls, but of these, 72 (25%) occurred on only one wall, with another 42 (14%) or only two walls. The species are listed below, with the number of walls on which they were seen.

COMPARISON WITH WALL FLORAS OF OTHER AREAS

The table below, which is based on a similar table in Kent (1961), compares the relative frequency of the 21 commonest wall plants in this survey with the frequency of some of them in earlier wall surveys in Wiltshire (Grose 1957), Middlesex (Kent 1961), London (Kent 1961), Essex (Payne 1978), Warwickshire (Cadbury et al 1971), Cambridge (Rishbeth 1948) and Durham (Woodell and Rossiter 1959).

Percentage occurrences (where known) in various surveys of the 21 commonest species in the Chew Valley survey

* Order of frequency

It is hardly surprising that the Wiltshire figures, though based on a very small sample of walls, come closer to those in the present survey than any from more distant parts of England.

The most striking absentee from a high place in the Chew Valley list is Poa annua, which occurred on only 13% of our walls, compared with 56% in Wilts, 54% in Essex, 48% in Middlesex, 40% in Durham and 38% in London, while it topped the Cambridge list. Other surprising species are Dryopteris filix-mas on only 6% (35% in Middlesex, 28% in London, 22% in Essex), Chamaenerion angustifolium on 2% (53% in Durham. 24% in Middlesex and 12% in Cambridge), Senecio squalidus on only 1 wall (29% in Essex, 25% in Middlesex, 22% in London) and Sagina procumbens also on only 1 wall (18% in Essex, 17% in Middlesex, 10% in London).

Poa annua, Chamaenerion angustifolium, Senecio squalidus and Sagina procumbens are all plants primarily of artificial, even urban, habitats, which are relatively infrequent in the pastoral Chew Valley, so there may be a comparatively small reservoir of populations of these species. Bolton (1985) found the Sagina abundant on walls in Exeter (an urban area), though Darlington (1981) refers to it as a common mural species generally.

The scarcity of Male Fern is harder to explain.

There may be two other factors to consider. The Chew Valley has a relatively heavy rainfall, which perhaps leads to annual and small species like P. annua and Sagina being overwhelmed by perennials, such as nos. 11, 12, 13, 14 and 21 on our list. And since many of the limestone walls in the lower parts of the Valley are surrounded by soils with low lime content, local races of plants may not readily adapt to the lime of the walls.

DRYSTONE WALLS

Drystone walls, which of course contain no mortar, initially offer little or no nutriment to the higher plants, but when they have been colonised by lichens and then mosses, some flowering plants and ferns soon appear and the drystone walls of the upper parts of the Chew basin support a flora which is to some extent distinctive. A factor in this distinctiveness may be the higher rainfall on Mendip.

The commonest species on the 40 drystone walls surveyed were as follows, with the percentage of such walls on which they were noted:-

Species which showed a statistically significant preference for drystone walls (on the basis of the test) were:-
chi² value

On the other hand, the following two species showed a marked relative aversion to
drystone walls :-

[The statistical tests were performed by constructing 2 x 2 contingency tables (species present or absent vs. selected wall category or other) and calculating chi² with one degree of freedom. Yates’ correction for continuity was applied, but in view of the large number of observations it made virtually no difference.]

RIVER WALLS

In this category are included vertical walls embanking the River Chew and its tributary streams, as well as revetments of stone bridges over streams. Only plants growing on the sides of these walls were recorded, ie those most likely to be influenced by the permanent proximity of water.

The commonest species on the 69 river walls were:-

Species which showed a significant preference for river walls were:-

Species with a relative aversion to river walls were:-

SHADED WALLS

In an attempt to show the influence of a shaded situation on the composition of a wall flora. 32 walls overhung by trees were selected. The selection was often difficult, in that a wall might well be under dense shade in summer but more or less exposed in winter when the overhanging trees were leafless. To that extent any conclusions drawn from this part of the survey are probably of less value than those relating to other, more clearly defined categories of walls.

The commonest species on the 32 shaded walls were:-

Species showing a significant preference for shaded walls were:-

Perhaps surprisingly, the species showing the most marked aversion to shaded walls was a fern:-

CHURCHES

All the old parish churches in the Chew Valley had plants growing on their outside walls: for the purpose of this survey each church was treated as a single unit. Church-yard walls were not included in this category. With the addition of five 19th-century nonconformist chapels, 27 churches were surveyed.

Of the 63 species noted on church walls (including their towers) the commonest was Sambucus nigra, which occurred on no less than 20 of the 27 churches. The dispersal of Elder by birds no doubt accounts for the frequent seedlings seen high up on church towers. Other common species were:-

Only Sambucus nigra showed a significant preference for church walls (chi² value 16.07). On the other hand, three species were significantly less frequent on
churches:-

OTHER BUILDINGS

Several farm buildings, mainly old barns, were included in the survey, together with two tall disused chimneys: 17 stone structures in all.

Of the 50 species noted from these buildings, the most frequent were:-

The data are insufficient to warrant any further comments.

RAILWAY WALLS

In a similar survey of the wall flora in Essex some years ago (Payne 1978) it was found that the walls of railway bridges and stations had a distinctive flora. In the Chew Valley, however, apart from the main Bristol to London line that runs through Keynsham at the extreme northern tip of our area, the one railway that formerly extended for some 4 miles through the Valley, the Bristol and North Somerset, was finally closed in 1968. Although every accessible bridge and other structure was examined - with the exception of Pensford Viaduct, access to which proved to be impracticable - there is little evidence that any distinctive flora has survived the 20 years of disuse. It may just be significant that the commonest plant overall was a fern, Asplenium ruta-muraria, which occurred on no less than 90% of those railway walls which supported any of the higher plants, and that one old bridge near Pensford still harbours one of the very few colonies of Asplenium adiantum-nigrum in the valley. (Hedera and Taraxacum came a poor joint second to Wall Rue). Railway walls, particularly those at stations, have long been known to botanists as a favourable habitat for ferns, presumably because of the damp micro-climate brought about by the frequent discharges of steam from locomotives in earlier times: once established, ferns tend to persist in such sites long after the demise of steam trains.

GARDEN WALLS

Some 66 stone garden walls were included in the survey. Only the outside faces of such walls were recorded, since it is unlikely that plants would have been deliberately introduced there. (In any case, as Clement (1984) has pointed out, it is far from easy to establish a plant on a wall). Even so, a high proportion of garden plants occurred on these walls, presumably self-seeded or dispersed from the adjacent gardens by birds. It is hardly surprising that all the occurrences of the following species were on walls backing on to gardens: -

Antirrhinum majus
Aquilegia vulgaris
Arabis caucasica
Aubrieta deltoidea
Campanula poscharskyana
Crataegus crus-galli
Euphorbia helioscopia [a common garden weed]
Hypericum calycinum
Juniperus communis
Kerria japonica
Linaria purpurea
Lonicera nitida
Lunaria annua
Lycium barbarum
Melissa officinalis
Nigella damascena
Papaver somniferum
Phygelius capensis
Sedum spurium
Sempervivum tectorum
Soleirolia soleirolii
Solidago gigantea
Syringa vulgaris

On the other hand, only half the records of Cheiranthus cheiri and Centranthus ruber were from garden walls, while in the genus Cotoneaster 11 out of the total of 29 records were on walls well away from gardens, doubtless dispersed through the agency of birds feeding on their berries. Other species originally introduced into England as garden plants but which are increasingly becoming established outside gardens, and which occurred on non-garden walls in this survey, are Buddleia davidii. Corydalis lutea. Leycesteria formosa and Meconopsis cambrica.

DIVIDING AND RETAINING WALLS

A comparison was made between the floras of dividing and retaining walls. Retaining walls, being backed up by a deep mass of earth, tend to retain moisture and nutrients to a much greater extent than isolated dividing walls, and might therefore be expected to harbour a different association of plants.

The following species showed a significant preference one way or the other:-

Dividing Walls

Retaining Walls

However, it should be noted that some of the species showing a preference for retaining walls are markedly associated with river walls, which of course were all of this type.

TOPS AND SIDES OF WALLS

The more or less flat top of a wall clearly receives more light and rain than the vertical sides, and accumulates more debris which leads to the formation of rudimentary soils. Fruits and seeds dropped or evacuated by birds are much more likely to arrive on the top of a wall than on its sides. It seemed probable, therefore, that certain species would tend to be found on the exposed tops of walls, while shade-loving plants and those not primarily dispersed by birds would be more at home on their sides. To this end, separate records were kept of occurrences of species on tops and sides, and the following species were found to show a significant preference for one or the other habitat:-

TOPS

SIDES

B Known to be dispersed by birds
W Known to be dispersed by wind
(Cannon 1957, Ridley 1930, Salisbury 1961)

The high proportion of grasses preferring wall tops is notable, while ferns dominate the other side of the table (with very high chi² values).

ASPECT

Darlington (1981) draws attention to ways in which the aspect of a wall may affect plant life growing on it. Thus a south-facing wall is exposed to desiccation by direct insolation, while on a north wall water will remain longer because the sun's rays reach it only peripherally and temperatures are less extreme on a north wall, sc plant life may often be more luxuriant. Records were kept of the aspect of each of the walls in the survey, but it is disappointing to have to admit that very few marked preferences appeared to be shown, perhaps because of the relatively high rainfall in the area. However, on the basis of the chi² test Dryopteris filix-mas, Asplenium trichomanes, A. scolopedrium and Angelica sylvestris showed a significant preference for north-facing walls as against all other aspects, while Sonchus oleraceus very markedly disliked the north aspect.

ALTITUDE

Although the height above sea-level of each wall was noted, the results of the survey suggested that the overall range of only some 270 metres was insufficient to constitute a significant factor in determining the wall flora.

COMMENTS ON PARTICULAR PLANTS

Ivy (Hedera helix), which more or less covers a high proportion of walls in the area, often smothering other plants, was only recorded when it appeared to be established in crevices without making contact with the ground.

Brambles (Rubus fruticosus agg) are always difficult for the non-specialist to identify, and the difficulty is made worse when they are growing on walls, because such plants normally fail to produce flowers or fruit. The same problems arise with wild roses, which I have treated as Rosa canina agg.

Special attention was paid to grasses and ferns. Poa compressa is generally regarded as a typical grass of old walls, and P. nemoralis as a woodland species: indeed Segal (1969) says that nemoralis is hardly ever found on walls. Nevertheless on Chew Valley walls nemoralis was almost twice as frequent as compressa, showing however some preference for the sides of walls (the more shaded position), while compressa evinced a marked preference for the tops. The surprising infrequency of Poa annua has already been commented on. Poa subcoerulea and P. angustifolia have both been recorded as typical wall species, the latter particularly in Eastern England, but neither was seen during the present survey.

Elymus caninus may seem a surprising colonist of walls, but each of the five walls on which it was noted adjoined either a wood or a shady hedgerow in which the grass was also growing. The seeds of Melica spp are dispersed by ants (Ridley 1930, Segal 1969), and this may explain the presence of M. uniflora on two walls.

It is difficult to account for the total absence of Desmazeria rigida from the Chew Valley walls, a grass described as common on limestone walls in Wiltshire (Grose 1957) and a wall-top plant as near to the survey area as Wells.

Asplenium adiantum-nigrum was relatively much scarcer in the Chew Valley than I had found it in Essex (Payne 1978), perhaps because it may be more partial to brick walls (Page 1988).

Polypody, unlike all the other ferns in the survey, seemed equally happy on the tops of walls as on their sides, although showing some preference for shaded sites. Because of the practical impossibility of separating in the field the three species (and three hybrids) into which this genus is now divided, I have treated all the wall Polypodies as P. interjectum, which is by far the most abundant of the segregates in this area.

ACKNOWLEDGEMENTS

Special thanks are due to Mr S.M. Taylor, who helped me greatly with statistical
calculations; Mr M.W.J. Paskin and Mr A.G. Smith also discussed statistical matters with me. I am grateful for help in identifying plants to Mrs J. Fryer and Mr J.R. Palmer (Cotoneaster), and to Mr J.G. Keylock and Mr R.D. Randall (mainly Hieracium). Mr I.M. Kinchin introduced me to several relevant papers dealing with wall flora. Dr K. Allen advised me on the geology of the area, and Mr R.A. Janes, Mr Paskin and Mr A.C. Titchen made useful comments on early drafts of the paper.

REFERENCES

Bolton, D.E. 1985 'Living Walls of Exeter', The Journal of the Devon Trust for Nature Conservation, 6, 51-66.
Cadbury, D.A. et al 1971 A Computer-Mapped Flora. London.
Cannon, J.F.M. & M. 1957 ‘The stability of the epiphytic flora of pollarded willows', Proc B.S.B.I., 2, 226-233.
Clapham, A.R. et al. 1987 Flora of the British Isles, 3rd edition. C.U.P.
Clement, E.J. 1984 ‘Alien Flora of Walls in the South West', BSBI News, No. 36, 23.
Darlington, A. 1981 Ecology of Walls. London.
Grose, D. 1957 The Flora of Wiltshire. Devizes.
Kent, D.H. 1961 ‘The flora of Middlesex walls', Lond. Nat., 40, 29-43.
Page, C.N. 1988 Ferns. London.
Payne, R.M. 1978 'The flora of walls in South-Eastern Essex', Watsonia 12, 41-46.
Ridley, H.N. 1930 The Dispersal of Plants throughout the World. Ashford, Kent.
Rishbeth, J. 1948 'The flora of Cambridge walls', J Ecol. 36, 136-148.
Salisbury, E. 1961 Weeds and Aliens. London.
Segal, S. 1969 Ecological Notes on Wall Vegetation. The Hague.
Thompson, H.S. 1925 'Flowering plants as Epiphytes on Willows and Alders', Nature CXVI, 710.
West, W. 1911 'Mural Ecology', Journal of Botany, XLIX, 59-61.
Woodell, S.R. and Rossiter, J. 1959 'The flora of Durham walls', Proc. Bot. Soc. Br. Isl. 2, 257-273.