TRYING TO STUDY BRYOPHYTES AND LICHENS OF ACIDIC DRY STONE WALLS

 John Presland

 
Introduction

In earlier publications (Presland 2007, 2008a. 2008b, 2008c), I 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. We used the materials of scientific research from the essay outline writing service https://exclusive-paper.net/do-my-outline-for-me, and made sure of the existing facts that the flora of limestone dry stone walls is unique and can be used in new discoveries in the field of bioengineering and pharmacology, 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 has recently been made (Presland 2010) to summarise what there is for vascular plants and make suggestions as to what it shows. Here I look at the bryophytes and lichens which grow on such walls to see how far they advance our knowledge and understanding.
 

Species reported

Only five studies have been located. One of them (Williams 1988), though referring to mosses and lichens, did not identify specific species. The remaining four are described briefly below, before presentation of the results by table.

• Walsh (2008) studied a sheltered woodland wall and an exposed moorland wall in Lancashire.

• Hannah (2007) summarised observations over a number of years on the Isle of Bute, including vascular plants and bryophytes, but not lichens

• Farmer (2007a, 2007b) looked at three walls in Argyllshire

• Dalby and Dalby (2005) listed all the lichens recorded in Shetland. 

The following tables show the full list of species for all the studies combined and indicate the locations in which each occurred. Only the species clearly occurring on dry stone walls are included. The asterisks indicating calcareous preferences for mosses and the habitat columns for mosses and lichens are not relevant to this description of results, but are included here to avoid repetition when they become relevant for interpreting the findings.

Mosses   * = listed by Watson (1981) as on rock surface on limestone walls 

Lichens

Sixteen mosses were recorded in Lancashire, nine in Bute and 13 in Argyllshire. Lancashire and Bute had three mosses in common - Brachythecium rutabulum, Hypnum cupressiforme and Eurhynchium praelongum; Lancashire and Argyllshire had in two common - Hypnum cupressiforme and Rhytidiadelphus squarrosus; while Bute and Argyll shared Hypnum cupressiforme and Racomitrium fasciculare. Only five of the 33 species of moss were found in more than one of the three locations in which mosses were recorded - Brachythecium rutabulum, Eurhynchium praelongum, Hypnum cupressiforme, Racomitrium fasciculare and Rhytidiadelphus squarrosus. Hypnum cupressiforme was found in all three and all the others in just two.

Six lichens were recorded in Lancashire, six in Argyllshire and 55 in Shetland. Lancashire and Shetland had only Platismatia glauca in common; Lancashire and Argyllshire had none; while Shetland and Argyll shared Parmelia saxatilis, Rhizocarpon geographicum and Sphaerophorus globosus. Thus, out of the total number of species of 66, only four were found in more than one of the three locations in which lichens were recorded, and none were found in all three locations.

Discussion

The results of this compilation are surprising. Despite apparently having acidic rock in common, the lists of species are markedly different in the different studies, with little overlap. There are a number of possible explanations of the discrepancies..

Firstly, only the Shetland study was a comprehensive survey. Many of the lichens occurring there may also be present in the other localities, but overlooked. Furthermore, Shetland is a much larger area than the other localities studied. Its geology is extremely varied, even including small limestone areas. The increased area and number of habitats involved would be expected to produce a larger number of species than would be found in the small areas studied in the other two sources. The following analyses support these suggestions. 

• The lichen table has a column giving the characteristic habitats described by Dobson for all the species found. Thirteen of the 55 species were more or less exclusively maritime or submaritime. They would not, therefore, be expected to occur in the places studied in Lancashire and Argyllshire, which were not near the sea.

• Six more Shetland species characteristically belong to calcareous habitats, so would not be expected in habitats selected as having acidic dry stone walls. Neither of the other two localities had any lichens in calcareous habitats.

• Three further Shetland species, though found on dry stone walls on Shetland, are not typically found in this habitat, but belong more to soil (including soil, trees, fences, mineral spoil heaps, the surface of mosses and other plants.

• Many of the Shetland species are simply notes as occurring on siliceous rocks and/or walls. Siliceous rocks tend to be acidic, but are not always so, since they often contain basic substance as well as silica compounds. This would lead to further discrepancies with reports from acidic habitats elsewhere.

This leaves 33 species known to occur on acidic or siliceous rocks, though not always exclusively. Even with this reduced number, however, the discrepancies between the three studies are still marked. 

Turning to the other studies, two of the Argyllshire lichens are not characteristically acidic dry stone wall species according to Dobson. The same applies to some of the mosses. Some of the walls studied, though appearing to be acidic, may occasionally incorporate limestone, if only in hidden fragments of mortar. This could explain why, as shown in the moss table, three of the Bute mosses (Grimmia pulvinata, Homalothecium sericeum and Tortula muralis) and the Argyllshire Tortella tortuosa are listed by Watson (1981) as characteristically growing on the rock surface of calcareous walls. Further indications of the problem come from a look at the moss habitats habitats from Atherton, Bosanquet and Lawley. These indicate a calcareous habitat for one moss from Lancashire, three from Bute and one from Argyll; and a habitat not normally including rocks or walls for four mosses from Lancashire, one from Bute and two from Argyll. 

The small numbers of walls studied in Argyllshire and Lancashire and the widespread limits to expertise in identifying mosses and lichens are likely to have led to species being overlooked, and may account for some of the differences between different locations.

Conclusions

The results may reflect the impression from the results for vascular plants - that the flora consists at least partly of species growing in different habitats nearby, the specific nature of the walls setting limits to the particular species which can colonise. The studies reported here do not allow us to claim that there is a common type of community occurring on acidic dry stone walls widespread. Further surveys would need to be looked at to see if more common elements can be identified.

These further investigations should, if possible, take account of the varying composition of siliceous rocks, ie, whether they are acidic, basic or neutral. This is likely to be difficult, since it would be hard to be sure of the nature of the precise piece of wall on which a bryophyte or lichen was growing without chemical tests for each instance. Perhaps a more practicable first step would be to study all dry stone walls that were not clearly made of limestone. The results might suggest more specific wall types which could be investigated to search for identifiable specific communities.

Local bryophyte and lichen floras may already exist that would be helpful for this purpose. They would, however, need to be analysed in some detail, because of the variations in rock type likely to be found, and the specific detail needed may well often be missing.

Certainly, bryophytes and lichens offer more promising prospects for further study of dry stone walls than do the vascular plants recorded. Even looking at vascular plants yields occasional treasures - like Cryptogramma crispa (Parsley Fern) and Cystopteris fragilis (Brittle Bladder Fern) in Lancashire.

References

ATHERTON, I., BOSANQUET, S. AND LAWLEY, M. (2010) Mosses and Liverworts of Britain and Ireland: A Field Guide. British Bryological Society.

DALBY, K. and C. (2005) Shetland Lichens. Shetland Amenity Trust.

DOBSON, F. S. (2005) Lichens: An Illustrated Guide to the British and Irish Species. 5^th edition. Richmond, Slough.

FARMER, C. (2007a), Sat 5 May 2007, Glen Lonan. Nature Notes from Argyll.

 www.nature-diary.co.uk/2007-05-05.htm

FARMER, C. (2007b) Some colonists of dry stone walls in Argyll. dry-stone-wall-flora.co.uk Gallery.

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 wall Community? BSBI News 109: 9-12.

RODWELL, J. S. (ed.) (2000) British Plant Communities Volume 5: Maritime Communities and Vegetation of Open Habitats. Cambridge: Cambridge University Press.1987.

WALSH, M. (2008) Two Drystone Walls in Lancashire: An Environmental Appraisal. dry-stone-wall-flora.co.uk.

WATSON, E. V. (1981) British Mosses and Liverworts . Cambridge University Press, Cambridge.

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 Leicester Polytechnic, Leicester.