Lichen Survey

Lichen Survey

A Lichen Survey of the Bell Tower at Evesham Abbey, by Ivan Pedley (August 18th 2015). This report reproduced by kind permission of Ivan Pedley

Summary: 28 Lichen species, 4 Bryophyte species, and 1 arthropod were recorded from the tower.

The lichen community on the tower is weak with only 28 species recorded—it lacks both diversity and abundance (cover). Many of the lichen species which are present are nitrophiles (lichens which are encouraged by atmospheric nitrogen compounds) indicating that they exist in an environment rich in these chemicals (see later discussion). The low diversity and incomplete stone cover is not typical of built structures of a similar geology in other parts of Worcestershire and the Midlands and indicates a lichen community recovering from serious atmospheric pollution. The cause of this pollution is almost certainly historic high levels of sulphur dioxide resulting from the combustion of coal in the surrounding villages and is corroborated by the presence of considerable areas of blackened stone on the tower. This is noticeable under “under-hangs” associated with the string courses and mouldings on the tracery panels, which are discoloured by carbon deposits. These deposits are not present on stone surfaces which have been exposed to rain where they have effectively been removed, exposing the pale limestone substratum.

In spite of the weak lichen “flora” one species of the genus Xanthoria was recorded which is probably new to the county. It is similar to a related species, Xanthoria candelaria, but to date, although recognised as a new entity, has not been named. It grows high up the tower on a north facing buttress; its position is not threatened by the cleaning regime and hence poses no conservation issues.

Most of the lichens which are present, being photosynthetic organisms, are found on well-lit surfaces–on the east, south, and west faces of the tower and particularly on the chamfers above buttresses, string courses and battlement ramparts.

Patches of an inert chalky white encrustation covers large areas of the vertical surface of the tracery panelling and other parts of the tower, particularly in shaded areas of the upper and middle stages. This is the inert chemical calcium oxalate, with smaller amounts of calcite and gypsum, deposited as a result of the metabolism of a lichen, Dirina massiliensis f. sorediata (photograph 2)—see later discussion. The living lichen is still present as a slightly pale mauve coloured thallus growing in crevices between the panelling and the vertical tracery (photograph 3) but is present in much reduced amounts. Its depauperate nature perhaps reflects the historic acidification of the local environment coupled with the recent increase in summer temperatures over the past decade and the associated reduced humidity. The extent of the white encrustation reflects the lichens position early in the towers history–in places the pale coloured calcium oxalate can be seen under the black carbon deposits which pre date the clean air acts of the 1950,s onwards.

Photograph 2 Inert calcium oxalate produced during the metabolism of the lichen Dirina massiliensis forma sorediata (now absent from the area)

Photograph 3. Dirina massiliensis form. Sorediata colonising the corner cracks on several levels.

Removal of the encrustation is probably not commercially viable and will result in damage to the patina of the stone surface. The encrustation, being pale in colour, is not particularly disfiguring and it could be argued that is presence is protecting the underlying stone from frost action and from re colonisation by Dirina—the appearance of this stone underneath removed sections of encrustation is remarkably stable and uniform, and has the appearance of being recently quarried.

The toxic effect of copper compounds leaching from the lightning conductors is very noticeable by the absence of lichens on either side of the conductors out to a distance of 40cm. (photograph 4)

Photograph 4 The toxic effects of copper on lichens (indicated by their absence adjacent to the strappings and conductors.

The conservation techniques being used to remove the carbon deposits i.e. soft brushing after a clay poultice are not dramatically compromising the growth of the lichens growing on the tower—small areas are being lost as the carbon discolouration is removed—but recolonization will occur in the future.

I must thank the site manager Mr Nicholas Sharland and members of Sally Strachey Historic Conservation for his help and interest shown during the survey and for the patience of the conservation operatives working on the tower.

  • Introduction: The commissioning agent was Tristan Evans Principal consultant, Turnstone Consultancy for Sally Strachey Historic Conservation.
  • Methods. The tower was surveyed from top down in excellent weather on the 18th August 2015.
  • Field and Laboratory Methods: A number of samples were collected for identification by microscopy and, in the case of the inert incrustations, chemically.
  • Lichen names: follow Smith et al. (2009), with recent updates in the Bulletin of the British Lichen Society. Bryophyte names: Follow Atherton et al. (2010)
  • Conservation and evaluation: The conservation status of each species of Lichen was assessed during the survey. All species of lichen are common on similar structures built of oolitic limestone in the Midland Counties.
  • Results: A total of 28 lichen species, 3 bryophyte species and a notable arthropod were recorded during the survey and are listed in the table below.

Discussion:

The amount of carbon deposited on the surface of the tower is remarkable given the lack of heavy industry in the immediate area and the size of the village. The deposits probably result from mists, developing close to the River Avon, mixing with smoke from the village chimneys and forming a highly polluting smog. This is supported by the position of the worst carbon deposits—under string courses, ornamental masonry etc. where rain deposition of the carbon is unlikely, and where the subsequent cleaning effect of rain is not felt.

This pollution, together with the associated acidification of the environment associated with fossil fuel combustion, has seriously depleted both the abundance and diversity of the lichens. Lichens are very sensitive to environmental change, particularly to the level of Sulphur Dioxide. The lichens which are present on the tower are, in the main, relatively recent colonizers [although the limestone of the tower would, to some extent, have buffered the worst effects of the acid rain and a very limited lichen community would have been present even during the height of the pollution]. They are also nitrophiles, responding positively to high concentrations of ammonium compounds in the environment (ammonia, nitrogen dioxide etc.) produced as a result of farming practices, car exhausts emissions, and bird excrement—the later is probably the dominant source of nitrogen compounds where the tower is concerned. Many of the lichen species comprising the genera Caloplaca, Physcia, and Xanthoria are often referred to as ornithocoprophilous species –literally, “lovers of bird excrement!”—and are typical of nutrient enriched bird perching sites.

Except for one species (see below) all the lichens on the tower are not having a significant effect on its stonework, indeed, they may well be protecting the surface from the worse effects of frost action.

  • The green alga Klebsormidium crenulatum also grows on the tower, particularly on the finials and weather vane (photograph 5). This is alga is also a nitrophile which has dramatically increased across the whole of Europe in the last decade. The growth of both these groups of organisms on the tower (lichens and algae) have previously been kept in check by high levels of Sulphur dioxide in the environment (acid rain) and consequently the levels of ammonia compounds also present in the environment have gone unnoticed.

Photograph 5. The alga Klebsormidium crenulatum indicating a nitrogen rich environment

  • The white deposits on the middle and upper stages is mainly calcium oxalate derived from the metabolic action of the lichen Dirina massiliensis forma. sorediata on the underlying limestone. Over much of the tower these deposits are inert i.e. the lichen which produced it is no longer present—indeed, the toxic nature of the oxalate may be preventing further lichen growth. Dirina produces oxalic acid as part of its metabolism and this causes chemical erosion of the cementing agents in the underlying limestone biotransforming the molecular structure of the surface. On certain structures, particularly frescoes and similar works of art, the effect of this aggressive lichen species can be dramatic and serious. A figure of 1kg of oxalate on 1 square metre of fresco over 12 years in Italy has been published ( Seaward in Springer 2015) However on buildings this biodeterioration of the stone is insignificant when compared to abiotic erosive elements—e.g. wetting and drying, heating and cooling, and of course, freezing and thawing. All the obvious deterioration of the surface layer of the stonework is mainly the result of these abiotic factors. P

Photograph 6. Calcium oxalate encrustation and living lichen Dirina massiliensis f. sorediata. The lichen is red in colour due to the addition of calcium hypochlorite (the chemical test for Dirina). The calcium oxalate stays clear when tested showing the extinction of Dirina at this position.

  • A survey of the bryophytes was not part of the contract relating to the survey but four bryophytes were recorded during the survey—growing mainly on the chamfers of the plinth and buttresses in shaded positions. All are common on stone throughout the UK and their removal, if thought necessary, does not pose a threat to the species generally. However bryophytes do root into the surface layers of stone and their removal by simply scraping, or pulling off disrupts the surface patina. A herbicide, used selectively, is the best course of action, or simply to leave them in situ. They seem to be causing little structural damage and will grow again if removed.
  • One notable arthropod was seen during the survey. This was Polyxenus ligurus, the Bristly Millipede. There are few records for Worcestershire for this, at 2mm long, our smallest millipede, but it is likely to be under recorded. The processes involved in the tower’s renovation are unlikely to endanger its continued presence.

Photograph 3 Bristly millipede Polyxenus ligurus

Species List: Lichens

  • “Levels” are determined by scaffolding floors.
  • Rare: Less than 10 recorded specimens on the tower
  • Occasional: between 11 and about 100 records for the tower
  • Frequent: over 100 records for the tower and about 500 records
  • Dominant: many hundreds of records for the tower.
Species Name Frequency Position and Comment
Aspicilia calcarea Rare Level 2 buttress chamfer > W
Caloplaca aurantia Occasional Many upper levels > S
Caloplaca citrina sensu str. Rare Level 10 > E
Caloplaca dalmatica Rare Level 10 buttress > S
Caloplaca dichroa Rare Level 3 chamfer > E
Caloplaca flavescens Dominant All levels and aspects
Caloplaca flavocitrina Rare Several levels Trefoil ornamentation > E,W
Caloplaca saxicola Dominant All levels and aspects
Caloplaca teicholyta Frequent All levels and aspects
Diploicia canescens Occasional Lower plinth
Diplotomma alboatrum Occasional Several levels following mortar courses > S
Dirina massiliensis f sorediata Occasional Several levels in vertical crevices where shaded. Large areas of moribund residue
Lecanora albescens Frequent All levels but mainly vertical faces all aspects
Lecanora campestris subsp. campestris Rare Chamfers to buttresses level 2 > W
Lecanora crenulata Occasional Most levels in scattered colonies
Lecanora dispersa Frequent All levels and all aspects
Phaeophyscia orbicularis Dominant All levels and all aspects
Physcia adscendens Occasional Most levels in scattered amounts
Physconia grisea Frequent All levels and all aspects
Psilolechia leprosa Rare Associated with copper run-off from L. conductor
Toninia aromatica Occasional Facing south
Verrucaria macrostoma f. furfuracea Occasional All levels facing west and south
Verrucaria macrostoma Rare Level 2
Verrucaria nigrescens Occasional All levels in small numbers
Verrucaria nigrescens f. tectorum Rare Level 11 chamfers > south
Xanthoria calcicola Dominant All levels and aspects
Xanthoria parietina Frequent Most levels with the above species
Xanthoria sp. Rare One level on buttress >north.

Bryophytes:

  • Bryum capillare
  • Campylopus introflexus
  • Grimmia pulvinata
  • Homalothecium sericeum

Bibliography:

  • Atherton, I. et al. (2010) Mosses and Liverworts of Great Britain and Ireland a field guide. British Bryological Society.
  • Seaward, M.R.D. Lichens as agents of Biodeterioration. In Springer India (2015). D.K.Upreti et al. (Eds.) Recent advances in Lichenology
  • Smith, C.W. et al. (2009) The Lichens of Great Britain and Ireland. The British Lichen Society

Click here for a PDF version of the Lichen Survey

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