What is the substratum, we often ask ourselves when discussing the identification of a lichen, is it basic or acid? Frank Dobson’s Key to Churchyard Lichens is arranged along the lines of substratum. Dobson’s Lichens and Smith et al.’s The Lichens of Great Britain and Ireland, however, also address the association of certain species with additional environmental conditions, such as lowland-upland gradients and relative pollution levels. I have often wondered about the relative importance of substratum and environmental conditions on the distribution of lichens, and how these conditions relate to each other. To learn more, I set out surveying twelve churchyards from the mouth to the source of the river Don in North-East Scotland, with the locations of the churchyards representing not only the gradient between lowland and upland, but also different levels of both (historic) sulphur dioxide pollution in a city and town, and nitrogen dioxide pollution in farmland areas.
Making sure I visited at least one church in each hectad, I thus surveyed the churchyards of St. Machars Cathedral in Aberdeen, Dyce Old Kirk, the cemetery and historic churchyards in Hatton of Fintray, and the churchyards in Kintore, Monymusk, Keig, Bridge of Alford, Kildrummy, Towie and Strathdon, followed by the cemetery in Corgarff. Though I did look at lichens on trees where present, here I focus on the saxicolous lichens as found on the gravestones, the church building and the boundary wall.
Each churchyard was visited for around an hour, during which I usually started with a detailed photographic record of the first three to five gravestones of each type (granite and sandstone, and tombstones), followed by a slow walk through the remainder of the churchyard to record any additional stones on which there appeared to be additional lichen species. Back at home, I would then identify the lichens (and make a list of unidentified species) on basis of good quality macrophotography, a combination of the above-mentioned guides, and photos on reliable online websites. A good knowledge of the lichen species, and lichen associations, found in NE Scotland helped with the confirmation of identifications.
In total, I found 75 lichen species, but this ranged from 12 to 34 saxicolous species per churchyard, with lowest numbers in churchyards near the mouth and highest numbers closer to the source of the river Don (see Table 1). When reading chapter on churchyard lichens in Oliver Gilbert’s Lichens, I was struck by the apparent diversity of substrata of gravestones and church buildings elsewhere in Britain. The rock type of gravestones did become more diverse as I moved upstream, but some churchyards consisted almost exclusively of granite. Though more abundant in historic churchyards, sandstone memorials were almost absent from modern graveyards. Only handful of instances marble or basalt gravestones were found. In addition, some of the churches were pebble-dashed in part or in whole.
Table 1: Churchyard lichens along the Don (the churchyards are arranged from the mouth of the river Don upstream: St. Marchar Cathedral (Aberdeen), Dyce Old Kirk, Hatton of Fintray Cemetery, Fintray Old Parish Church, Kintore, Monymusk, Keig, Bridge of Alford, Kildrummy, Towie, Strathdon, Corgarff).
This relative lack of diversity of saxicolous substrata in churchyards may well have been responsible for the relatively low numbers of species found in this survey. What’s more, the churchyards of St Machar and Kintore were clearly influenced by (historic) sulphur dioxide pollution and many of the lichen species here were no more than thin stains that could not be identified through photography as they lacked well-developed crusts and reproductive features. The churchyard of Towie stood out as it was almost entirely sandstone.
Along the Don there were a few clear shifts in the landscape. Somewhere between Kintore and Monymusk one moves from a more open lowland landscape with intensive arable and livestock farming to a mixed landscape with low hills and a combination of conifer plantations, estates and more or less intensive farming. Further upstream, between Towie and Strathdon, the hills become more exposed, still with conifer plantations, but farming lower down the slopes becoming more extensive. Interestingly, this seems to be reflected in the lichen species found. Nine out of the thirteen species found in St Machar churchyard are common species that were found along the full course of the river upstream. The first new introduction of species, those commonly found on bark of conifer trees as well, is in the historic churchyard of Fintray, which is surrounded by woodland. Then, at Bridge of Alford, another assemblage is introduced, with species that are also associated with conifer trees in more upland conditions. From Strathdon upstream, lichen species associated with upland landscapes are introduced. In a way, this stepwise introduction of species when one moves upstream is captured in the three Umbilicaria species found in this study. Umbilicaria polyphylla, first found in Bridge of Alford, is a species that can be encountered on dykes in the mixed farmland and conifer landscape in the middle reaches of the river Don. For Umbilicaria cylindrica, first encountered in Strathdon, one tends to have to travel well to the middle reaches of the river, and be in more upland settings. Umbilicaria proboscidea, only found near the source of the river in Corgarff, one would usually only encounter in the hills.
So what did I learn from this exploratory study regarding the relative importance of substratum and environmental conditions in determining lichen assemblages in churchyards? At the moment, my thinking goes along the lines of the substratum being an enabler, determining the range of species that one can possibly find.
Environmental factors such as the lowland-upland gradient, and associated landscapes and pollution levels, however, seem to put limitations on this range, shrinking that vast number of possible species on a certain substratum to a much smaller number. This exploratory and descriptive study is not the place to quantify the relative importance of substratum and environmental conditions for lichen assemblages found in churchyards, but it has shown the complex interplay between the factors that is difficult to reduce to substratum alone.
This article was first published in the winter 2022 edition of The British Lichen Society Bulletin.
This blog builds on the earlier blog post Churchyard Lichens along the Don.
scottishlichens.co.uk 