Some lichens require a specific substrate and will grow on no other; some will grow on just about anything. These latter are the ‘weeds’ of the lichen world. It is worth any beginner lichenologist spending time getting to know them.
The main substrates are rocks and wood. Some lichens grow on or among mosses and on soil.
Silica Rich (SR)
Geologists have classified igneous rocks on the amount of silica they contain. The terms ‘acid’ and ‘basic’ have traditionally be used to describe the rock’s silica content. Acid rocks (silica rich or SR rocks) have >60% silica in the form of silicate minerals and > 10% free quartz. Granites, granodorites and rhyolites are examples of SR rocks.
Instead of the terms ‘acid’ and ‘basic’ to describe igneous rock substrate, this website uses the term SR (silica rich) and SD silica deficient.
The main geographical areas of Ireland with SR rocks are Donegal (granites), Connemara & Mayo (gneisses and schist’s), Wicklow Mts. (granites), Mourne Mts. (granites). Lichens seem to avoid large outcrops of quartzite, probably because it is difficult for them to get a foothold. Good quality quartzite has a greasy ‘impossible-to-adhere-to’ texture.
Silica Deficient (SD)
Basic rocks, or silica deficient rocks (SD) are composed mainly of iron, magnesium and calcium with 45% to 53% silicates and a very minor amount of free quartz. Basalts (Antrim Plateau) and Gabbros are examples of SD igneous rocks.
Limestone is the most common sedimentary rock in Ireland. Ireland ‘sits’ on a bed of limestone that is exposed throughout the centre of the country and in a few coastal regions such as Clew Bay, Dublin city and the Burren. The dominant lichen type on rocks is crustose.
Mortar is rich in lime and is a silica deficient ‘rock’ substrate. Granite walls with old mortar can host a great diversity of lichen species.
Tress are a primary substrate for all lichens types (crustose, foliose and fruticose). One of the principal limiting factors is the pH of the bark. Bark pH is described using the terms acid and basic. A bark’s pH is dictated to some extent by the pH of the soil the tree is growing in, although coniferous trees are always acid. The lichen flora on a tree also varies from ground level to the canopy.
Lichen flora on deciduous and coniferous trees can vary considerably, even within the same woodland.
As the term twig is ill-defined in lichen literature, we take it here to apply to recently fallen ‘small’ ‘branches’ of trees found in the litter layer. The pH of a twig changes after it has left the parent tree.
Old gate or fence posts can become a wonderful habitat for lichens, particularly if they are not chemically treated. In areas where rotten posts containing lichens are being replaced with new ones, try leaving the olds posts also to allow their lichens to colonise the new posts.