Very slow progress was made the other day while I was cutting down the remains of a dead oak tree as I kept getting distracted by the diversity of mosses and lichens growing on the dead bark.
Lichens are amazing plants. Mark Seaward's census catalogue lists 1,134 species recorded growing in Ireland, so they are a pretty significant aspect of our natural heritage. While they are regarded as individual 'plants' and 'species', lichens are unique in that they are the only known life forms on the planet that are composed of two or three different organisms living together.
The two constituent organisms that form most lichens are either a fungus and an alga or a fungus and a bacterium. Sometimes all three live together: fungus, alga and bacterium. The two or three constituent organisms live together in a close and intimate biological relationship.
With so many species there are several different domestic arrangements, but the general rule is that the algal partner in the relationship makes food using sunlight in the same way that grass and all other green plants grow. Photosynthesis is the chemical reaction that drives the continuation of so much of live on Earth.
What the fungal partner brings to the relationship is its ability to absorb nutrients from the environment. Sometimes the fungus has to absorb nutrients either directly from the air or from rainwater when the lichen is growing in such a challenging desert environment as the bare vertical rock face of a headstone in an old graveyard.
Tree lichens thrive better on dead wood than on the wood of a live tree. The most abundant species on my dead tree was the very common Hammered Shield Lichen, the one pictured above. It is an easily identified species. Its body is made up of overlapping leafy lobes each with a squarish end. These lobes are a very pale silvery, bluish-grey colour above and a dark treacle-brown colour underneath.
The dark colour underneath borders the pale lobes giving them distinctive brown tips. Its fruiting bodies are shield-shaped. The leafy lobes have a network of sharp ridges and depressions giving the lichen a hammered appearance, hence its English name. Hammered Shield Lichen comes in a number of forms. It is very common and widespread and since it is one of the lichens that is very tolerant of pollution it is no stranger to tree trunks in parks and gardens in built-up areas, villages and towns.