Country diary: tiny shield lichen tell a big story

In the trash under trees battered by storms lie sticks covered with what looks like flaky aluminium. This stuff is shield lichen, Parmelia sulcata, which is still alive, even though the branches it grew on are now rotting on the ground. Shield lichen is a foliose lichen, the ones that form leafy lobes called thalli. These are about 5mm wide, loosely attached in sprawly rosettes, and grow on branches, twigs and tree trunks. From a distance, the lichen is silvery grey but looking closer, the thalli have a bluey-green cast.

Lichens are an intimate association, a symbiosis, between different and unrelated microorganisms – alga and fungus. The shield lichen is a community that includes Marchandiomyces and Nesolechia fungi, which create the architecture that harbours the single-celled green alga Trebouxia – a photobiont, turning sunlight into carbohydrates for the fungi and receiving some nutrients and vitamins in return. Shield lichen are fairly resistant to pollution and so are cosmopolitan and common, and their thalli grow 2mm-5mm in a year. There are some species, particularly those growing on rock in the Arctic, that only grow 0.5mm a year and there are individuals that are 5,000 years old.

The study of lichenometry over the past 25 years uses the growth rates of lichens to tell the story of climatic change and how long rock surfaces and glacial deposits in the Arctic have been exposed. That these complex and sophisticated living communities of very different organisms can be read as historical documents is fascinating. Looking at the shield lichen under the trees growing on something that is already dead, I wonder what it can tell us. Will the chronicles in its thalli be read like ancient illuminated texts that hold secrets about what lies beyond the world as we see it? Is it sensitive to extra-terrestrial forces? Did it pick up anything from the “heatwave” of masers – microwave lasers – that burst from the new star, called G358-MM1, 22,000 light years away last year?

We are, despite our destructive self-obsessions, communities of organisms affected by the universe. With only this rapidly changing planet to cling to like a dead stick, perhaps lichens can also be read as the poetry of peaceful coexistence.