A couple of weeks ago we visited Karin’s family in Jutland and went for a couple of long walks around the area. One of these took us through some very nice mixed pine, oak, and birch forest close to a river. The forest was anchored into a thin horizon of mulchy topsoil, beneath which was almost pure sand, a post-glacial legacy of the wider, wilder rivers that ran through the region at the end of the last Ice Age.
Where our path ran parallel to the river I noticed that the exposed vertical sections were far from lifeless: the sandy faces had been colonised by algae, lichens, fungi, cyanobacteria, and mosses. These biological crusts had stabilised the sand and prevented it from eroding further back into the bank. On a miniature scale they were doing what forests and other vegetation does in mountainous areas all over the world: preventing landslides.
Biological crusts in turn provide opportunities for ferns and seed plants to germinate and gain a foothold: they are often the starting point for further ecological succession.
Not only are these crusts acting as substrate stabilisers and seed beds, but all of the usual ecological processes of photosynthesis, nutrient acquisition, decomposition, carbon storage, symbiosis and competition are taking place in just a few millimetres of biodiversity. There’s a lot going on in these thin veneers of life.