During the autumn and winter, I have had a lot of opportunities to observe lichens. With most of our flowering plants dead or dormant, lichens are one of the few things that are out and obvious.
One of my questions about lichen is why there is such a diversity of them, often growing in the same conditions. Most of the trees in my area, whether they are growing on the street or in a natural area, easily have four or five types of lichen growing on them: shield lichens, oakmoss, sunburst lichens, lung lichen, old-mans beard, whitewash lichen, wreathe lichens, to name just some of them. Some of these lichen have obvious morphological adaptations that make them very difference: the thick, “leafy” lung lichen in opposition to the much smaller sunburst lichen, for example. But I wonder why there is so much specialization in lichen, if the lichen that are obviously different are competing with each other, or if they are filling different niches. Do they have mechanisms to discourage other lichen from growing on the same tree? Along with other lichens, they also compete with moss, ferns, free-living algae, and flowering plants, all on the same tree.
I know there is probably a lot of technical works written on this, but from a practical point of view, is there an easy rule to explain what lichen grows where, and why?