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, âleafyâ 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?
To just about everything you wrote, one could say, âYes!â Many lichens to have different niches, and that reduces competition. Some of course, have the same or similar niches. All those lichens on one branches are competing. The competition maybe minimized â or won! â by different growth forms, growth rates, chemicals. If you look along an old branch that fell, youâll see evidence of succession, gradual replacement of some lichens by others. Many lichens have chemicals that prevent overgrowth by other lichens or by mosses and liverworts, but lichens, mosses, and liverworts may overcome those barriers. Itâs a wild (though very slow) world out there in lichen land.
A quick look at some of the research papers suggests that there is niche partitioning and competition both taking place.
In Habitat overlap/niche segregation in two Umbilicaria lichens: a possible mechanism ( Larson 1984 ) the two similar species appear to have slightly different ways of getting water:
Controlled experiments showed that the upper and lower cortical surfaces of the two species functioned in very different ways so that U. muhlenbergii is wetted most rapidly by runoff and mist-like precipitation whereas U. papulosa is wetted most rapidly by large raindrops.
In Competition in lichen communities ( Armstrong & Welch 2007 ) suggest that there is a moving system of competition and symbiosis between members of lichen communities depending on the immediate resource needs. This seems to be a good mechanism for establishing the sort of complex patchwork communities seen in lichen dominated areas.
In both Quantifying the climatic niche of symbiont partners in a lichen symbiosis indicates mutualistâmediated niche expansions and Niche partitioning and photosynthetic response of alectorioid lichens from subalpine spruceâfir forest in north-central British Columbia, Canada: the role of canopy microclimate gradients ( Rolshausen, et al 2017 and Coxson & Coyle 2003 respectively ) indicate that there is quite a bit of importance placed on microclimates in determining where lichens will grow in a community.
There was a paper I read quite a while ago that looked at microclimates and small variations on the surface topography of boulders and exposed rock faces and found that even those very small differences were important in determining both type and distribution of lichens.
Iâd expect that differences in nutrients on the surface would also make a difference. On igneous rocks, for example, there are exposed mineral crystals representing different resources, and even on tree bark differences in texture lead to differences in moisture and debris accumulation.
Someone who is more familiar with lichens can provide a much better response, Iâm sure, but maybe some of the linked papers will help provide some useful insight.
I actually got a book from the library about lichens, and it is full of information, but it is a field guide: basically a list of statistics and features.
I guess the problem for me is, that with plants, I have a pretty intuitive feeling for what they are like and what they are doing. I know that members of the mint family like shady, damp and often riparian habitats, and that they have square stems and opposite leaves, and they have a pleasant aroma. There is lots of cultural mythology and significance to mint plants. I can look at them and describe them with my casual, non-technical vocabulary.
But with lichens? The âwhyâ of what they are doing is not apparent from their form or habitat. They donât have common names, for the most part, they arenât used in cooking very often, and they donât have an obvious growth and reproduction cycle I can tap into. Flowers are one of the first things that children learn to draw, but kids donât really learn to draw lichens!
So I guess my problem is, even though I can learn the facts about lichen, I canât tap into them on an âanthropicâ level the way I can with flowering plants.
Organisms can coexist in space and time if resources are limiting when they partition resources among themselves and so limit interspecific competition. The simplest example of niche segregation in lichens is soil/rock/tree. Some lichens are found on more than one (generalists) and some are found on only one (specialists). Within each of those ânichesâ there are subniches (soil type, rock type, tree species, shade/light, wet/dry, south facing/north facing, etc., etc, etc. Niches change over time as pioneer species prepare the way for successional waves of new niches.