Lichen Distribution

Lichen Growth and Distribution [Part I: In disturbed areas]

On a university campus, there are a few interesting trends of lichen growth on trees. A university campus is a location of moderate disturbance; having high vehicle and foot traffic, one sees a smaller diversity of lichens than in, say, a forest preserve or a park. And yet, looking closely at the trees, one finds that there are still lots of lichens growing there.

The way that these fewer species of lichens are distributed on their tree substrates varies by lichen species and by tree species.

In terms of trees…

For instance, on smooth-barked trees, lichens can have a hard time establishing. Yet, looking closely, one finds a pattern of Elbows and Armpits; lichens are able to establish/thrive in the nooks of a growing tree.

This also applies to wrinkles or cracks in the bark.

On rough-barked trees, one typically sees a larger abundance of lichens, all over the tree [trunk, branches, etc]. On these trees, lichens tend to be the most abundant on the oldest part of the tree [trunk], unless the trunk is exposed to a high amount of disturbance [people peeling bark, air pollution from road etc, in which case the branches have a thicker density of lichens].

Some trees, as they grow, transition from smooth bark to rough/flaky bark. On these trees, lichens are actually more abundant on the branches, as the flaking bark of the older part of the tree prevents long-term lichen establishment.

It is not just the roughness or smoothness of bark that determines where lichens grow and how abundant they become. Bark pH, the number of other epiphytes [mosses, liverworts, algae, cyanobacteria, fungi], and availability of light and moisture also play a role in the distribution of lichens on a given tree.

When it rains, take a look at the trees – often you will see brighter greens, blues and yellows on the bark than what you see when the tree is dry. These are the lichens; the lichenized fungi and their algae swelling happily with water. Take a look at the patterns of these bright colors; often you will notice they follow the “drip line” of the tree.

In terms of lichens…

The lichens themselves also influence their own distribution, depending upon how they propagate and how fast they grow. Smaller lichen propagules are more likely to be picked up by the wind and taken to another part of the same tree or to a different tree altogether. Lichens that make larger propagules, either larger [heavier] fungal spores, large soralia [loose associations of the parental fungal hyphae and algal cells] or fragmentation, may be more susceptible to gravity and water flow, in which case you can often see lines of the same species of lichen running down the side of a tree.

These patterns may differ in less disturbed environments, given the higher number of lichens and other epiphytes competing for space. In addition, these patterns may differ for lichens that grow on rocks [saxicolous] or as a part of soil crust communities [terricolous]. Trees are a unique substrate for lichens and other epiphytes, and much research is still needed to determine the many factors influencing lichen distribution and abundance on trees.