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.
