Overview of Lichens
Humans and Lichens
Although lichens are a conspicious and significant component of the
ecology of Sky Meadows State Park, they have not been systemically
studied. Future research into the various lichen "species" inhabiting
the park needs to be conducted.
Overview of Lichens
The following is an edited version of the Wikipedia entry for "Lich-
A lichen is a composite organism that emerges from algae or cyano-
bacteria (or both) living among filaments of a fungus in a mutually
beneficial (symbiotic) relationship. The whole combined life form
has properties that are very different from properties of its compon-
ent organisms. Lichens come in many colors, sizes, and forms. The
properties are sometimes plant-like, but lichens are not plants. Lich-
ens may have tiny, leafless branches (fruticose), flat leaf-like struc-
tures (foliose), flakes that lie on the surface like peeling paint (crust-
ose), or other growth forms. A macrolichen is a lichen that is either
bush-like or leafy. A microlichen is everything else. Here, "macro"
and "micro" do not refer to size, but to the growth form. Common
names for lichens may contain the word "moss" (e.g., "Reindeer
moss", "Iceland moss"), and lichens may superficially look like and
grow with mosses, but lichens are not related to mosses or any plant.
Lichens do not have roots that absorb water and nutrients as plants
do. Instead they produce their own food from sunlight, air, water,
and minerals in their environment. When they grow on plants, they
do not live as parasites but use the plants only as a substrate.
Lichens occur from sea level to high alpine elevations, in a very wide
range of environmental conditions, and can grow on almost any surface.
Lichens are abundant growing on bark, leaves, mosses, on other lichens,
and hanging from branches "living on thin air" (epiphytes) in rain forests
and in temperate woodland. They grow on bare rock, walls, gravestones,
roofs, exposed soil surfaces, and in the soil as part of a biological soil
crust. They can survive in some of the most extreme environments on
Earth: arctic tundra, hot dry deserts, rocky coasts, and toxic slag heaps.
They can even live inside solid rock, growing between the grains. Some
lichens do not grow on anything, living out their lives blowing about the environment. It is estimated that 6% of Earth's land surface is covered by
lichen. Colonies of lichens may be spectacular in appearance, dominating
much of the surface of the visual landscape in forests and natural places,
such as the vertical "paint" covering the vast rock faces of Yosemite
The fungus benefits from the symbiotic relation because algae or cyano-
bacteria produce food by photosynthesis. The algae or cyanobacteria bene-
fit by being protected from the environment by the filaments of the fungus,
which also gather moisture and nutrients from the environment, and (usual-
ly) provide an anchor to it. Lichenized fungus may refer to the entire lichen,
or to the fungus growing in it. The lichen combination of fungus with algae
and/or cyanobacteria has a very different form (morphology), physiology,
and biochemistry than the parts growing by themselves. Lichens are said
to be "species", but what is meant by "species" is different from what is
meant for plants, animals, and fungi, for which "species" implies a com-
mon ancestral lineage. Lichens are really combinations of species from
two or three different biological kingdoms, so there is no common lineage.
By convention, lichens have the same scientific name as the fungus in them,
and are not classified according to the species of the algae and/or cyano-
bacteria growing in them. The algae or cyanobacteria has its own, unique, scientific name (binomial name). There are about 20,000 known species
of lichens. Some lichens have lost the ability to reproduce sexually, yet
continue to speciate. Recent perspectives on lichens include that they are
relatively self-contained miniature ecosystems in and of themselves, possi-
bly with more microorganisms living with the fungi, algae, and/or cyano-
bacteria, performing other functions as partners in a system that evolves
as an even more complex composite organism (holobiont).
Lichens may be long-lived, with some considered to be among the oldest
living things. They are among the first living things to grow on fresh rock
exposed after an event such as a landslide. The long life-span and slow
and regular growth rate of some lichens can be used to date events (lichen-
ometry). Many lichens are very sensitive to environmental disturbances
and can be used in cheaply assessing air pollution, ozone depletion, and
metal contamination. Lichens have been used in making dyes, perfumes,
and in traditional medicines. Few lichen species are eaten by insects or
Lichens are pioneer species, among the first living things to grow on bare
rock or areas denuded of life by a disaster. Lichens may have to compete
with plants for access to sunlight, but because of their small size and slow
growth, they thrive in places where higher plants have difficulty growing.
Lichens are often the first to settle in places lacking soil, constituting the
sole vegetation in some extreme environments such as those found at high mountain elevations and at high latitudes. Some survive in the tough con-
ditions of deserts, and others on frozen soil of the Arctic regions.
A major ecophysiological advantage of lichens is that they are poikilo-
hydric (poikilo- variable, hydric- relating to water), meaning that though
they have little control over the status of their hydration, they can tolerate
irregular and extended periods of severe desiccation. Like some mosses, liverworts, ferns, and a few "resurrection plants", upon desiccation, lichens
enter a metabolic suspension or stasis (known as cryptobiosis) in which
the cells of the lichen symbionts are dehydrated to a degree that halts most biochemical activity. In this cryptobiotic state, lichens can survive wider
extremes of temperature, radiation and drought in the harsh environments
they often inhabit.
Lichens do not have roots and do not need to tap continuous reservoirs of
water like most higher plants, thus they can grow in locations impossible
for most plants, such as bare rock, sterile soil or sand, and various artificial structures such as walls, roofs and monuments. Many lichens also grow as epiphytes (epi- on the surface, phyte- plant) on plants, particularly on the
trunks and branches of trees. When growing on plants, lichens are not para-
sites; they do not consume any part of the plant nor poison it. Some ground-dwelling lichens, such as members of the Subgenus Cladina (reindeer lich-
ens), however, produce allelopathic chemicals that leach into the soil and
inhibit the germination of plant seeds and growth of young plants. Stability
(that is, longevity) of their substrate is a major factor of lichen habitats.
Most lichens grow on stable rock surfaces or the bark of old trees, but
many others grow on soil and sand. In these latter cases, lichens are often
an important part of soil stabilization; indeed, in some desert ecosystems,
vascular (higher) plant seeds cannot become established except in places
where lichen crusts stabilize the sand and help retain water.
Lichens may be eaten by some animals, such as reindeer, living in arctic
regions. The larvae of a number of Lepidoptera species feed exclusively
on lichens. These include common footman and marbled beauty. However,
lichens are very low in protein and high in carbohydrates, making them
unsuitable for some animals. Lichens are also used by the northern flying
squirrel for nesting, food, and a water source during winter.
Substrates and habitats
Lichens grow in a wide range of substrates and habitats, including some
of the most extreme conditions on earth. They are abundant growing on
bark, leaves, and hanging from branches "living on thin air" (epiphytes)
in rain forests and in temperate woodland. They grow on bare rock, walls, gravestones, roofs, exposed soil surfaces. They can survive in some of the
most extreme environments on Earth: arctic tundra, hot dry deserts, rocky
coasts, and toxic slag heaps. They can even live inside solid rock, growing between the grains, and in the soil as part of a biological soil crust in arid
habitats such as deserts. Some lichens do not grow on anything, living out
their lives blowing about the environment.
They are not parasites on the plants they may grow on, but only use them
as a substrate to grow on. The fungi of some lichen species may "take over"
the algae of other lichen species.
When growing on mineral surfaces, some lichens slowly decompose their substrate by chemically degrading and physically disrupting the minerals, contributing to the process of weathering by which rocks are gradually
turned into soil. While this contribution to weathering is usually benign,
it can cause problems for artificial stone structures. For example, there is
an ongoing lichen growth problem on Mount Rushmore National Memorial
that requires the employment of mountain-climbing conservators to clean
Lichens are not parasites on the trees they grow on, but only use them as
a surface for anchoring themselves. Lichens make their own food from
their photosynthetic parts and by absorbing minerals from the environment. Lichens growing on leaves may have the appearance of being parasites on
the leaves, but they are not.
In the arctic tundra, lichens, together with mosses and liverworts, make up
the majority of the ground cover, which helps insulate the ground and may
provide forage for grazing animals. An example is "Reindeer moss", which
is a lichen, not a moss.
A crustose lichen that grows on rock is called a saxicolous lichen. Crustose
lichens that grow on the rock are epilithic, and those that grow immersed
inside rock, growing between the crystals with only their fruiting bodies
exposed to the air, are called endolithic lichens. A crustose lichen that
grows on bark is called a corticolous lichen. A lichen that grows on wood
from which the bark has been stripped is called a lignicolous lichen. Lich-
ens that grow immersed inside plant tissues are called endophloidic lichens
or endophloidal lichens. Lichens that use leaves as substrates, whether the
leaf is still on the tree or on the ground, are called epiphyllous or foliicol-
ous. A terricolous lichen grows on the soil as a substrate. Many squamul-
ous lichens are terricolous. Umbillicate lichens are foliose lichens that are
attached to the substrate at only one point. A vagrant lichen is not attached
to a substrate at all, and lives its life being blown around by the wind.
Lichens and soils
In addition to distinct physical mechanisms by which lichens break down
raw stone, recent studies indicate lichens attack stone chemically, entering
newly chelated minerals into the ecology. Over time, this activity creates
new fertile soil from lifeless stone.
Lichens may be important in contributing nitrogen to soils in some deserts
through being eaten, along with their rock substrate, by snails, which then defecate, putting the nitrogen into the soils. Lichens help bind and stabilize
soil sand in dunes. In deserts and semi-arid areas, lichens are part of exten-
sive, living biological soil crusts, essential for maintaining the soil struc-
Crooked Run Valley