Lichens

 

Section Overview

Overview of Lichens

 

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Humans and Lichens

 

Section Overview

 

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-

ens".

 

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

National Park.

 

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

larger animals.

 

Ecological interactions

 

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

the monument.

 

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-

ture.

 

 

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