Ents, Christmas decorations and a nest: A lichen story

Lichens are fascinating.  Slow growing, pulling nutrients from the air and converting rocks into soil these consummate symbionts are found on all continents, grow from sea level to mountaintops and from the tropics to the arctic.  Their colors range from red and orange to gray-green to bright yellow.  Lichens are a combination of a fungus, and a photosynthetic microbe, either a green alga or a  cyanobacterium (formerly know as blue-green algae). 

Lichens on a sea cliff in Wales

Lichens are classified according to the fungus they contain rather than the by their photosynthetic partner.  The body of the lichen, the thallus, is made of fungal filaments called hyphae.  Within the thallus is the photosynthetic alga or cyanobacterium.  The photosynthetic cell produces sugars for the fungus and the fungus provides water and nutrients to the alga.   A lichen’s growth form is different from either the fungus or the alga when they live independent of each other. 

Lichens can live in very harsh environments and are among the first colonizers of bare rock.  Lichens secrete acids that begin the chemical weathering of rock to produce soil.  Lichens also commonly grow on trees trunks and branches taking water from rain and dew and inorganic nutrients from dust and bird droppings.  Lichens are very slow growing, a few millimeters per year in some cases, because optimal growth conditions may only occur a few hours a day.  In the high arctic lichens grow on the ground and are grazed by large mammals.  This is why one of the arctic lichens has the common name reindeer moss. 

Despite living in forbidding environments lichens are very susceptible to air pollution, particularly sulfur dioxide.   In urban Washington, DC parks, the lichen diversity is much lower than in parks in the surrounding area.  This difference in lichen distribution is due to the air pollution in town.

Lichens exhibit a number of different growth forms.  Some grow as a thin layer tightly appressed to rock, cemetery headstone or tree trunk.  These are the crustose lichens since they form a crust on their substrate. 

 

A crustose lichens in south Florida

Foliose lichens have flat, leaf-like thalli whose edges are not attached to the surface on which they grow. 

 

Parmotrema sp. a foliose lichen in North Carolina

Fruticose lichens are three-dimensional and branched.  Fruticose lichens resemble little trees or bushes and are used by model railroaders to add tiny trees to their displays.  Treebeard, the Ent in J.R.R. Tolkein’s The Lord of the Rings, had a beard of fruticose lichens

Cryptothecia rubrocincta or Christmas lichen is one of the most beautiful of all the lichens.  It grows throughout tropical America and extends its range into the subtropical regions of the United States.  You can find it growing near the coast from Texas to Florida and into North Carolina.  The Christmas lichen’s range tracks very closely the distribution of the cabbage palmetto.  This crustose lichen stands out on the tree trunks that support it because it is bright red.  The color of this lichen varies from red and pink to white and looks much like a Christmas decoration on the tree.  Two chemicals, beta-carotene and chiodectonic acid cause this lichen’s red color.  The algal symbiont produces the pigment beta-carotene and chiodectonic acid is produced by the fungus.  Both these compounds probably protect the lichen from ultraviolet damage and other environmental stresses. 

 

Cryptothecia rubrocinata, Christmas lichen in a Florida cypress swamp

Members of the foliose lichen genus Usnea is widespread and grows on tree trunks and small branches.  Usnea is gray-green in color and resembles small plants of Spanish moss.  Usnea strigosa has reproductive structures found on fungi that are not lichenized.  These flattened cups (apothecia) produce spores and are typical of cup fungi.  The apothecia release spores that germinate and the fungal hyphae must find their particular species of alga to reestablish the lichen symbiosis.

Usnea strigosa, a fruticose lichen with apothecia in North Carolina

 The scientific name of Spanish moss is Tillandsia usneoides, so-named for its resemblance to the lichen.  The Northern Parula is a warbler that winters in Central America and on Caribbean islands. In spring, Parulas fly to eastern North America where they nest.  On the coastal plain of the southeastern United States, the Northern Parulas make their nests of Spanish moss while further north they use Usnea for nest material.  Even this warbler notes the resemblance between the Usnea and Spanish moss.