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.    

Winter Decomposers

Fungi make their living a variety of ways.  Some are parasites taking nutrients from living organisms.  Some are symbionts associated with the roots of plants that transfer inorganic nutrients to the plant in exchange for sugar and other organic molecules.  And some fungi decompose formerly living material.  These decomposers mainly break down dead plant parts. Without the decomposers, a walk in the woods would be wading through a sea of dropped leaves, a tangle of downed branches and impassable barriers of dead tree trunks.   We usually don’t see these vital components of the ecosystem except when they make their reproductive structures called fruiting bodies or more familiarly, mushrooms.  Most of the fungal biomass is in microscopic threads of cells called hyphae.  The hyphae of the decomposers growth throughout the dead plant parts and release enzymes that break down highly resistant plant polymers like cellulose and lignin.  The fungus then absorbs the breakdown products of these polymers and the plant parts are recycled.  Winter is not typically a good time for finding mushrooms in this area but the decomposers are an exception. 

Turkey tail fungus, Trametes versicolor, on a rotting log

On some recent hikes, we found several of these winter decomposers. One prominent fungus on dead tree trunks is the turkey tail (Trametes versicolor).  The flat caps of the turkey tail grow in large groups on dead trees.  They are quite striking with alternating bands of varying shades of brown, gray and white that resembles the tail of a turkey.  On the underside of the cap are small oval pores that produce the spores of the fungus. These pore-bearing fungi are Polypores, classified in the family Polyporaceae. The spores float through the air and if lucky enough to land on a dead tree, can establish another colony of turkey tail fungus. 

Fruiting bodies of Poronidulus conchifer

Small white mushrooms growing on small dead branches is Poronidulus conchifer another Polypore. The pores are found on the lower side of the fruiting body.  This fungus is closely related to the turkey tail.  The scientific name quite descriptive. The genus, Poronidulus, is from the Latin meaning “little nest with pores”.  When the fruiting body is small it is curled up and look like little bird's nests.  Since it is a The species name, conchifer, is Latin for “conch bearing” because the larger fruiting bodies look very shell-like. 

A common large mushrooms growing on downed logs and on dying live trees is the oyster mushroom Pleurotus ostreatus. This fungus is has a worldwide distribution and is a major decomposer of wood.  It is also a choice, edible mushroom.  The whitish caps of the fruiting body resemble oysters and that gives it both its common name and the species name (ostreatus is from Latin for oyster).  The fruiting body usually grows in groups on a tree trunk or log.  The underside of the mushroom bears spore-producing gills typical of members of the order Agaricales, the gilled mushrooms.  Oyster mushrooms make their living decomposing wood but have a more ominous source of nutrients.  Pleurotus ostreatus is also a carnivore.  Hyphae of the oyster mushroom produce droplets of a toxic protein that paralyzes nematodes found in and on the rotting wood.  Oyster mushroom hyphae then invade the body of the paralyzed nematode and consume the unfortunate worm from the inside. 

 

Oyster mushroom (Pleurotus ostreatus) showing spore-producing gills

Oyster mushroom on log with their typical oyster shape

Even in winter, the process of decomposition goes on, carried out by a group of fascinating fungi.