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Saturday, December 12, 2015

Likin' Lichens

A fruticose Slender Orange Bush Lichen, Teloschistes exilis, makes
its home among various foliose lichens.

What lies beneath our feet and in front of our eyes, typically small and unassuming, sometimes brightly colored, but always waiting patiently for their contributions to take hold?  Lichens! Composite organisms made up of a fungus and a photosynthetic partner, most commonly an algae, lichens are ecosystem pioneers.  They break down rock surfaces and prepare areas for mosses, grasses, and trees to follow.  Certain species that also contain cyanobacteria can improve the fertility of the soil by adding necessary, usable nitrogen, and many species are used as bio-indicators of air quality.  They are also highly efficient collectors of airborne substances, and recycle these substances into the soil.  Historical uses for lichens include food, natural dyes, and herbal remedies. 

Mexican Yolk Lichen, Candelina submexicana, is a good example of a crustose lichen.

Lichens can be observed in many places, including on the ground, on rocks, and on trees.  The surface, or substrate, on which they are found provides a place for them to attach and grow.  Some species are substrate-specific while others can grow on a wide variety of substrates.  Ground substrates can include sand, soil, mosses, and downed decomposing logs.  These lichens act as soil stabilizers and contribute to soil fertility.  Rock substrates include natural substances like cliffs, talus slopes, boulders, and pebbles, and man-made substances such as concrete and roof shingles.  Lichens on rock substrates often colonize the rock and act as decomposers, turning it into soil.  Tree substrates include both deciduous trees such as oaks and conifers such as firs, cypresses, junipers, and pines.  Older trees often have a greater diversity of lichens, partly due to a more fissured bark that has a specific texture, chemistry, and moisture-holding capacity.  

Cumberland Rock-shield Lichen, Xanthoparmelia cumberlandia, is a type of foliose lichen. 

Lichens are mostly commonly divided into three growth forms: crustose, foliose, and fruticose. Crustose lichens look like they are spray-painted on their substrate, and their lower surface or ‘medulla’ is the same color as and blends into that substrate.  Foliose lichens are leafy growths with distinct lobes, and their medulla is a different color.  Fruticose lichens are often bushy or shrubby growths, and can be highly variable.  They often form obvious disk or cup-shaped structures called ‘apothecia.’  

Western Antler Lichen, Pseudodevernia intensa,
is a fruticose lichen.
Sundew Beard Lichen, Usnea cirrosa, shows
off its apothecia.

As composite organisms, lichens have been used worldwide as indicators of air quality.  They are very sensitive to the presence of low levels of sulfur, nitrogen, and fluorine-containing pollutants that adversely affect their community composition, growth rates, reproduction, biomechanics, and appearance.  By concentrating a wide variety of pollutants in their tissues, they act as pollution monitors, and are the subjects of many biomonitoring studies regarding air quality and climate change being conducted jointly by the U.S. Forest Service and the Environmental Protection Agency.  Since lichens are long-lived, can be observed in the field in any season, and with many species having extensive geographical ranges, they allow for changes in pollution to be studied over large areas and long periods of time.

As you can see, the lichen has almost all it needs to survive.   While most species of lichens grow very slowly, they all require a proper substrate, clean air, moisture, sunlight, and warmth to thrive. Turning air into life, they are truly one of nature’s alchemists!