Species of the Week
Number 32--
March 19, 2007

In the Species of the Week feature of the Wildwood Web we took a close look at one of the species that lives in Wildwood.  To see the earlier featured species check the Species of the Week archives.


Common Green Shield Lichen

Flavoparmelia caperata

With the return of winter this past week, the flowers have gone back into waiting mode; even the coltsfoots were closed up when I went through on Sunday.  This week, then, we will return to looking at evergreens.  As we saw earlier, not everything that is evergreen in the park is a conifer; some ferns are evergreen.  This week we will meet an evergreen that is not even a plant, common green shield lichen, which is one of the most common and noticeable lichens in Wildwood Park.

Lichens grow on tree bark, on branches and twigs, on soil, and even on bare rock.  In Wildwood, lichens grow mostly on tree branches.  Paradoxically, one of the best ways to see lichens in the park is to look down, where you are apt to spot fallen branches and twigs covered with lichens.  All the pictures on this page were taken of lichens on fallen branches and toppled trees.

As we mentioned above, common green shield lichen is not a plant.  In fact, all lichens are combinations of two very different organisms, a fungus and either an alga or a photosynthetic bacterium.  The lichen's body is made of fungal strands, but under the upper surface there is a layer of either one-celled green algae or bacteria.  Most lichens, including green shield lichen have green algae.  The algal cells are able to photosynthesize, using the energy of sunlight to provide sugars both for themselves and for the fungus.  Lichen fungi are never found growing alone, without their algae.  They are thus dependent on the algae for survival and cannot get food any other way.  The algae, on the other hand, are perfectly capable of living on their own.  Inside the lichen, the algae are somewhat protected, and so do get some benefit.  Lichens can often grow in habitats much drier or more exposed than the algae can survive in alone. 


Close-up of surface.

  Lichens tend to be very sensitive to pollution, absorbing toxic compounds that fall on the them in polluted rain or fog.  Urban areas tend to be nearly devoid of lichens for that reason.  The air in the city of Radford is apparently not too bad, since a number of lichen species grow in Wildwood Park, on the University campus, and on trees generally.  However, the number of species that can survive here is but a small fraction of the diversity you can find in the national forests of the nearby mountains.

Green shield lichens, the genus Flavoparmelia, are easily recognized as round to oval rosettes clinging to tree bark or rocks and spreading over them.  The rosettes are made of flattened fungal bodies which branch at the ends into rounded lobes.  The surface is yellowish green, becoming greener when wet as the algae inside migrate closer to the surface.  The older parts often become wrinkled, and covered in places with a powdery material, called soredia, as seen in the close-up at right.   The underside of the lichen is black except for a brown outer edge, as seen below.  The undersurface is covered with short black root-like structures which help attach it to the branch.  If you break the lichen open you can see that the inside is made of pure white cottony strands of fungal material, as seen below where the fragment was broken off a rosette for examination.  The powdery soredia on the upper surface are reproductive structures, tiny balls of fungal strands wrapped around algal cells.  They can blow or wash away to a new location and start growing into a new lichen.

Common green shield lichen grows on bark on the trunks and branches of all kinds of trees; it does not harm the trees.  In Wildwood it seems to be fond of white pines.  It can also sometimes grow on rocks.  It can be found from Nova Scotia to Manitoba, south to Oklahoma, Texas and Georgia.  It also occurs in the west, coastal Oregon and California, Arizona, New Mexico, Colorado, and south into Mexico.  Being relatively insensitive to pollution, compared to most other lichens, it is one of the most common lichens in America, often easily spotted along roadsides.  It is easily found in Wildwood by looking on the ground for fallen branches, especially on the east slope of the park.  There is a large tree recently down over the main trail on the east side which has many colonies of green shields on its branches and on the broken branches littering the ground around it.  The second picture above is of a colony on a broken branch of this fallen tree.  

Underside of lichen

Irwin Brodo and his collaborators report in Lichens of North America (2001, Yale University Press) that the Tarahumar people of Mexico have dried and crushed common green shields and used the powder to treat burns.

Green shield lichens are in the Parmeliaceae, the Shield Lichen Family.  They used to be in the genus Parmelia, but that genus was considered too large and unwieldy and was broken down into many smaller genera.  The name Parmelia comes from the Latin parma, which means "shield;" hence "little shield."  The prefix flavo- means "yellow" and alludes to the yellow-green color.  The species name caperata is Latin for wrinkled.

Lichens are fascinating in their shapes and colors.  Look for common green shields on fallen branches in the Park, and look also for other lichens that can be found there.  When visiting the national forests look for the many other species of lichens too sensitive to live in the city.


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