My neighbor recently told me that she was concerned about the greenish patches she had noticed on the trunks of many of the trees in the forest here.  She feared these green growths might somehow be harming the trees. 

     Fresh from my Master Naturalist training, I was able to assure her that the pale green–dotting some trunks and branches and practically enveloping others–is nothing to worry about.  Called greenshield, these greenish patches are a common lichen that is harmless to trees.  In fact, greenshield on trees is a good thing because it is a sign that the surrounding air is clean.  Like all other lichens, greenshield is highly sensitive to the toxic compounds in air pollution.  Hence, greenshield and other lichens abound in forests but are mostly absent from polluted cities.  

     With its different looks on different trees, greenshield fascinates me.  For example, the two giant chestnut oaks I see outside my window right now are almost entirely a mottled pale green.  Only the youngest, thinnest branches of these trees are a solid brown.  Farther down the hill I see a corkscrew willow with several roundish green splotches about the size of coffee saucers here and there along the tree’s main branches. 

     Then on a tupelo tree near the other side of the house is one particularly interesting greenish splotch.  As I observe it through the window while I wash dishes, I am reminded of a toupee.  About head-size, thicker and much more textured than most of the other greenshield I see, it could have been parked there on the tree between wearings.  

     As with most aspects of nature, even a tiny bit of research reveals highly complex processes taking place.  And lichens are no exception.  A lichen is actually a combination of fungus and algae.  The lichen takes the form of the fungus, and the algae lives inside it.  The algae makes food directly from sunlight, and that food nourishes the fungus as well.  And the fungus, by surrounding the algae, protects the algae from drying out.  The fungus and the algae grow together as a single being, spreading very slowly over many decades.

     The greenshield’s powdery outer surface is the lichen’s reproductive system.  The ‘powder’ is made of spores, which are a combination of fungus and algae.  Tiny balls of fungal strands wrapped around algal cells are blown or washed away to a new location, where they may start growing into new greenshield.  Greenshield attaches itself to a tree with black, root-like structures on its undersurface.

     Knowing just a little more about greenshield than I knew yesterday, my pleasure in seeing these clean air-loving lichens on the trees all around me is a joy.–April Moore 

chestnut oak mottled with greenshield

  

'toupee' on the side of a tupelo tree

  

small round splotches of greenshield

 

    

   

This entry was posted on Tuesday, August 3rd, 2010 at 1:51 pm and is filed under Insights and Visions on the State of the Earth. You can follow any responses to this entry through the RSS 2.0 feed. You can leave a response, or trackback from your own site.