The Secret Life of Greenshield

     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

chestnut oak mottled with greenshield


'toupee' on the side of a tupelo tree

'toupee' on the side of a tupelo tree


small round splotches of greenshield

small round splotches of greenshield




7 Responses to “The Secret Life of Greenshield”

  1. John Cochrane Says:

    What I called moss is more complicated than I thought.

    Thanks for the lucid explanation.


  2. Todd Says:

    what is “master naturalist training”?

  3. Judy Muller Says:

    This is fascinating, April. Just last Saturday I hiked in the Sandias and one of my friends was intrigued with the mosses and algae and lichens and mushrooms, stopping to photograph them all. His curiosity and interest allowed me to see more than I normally would have. And now I will notice the greenshield and know that the forest is thriving.

  4. Joan Brundage Says:

    Thanks, April, for sharing this. I always wondered what those patches on trees are about. What is the Master Naturalist training you did? Sounds fascinating. I wonder if I could do that here in Tucson?

  5. April Says:

    Many (but not all) states have the Master Naturalist program. It’s modeled after the Master Gardener program. To be certified as a Master Naturalist, one must take a training of 10 weeks or so, followed by some advanced training and 40 hours of service. To retain certification, one must do advanced training and service every year.

    Master Naturalist programs are offered through a university extension service or through the state government, or a combination.

    I have completed the formal training and am in the process of earning my certification. The program is great. I’ve learned so much, and the learning continues.

  6. Elizabeth Cottrell Says:

    Congratulations on your progress in the Master Naturalist program, April! I already consider you a master naturalist, but of course there’s always more to learn!

  7. Earline Walker Says:

    I’m pleased to read your observations on Common Greenshield. Because it is an environvemtal indicator, I’m always pleased and thankful for its message. You tweaked my interest and I spent a good amount of time refreshing my memory about the life and times of this lichen. Never underestimate the inquisitiveness of a Master Naturalist!

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