Hammered Shield Lichen

When you are out skiing, snowshoeing or hiking in the winter, or tramping in the woods in the summer, be on the watch for lichens growing on the trunks or branches of aspen and other deciduous trees, also on the branches of spruce. If you look closely, there are quite a few foliose and crustose species. One of the commonest and most noticeable is Parmelia sulcata, the whitish to mineral gray, foliose lichen in the center of the following image. A foliose lichen has leaf-like thalli that are loosely attached to the substrate.



Lichens are species that are actually made up of two separate species, a fungus and an alga. The fungus makes up the body that we can see but, if you section it and look at the cross section under a compound microscope you will see that, in this case, it encloses single-celled green algae. These belong to the algal genus Trebouxia.

In the close-up image below, you can see that the upper surface and margins of the lobes have numerous ridges and grooves that contain gray, powdery structures. The latter are called soredia and they occur in laminal soralia. The soredia are made up of fungal hyphae and algal cells. When the soredia come loose and are carried by wind they may land on the bark or branch of a tree, lodge there, then grow into what we see as the lichen.

Image 1

Lichens are not parasites, drawing nutrients from the substrate. Instead, they are simply growing on the bark of the substrate. They absorb moisture from the air like a blotter. They also obtain nutrients from wind-born dust.

Parmelia sulcata is one of our commonest lichens and it is cosmopolitan in distribution. You can read more about the species at

You can find additional images at

This is yet another example of the many fascinating species that we have. We just need to get outside, be observant, and have an inquisitive mind. They also fun to photograph. If you want to see a list of the lichen species found at the J.J. Collett Natural Area, go to checklists on their website –