Shield Lichens at West Woods, Geauga County

Lichen-covered tree, West Woods Nature Center, Geauga County, Ohio

Linda Gilbert, who is a naturalist at West Woods Nature Center in Geauga County, Ohio texted me a photograph of the tree shown above, which stands in the center of one of the parking lots near the nature center and is covered with more lichens than I have ever seen on a single tree in northeast Ohio. Almost every square inch of the bark on the trunk of this red maple up to a height of 10-15 feet from the ground is covered with lichens. (I think it is a red maple – it’s hard to see the bark on this tree because of the lichens covering it)

Shield Lichens on red maple bark, West Woods, Geauga County

Virtually all the multi-colored lichens on the bark of this tree are foliose lichens, which have a leaf-like structure called a thallus with an upper and lower surface. The lower surface of the lichen thallus adheres to the bark of the tree, anchored by root-like structures called rhizines. The lower surface of the thallus is often a different color than the upper surface. Lichens do not penetrate the bark and harm the tree in any way – the bark is used solely as a substrate for supporting the lichen.

Most of the lichens covering the bark of this tree are shield lichens, which are often roughly circular and shield-like in appearance. Shield lichens are the first organisms to colonize tree bark, and most of the foliose lichens on this tree are either Common Greenshield, Rough Speckled Shield, or Hammered Shield lichens. These are three of the most common lichens throughout the Buckeye State, and in the rest of this article I’ll share some tips on how to recognize and identify them.     

Common Greenshield, Flavoparmelia caperata

The most conspicuous lichens on this maple tree are roughly circular, up to several inches in diameter, and are a distinctive pale yellow-green in color. These lichens are the Common Greenshield, Flavoparmelia caperata. The lower surface of the lobes, which are 3-8mm wide with wavy margins, is coffee-black, with brown edges. The upper surface of the lobes away from the edges is often wrinkled. The center of the thallus is covered with a mass of tiny granular structures called soredia, each of which is a tiny ball of fungal filaments surrounding algae. Soredia, which are the primary means of asexual reproduction for Common Greenshield, are easily rubbed off and dispersed by birds, mammals, insects, and wind currents. Common Greenshield is found almost exclusively on bark and weathered wood, including old fences and picnic tables.  

Common Greenshield is very sensitive to sulphur dioxide emissions from coal-fired power plants, and during most of the 20th century this striking lichen was extirpated from the industrial areas of northeast Ohio. With improved air quality since the passage of the 1970 Clean Air Act, Common Greenshield has reappeared and largely recolonized the affected areas.            

Common Greenshield, Rough Speckled Shield, and Hammered Shield Lichens

The photo above shows all three of the shield lichens that cover the tree. On the left is a section of the pale yellow-green thallus of Common Greenshield. In the center is a lichen with slightly darker green lobes and a large central mound of  rough granular material. This is Rough Speckled Shield Lichen, Punctelia rudecta, described in more detail below. Near the top left edge and right edge of the Punctelia rudecta are some narrow, blue-gray lobes of Hammered Shield Lichen, Parmelia sulcata, which is also described in more detail later in this article.    

Rough Speckled Shield Lichen – Isidia and Pseudocyphellae

The photo above shows an enlarged section of a Punctelia rudecta thallus. Notice that the lobes of the thallus are a darker gray-green in color, and the lobes are covered in white speckles and streaks. The top section of the thallus is covered with tiny, rough, granular structures which are called isidia. Isidia are fragile, asexual reproductive structures that break off and may be dispersed by wind, animals, and splashing raindrops.  Structurally, isidia may be warty, cylindrical,  club-shaped, scale-like, coral-shaped, peg-like, or branched, depending on the lichen species. The isidia erupt from pseudocyphellae, which are the white speckles and streaks on the surface of the lobes of the thallus.  

Parmelia sulcata with apothecia, West Woods Nature Center

The third type of shield lichen that may be found on the tree is Hammered Shield Lichen, Parmelia sulcata, which is shown in the above photo. Notice that the lobes of this lichen are narrow and strap-shaped, and blue-gray in color when dry. When the lobes are wet, their upper surface, called the cortex, becomes semi-transparent, allowing the green color of the algal layer to become more noticeable.

If you examine the photo above you will also see a number of small, circular cup-shaped structures with a brown interior and a white rim. These are sexual reproductive structures containing spores called apothecia. Apothecia are rarely found in Parmelia sulcata, but they are very common in many other lichen species. Unlike soredia and isidia, which are miniature fragments of the lichen containing both the fungal and algal components, apothecia discharge fungal spores that must land on a suitable substrate (e.g. bark) and quickly find an algal partner in order to create a new lichen.       

Another feature of the lobes of Parmelia sulcata is a pattern of white ridges and concavities that looks like the lobes have been beaten with a tiny ball-peen hammer, hence this lichen’s common name, Hammered Shield. Patches of soredia cover some of the lobes of this lichen. A close-up photo of the lobes of Parmelia sulcata is shown below.  

Hammered Greenshield detail – ridging and soredia

In order to examine the details of tiny lichen structures such as apothecia, soredia, isidia, and rhizines, you will need a magnifying glass, preferably 10x in power – don’t leave home without it! For in-depth study at home, a dissecting microscope is a necessary investment, but a compound microscope isn’t needed unless you wish to study lichen spores in detail. You will also need one or more lichen field guides and/or technical guides, which are described and reviewed in my recent blog article:

If you would like to try your hand at lichen photography, you can get excellent results with your smartphone. Check out my article, Lichen Photography with a Smartphone, in the Lichen section of the Ohio Moss & Lichen Association (OMLA) website:

Most fungi, including mushrooms, fruit for only a day or two before decaying and disappearing. Lichens, however, are long-lived and can be found and studied at any time of the year. 

I’m working with Linda Gilbert and Jennifer Windus, Vice President of the Ohio Natural Areas & Preserves Association (ONAPA), to plan ONAPA’s annual meeting, which will be held on Saturday, August 22 at Geauga County Parks’ West Woods Nature Center. One of the activities in the afternoon will be a lichen hike, and we will be sure to visit the lichen-covered red maple that is the subject of this article.

Let’s hope that we are well past the peak of the Covid-(19) pandemic by late summer, and able to once again gather in groups to learn from some great speakers and visit a few of Geauga County’s finest natural areas as part of our ONAPA annual meeting.

In the meantime, be well and stay safe. 


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