The Rock Pioneers: Lichens and Mosses

March 17, 2024

We don’t have many big rocks throughout the grasslands of the Mitigation Lands, so the boulder on the north end of the valley as we enter the wetlands is something I like to look at very closely. This boulder is covered in lichens and mosses, which are specific to the environment of noncalcareous rock. This is a much-desired habitat for them since we have so few surface areas like it.

The number of different lichens is mind-boggling. The most common lichen covering the rock is an interesting foliose lichen called Emery rock tripe (Umbilicaria phaea). This unusual-looking lichen is commonly found in every county of California on boulders in open areas. According to the book Lichens of California by Hale and Cole, the genus Umbilicaria is attached to rocks by a central cord. This was likened to an umbilical cord and was thus named “Umbilicaria.” It has black apothecia, small cup-shaped structures on top of lichens that release sexually produced spores, which are deeply fissured in concentric circles. As you can see from the photographs, this lichen looks like an undersea world of coral. Its patterns of spots, apothecia, and array of colors, from beige to dark brown, make it rich and magnificent.

Another lichen, a light sage green color, stands out on this boulder because it is foliose, not crustose. In an earlier blog post, I discussed the difference between the two. Foliose lichens are leafy and lobed-shaped, and crustose lichens are flat and crust-like. This is one of the larger species of rock-shield lichens (Xanthoparmelia cumberlandia) and is commonly found on rocks. There are many places where these two lichens are growing into each other, and I wonder if there is some competition for space happening on this boulder. Other crustose lichens cover all the possible bare spaces not being utilized by the mosses and the foliose lichens already pointed out. Crustose lichens form crust-like patches on whatever they are growing on. There is much variation in how these lichens present themselves; they can have rough worts, a cracked and thin surface, or no visible thallus (the lichen’s main body). Their apothecia are delightful to look at with a hand lens or a dissecting scope. I found some rim lichen (Lecanora sp.) and maybe a disk lichen (Lecidella sp.). Many lichens need to be keyed using chemicals such as potassium or iodine.

The north and south side of this large boulder look completely different. Exposure is everything for rock-dwelling plants and organisms. Think of the difference between a north-facing garden and a south-facing garden. What does well on that cold, shady north side compared to the hot, exposed south side? The photos show that the north-facing part of the rock is more than 50% covered in mosses. On the south side, it is 100% covered in lichens. The east and west sides are more diverse, with the east side showing the most species of mosses and lichens together. The mosses on this boulder like to grow on rocks rather than soil or trees. They have specialized root structures that can hold onto the rock’s surface. Grimmia laevigata is one of the hardy, drought-tolerant mosses on the south-facing side of this boulder.

Focusing on one small area to thoroughly investigate the diversity of life, such as a large single boulder, feels very satisfying and eye-opening. This fills me with wonder at how complex and beautiful the natural world is!