Desert Lichen Microforests

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Lichen, rock, grass – Sonoran desert

Deserts and northern latitude terrains seem to be where most of the world’s lichens live. During my visits to the arid Sonoran desert, desert lichens have my attention. I’ve always loved deserts. Mystery and history sum it up. Desert land is the bones of the earth exposed; rock in endless arrangements and forms that are invisible in vegetation covered lands tell of how the earth has moved and moved, and moved again. Earth rises, sinks, slowly flows from one place to another, melting and cooling in an endless cycle. In this part of the Sonoran desert, the geology in some mountain areas is very complex because of all the different processes and movements over time. Geologists call one mountain range here the ‘Tucson Mountains Mess’, and it is a mess. All sorts of rocks are mixed together in improbable layers and configurations. I often stand in one spot and pick up 15-20 different types of rock without moving a step.

Samples of rock from a Sonoran wash

All these rocks were made in a different place, in a different way and at a different time from each other, but they all ended up on the top of the ground in the same place. Some geological processes are still not well understood, and one of the mysteries is how some rocks got in between other layers or areas of rocks that have nothing to do with each other.

The first book about deserts that influenced me was In_the_Deserts_of_This_Earth by Uwe George, a German naturalist. He described stones in the desert covered with what is called ‘desert varnish’, a thin layer of red or black that is from metallic oxides migrating to the surface of rocks.

Broken rock showing desert varnish on outer surface

It was long believed that weathering caused the varnish to form. Israeli scientists were studying the chemical composition of desert stones with desert varnish on them, when they accidentally found that the stones gave off carbon dioxide at night, when covered with dew, and reversed the process in the day, taking in carbon dioxide and giving off oxygen. The process stopped when the sun dried the dew. The rocks were photosynthesizing. Under magnification, the stones were found to be covered with many forms of life; algae, bacteria, fungi and of course our little friends the lichens. It was a microforest, invisible to the eye. While photosynthesizing, these life forms were removing minerals from the dust in the air, fixing them to the surface of the rock and changing the surface of the desert.

Wolfgang Wunderlich, a German geologist, studied these microflora in the Negev desert in Israel and found that the various species of bacteria and lichens were responsible for transforming the metallic oxides from the air and in the rock to the surface of the rock, and so were instrumental in creating the ‘varnish’, as well as photosynthesizing, which changed the chemical structure and composition of the rocks and the surface of the desert. A more complete explanation of how lichens and bacteria make desert varnish can be found in papers from university published papers.

An excerpt from: describes the process of making desert varnish:

“One of the most remarkable biogeochemical phenomena in arid desert regions of the world is desert varnish. Although it may be only a hundredth of a millimeter in thickness, desert varnish often colors entire desert mountain ranges black or reddish brown.

Several genera of bacteria are known to produce desert varnish, including Metallogenium and Pedomicrobium. They consist of minute spherical, rod- shaped or pear-shaped cells only 0.4 to 2 micrometers long, with peculiar cellular extensions. In fact, the individual cells are smaller than human red blood cells which are about 7.5 micrometers in diameter. Because of the radiating filaments from individual cells and colonies, they are called appendaged bacteria. All living systems require the vital energy molecule ATP (adenosine triphosphate) in order to function. In our cells ATP is constantly produced within minute bodies called mitochondria. As electrons flow along the membranes of our mitochondria, molecules of ATP are generated. The electrons come from the breakdown (oxidation) of glucose from our diet. Although varnish bacteria do not have mitochondria, they do have a similar inner membrane structure through which electrons flow to generate ATP. However, in varnish bacteria the electrons come from the oxidation of manganese and iron rather than glucose. Herein lies the marvelous adaptive advantage for producing a layer of black and red varnish on desert boulders.”

Desert lichen on granite in Picacho Peak State Park, Arizona

In an earlier post, there is an image of lichen buried inside rock. They are only millimeters below the surface but that is enough for them to do their work of changing the rock and bringing nutrients to the surface that are the basis for the development of all other plants.

Weather does play a part in bringing the metallic substances to the rock surface, but lichens and their companions do a lot of the work when they are present. Lichens, in a healthy landscape, cover about 8% of the land surface, so they instigate a significant amount of the chemical interactions on earth. Other than changing the rock itself, lichens fix up to 20% of the nitrogen in the soil.

Lichens on granite rock in Santa Rita Mountains
One of the tiny gardens of foliose and crustose lichens, on granite rock, Santa Rita Mountains

What looks inconsequential to us as we walk across the ground, whether in the forest or in the desert, is actually a delicate and intricate web of life that literally keeps the earth from blowing away and creates the nutrient chain that supports the whole Web of Life. If it sounds crunchy or you feel little things breaking underfoot, you are walking on a living, working part of the environment that is unnoticed but essential. Please stay on the trail and protect the biofilm. The life crunched underfoot will take decades to regrow.

If you carry a hand lens, you can stop once in a while (a very good idea anyway), get down on your hands and knees and look around through the lens. Besides being able to see the lichens in detail, you might find other mysteries and wonders.

The delicate biocrust from a hiker’s perspective
Most of the desert floor is covered in seeds, twigs, insect shells, scat, dry plant material and lichens
A tiny Arizona pincushion cactus
Mud tunnels of a translucent white ant

That’s just a few of the surprises found on a trail through that barren looking stretch of gravel. Wherever you are, stop and look down once in a while, get close, and then use the magic key of the hand lens to enter the miniature forests underfoot. There’s always more going on than noticed at first glance.

Please remember, when you are exploring to stay on the trail. Learning to walk on the living layer of biocrust without killing it is a skill that takes time and attention. Here’s a friendly challenge: spend one season of hiking observing everything possible from the vantage point of standing on the trail. Closer observation leads to realizing you will never see everything that is there! No need to wander off trail to find something new.

Sonoran sunrise
Sonoran sunset

Thank you for wandering in Lichen Land with me. Please share and like this blog to help lichens become a familiar and valued part of how we understand our Life Support System.

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