Have you ever seen pale whitish-green dusty patches on the undersides of stumps and boulders - the places that stay dry even on a rainy day? These dusty patches are very common in every forest. They are the dust lichens (Lepraria sp.), and form a whole unique ecosystem in places that are too extreme for anything else. The sites where they grow are micro-deserts within the rainforest. These are the spots that are so dry that mosses and other lichens cannot live here. Such micro-sites are on the lower sides of branches, boulders and stumps, as well as areas on tree trunks that are sheltered from rain or flowing water.
To live in very hostile environments requires special adaptations and the dust lichen has a very special way of dealing with this difficulty. Actually there are a number of different kinds of dust lichens, but many of them cannot be told from each other without complex chemical tests, so it is easier to think of them as one type of organism, unless you have access to a chemistry laboratory. The dust lichen has simplified its structure and life style about as much as it is possible to simplify a structure or life style. All parts of the dust lichen are the same - dust. Rub your finger across the surface of one and it will be covered by a fine layer of dusty particles. All the particles are exactly the same, and are called soredia. An individual soredium is so small we cannot see it without a magnifying lens. What we see as dust are thousands of these particles. Each one is a ball of fungus threads and in the centre of the ball are a few single-celled microscopic plants - green algae. The deep green coatings often seen on rocks and tree trunks are colonies of green algae that are closely related to those in the dust lichen. The little balls bud off other little balls and the whole lichen is entirely made of little balls. This is the way dust lichens live and reproduce. They have no form of sexual reproduction. There are millions of these little particles, each one a simple ecosystem of two organisms - a fungus and a plant. When we walk through the woods we disturb myriads of them, and carry them with us to other sites. Birds, insects, and breezes do the same. Most of the soredia do not find a favorable place to grow, but occasionally a few of them do find a newly created bare surface, and start a new dust lichen. This process of initiation, however, is well below our level of awareness and has never been directly observed in a natural environment.
How do dust lichens live in dark, dry places? They grow very slowly, and do not need much light. Because other organisms cannot grow in these dry spots the dust lichen does not need to worry about competition, and can take its time, depending on how much light it has available. The answer to the second question is very strange indeed. The dust lichen does not need liquid water. Unlike its neighbors it uses water vapor. In the forest during most of the year the air is very humid and this lichen can use the little bit of moisture that is delivered to it in the air. In fact dust lichens repel liquid water. Vapor can enter them but liquid cannot. You can see this quite easily. Pour a little water on a dust lichen and it rapidly runs off. What little remains on the surface pools in small droplets, and does not soak in. Whatever substance is responsible seems to be more effective than many rain-clothes. When viewed under a microscope the dust lichen fungus threads look quite different from the threads of other fungi. Other fungi are transparent and you can see right through them. The dust lichen is opaque. The algae inside its small particles cannot be seen unless the particle is broken open.
Wherever you walk in the woods you will encounter this unassuming little organism with its secret life style. The little lichen that makes its own simple ecosystem, one that is as simple as an ecosystem can be.