I get letters and the occasional phone call from people thinking that lichens are doing something bad to their apple tree azaleas, or camellia.
When, in point of fact, they're harmless.
Lichens are unique organisms. But, they do pose a significant problem to scientists whose job is to classify them. This is because lichens aren't a single living thing. They’ve been around over 500 million years and over that time they’ve become symbiotic with a fungus and an alga working together to benefit each other.
They are so closely interconnected with each other that they seem to be a single organism. Curiously, the fungal side of the species has to have an algal partner to survive. Whereas, the algal partners can be found as free-living species. The same fungus can form different lichens depending on which of the algae they incorporate.
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In some instances, there are more than one algal partner associated with the lichen. To make it even more difficult, scientists have found a second fungus mixed up in the relationship. One of these is a yeast that showed up around 200 million years ago. For thousands of years, people around the world have used these particular lichens to leaven bread and make alcoholic beverages.
It's this partnership that has made lichens so successful. The algal cells provide the fungus with vital organic nutrients through the process of photosynthesis. In turn, the fungus gives the algae water, gases and chemical nutrients it needs. The fungus also supplies a physical structure on which the algae can safely grow; sheltered from an aggressive sun and the subsequent water loss.
There are around 14,000 species of lichens in the world. Here, in North America, we've manage to identify a little under 4,000. Lichens can be found from the extreme cold of the Arctic to the driest of deserts. Extreme environments don't seem that extreme to lichens. In some instances, lichens may be the only "plants" in some areas of the world.
Lichens aren't particularly competitive with plants or even other fungi. They grow very slowly. Some species grow less than 1 mm a year. A few species, under good conditions, may grow a centimeter in twelve months. This slow growth makes lichens long-lived. One lichen colony in northern Canada has been estimated to be over 9,000 years old.
Because lichens are slow growing, they require a stable surface on which to develop. This normally means a rock or a tombstone. But, they'll grow on living plants as well. When you see lichens growing on your tree or shrub, it means that your plant isn't growing at the rate it used to grow. That may or not mean there's a problem with the plant.
The simplest thing to do is to check the growth of the plant. This spring, mark a couple of branches at or near the tip. Come fall, look to see how much new growth there is. If your satisfied with the growth, don't worry about it. If it's less than you think it ought to be, fertilize it next spring. If your shrub or tree starts to grow at a faster rate, over time the lichens will disappear.
Tim Lockley, a specialist in entomology, is retired from a 30-year career as a research scientist for the U.S. Department of Agriculture. For answers to individual questions, please send a stamped, self-addressed envelope to Tim Lockley, c/o Sun Herald, P.O. Box 4567, Biloxi MS 39535.