ENHS Nature Trails December 2001 Article 2

Natural History and You - The President's Forum
by Dave Wagner

Why I Like Mosses. Part I.

People who know me and who have gone on my plant walks at this time of the year have heard me go on and on about mosses. I love mosses. I love mosses and liverworts. These are the fluffy green plants that put a shag on the trunks and drape from the branches of trees in the Pacific Northwest. I like the lichens, too, the plant-like things that put a colorful crust on the bark and give the oak and ash branch tips their gray-green cast in the rainy season.

I like mosses not merely because of how they look. They are pretty enough, of course, especially if you use a lens or microscope. I like mosses and their ilk as much because of what they do, not merely how they look. There are two special things about them that impress me more than anything. One is how they deal with the dry season and the second is how they deal with the rainy season.

This month I'll write about how they deal with the dry season typical of our area. In a normal year, there is a six week period with very little rain at the end of the summer. Right now our dry season has just ended and a truly dry season it has been. It came at the end of two years of drought. The mosses, liverworts and lichens on the branches of the oaks and ashes out at Mount Pisgah, the mosses in the big trees up in the national forest, have been just as crispy as could be.

These plants are epiphytes and not parasites. Epiphyte means, "upon the plant." Mosses perched on the branches are attached to the bark but not penetrating the conducting system of the trees. This is a desiccating habitat, just like a bare rock face. It dries anything out very rapidly. One researcher said that a moss growing on bare rock in the sun is drier, on a percent moisture basis, than the USDA specifications for flour on a grocery store shelf. Dry but alive. Mosses and liverworts (and lichens) on the oak and ash branches are both dry and alive. They are desiccation tolerant. Most flowering plants and conifers are not desiccation tolerant. Things like cacti and succulents are desiccation resistant, meaning that they store water extremely well so they can last through a drought without completely drying. But if they actually dried out so their tissues were as dry as a moss on an oak tree, they would die. Only about twenty kinds of flowering plants are truly desiccation tolerant.

According to a recent review of the phenomenon, desiccation tolerance was a characteristic of the very first land plants. These appeared in earth's history around 500 million years ago. As plants evolved internal water conducting systems they lost the ability to survive desiccation. Apparently this is because moisture retaining tissue is more efficient at growing. The few flowering plants that have evolved (or reverted back to) desiccation tolerance since then have done it at least eight different times. It's never proven to be a very popular way to go.

Things are different for mosses and liverworts. They are closely related to the first land plants. Among plants living today, they are most similar to the earliest of land plants. They have retained their ability to survive drying out. This desiccation tolerance is what allows them to occupy micro habitats that the "higher" plants ignore: on open rock surfaces and on branches of trees in climates with frequent, long dry spells.

What's amazing about these desiccation tolerant mosses, is that they can start their life processes in a very few minutes after they are rehydrated. Within a short period of time of the cells' absorbing water, they start making food. They combine carbon dioxide from the atmosphere with the water and, using energy from the sun, synthesize sugars. This is photosynthesis, the process of life that gives energy for growing to all living things. In most higher plants the enzymes which do this photosynthesis will disintegrate when the cells dry out. These are the desiccation intolerant plants, those that die when they dry. And this is why desiccation tolerance is so wonderful. Somehow, mosses are able to protect the photosynthetic enzymes in their cells from disintegrating. They're ready to go to work immediately upon rehydration. The cells don't have to make new enzymes.

Nobody has yet figured out how mosses do this. There's exciting work to be done in this area. Look for the genetic engineers of the next generation to unlock the secrets of the mosses and put the genes for desiccation tolerance into crop plants. They will be valuable in the drier parts of the earth. Imagine an annual crop that you don't have to water; just plant and wait for the rain. After the soil dries out again, the plants merely shrivel and go dormant instead of drying and dying. With the next rain they'll spring to life again and grow further until that dose of water is used, and so on until ready for harvest.

What is a fantasy for genetic engineers is a part of everyday life for the mosses on the oaks around us. Step outside and give them another look. Gives you a new respect for them, doesn't it!

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