ENHS Nature Trails January 2002 Article 2

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

Why I Like Mosses. Part II.

Last month I explained the first half of why I love mosses: their survival techniques to get through dry spells. This month I'll explain how mosses deal with wet periods. Both add up to the original statement: I like mosses because of what they do as much as what they are.

That mosses like wet weather is well understood by most Oregonians. This is the time of the year when mosses are most prominent; this is the time of the year when mosses grow the most and fastest; this is the RAINY SEASON.

Last month I explained that mosses on the branches of trees dry out during drought periods because they are not parasitic on the trees. The branch mosses cannot get moisture from the host trees; nor can they get essential nutrients from the host trees. The essential elements are to mosses as vitamins are to us: not a lot needed, but that little needed is absolutely necessary for life.

Some essential elements are easy to get, like carbon from carbon dioxide in the atmosphere. Many others are quite scarce in the environment, especially metals which need to be absorbed as ions of dissolved metallic salts. Mosses must obtain all the elements necessary for growth from the only source available, the air around them. A certain amount is blown in as dust during dry weather and made available when the dust is wetted by the rain so it can be absorbed by the moss tissues. But the nutrient elements present in dust are just as likely to be washed away by the rain as absorbed by the plant tissue. Furthermore, there's not likely to be an adequate supply of essential elements in dust and it's hardly uniformly distributed over the tree branches. So where in air do mosses get their essential elements? From the rain! The rain that comes from the air around them.

Rain water does not have a lot of dissolved elements in it but it always has some. What is special about mosses is that their cell walls have a special affinity for the dissolved elements. They are able to pick out what they need and bind them into their biological processes. Moss cells use a process called ion exchange to get the rare elements in ionic form. Moss cells literally scavenge essential elements from the most dilute of solutions, even rain water. Any nutrients that come from dust present on mosses and wetted by the rain are also snapped up before they can wash away. In this way mosses get what they need--not very much, just enough to grow slowly. Slow growing seems to be just fine for mosses.

This ability to scavenge elements from dilute solutions is much better developed in mosses than in flowering plants. That's why mosses do so well in your lawn during the winter. That's why mosses, not grasses, grow on your roof. The winter rains wash away all the nutrients that grasses need to grow, but provide enough for the lawn mosses to thrive. This special ability to scavenge elements is used against mosses in the Moss Kill formulations people use to eliminate mosses from lawns and roofs. The Moss Kill compounds simply rely on salts of metals like zinc and copper. Zinc and copper are actually essential elements for mosses. The special ion exchange capacity of moss cell walls pulls out the copper and zinc ions they need from rain water. When there is an excess of these metals, however, too much is pulled into moss cells. Too much copper or zinc in moss cells poisons them. Even slightly elevated levels of zinc and copper, harmless to most flowering plants, are toxic to mosses.

The use of copper or zinc to poison mosses demonstrates that the ion exchange capacity of mosses is well known and put to economic advantage. Surprisingly, hardly anybody mentions that this special ability of mosses might be extremely important to one of our most prominent ecosystems. I refer to the rain forests found along the Pacific Coast from northern California to Alaska.

The rain forests of the Pacific Coast are characterized by the abundance of mosses festooning the branches of every tree and shrub as well as carpeting every open square foot of forest floor. The mosses are what tell you this is a rain forest. So, are the mosses just there for decoration? Are they there just to take advantage of the situation, because they can grow in rainy forests better than in the inland forests? I think most ecologists have been satisfied to accept this explanation. What they are overlooking is that mosses are an essential element of the rain forests, that the rain forests would not be the forests they are without the mosses.

It's the rain, remember--Rain which washes away the dissolved nutrients in the soil, away from the roots of the trees and flowers. It is the moss component of the ecosystem that captures the essential elements from the rain water and fixes them into biological compounds available for all plants of the ecosystem. That's my theory anyway. You read it here first. It needs to be proven, but good ecologists have opined that it is an idea worthy of study. Mosses are responsible for the nutrient health of our rain forest ecosystems. And that is why I like mosses.

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