Mosses: More than just green forest dwellers

Prevalent Southeast species play home to fungi, bacteria, other tiny, specified organisms

Posted: Friday, November 06, 2009

Moss - it's all around us here in Juneau. We trample it underfoot when we walk in the woods. We clean it off our roofs and sometimes try to eradicate it from our lawns and sidewalks.

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Abby Lowell / Juneau Empire File
Abby Lowell / Juneau Empire File

In special circumstances we like mosses; the best camp bed I ever had was a deep carpet of moss under a big spruce tree. In some regions of the world, sphagnum moss is mined and forest mosses are harvested commercially (to the detriment of wild creatures that live in or use moss). Except for such things, we seem to spend a certain amount of effort getting rid of mosses and a good deal of time just ignoring them altogether.

But that attitude indicates that we are failing to appreciate an amazing little world - a microcosm of fascinating biology.

By way of introduction, let's start with the mosses themselves. There are over 20,000 species, which have in common their lack of support tissues and a resulting small stature. They inhabit various surfaces, seldom extending more than a couple of inches above that surface. Close to the surface of logs, tree trunks, branches, or rocks, they inhabit a zone called the boundary layer, within which airflow is slowed. This allows for the build-up of carbon dioxide concentrations, which can be several times higher here than just above the moss plants. Carbon dioxide, resulting from the metabolism of fungi, bacteria, and other organisms below the moss mat, is the raw material needed for photosynthesis of sugars, which the moss needs for energy.

Water vapor also accumulates within the boundary layer. Mosses have no roots, and their leaves are only one cell thick, enabling them to get water directly from rain, mist, dew, and evaporation from their substrate. Water on moss leaves allows carbon dioxide to enter the leaves and oxygen, one of the products of photosynthesis, to exit. Water is also necessary for mosses to reproduce sexually, because sperm have to swim in a film of water in order to reach a female shoot and thus fertilize an egg.

In dry conditions, mosses can lose up to 98 percent of their water, shut down all activity, and become dormant. Some mosses can remain desiccated and dormant for years, and then revive when wetted again. They can't wait forever, though; if a mossy tree becomes exposed to sun and wind for many years, its moss cover will die.

Mosses also perform ecosystems services by holding water, sometimes lots of water, in the tiny spaces among their leaves and stems. They can help slow the runoff from rain and help prevent erosion. Mosses also provide a nursery for seeds and spores of other plants - a moist germination site for propagules that happen to land there at the right time.

Mosses are also home to thousands of microscopic organisms. In some cases, a handful of moss may harbor several hundred thousand tiny creatures. Some of these are bacteria or one-celled protozoa. Some are rotifers: small invertebrates that feed on the smaller organisms by collecting them in their whirling, hair-like cilia. Arguably the most interesting of all the wee creatures are called tardigrades - the "slow-walkers" or "water bears."

Tardigrades are not insects and not worms. They're in their own group somewhere in between. There are about a thousand species in the world, living from high mountain peaks to the deep sea. It is a very, very old group, way older than mammals and birds or even fish, dating back to Cambrian times, over 500 million years ago. Water bears are commonly only a millimeter or so long, and often less, but they have four pairs of stubby legs, a brain, and sucking mouthparts for piercing and feeding from plant (moss) cells. Tardigrades, like the mosses where they often live, can tolerate extreme desiccation. When dehydrated, they lose 99 percent of their water and shrivel up into a speck-like nurdle. They can then put their metabolism and life on hold for up to ten years.

Water bears are very tough little things in other ways too. They can tolerate environmental extremes, in addition to drought, that would kill most other creatures. Some can survive very high pressure, as in the deep sea. Others can withstand near-vacuum conditions. Some survive temperatures near absolute zero (-273 degrees centigrade) and others can handle temperatures well above boiling. Some can tolerate levels of radiation that would kill a human. And these extraordinary little beasts conduct their lives beneath our usual notice.

The mosses that harbor these tiny lives have varied ways of living. Some can live only where continual disturbance opens up new living space free of competitors. Others can't handle disturbance, but can push out everybody else if left undisturbed. Some can occupy various habitats, while others are extremely specific in their habitat requirements; for example, one species lives only on moose dung, and another one only on carnivore dung.

Life in the boundary layer can be good for not only the mosses that reside there, but also for the tiny Tardigrades within.

• Mary F. Willson is a retired professor of ecology.

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