California mosses in winter

By Dan Norris, research botanist, University of California, Berkeley

In winter, when flowering plants slow or cease growth, California mosses begin their main growing season. This seems to be a survival strategy that avoids shading by the larger associated flowering plants. (Our California trunk-growing mosses virtually never grow on the evergreen conifers.)

This strategy is possible because the moss photosynthetic system can harvest net energy at temperatures very close to freezing. In contrast, the growth of flowering plants usually requires temperatures above 50° F.

In the winter, mosses at low elevations in the moister parts of California cloak the forest trees in a manner reminiscent of Tolkien's novels. In spring and summer, these same mosses lose their intense greens, partly because drying contorts them and also because most or all of the annual growth has ceased.

These dried mosses are not, however, dead, but dormant, and will resume growth when they again become wet. This capacity, almost unique among plants, allows those mosses to live through even very long or very frequent periods of dryness. Some specimens have been revived decades after collection and with the addition of water, proved capable of resuming growth. It is interesting in this connection that dry mosses are seldom damaged by temperature extremes, but wet mosses may be killed after a short time at temperatures in the 80s.

Most mosses are perennial, with leaves and stems living indefinitely. The cascading tresses of mosses on the trees of northwestern California will only get longer and more beautiful through the years - until their weight pulls them down from the trees. In grasslands and disturbed areas, however, grow a large number of seemingly annual species. These so-called "ephemeral mosses" render the soil surface a green hue markedly more attractive than the drought-cracked clay of the previous summer. Many of these mosses may live for only the two months of the winter, and will be regenerated by spores with the wet weather of the next fall. Some are so small that the leafy plant and its spore-producing capsule are approximately the size of a pinhead. In January or February, when you see an old man crawling on his knees in a grassland, it may be I, in search of the hidden ephemeral.

Mosses are unique among land plants in their total lack of a root system, even in embryonic development. They have instead evolved the capacity to take in water through all parts of their bodies - a rapid absorption accompanied by ease of evaporation. The growth of the tiny mosses in grassland does not even depend upon an autumn rain. The same dew that we wipe off our windshields in October is enough to begin the growth of ephemerals. It does not get into the soil to restart the flowering plants but is sufficient for the soil mosses.

Even the larger mosses on the bark of trees may benefit from the slightly wetter nights of fall. In coastal California these mosses show the first signs of growth in October, when mist and dew, intercepted by tree canopies, flows slowly down the trunks, where it is absorbed by the mosses. Without roots, the erect stems of some of these mosses can be spaced much more closely than among flowering plants with their sensitivity to root competition. Such closely spaced mosses move water out from the tree trunk almost like the wick of a candle.

Mosses (including hornworts and liverworts) are abundant in all parts of California, including its deserts. More than 750 species are documented in the state, and new ones are regularly discovered. For more on California's mosses, see the July 2003 issue of Fremontia, the publication of the California Native Plant Society

Gathering momentum for conservation of moss

I have spent 52 years studying mosses, liverworts, and hornworts (collectively here called mosses). During my early years in the profession, mosses were mostly considered unworthy of attention. Some change began in 1974, when I was asked by authorities in a California county to evaluate a land-use project. My report on the presence of a moss species then known only from two sites in California created a storm of protest when the project was denied. The resultant outrage made me unwelcome on some private lands.

With increased official interest in mosses, I found myself participating in the development of the "Northwest Forest Plan" (the spotted-owl conservation program of 1993). My list of species of mosses whose rarity demanded special attention contributed to the inclusion of mosses in subsequent field surveys and Habitat Conservation Plans. More recently mosses were included in the sixth edition of the California Native Plant Society's Inventory of Rare and Endangered Plants of California and in the California Natural Diversity Database compiled by the California Department of Fish and Game.

Habitats especially rich in rare mosses in California are the alpine regions and the peatlands in the state.

Dan Norris