Lichens or Not - A Perspective
Frances Burns
Vida, Oregon
(Reprinted from the April 2009 Eugene Chapter newsletter, which was sourced from http://oregonstate.edu/~mccuneb/lichenharm.htm)

        Lichens are among the most beautiful and valuable gifts of our climate and vegetation to those of us lucky enough to live in Oregon. As you've seen, they grow on trees; they also inhabit rocks and soils, and a few grow on less expected substrates, such as mosses. Lichens have no specialized structures for absorbing substances from their environment. They have rhizines, which are little projections from their bases that adhere to surfaces on which they grow. Rhizines do not conduct water, minerals, or food substances. They just grow into tiny cracks in the bark, rock, or soil surface enough to anchor the lichen body. On the other hand, the roots of vascular plants actually conduct water through tubes formed by plant cells. In lichens and bryophytes, however, water movement occurs through cell-to-cell absorption only.
        Lichens are formed by two organisms - a fungus and an alga. The fungus provides the shape, and a lichen species' name is the name of the fungus that shapes it. The algal cells grow in a layer near the top of the fungal structure. They photosynthesize - they manufacture food - for both the fungus and themselves.
        Ecosystems are served by lichens in several ways. Bryophytes, which are mosses and similar-looking small green plants (liverworts and hornworts), perform many of the same functions. One important function is water absorption. During a rainstorm, lichens and bryophytes soak up a lot of water. Forest mosses can absorb at least three times their body weight in water, so far more water is retained in a system that includes lichens and bryophytes than in a similar system without them. Water is then released slowly by the soaked-up lichens and bryophytes, rendering the ecosystem (forest, grassland, shrub-land, temporary or permanent water body) humid for a long time after rain has stopped. Nutrients are retained and slowly released in this way as well. The alternative to this retention and slow release is fast runoff and thus loss of water and nutrients during precipitation events.
        Lichens and bryophytes provide habitat for a large and intricate community of algae and invertebrate animals. These small organisms all take in water and nutrients in the courses of their life cycles. They, too, assist in holding water and nutrients for a longer period than if they were absent. They also break down complex molecules into small molecules during their various means of obtaining energy to fuel their metabolic processes. Plants require simple nutrient molecules for their metabolic processes. Because of nutrient cycling and breakdown by lichen and bryophyte dwelling organisms, more usable nutrient molecules are available to plants than would be available if the bark, soil and rocks were barren.

        A byproduct of the existence of lichen and bryophyte algae - invertebrate communities is that food for other animals is more plentiful than if these organisms were absent. For example, many bark-foraging birds, such as juncos, woodpeckers, nuthatches, and bushtits, eat arthropods that inhabit lichens and bryophytes or else arthropods that feed on other arthropods that inhabit them. In my own yard, I've noticed that these bark-foraging birds tend to spend more time foraging on trunks and branches covered with lichens and bryophytes than on bare bark. Some kinds of trees in western Oregon are good substrates for lichens, and they can have a pretty dense lichen growth on them. Strong trees built of dense, entwined wood cells like oaks and big-leaf maples can support a lot of weight beyond their own, so lichen and bryophyte growth there is fine, even though it looks like a lot. Native trees and the native lichens and bryophytes have the advantage of having evolved together over the past several million years, so that trees with bark that lichens can hold on to will have evolved strength enough to support them. Potentially some non-native trees, e.g., ornamentals in urban plantings, could have some problems and break under the load, but compared to the weight of branches themselves, generally epiphytic lichens and bryophytes don't weigh that much, even when wet. Conifer species usually have fewer lichen and bryophytes species growing on their bark because their bark tends to chip off in little plates, whereas broadleaf tree bark often builds up into a thick layer. There are exceptions, such as cherry (we have two native species, and at least one non-native also grows in west coast forests) which has smooth bark that renews its outer layer periodically by peeling, and madrone (native), which also repeatedly exfoliates bark layers.
        Last meeting the subject of lichens came up when a dire plan to eradicate them on some rhododendrons with a chemical was suggested. I have lichens on a 50+ year-old Rhododendron schlippenbachii, which hasn't been pruned for several years, and it still blooms beautifully every spring, so the lichens have done no harm. Even in full sun azaleas often get lichens, and in my experience, pruning to encourage new growth is the answer if one does not like the look of lichens, as new growth may not acquire lichens for a while. Chemical use is undesirable. As Merle Sanders says, "Prune, prune, prune!"

Frances Burns is a member of the Eugene ARS Chapter.