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Lichens of Subtropical Queensland R.W.Rogers Lichens What is a Lichen? A lichen is complex community having a fungus and a photobiont as its main components. The photobiont (a green alga or a cyanobacterium) provides the energy source for the lichen and the fungus the structured habitat with its the absorptive and protective structures. Some cyanobacteria also provide the nitrogen for the lichen. Some lichens have both green algae and cyanobacteria in the one thallus, in this case the cyanobacteria are usually in structures called cephalodia. Lichens are capable of inhabiting extreme environments, and it may be true that a lichen can only survive in an environment that would support neither of the separate components. The close mutuality of the lichen thallus may confer great stress tolerance on a lichen, but the delicate balance between the components makes lichens very susceptible to damage. Lichens are generally intolerant of all but the most chemically infertile environments, and are therefore destroyed by air pollution. This delicate balance maintained by adversity means that lichens are usually extremely slow growing and very long lived. It is easy to destroy a decade of growth by carelessly walking on a lichen or collecting it. It may take several years for a new thallus to reach the size of a pinhead. Lichens come in many shapes, sizes colours. The larger lichens, the shrubby and leafy lichens, are visually striking in many parts of subtropical Queensland. They occur on tree trunks and branches, on rocks and on soil. They occur in rainforests, in eucalypt woodlands and out into the driest parts of the state. Lichens are sensitive indicators of microclimate and of air quality. History The lichens of subtropical Queensland have been the subject of study for over one hundred years. John Shirley (an Inspector of Schools) published the first taxonomic account of the lichens of Queensland in 1888-89. Little more published information about Queensland lichens then appeared for about 80 years, when work on lichen biology was taken up in the University of Queensland. This was coincident with a burst of activity around the country. In the last thirty years Australian lichens have attracted attention within Australia and around the world. As a result we have seen the production of four lichen volumes in the Flora of Australia series, with several more yet to come. Because of this history, it is now possible to produce an account of the larger lichens in subtropical Queensland. The larger lichens, those that are shrubby (fruticose) and leafy (foliose) are the most obvious and the easiest to determine to species. The crustose lichens are still relatively poorly understood in Queensland, and their identification requires great patience, considerable skill and a good microscope. Identifying the shrubby and leafy lichens does require patience too, especially as the set of new terms that must be understood in 6
Lichens of Subtropical Queensland R.W.Rogers identification is learned. Like all fields of studies, lichenology has its own language that, once learned, makes the task easier. Parts of a Lichen The Lichen Body The lichen body is termed a thallus. The thallus contains two main components, a fungus and a photosynthetic organism, the photobiont. Green algal photobionts are usually present as single, bright green cells in a layer just below the cortex. Cyanobacterial photobionts are usually present as clumps of blue-black cells immediately below the cortex. The name of a lichen applies only to its fungal component. The photobiont has its own name. Almost all lichens have a fungal component from the class Ascomycota. The thallus of a leafy (foliose) or shrubby (fruticose) lichen usually has three distinct tissues. The outer layer is a cortex composed of tightly packed fungal cells that forms a protective layer around the thallus. In leafy lichens the upper and lower cortex are usually sharply differentiated. In shrubby lichens there is usually no such differentiation. Inside the cortex (upper only) is found a layer of less dense fungal tissue (the medulla) with the photobionts incorporated into it (see illustrations below) The colour of the lower cortex varies from white to jet-black: colour variation is taxonomically significant in some genera. Colour of the upper surface is also taxonomically significant because of the link between colour and chemical constituents. Peculiar Lichen Structures Lichens have some distinctive asexual reproductive structures known as soredia and isidia. These are easily detached from the lichen, especially by raindrops, and when scattered may grow into a new thallus. Soredia are made up of groups of algal (photobiont) cells loosely entangled with fungal hyphae. Soredia are gathered into restricted areas known as soralia. Soralia can be pin points, small globular structures, or linear structures along the edge of lobes. Isidia are more complex structures that have a well-developed outer wall that encloses a central core of algae and fungi. Isidia are often finger-like, and a about 1mm tall, but they do vary considerably. Isidia have an abscission zone at their base, making their detachment from the thallus easy. In some case the isidia are hollow and swollen at the tip, and may even burst and become pustulate. Rhizines are commonly present on the lower surface of foliose lichens. Rhizines may take a variety of forms (see below) that are taxonomically significant. In some cases the lower surface is quite naked, in others it is covered by a dense carpet-like tomentum, and in yet others by a light tomentum of hypha-like hairs. The distribution of rhizines on the lower 7
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