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Case StudiesBaeomyces rufusThe German lichenologist HM Jahns studied the interaction between the lichen Baeomyces rufus and the bryophytes along a half kilometre roadside bank near Frankfurt in Germany. He found an interesting cyclic swapping of dominance between bryophyte and lichen. Along the roadbank in question Jahns noted the following bryophytes: Bazzania triloba, Dicranella heteromella, Dicranum scoparium, Diplophyllum albicans, Hypnum cupressiforme, Polytrichum commune, Polytrichum juniperinum, Rhytidiadelphus loreus and Rhytidiadelphus squarrosus. Of these, Bazzania and Diplophyllum are leafy liverworts and the rest are mosses. Apart from Baeomyces rufus the only other lichen present was Cladonia digitata. Baeomyces rufus reproduces vegetatively by schizidia or by soredia. The former are large, scale-like propagules that detach themselves from the lichen thallus. The latter are granular propagules and are formed in much greater numbers than the schizidia. They are also more easily dispersed because of their very small size. Near the older Baeomyces thalli Jahns found bryophyte mats or cushions thickly powdered with lichen soredia, the soredia adhering firmly, apparently because of a gelatinous texture. He found none of the fungal hyphal outgrowths that are the typical means of attachment for lichen soredia. Though nine bryophyte species were present Jahns found soredia mostly between the narrow, hair-like leaves of Dicranella heteromella or amongst the rhizoids on the underside of Diplophyllum albicans and rarely between the leaves of the latter. Soredia were absent from the other bryophytes, seemingly because the others were larger, more open in growth and without the fine niches to trap the soredia. The individual soredia grow by an outward growth of fungal hyphae and this leads to the amalgamation of neighbouring soredia. During this process the hyphae do not penetrate the bryophytes so there is no direct damage to the underlying bryophytes. Individual soredia may not grow much but there are numerous soredia. Once many soredia have amalgamated they have created a large lichen thallus which deprives the underlying bryophyte of light. Such bryophytes then quickly die and rot. Jahns could not say whether there was also any harmful chemical produced by the lichen, but it is a plausible hypothesis. Once the bryophyte has rotted it is very difficult to see that a given lichen thallus had originally established on bryophyte rather than on the earth. During the first two years of Baeomyces thallus growth the lichen produces sexual spores in apothecia and vegetative reproduction by schizidia is more common than by soredia. In the third year apothecia are rarely formed and production of soredia increases, initially marginally but eventually much of the thallus breaks down into soredia. Such a decomposing lichen thallus becomes unstable and incapable of holding the soil. Now fragments of thallus or soil fall from the roadbank and in these disturbed areas bryophytes re-establish and overgrow the disintegrating lichen thalli. While some Baeomyces thalli are disappearing, soredia are coalescing on bryophyte mats or cushions elsewhere along the roadbank. At some later time, Baeomyces may again colonize an area where bryophytes have displaced old Baeomyces thalli. Though individual Baeomyces rufus thalli live only for about three years the bryophyte-lichen change is cyclic and repeated. Therefore, when looked at as a whole, the roadbank is a stable ecosystem. Baeomyces rufus is known from widespread localities in the northern hemisphere and is able to colonise bare soil as well. Along the roadbank studied by Jahns colonization on bryophytes was more common than on bare soil.
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