Disjunctive Distributions in the Lichen-Forming Fungi

Abstract

There is a widespread notion among botanists that the nonvascular cryptogams are easily disseminated, ubiquitous plants without well defined geographic ranges. Certainly for many groups, such as the fresh-water algae and the nonpathogenic or nonsubstrate-specific microfungi, this view seems to be justified, for few meaningful geographic-to-taxonomic correlations among these organisms have been perceived. In other groups, for example the higher Basidiomycetes, our ignorance of phytogeographically significant ranges may reflect only the magnitude of the problems inherent in the study of organisms known only from ephemeral sporocarps. There are, however, two large groups of cryptogams in which species and taxa of higher than specific rank show well marked geographic distributions, including disjunctive ones, like those known in the vascular plants. These are the bryophytes and the lichens, the only major groups of terrestrial nonvascular cryptogams in which the vegetative plant body is exposed and perennating. The aim of this paper is to examine disjunction as it is known with regard to the second group, the lichen-forming fungi. Reliable distribution maps have been published for few (only hundreds) of the approximately 20,000 recognized species of lichen fungi. Most of the best substantiated distributions are for conspicuous foliose or fruticose species from temperate regions of the northern hemisphere. To underline the point that lichen fungi may have geographic ranges comparable to those of vascular plants, Table 1 gives eight pairs of common, locally abundant, native North American plants, in each case a lichen fungus and a tree, for which the total geographic ranges are almost congruent or very highly comparable. It should be pointed out that although all but one of the lichen examples are epiphytes, none is ecologically restricted to the bark of the tree species to which its range is compared. Many more such examples could be given from the flora of North America and the floras of Europe and the Far East. So similar in fact are the well documented ranges of lichens to the ranges of vascular plants that the conclusion that both result from the same physioecological and historical factors is inescapable.