A high percentage of lichen-forming ascomycetes reproduce sexually and thus are assumed to disperse primarily via ascospores, which have to relichenize. However, one should keep in mind that even fertile lichens have options for vegetative dispersal in the symbiotic state, either via symbiotic propagules such as soredia, blastidia or isidia, or via thallus fragmentation. Viable fungal and algal cells were shown to be contained in fecal pellets of lichenivorous slugs (McCarthy and Healey 1978; Fröberg et al. 2001) and of the ever-present lichenivorous mites (Meier et al. 2002). Thus, it is not known how often relichenization occurs in natural habitats.

A detailed knowledge of sexual reproductive strategies is required for understanding evolutionary traits and population genetics. Zoller et al. (1999) were the first to recognize that lack of ascomata in strongly fragmented and geographically isolated populations of Lobaria pulmonaria (“lungwort”) might be due to missing mating partners. As this species produces abundant isidiate soredia, one might conclude that ascospores are unnecessary. However, recombination as the centrally important element of sexual reproduction has an impact on genetic stability, whereas favorable and unfavorable mutations are transmitted to the offspring in clonal (vegetative) dispersal. As pointed out by Hestmark (1992), sexual reproduction may often be a mode of escape from old, severely parasitized thalli (Seymour et al. 2005b). It remains to be seen how often new thalli are formed from germinating ascospores in rarely fertile species with efficient dispersal via vegetative symbiotic propagules, such as Pseudevernia furfuracea, Hypogymnia physodes and others.