Recording the epiphytic lichen flora in Amsterdam on 576 trees distributed over eight common lane tree species shows that the urban epiphytic diversity alone is considerable, representing 15.2%, or 100 species, of the total lichen diversity in the Netherlands. The species recorded include many rarities and some that can be viewed as urban opportunists. Trees bear 15 lichen species on average but are greatly influenced by local factors. Species-specific bark qualities such as water-holding capacity, texture and bark-shedding, influence species richness greatly but are often overshadowed by dominant environmental factors. Tree species with a higher water-holding capacity and texture generally bear the highest species richness. Bark qualities are more indicative of species richness than tree species, showing few significant differences between species richness linked to tree species. Platanus × hispanica is the only observed species whose frequent bark shedding causes it to consistently have the lowest lichen species richness, regardless of environmental factors. In general, bark desiccation and eutrophication are the most dominant factors in influencing urban epiphytic lichen diversity, resulting in xerophytic and nitrophytic lichen species being the most common. Pollution is no longer observed to be the main limiting factor for urban lichen diversity as it was in the past. Instead, bark desiccation associated with the Urban Heat Island (UHI) and low air humidity (drought) is the most damaging factor in contemporary urban conditions in Amsterdam, but it rarely reduces species richness to zero or near zero levels. Areas in which eutrophication and desiccation are much less dominant were repeatedly observed. Such areas sometimes showed local dominance of acidophytes or other distinctive communities. In line with long-term improvements to Dutch air quality, the city now offers a niche to a wider range of species. Three ecological groups (acidophytic, lithophytic-minerotrophic, xerophytic-nitrophytic) are described in this context to characterize reoccurring lichen communities in the city that are indicative of contemporary urban conditions. The term ‘lithification’ is proposed in an ecological context to describe the frequently observed urban phenomenon of tree bark taking on the properties of rock and consequently bearing lithophytic communities. Additionally, we show the potential use of lichen species and ecological groups to monitor urban climate factors such as the UHI on a very local and accurate scale.

A saxicolous species of Chiodecton and two corticolous species of Enterographa are described as new to science. Chiodecton submontanum is characterized by a saxicolous habitat, irregularly verrucose thallus, inspersed hymenium, ascospores usually exceeding 50 μm in length with 6–10 septa and the presence of roccellic acid. Enterographa sparrii has immersed, perithecioid ascomata in indistinct to slightly raised pseudostromata, 40–55 μm long ascospores with 6–9 septa and contains roccellic acid. Enterographa subcaudata has immersed, more or less round ascomata with a black disc, 35–58 μm long ascospores with 6–12(–15) septa and schizopeltic acid in its chemistry. Additionally, an identification key to the members of Roccellaceae reported so far from India is provided.

Laetisaria macrospora is a new species of lichenicolous Corticiaceae forming light pink to coral coloured basidiomata on Physcia adscendens and P. stellaris. The basidiomata are usually located on the lower surface of the host thallus and produce basidia with four sterigmata and relatively large basidiospores, 23–34 × 11–18 μm. Phylogenetic analyses using nuITS sequences fully support the placement of the new species in Laetisaria, a genus that is recovered as monophyletic. Laetisaria macrospora was discovered in the Massif Central (France) in lichen communities growing on branches of Juniperus and Genista at the summit of the Puy de Manson.

This work attempts to better understand the significance of morphological diversity among fungal-algal contact zones present in lichens. We used TEM to examine a variety of lichen symbioses involving non-trebouxialean green algae that show intraparietal penetration by the mycobiont. A principal focus was on Endocarpon pusillum, a well-known member of a family (Verrucariaceae; Eurotiomycetes) previously reported to be characterized by unwalled haustoria exposing a naked fungal protoplast. Peg-like haustoria arose from an inner layer(s) of the mycobiont cell wall that broke through outer layers and penetrated a short distance into the wall of the green algal symbiont (Diplosphaera). In both fungal and algal cells at the contact interface, lomasome-like vesicles and tubules occurred as modifications of the plasmalemma intermixed with wall materials at the inner surface of the cell wall. A fungal cell wall was consistently present around the haustorium, which resembled those depicted in earlier TEM studies of Verrucariaceae. Previously published micrographs of Verrucariaceae purporting to show wall-less haustoria surrounded by an empty space are believed to have been misinterpreted. However, in the isidiose Porina and foliicolous Calopadia, Byssoloma and Fellhanera species (Lecanoromycetes), we did observe extreme degrees of reduction in the mycobiont cell wall at symbiont contact interfaces. In those lichens, a broad area of the fungal cell bulged into the adjacent algal symbiont, broadly invaginating the wall of the latter and penetrating it intraparietally without differentiation of a distinct haustorial structure. The mycobiont wall surrounding such protrusions often thinned to near indistinguishability towards its extremity. The protrusion made direct contact with the algal cell wall; no empty space occurred between them. We propose that the short, peg-like intraparietal haustoria bind the symbionts and help maintain cell contacts amid the stresses of tissue expansion and shrinkage, thereby avoiding disruption of the continuous hydrophobic coating that facilitates transfer between them. Broader contact interfaces with extremely thin adjacent walls may facilitate solute flow between symbionts. Reciprocal penetration of algal protrusions into mycobiont cells, noted in Porina as well as other lichens studied previously, is a neglected but potentially significant indication that both symbionts may actively work to maintain functional contact interfaces.