Lichens, renowned for their resilience in extreme environments, serve as valuable bio-indicators of environmental conditions. Despite this recognition, environmental influences on lichen ecophysiology are not well understood in urban environments. In this study, we explore the use of functional traits in analyzing the impact of the urban heat island (UHI) on epiphytic chlorolichens. Lichen material was collected from 12 sites across an UHI gradient in Amsterdam and Leiden, the Netherlands. For each lichen specimen, the specific thallus mass (STM) and water-holding capacity (WHC) were calculated. The relationship between the UHI and STM/WHC was assessed using linear mixed models and ANOVA. Our study provides functional trait values (STM and WHC) for 18 species for which no prior data were available. Furthermore, our findings reveal a significant correlation between the UHI and the STM, which suggests STM as a potential indicator for the UHI.

Rapid advances in species distribution modelling have been facilitated by open availability of ‘big data’ and powerful statistical methods. A key consideration remains the time window over which field recorded occurrence data are sampled to develop a baseline species distribution. Too narrow, and distributions are incomplete and affected by sampling bias, too broad and distributions may fail to meet an assumption of equilibrium, having been affected by dynamic change across a range of different predictors. Lichens are a case in point; being diverse, functionally important and the subject of bioclimatic modelling for conservation assessment, they are nevertheless a specialist taxonomic group that is comparatively less well recorded compared to birds, mammals or vascular plants, for example. In this study, we examined the distribution of the ‘hair-lichen’ Bryoria fuscescens, based on UK record data. We partitioned records into sub-decadal periods (1970s, 1990s, 2010s), and accounting for recording effort, we compared these distributions to three predictors: an historical reconstruction of two different pollutants (sulphur dioxide and nitrogen deposition), and the climate (minimum mean temperature). We asked whether the strength of evidence for the effect of environmental predictors on Bryoria fuscescens distribution varied among the different decades, while also considering a potential for lag-effects. We show that a Bryoria fuscescens distribution that appears static, is dynamic when referenced against patterns of field recording effort. Climate was consistently important in explaining Bryoria fuscescens distribution, which was also affected by the changing pattern of pollution over time. This included a lag-effect of peak sulphur dioxide in the 1970s, and accrued effects of nitrogen deposition that strengthen over time. Overall, we conclude that Bryoria fuscescens has undergone a long-term decline in extent over the last six decades, caused by complex multivariate effects of air pollution, probably combined with climate warming. The ability to resolve these trends for assessment against future conservation targets depends critically on maintaining field identification skills and a sufficiently robust recording effort.

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.

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.