Effectiveness of aerobiological dispersal and microenvironmental requirements together influence spatial colonization patterns of lichen species on the stone cultural heritage

Dispersal patterns of lichen species in monumental and archaeological sites and their relationships with spatial population structure are almost unknown, hampering predictions on colonization dynamics that are fundamental for planning conservation strategies. In this work, we tested if the local abundance and distribution pattern of some common lichen species on carbonate stones of heritage sites may be related to their patterns of propagule dispersal. We combined analyses of the spatial population structure of eight species on the calcareous balustrade of a heritage site in Torino (NW Italy) with aerobiological analyses. In situ and laboratory analyses were mainly focused on the ejection of ascospores and their air take-off and potential dispersal at short and long distance. Results indicate that the spatial distribution of lichens on the stone surfaces is influenced by both species-specific patterns of propagule dispersal and microenvironmental requirements. In particular, apotheciate species that have a higher ejection of ascospores with higher potential for long range dispersal are candidate for a much aggressive spreading on the monumental surfaces. Moreover, their occurrence on natural or artificial stone surfaces in the surroundings of the stone monumental surface may easily support recolonization dynamics after cleaning interventions, as an effective supply of propagules is expected. On the other hand, species with a lower dispersal rate have a more clustered distribution and are less effective in rapid recolonization, thus representing a minor threat for cultural heritage conservation. These results support the idea that information on the reproductive strategy and dispersal patterns of lichens should be coupled with traditional analyses on stone bioreceptivity and microclimatic conditions to plan effective restoration interventions of stone surfaces.