Recolonization dynamics of lichens after biocidal treatments: A limitation in the spreading of water-based solutions beneath thalli as species-specific influencing factor

• Lichen monitoring on heritage marble surfaces of Savoy Residences (UNESCO, NW Italy). • Immediate recolonisation after conservation interventions evaluated by lichenometry. • Spread and penetration of water-based solutions beneath lichens visualized by dyes. • Hydrophobic rock volumes inaccessible by biocides beneath thalli of certain species. • Non-devitalized remnants restarting colonization and limiting conservation efficacy. Recolonisation by lichens of recently cleaned heritage surfaces is a major issue for conservation, but subtending processes were only partially unveiled, particularly with respect to species with a low potential of propagule dispersal, for which a rapid ingress from external sources is rather unlikely. In this work, recolonisation by lichen species of Megasporaceae and Verrucariaceae was investigated on the marble balustrades of two residences of the Royal House of Savoy (UNESCO-WHS 823bis; NW Italy), cleaned approx. 20 and 12 years ago, respectively. The work aimed to verify if (i) lichen recolonisation started immediately after the conservation interventions and if (ii) this may depend on a scarce effectiveness of devitalization treatments. With regard to this latter point, a possible specific resilience due to a high depth of hyphal penetration within the substrate and/or a limitation in the biocide spread and penetration beneath thalli -due to stone properties and/or a lichen-related effect- were experimentally evaluated. The calculation of growth rates and minimum age of thalli on the balustrades indicated that recolonisation started immediately after the conservation interventions. Blocks of marble from the ancient quarry sites were used to verify the spread and penetration of water-based solutions, mimicking those of quaternary ammonium salt biocides, in presence and absence of Megasporaceae and Verrucariaceae lichens. Lichen cover was shown to influence the capillary water absorption, but the depth reached by the water-based solution (visualised with fluorescein dye) was still higher than that of hyphal penetration structures. On the other hand, some hydrophobic volumes were detected just beneath/below lichen thalli. The applications of water-based biocides do not likely affect lichen structures observed in these rock volumes, which may thus act as viable remnants rapidly starting recolonisation process. Such scenario may thus account for the frequent occurrence of species of Megasporaceae and Verrucariaceae, having a low potential of propagule dispersal, on recently cleaned marble heritage surfaces.