Dispersal patterns of meiospores shape population spatial structure of saxicolous lichens

Relationships between reproductive strategies and population spatial structure have often been suggested for lichens, but there is a lack of supporting aerobiological data. For the first time, this study couples aerobiological investigations on meiospore dispersal by Caloplaca crenulatella (Nyl.) H. Olivier and Rhizocarpon geographicum (L.) DC. with analysis of local spatial patterns of thalli of both species. During a two-year monitoring period carried out on the walls of a medieval castle in NW Italy, a total of 169 polar diblastic spores, 20% of which were morphologically attributable to C. crenulatella, was detected in the mycoareosol, while muriform spores of R. geographicum were never found. Laboratory experiments confirmed that different dispersal patterns characterize the two species, the meiospores of R. geographicum being poorly discharged and only recovered at a short distance from thalli, whereas those of C. crenulatella were more abundantly discharged, suspended and better dispersed by a moderate air flow. This difference was reflected on the castle walls by the random spatial pattern of C. crenulatella, while R. geographicum showed a clustered distribution. Different discharge rates and take-off limitations, possibly related to size differences between the spores, are not sufficient to explain the different colonization patterns and dynamics of the two species. Additional intrinsic and extrinsic factors are likely to drive their dispersal and establishment success. Nevertheless, information on the relationships between different dispersal patterns of the species and the local spatial structure of their populations might help to predict the recovery potential of lichen species exposed to habitat loss or disturbance, or encrusting monument surfaces.