Dispersal patterns of meiospores shape population spatial structure of saxicolous lichens

Relationships between reproductive strategies and population spatial structure have been often suggested for lichens, but still not supported with experimental 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 the analysis of their local spatial patterns on the walls of a medieval castle in NW-Italy. During a two-year monitoring period carried out in the castle courtyard, 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 the thalli, whereas those of C. crenulatella were more abundantly discharged, suspended and better dispersed by a moderate air flow. Such a difference was reflected on the castle walls by a 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, and additional intrinsic and extrinsic factors likely drive the dispersal and establishment success. Nevertheless, information on the relationships between different dispersal patterns of the species and the local spatial structure of their populations may contribute to predict the recovery potential of lichen species exposed to habitat loss or disturbance, or encrusting monumental surfaces.