Staying alive on an active volcano: 80 years population dynamics of Cytisus aeolicus (Fabaceae) from Stromboli (Aeolian Islands, Italy)

Abstract

Cytisus aeolicus is a narrow endemic species restricted to the Aeolian archipelago (SE Tyrrhenian Sea, Italy) and it is one of the most evolutionarily isolated plants in the Mediterranean flora. Historical and literature data suggest that both metapopulations and isolated individuals of C. aeolicus are gradually shrinking. Field investigations and drone images demonstrate that the C. aeolicus metapopulation from Stromboli experienced a strikingly fast increase during the last decades. As of 2019, more than 7000 ± 3000 mature individuals occur on Stromboli, i.e. 14 to 20 times more than those counted during the last census, 25 years ago. The diachronic analysis of aerial photos concerning last 80 years and the analysis of the growth rings of some selected plants pointed out that the surface occupied, the demographic structure and the distribution pattern of the subpopulations of Stromboli has been highly fluctuating during last decades. Moreover, data issuing from field observations in permanent plots placed in a transect between two isolated mature individuals showed that, under natural conditions, the germination rate of the seedlings of C. aeolicus can be very high and their establishment rate may exceed 40%. By contrast, seedlings mortality is subject to strong annual fluctuations. Additionally, the pollen morphology of the Strombolian metapopulation of this rare and isolated species is studied here for the first time. Contrary to what is stated in recent literature, the C. aeolicus metapopulation from Stromboli is healthy and very dynamic, albeit frequently damaged by the volcanic activity. Regular and repeated field surveys carried out during 3 years (2017–2019) allowed improving our knowledge on the life cycle of C. aeolicus and a new extinction risk assessment of the species, according to IUCN criteria, is presented.