Disentangling the effect of climatic and genetic factors contributing to Abies alba tree-ring growth variation along the Italian peninsula

Silver fir (Abies alba) is an indigenous tree species present on several central and southern European mountain ranges. Its fragmented distribution along the Italian peninsula, where climatic conditions are expected to markedly change, brought both population geneticists and dendrochronologists to investigate the adaptive responses of silver fir in terms of growth dynamics. We used a dataset based on 62 site chronologies, built on about 1300 trees, to cover the entire distribution range of silver fir in Italy. Half of the sites were also genetically characterized by nuclear and chloroplast microsatellite markers. Principal component analyses of the tree-ring width site chronologies and of climate-growth correlation functions were applied: i) to extract common variability in annual radial growth among the chronologies and climate zones, and to ii) assess the climate-growth relationships of the site chronologies. Dendrochronological and climatic responses reveal a clear distinction between the western and the eastern sectors of the Alps. In the Apennines, the northern sites are similar to the eastern Alps sites, and differ from the central and southern ones. These results were generally consistent with the biogeographical patterns detected by using clustering of genetic data, with the noteworthy exception of the central Apennines populations, showing genetic similarity with the northern Apennines ones. Despite the highly fragmented distribution of silver fir populations along the Italian peninsula, a genetic drift has apparently eroded neither in genetic variation nor in the genetic signature of post-glacial recolonization dynamics on their spatial genetic structure. Our results suggest a combined effect of climate and genetic population structure to explain the different geographical responses. The spatially different responses of silver fir tree-ring growth and climate sensitivity between eastern and western Alps and northern, central and southern Apennines could be related to both post-glacial recolonization dynamics and strong local selective pressures particularly in specific areas of the Apennines.