What Shapes the Genetic Diversity of the Alnus cordata Species Across Its Italian Native Range? Informing Conservation Strategies

Alnus cordata is an endemic tree species native to the Southern Italian Apennines and north-eastern Corsica, renowned for its ecological significance. Climate change projections for the Mediterranean basin indicate range shifts and increased fragmentation for many forest trees, including A. cordata. Hybridization with the sympatric A. glutinosa in the central part of its Italian native range may also influence the genetic structure and conservation priorities for A. cordata. A comprehensive conservation strategy is needed to preserve its genetic resources in Italy. We analyzed the genetic diversity, population structure, and extent of hybridization with A. glutinosa in 23 A. cordata forest stands across its native Italian range using nuclear microsatellites. Habitat suitability was modeled under current and future climate scenarios using an ensemble forecasting approach. Conservation prioritization was guided by a genetically informed Reserve Selection analysis in DIVA-GIS to identify areas of high conservation value and address gaps in genetic resource representation. Italian alder populations exhibit low genetic diversity, which decreases towards the southern latitudinal margins of the peninsula. Hybridization and introgression with Alnus glutinosa have a geographically localized impact on the genetic variation within A. cordata populations. Local increases in private allelic richness do not alter the spatial genetic structure of A. cordata, but they help mitigate the risk of severe genetic erosion. A significant proportion of the species' genetic diversity is effectively preserved through in situ conservation. Model projections under future climate scenarios indicate a substantial decline in habitat suitability for A. cordata stands with high priority for in situ conservation. This highlights the need for complementary strategies, including ex situ conservation measures. Our study highlights the importance of integrating genetic analyses, habitat suitability modeling, and spatial prioritization techniques for effective conservation planning of A. cordata in the face of climate change across the Mediterranean.