Genetic and environmental characterization of Abies alba Mill. populations at its western rear edge

The genetic structure of ten Abies alba populations at the western rear edge in the Spanish Pyrenees was characterized and compared with two German populations, looking for the role of climatic factors in the fir decline. Growth, defoliation, aridity and cumulative summer water deficit summer were also characterized. Spanish populations show a lower genetic diversity and a high genetic differentiation than German ones, mainly established by an East (higher diversity)-West (lower diversity) gradient. The three defoliated populations are the western ones, with higher summer aridity. Contrastingly, the southern population (Guara) shows climatic variables close to western sites but without defoliation and with a higher genetic diversity, indicating a possible adaptation to the sub-Mediterranean conditions. Silver fir in the Spanish Pyrenees constitutes a “stable” rear edge because of their isolation, small sized and small genetic diversity. Western Pyrenean sites subjected to dryer conditions and presenting lower genetic diversity are prone to drought-induced mortality in the context of global warming.