Buxus in Europe: Late Quaternary dynamics and modern vulnerability

The suggested location of broadleaved evergreen trees in Europe during the last full-glacial has traditionally favoured a southerly refugial model, which proposes survival in the Mediterranean peninsulas and recolonization of central and northern Europe during the Holocene. This hypothesis is not always substantiated by thorough reviews of original past and modern occurrence data, or considered in the light of plant traits and autoecology. Our approach focuses on the genus Buxus with the aim of exploring (i) the relationship between the location of refugia and post-glacial population dynamics, (ii) past processes determining density, fragmentation and local extinctions of modern populations, and (iii) the vulnerability of Buxus in the context of the undergoing environmental changes. We compiled a database of over 3600 modern occurrences and 676 fossil sites to reconstruct the distribution of Buxus in Europe since 30 ka cal BP. The location of fossil finds and the plant traits of Buxus indicate that it persisted widely across its modern distribution through the last glacial period with modes varying from region to region. The E Pyrenees, W Alps, and Jura Mts hosted dense populations, which expanded exponentially during the whole Holocene, and resulted in a modern continuous distribution area. In contrast, the Mediterranean Peninsulas hosted sparse populations, which increased exponentially only during the first half of the Holocene, clearly decreased in the last 4.5 ka BP and resulted in a highly fragmented modern distribution area, most likely in relation to the climate trends towards dry conditions of the last few millennia. These results challenge the common view that the Mediterranean regions are the exclusive and most important refuge areas for evergreen broadleaved trees and stress the importance of considering long-term population dynamics based on fossil data to evaluate the vulnerability of modern fragmented plant populations in view of conservation actions.