Performance and conservation of a refugial beech (Fagus sylvatica) stand in SW France

Abstract

The identification and conservation of microrefugia that allow local species persistence in otherwise inhospitable landscapes is now receiving great attention. Yet it remains little understood how the interplay of the local abiotic and biotic environment influences the dynamics, performance and perspectives of refugial populations, especially for large keystone species such as forest trees. Here we present a case study from an emblematic refugial stand of European beech (Fagus sylvatica) in SW France. The gorges of the Ciron River serve today as interglacial refugium to a small and isolated beech population at precisely the same place that already served as a putative glacial refugium to the species. Microclimate varies considerably between the central and the peripheral parts of the valley, where the species is restricted. Genetic analyses show that the beech population experiences very limited gene flow and an unprecedented frequency of sib mating. These processes, together with an apparently great historical stability of the refugial population, have resulted in an outstandingly strong and extensive spatial genetic structure. Dendroecological studies indicate that interannual variation in tree growth is primarily triggered by the water balance during the growing period. However, the population has so far been relatively little affected by modern climate warming and projections do not suggest a major drop in tree growth over the coming few decades. Overall, the Ciron refugial beech stand represents a highly suited model for monitoring fine-scale ecological processes and their relationship with microclimate in a refugial setting. A detailed understanding of its functioning should help inform strategies for conserving and managing refugial forest tree populations, a major challenge in southern and central European mountain ranges.