The expansion of hazel (Corylus avellana L.) in the southern Alps: a key for understanding its early Holocene history in Europe?

Abstract

In Northwestern and Central Europe the Holocene expansion of Corylus occurred before or at the same time as that of other thermophilous trees (e.g. Quercus). This sequence of expansion has been explained by migrational lag, competition, climatic changes, human assistance, or disturbance by fire. In the southern Alps, however, hazel expanded around 10,500 cal yr BP, more than two millennia after oak had become important. This delayed expansion is in contrast with the rapid expansion often assumed for hazel in central Northern Europe. We use two well-dated pollen and charcoal records from the southern forelands of the Alps: Lago Piccolo di Avigliana and Lago di Origlio. We conclude that distance of refugia, speed of seed dispersal, and competition cannot sufficiently explain the absence of the hazel expansion prior to the establishment of mixed oak forests in the southern Alps. Instead our records indicate that higher moisture availability and low temperatures inhibited hazel and favoured the establishment of pine and mixed oak forests during the Allerød. The expansion of hazel 11,000–10,500 cal yr BP was favoured by a combination of high seasonality, summer drought and frequent fires, which helped hazel to out-compete oak in the south as well as north of the Alps.