The early-Holocene spread of hazel (Corylus avellana L.) in Europe north and west of the Alps: an ecological hypothesis

Abstract

The northward spread of hazel (Corylus avellana L.) from its glacial period refuges has been studied for over a century. North of the Alps the course of its pollen curve, in both relative and absolute pollen diagrams, differs from those of other early immigrant trees. There is no long ‘tail' of low values before the marked ‘rise’ to high values. The hypothesis presented is that hazel was present as non-‘ owering trees before hand, with full pollen production setting in after a stabilization and amelioration of the climate in the early Holocene. This view is based on a comprehensive survey of all the life history stages of hazel in relation to climatic and other factors, together with indications from radiocarbon dates of the ‘rise’ phenomenon, not of arbitrary pollen percentage values, for sites north of the Alps. The regional and altitudinal differences in 14C dates, with due regard to the existence of 14C plateaux, support the hypothesis, as do other independent estimates of the early-Holocene climate in northern Europe. The role of ‘ uctuating seasonal air temperatures and precipi tation is noted and the need to consider the ecological requirements of all stages of any plant species' life history is stressed.