Problems of inertia and threshold related to post-Glacial habitat changes

Most of the major late-Quaternary vegetational changes deduced from the study of pollen diagrams have generally been supposed to have been brought about by climatic change. The assumption has also been made that widespread climatically controlled vegetational changes are likely to have been broadly synchronous (cf. for instance Godwin 1956, p. 57). Recently, however, it has become clear that differences of migration rate and the rates of pedogenesis should be given more attention; Faegri, in particular, has made this point very strongly with reference to the history of the Scandinavian flora (Faegri 1963). In addition, it is appropriate to note that Iversen (1960, p. 9) has questioned whether we can attach any climatic significance to the pollen zone transitions up to the beginning of the Atlantic period and whether they will prove to be synchronous over any great distance. Nevertheless, the assumption that the well-marked vegetational changes in a small area such as the British Isles are likely to have been synchronous has sufficed to allow the establishment of a useful relative chronology for archaeological and other purposes. While the radiocarbon age estimations so far obtained do not on the whole confute the assumptions, there are a number of exceptions (cf. Godwin 1960; Godwin & Willis 1959, 1962; McAulay & Watts 1961, etc.). For phytogeographical purposes, however, it is clearly to argue in a circle to use a chronology based on vegetational evidence; we must equally be on our guard against the circular argument in discussing the role of habitat changes in palaeoecology.