Assessment of the impact of climate and anthropogenic factors on Holocene Mediterranean vegetation in Europe on the basis of palaeohydrological records

Correlations between lake-level and river-activity records from the western Mediterranean and palaeohydrological records from northern Africa and central Europe suggest a distinction, in the western Mediterranean region, between an early Holocene period characterised by cooler and moister conditions than at present, favourable to temperate deciduous trees, and, after 5000 yr BP, a later Holocene period with a warmer and drier climate. The second phase was favourable to the extension of evergreen sclerophyllous trees possibly reinforced by human activities. These two successive Holocene periods reflect orbitally induced changes in summer insolation. Furthermore, superimposed on this general climatic trend, century-scale climatic oscillations punctuated the whole Holocene period. Decreases in river activity in the western Mediterranean region occurred at ca. 11 500, 10 500, 9000, 7000, 4000, 3000, 2000 and 800 cal yr BP. These decreases coincided with lake-level lowering in Jura, eastern France, and glacier retreat in the northern Alps and could be associated with a temporary expansion of sclerophyllous trees. This general pattern could have resulted from an alternatingly southward/northward displacement of the Atlantic Westerly Jet.