CENTENARY CONGRESS ARTICLE Vegetation dynamics in north-western Mediterranean regions: Instability of the Mediterranean bioclimate J.-L. DE BEAULIEU 1 , Y. MIRAS 1,2 , V. ANDRIEU-PONEL 1 & F. GUITER 1 1 Universite ´ Paul Cezanne, Aix- en-Provence, France, and 2 Laboratoire de Chrono-e ´cologie, Universite ´ de Franche-Comte ´, Besanc ¸on cedex, France Abstract Pollen and plant macrofossil records from a selection of sites in Spain, France and Italy are used to explore the origin and the development of the Mediterranean vegetation. The role of evergreen and summergreen taxa is discussed. The expansion of sclerophyllous trees during the Pleistocene interglacials is well correlated with the orbital forcing. During the Holocene, the large number of data illustrates latitudinal and longitudinal differences in vegetation dynamics. Multidisciplinary studies identify short terms climatic fluctuations. The debate is still open between those who attribute an increase of sclerophyllous forests during the late Holocene to a trend toward aridity and those who consider this dynamic as human-induced. Key words: Mediterranean ecosystems, palynology, palaeoclimates, human impact, review. Introduction The evidence of the effect of climate on Mediterra- nean flora appears instantaneous and limpid: a summer drought, more or less pronounced, and an adaptation of plants to this seasonal stress, notably by sclerophyllous leafs. This summer drought is not specific to the Mediterranean Sea borders only, but is encountered in several subtropical zones of the world (north of Australia, Cape Region, Chile and Cali- fornia). Striking similarities exist between the ecosystems from these distant regions, pointing to similar physiological constraints (Que ´zel & Taylor, 1984). Here we shall concentrate on north-western Mediterranean regions. The concept of Mediterranean bioclimate and vegetation appears thus very simple, but as soon as you want to get deeper and try to delimitate them, difficulties arise. In their key book ‘Ecologie et bioge ´ographie des fore ˆts du bassin me ´diterrane ´en’, Que ´zel & Me ´dail (2003) analyse the thermic and hydric parameters that determine the Mediterranean zones and give a list of the main bioclimatic indices proposed by different authors. The surfaces attrib- uted to the Mediterranean regions may dramatically differ. In terms of flora, the range of some key species may extend far beyond the Mediterranean area. For instance the emblematic species, Quercus ilex, is present along the wet French Atlantic coast. Another difficult question is the altitudinal range of the Mediterranean vegetation: a subdivision in altitudi- nal belts has been proposed by different authors (see Quezel & Medail, 2003) but as the summer drought decreases with altitude, some mountains could be excluded from the Mediterranean regions, as pro- posed by Bagnouls & Gaussen (1953). However, the status of Mediterranean mountain vegetation is not only determined by a collection of strict climatic parameters, but also by the connection with adjacent Mediterranean lowlands and by historical setting illustrated by the status of ‘biodiversity hotspots’ attributed to most of them (Me ´dail & Quezel, 1999). This setting at the interface between tropical and temperate worlds occurred during a long geological time, and the cyclic climate changes during the Pleistocene played a major role. How can we speak of a Mediterranean climate during the glacial times when most of the regions were covered by steppe vegetation? Referring to the expansion of sclerophyl- lous oaks, some authors have suggested a very recent appearance of the Mediterranean climate during the present interglacial (Jalut et al., 1997). But others Correspondence: Jacques-louis de Beaulieu, IMEP-UMR CNRS 6116, Univ. Paul Cezanne, Europo ˆ le de l’Arbois, B .P. 80, 13545 Aix- en-Provence, France. E-mail: Jacques-Louis.de-Beaulieu@univ.u-3mrs.fr Plant Biosystems, Vol. 139, No. 2, July 2005, pp. 114 – 126 ISSN 1126-3504 print/ISSN 1724-5575 online ª 2005 Societa ` Botanica Italiana DOI: 10.1080/11263500500197858