Vegetation change through glacial—interglacial cycles: a long pollen sequence perspective

Abstract

A core recovered from a thick sedimentary sequence in the Ioannina basin, on the western flank of the Pindus M ountain Range, northwest Greece, presents the opportunity to observe multiple changes in vegetational communities at one locality through a series of glacial-interglacial Q uaternary cycles. The Ioannina 249 record adds to the knowledge of vegetation history of areas of increased topographical variability and precipitation of the western Balkans and provides a complete stratigraphical record that can be compared with that of other long terrestrial sequences and with the marine record. Pollen analytical results are presented as percentages and concentrations, the former providing information on the composition and structure of vegetation, while the latter is considered here to be a reliable indication of vegetation density when changes differing by an order of magnitude are documented. The record shows an hierarchical order of variation in the response of vegetation to environmental change. Higher order of magnitude changes are alternations between forest and open vegetation communities, a reflection of major climatic shifts from interglacial to glacial modes. Superimposed on these oscillations is a lower order variability associated with vegetation changes within interglacial and glacial periods. During forest periods a succession is recorded withQuercusandUlmus/Zelkovaexpanding early, followed byCarpinus betulusand alsoOstrya carpinifolia/Carpinus orientalis, and finallyAbiesoften accompanied byFagus. Although individual periods may be characterized by dominance of one or more taxa, the underlying pattern of differential expansion is usually distinct and consistent. Nine forested intervals are distinguished and are assigned local names to facilitate longdistance comparisons and correlations. During open vegetation periods a series of changes is also observed from transitional steppe—forest or forest-steppe vegetation, through grassland steppe communities, culminating in a discontinuous desert-steppe vegetation. In addition to the two ends of the spectrum (forest and desert—steppe), attention is drawn to the intermediate phases representing ‘average’ Quaternary conditions. The Ioannina record is correlated with that of other long sequences from Europe and variation in the response of vegetation with site characteristics is considered. A strategy for long-distance correlations relying on the primary structure of vegetation and relative stratigraphical position of individual periods is described. The last interglacial period followed by two interstadials is recorded in much the same fashion in all records. Correlation of earlier periods was also in general agreement although only two continuous records that extend beyond the last interglacial are at present available for comparison. To minimize elements of circularity, similarities in the behaviour of individual taxa during particular periods are not part of the correlation criteria so that if their chronostratigraphical equivalence is independently corroborated their significance can be examined. On this basis, the importance ofCarpinus betulusand the almost complete absence ofFaguson a subcontinental scale during the last interglacial are noted. Possible effects of climate, competition and disease are discussed. Cross-correlation with the deepsea oxygen isotope record provides a tentative chronology for the Ioannina record. Based on this, the sequence down to a depth of 162.75 m is considered to represent a record of approximately the past 423 000 years. Aspects of land-sea correlations are discussed in the light of the Ioannina 249 record and the importance of long sequences in the development of European Quaternary stratigraphy is emphasized.