Palynology and Mediterranean vegetation history

The history of Mediterranean vegetation can be outlined using pollen grains contained in lacustrine, marine and other sediments. These sediments have recorded very important vegetation changes during recent geological times. For example, during the last 6 Ma (million years), the effects of different events acting at regional (e.g. the Messinian salinity crisis between 5.96 Ma and 5.33 Ma) and global (expansion of the Arctic ice at ca 2.6 Ma) scales produced a progressive decrease and final disappearance of tropical and subtropical taxa. However, prior to the start of the Quaternary the Mediterranean flora still included a consistent number of tropical and subtropical arboreal taxa accompanying deciduous and partly evergreen trees that have persisted until today. The most important features of the vegetation history of the Quaternary consist in the fact that vegetation adapted to climate changes due to changes in orbital cyclicity, alternating between glacial and interglacial periods. The more widespread vegetation types were steppe and grassland formations during the dry and cold glacial periods whereas either deciduous or evergreen forests were characteristic of interglacial periods. These cold-dry to warmhumid climate cycles became more and more intense towards the present. During the second half of the present interglacial, after the mid-Holocene, joint actions of increasing dryness, climate oscillations and human impact led to the present day Mediterranean plant landscape. It is however not clear how far the causation of this spread of evergreen taxa was climatic or human. One of the most exciting challenges is the prediction of the future course of Mediterranean vegetation. In this perspective a consistent help, not fully explored yet, can be found in aeropalynology, recording the pollen transported in the air. Together with modern surface samples, these data act as modern analogues. Though it probably does not represent the same past vegetational composition, the current pollen rain is the only basic reference on which our comparative approach can rely. Present trends are interpreted and future scenarios can be hypothesized just using a combination of aero- and archaeo-/palaeo-palynological approaches.