Climatic change in Mediterranean area and pollen monitoring

The weather conditions directly influence pollination by determining time of onset of flowering, the number of pollen grains produced, and by controlling the amount of pollen that is discharged into the air from day to day. The climate in the Mediterranean is mild and wet during the winter and hot and dry during summer. In this area plant physiology is controlled by temperature, but some plants flower when water is available or as day length increases in spring or decreases in summer. Some species respond to a combination of these environmental parameters which interact with both the initial steps of the flower initiation and the speed of the flower development. In the last decades, it has been an evidence of a significant increase of air temperature with consequent changes on the airborne presence of anemophilous pollen. The changes recorded involve, above all, the timing in which occur the pollination, the peak date, the behaviour of pollen release, whereas any significant influence on the total pollen emission is found. Many studies carried out on long time series of airborne pollen counts have shown an earlier start of pollination in spring flowering trees, such as Platanus, Quercus, Betula, Pinus, Olea. For taxa flowering in winter, such as Corylus, Alnus and Ulmus, this trend is not linear and could even be delayed by autumn higher temperatures which determine insufficient chilling accumulation and later start of the season. On the contrary of areas of central and northern Europe, in the Mediterranean area more or less stable trends of weeds or grasses have been observed, with a slight tendency to advance or delay of the onset of pollination, depending on the species. There is also evidence that the length of the pollen season could show significant variations: in general, in the warmest years the pollination is shorter that in the years with lower temperature; in some cases it is also extended, especially in late flowering species or species which start to growth once there is sufficient water availability. Changes in the parameters which characterize the presence of airborne pollen in the atmosphere of Mediterranean areas appear to be a relatively considerable response to climate change although there are both species and regional differences in the examined models.