THE RELATIONSHIP BETWEEN AIRBORNE POLLEN GRAINS AND UNUSUAL WEATHER CONDITIONS IN TURIN (ITALY) IN 1989, 1990 AND 1991

A quantitative and qualitative study was made of the pollination patterns of 14 taxa of special significance for the atmosphere of Turin (northern Italy). Particular attention was paid to the years 1989, 1990 and 1991, since there were marked climatic abnormalities during this period, especially in the form of an increase in the mean temperature for February and March compared with the means for 1803-1988. The onset dates of the principal pollination period (PPP) and the thermal sums (T sums) for 1989-91, both from 1st January and over the 30 days prior to the onset of pollination, were compared with the corresponding data for 1984-1988, published in a previous study. The amount of pollen collected for each taxon in each year was calculated. In 1989-91, the dates for the onset of pollination were almost always earlier than in the years when temperatures lay within the mean range, although they were not always accompanied by constant T sum values close to those of 1984-88. In some cases, the T sums were higher (Alnus, Betula, Cupressaceae, Oleaceae, Populus). These taxa would seem to be able to begin flowering when a particular threshold is reached. The extent to which this critical value is exceeded, however, does not appear to be in proportion to the time taken to reach the moment when pollen is first released. By contrast, for Platanus and Juglans, a close correspondence between the T sums and PPP onset was noted. Late flowering taxa (Castanea, Artemisia and Ambrosia) were more constant with respect to their pollen distribution periods and T sums.