Fluctuations and trends in airborne concentrations of some abundant pollen types, monitored at Vienna, Leiden, and Brussels

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Abstract
The annual sums of daily airborne pollen concentrations fluctuate from year to year. It has been suggested that for some taxa there is a regular or alternating pattern in these fluctuations. On the other hand, environmental changes may lead to decreasing or increasing trends in airborne pollen concentrations. These two phenomena can only be studied reliably on the basis of long-term volumetric observations of abundantly occurring pollen types. For this study two arboreal (Betula and Quercus) and two herbaceous (Poaceae and Urtica) types were chosen. For some of these pollen types a weak but significant trend is observed. For the arboreal types (Quercus and Betula) there appears to be a rather constant biennial fluctuating rhythm.