Relationships between flowering development of Ulmus pumila and Fraxinus excelsior and their airborne pollen

The flowering patterns of Ulmus pumila and Fraxinus excelsior were studied during the 1990 growing season in order to investigate their pollen dispersion curves. The use of airborne pollen records as predictors of flowering of allergenic trees was evaluated. A sampling method to describe quantitative flowering phenophases was applied. A Burkard trap recorded airborne pollen in the city of Mar del Plata. The phenological sampling method showed that floral phenophase development is not always synchronous between trees. The variability registered was larger for F. excelsior than for U. pumila. A delay between flowering and the airborne pollen recorded was not noticed for Ulmus. The greatest amount of Ulmus pollen came from the local vegetation, mainly from U. pumila. Fraxinus pollen was recorded for a longer period after the F. excelsior flowering season the ended. Fraxinus pollen income came from F. excelsior but also from F. americana and F. excelsior var. aurea that flower later. An immediate decrease in airborne concentration was noticed for both pollen types following rain. When relatively strong rainfall was registered a decrease in the number of open flowers occurred only in F. excelsior.