Trends of some airborne tree pollen in the Nordic countries and Austria, 1980—1993

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Abstract
Long term trends in the occurrence of airborne pollen might help in elucidating the probability or unlikelihood of developments which have been predicted in theory such as global warming or forest decline. We have investigated and compared the trends over a 14 year period (1980–1993) of five selected common tree pollen types (Alnus, Corylus, Betula, Pinus, and Ulmus) from three sites in North Europe (Stockholm, Trondheim, Turku) and one in Central Europe (Vienna) in terms of the start, peak, end, and duration of the season, peak values and annual totals, using data from the European pollen data bank. The existence of trends has been investigated by calculating correlation against years and the statistical significance of the regression lines has been examined at the 5% probability level (p<0.05). A consistent trend in the earlier commencement of the flowering season is observed in all cases, as well as an earlier incidence of peak days. A marked trend to an earlier end of the season was also found in the majority of cases. In most cases, the length of the season showed a trend towards prolongation although this was not significant. Both the peak values and the annual totals exhibit clear trends towards higher values. Since both the start of the season and peak days are primarily temperature-dependant, the clear trends towards an earlier start and peak day in practically all the pollen types and sites examined supports the theory of global warming or, at least, a temporary rise in air temperature over the seasons covered by this investigation. The same conclusion can be drawn from the fact, that most of the trends of pollen frequency show an increase in the number of airborne pollen grains. It was not possible to comment on the possibility of forest decline, as there is insufficient knowledge on the influence of environmental factors on pollen production. Our material shows no trends which are distinct and consistent enough in terms of either decreasing or increasing annual totals or peak values to throw light on the question of forest decline.