Factors Involved in the Phenological Mechanism of Alnus Flowering in Central Europe

The objectives of this paper are to ascertain the main factors involved in the
phenological mechanism of alder flowering in Central Europe by understanding the in
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fluence of the main meteorological parameters, the North Atlantic Oscillation (NAO)
effect and the study of the Chill and Heat requirements to overcome dormancy. Airborne
pollen (1995–2007) was collected in Poznań (Poland) by means a volumetric spore trap.
Temperatures for February, and January and February averages of the NAO are generally
key factors affecting the timing of the alder pollen seasons. Chilling accumulation (which
started in Poznań at the beginning of November, while the end took place during the
month of January) of 985 CH with a threshold temperature of -0.25ºC, followed by 118
GDDºC with a threshold temperature of 0.5ºC, were necessary to overcome dormancy
and produce the onset of flowering. The calculated dormancy requirements, mean tem
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peratures of the four decades of the year, and January and February average NAO index
recorded during the period before flowering, were used to construct linear and multiple
regression models in order to forecast the start date of the alder pollen seasons Its ac
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curacy was tested using data from 2007, and the difference between the predicted and
observed dates ranged from 3–7 days