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 influence 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 Poznan (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 Poznan at the beginning of November, while the end took place during the month of January) of 985 CH with a threshold temperature of -0.25 degrees C, followed by 118 GDD degrees C with a threshold temperature of 0.5 degrees C, were necessary to overcome dormancy and produce the onset of flowering. The calculated dormancy requirements, mean temperatures 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 accuracy was tested using data from 2007, and the difference between the predicted and observed dates ranged from 3-7 days.