A MODEL TO PREDICT THE BEGINNING OF THE POLLEN SEASON

In order to predict the beginning of the pollen season, a model comprising the Utah phenoclimatography Chill Unit (CU) and ASYMCUR-Growing Degree Hour (GDH) submodels were used to predict the first bloom in Alnus, Ulmus and Betula. The model relates environmental temperatures to rest completion and bud development. As phenologic parameter 14 years of pollen counts were used. The observed dates for the beginning of the pollen seasons were defined from the pollen counts and compared with the model prediction. The CU and GDH submodels were used as: 1. A fixed day model, using only the GDH model with 1st January as fixed initiation point. 2. A CU/GDH model, with a fixed sum of Chill Unit requirement as initiation point for the subsequent GDH accumulation. 3. A dynamic CU/GDH model, based on a dynamic relationship between CU and GDH. It is concluded that the CU and GDH relationships defined for fruit trees are generally applicable, and give a reasonable description of the growth processes of other trees. This type of model can therefore be of value in predicting the start of the pollen season. The predicted dates were generally within 3-5 days of the observed. Finally the possibility of frost damage is discussed in relation to the great variation in the total pollen counts observed from one year to the other.