Thermal requirements and forecasting the start of the plane tree pollen season in Tétouan (NW Morocco)

Chilling and heat requirements are important for breaking dormancy and initiating blooming in temperate deciduous trees. To establish these requirements for Platanus trees, and predict the start of pollination in a given year, we performed a study based on aerobiological data from 2013 to 2024 in Tétouan (NW Morocco). Chilling requirements were established using two methods: the Aron Model (AM) and the Dynamic Model (DM), which estimate chill hours (CH) and chill portions (CP), respectively. As for heat requirements, we used Ring's Growing Degree Days (GDD) and Growing Degree Hours (GDH) methods. These various approaches generated four combinations: Aron's model with GDD (GDD_AM), Aron's model with GDH (GDH_AM), Dynamic model with GDD (GDD_DM), Dynamic model with GDH (GDH_DM). Our findings showed that the chilling requirements accumulated a mean of 663.14 CH and 23.9 CP, while heat requirements showed a mean value of 180.5 GDD_AM, 8207.1 GDH_AM, 129.19 GDD_DM, and 6742.93 GDH_DM. The leave-one-out cross-validation (LOOCV) allowed a generally satisfactory forecast for all combinations and exhibited minor differences between predicted and observed pollination dates, ranging from - 3 to 0 days. Following a thorough analysis based on linear regression analysis and other considerations detailed in the discussion, heat requirements using GDH_DM showed a closer relationship with the start date of the pollen season and could be preferred in this case to develop forecasting models in Tétouan. However, it would be interesting to employ other approaches and to use more extended data series to strengthen this conclusion.