Abstract
This study aimed to improve the modelling of flowering time in fruit trees and to understand to what extent global warming has affected this trait since the end of the 1980s. The onset of flowering time (F1 stage) in apple trees has advanced by 7 – 8 d in France since the late 1980s. In this context, a sequential model composed of a chilling sub-model and a heat sub-model was considered. The input data consisted of F1 dates for ‘Golden Delicious’ apple in three French cropping areas from the North-West to South-East over the period 1976 – 2002 (81 F1 dates). A user-oriented software package, called ‘Pollenoscope’, automatically optimised combinations between seven chilling and three heat temperature functions.This was achieved by maximizing the R2 values between the observed and simulated flowering dates. The study provided comparative information for assessing the respective effects of temperature functions commonly used for modelling flowering time in temperate trees. Three selected models explained 82 – 86% of the observed variability in flowering. Their fitness for an accurate prediction of the F1 date was validated using independent flowering datasets. All three models simulated similar time-course changes in the duration of the chilling effect at all three locations [i.e., a mean increase in the duration of this effect (by 3 – 5 d) since the end of the 1980s]. Consequently, it suggested that the duration of the heat effect had decreased (10 – 13 d) to explain the advance in flowering time. Hence, our results support the idea that global warming has, simultaneously, exerted two opposing effects in France between 1976 – 2002: (i) a slower mean rate of completion for the chilling requirement, and (ii) a higher mean rate of completion for the heat requirement. A more marked effect on completion of the heat requirement may have resulted from more pronounced warming from January to April, corresponding to the active growth phase of floral primordia, than from October to January, corresponding to the dormancy-breaking phase