Analyzing the time-course variation of apple and pear tree dates of flowering stages in the global warming context

Over the last 40 years, perceptible advances in dates of flowering stages have been observed in apple and pear trees growing in three cropping areas in France and one in Switzerland. The time-course variation of dates of flowering stages was established for eight chronological sequences. Our aim was to propose a statistical modelling framework for such sequences with the objective of characterizing the relationship between flowering advances in fruit trees and global warming. After an exploratory analysis, change-point models were applied to multivariate and univariate sequences. The results clearly support the occurrence of a significant abrupt change in the time-course variation of flowering dates at the end of the 1980s toward more frequent early dates, the most probable change instant being between 1988 and 1989. The coincidence between this abrupt change in phenological variations and marked increases in temperature recorded particularly in France at the end of the 1980s led us to consider the flowering advances in apple and pear trees as impacts of global warming. The suddenness in the response to global warming could be explained by changes in rates for completion of chilling and heat requirements, successively essential to the development of floral primordia within buds. In all cropping areas, annual mean temperatures had suddenly increased since 1988 (1.1-1.3 °C), but including noticeable monthly differences. Particularly, warming was clearly more pronounced in February and March (mean temperature increases of 1.6 °C) corresponding to the main period of heat requirements, than in November and December (0.8 °C) corresponding to the main period of chilling requirements. So marked temperature increases during the heat phase would have suddenly resulted in more frequent years with relatively short duration for completion of the heat requirements and consequently more frequent early flowering years, despite some years with relatively long duration of chilling requirements.