Analyzing the time - course variation of apple and pear tree dates of flowering 1 stages in the global warming context

14 Over the last 40 years, perceptible advances in dates of flowering stages have been observed 15 in apple and pear trees growing in three cropping areas in France and one in Switzerland. The 16 time-course variation of dates of flowering stages was established for eight chronological 17 sequences. Our aim was to propose a statistical modelling framework for such sequences with 18 the objective of characterizing the relationship between flowering advances in fruit trees and 19 global warming. After an exploratory analysis, change-point models were applied to 20 multivariate and univariate sequences. The results clearly support the occurrence of a 21 significant abrupt change in the time-course variation of flowering dates at the end of the 22 1980s toward more frequent early dates, the most probable change instant being between 23 1988 and 1989. The coincidence between this abrupt change in phenological variations and 24 marked increases in temperature recorded particularly in France at the end of the 1980s led us 252 to consider the flowering advances in apple and pear trees as impacts of global warming. The 26 suddenness in the response to global warming could be explained by changes in rates for 27 completion of chilling and heat requirements, successively essential to the development of 28 floral primordia within buds. In all cropping areas, annual mean temperatures had suddenly 29 increased since 1988 (1.1-1.3°C), but including noticeable monthly differences. Particularly, 30 warming was clearly more pronounced in February and March (mean temperature increases 31 of 1.6°C) corresponding to the main period of heat requirements, than in November and 32 December (0.8°C) corresponding to the main period of chilling requirements. So marked 33 temperature increases during the heat phase would have suddenly resulted in more frequent 34 years with relatively short duration for completion of the heat requirements and consequently 35 more frequent early flowering years, despite some years with relatively long duration of 36 chilling requirements. 37 38