It now seems generally accepted that average air temperatures are rising and that many perennial plants are flowering earlier than before. The research reported here used records of when Golden Delicious apples reached the F1 flowering stage (10 per cent of flowers open) at three locations in France, together with records of monthly mean temperatures.
The flowering data showed that in the period 1989-2002, trees flowered seven to eight days earlier on average than in the period 1976-1988. Furthermore, average temperatures were 1 degsC higher in the period 1989-2002. Apple buds enter dormancy in autumn and this dormancy is not broken until the buds have accumulated sufficient chilling.
They then grow out as temperatures rise in spring. A sequential model consisting of a chilling sub-model and a heat sub-model was used to assess the impact of various mathematical functions on the relationships between temperature and flowering.
Three of the resulting models were good enough to predict flowering times accurately at another location. These models suggest that when average temperatures rise, dormant buds will take longer to accumulate enough chilling to break dormancy. However, once broken, the buds will reach the F1 flowering stage more quickly and this will more than offset the earlier delay.
Selecting Models of Apple Flowering Time and Understanding How Global Warming Has Had an Impact on this Trait by Legave, Farrera, Almeras and Calleja (2008). Journal of Horticultural Science & Biotechnology 83 (1): 76-84. The authors' abstract of their manuscript can be seen in full at www.jhortscib.com.