Evaluation and fitting of models for determining peach phenological stages at a regional scale

Phenological models provide an assessment on the adaptation of species and cultivars to current conditions as well as the on the impact climate change may have on their performance in the future. The aim of this study was to obtain phenological models that predict accurately peach bud development for reference cultivars in temperate growing areas. Model testing and validation were done using phenological observations from orchards in Ebro and Guadiana Valleys, two of the main peach growing areas in Spain, where chilling accumulation ranges from medium to high. Data were acquired during 6 years for three widely grown cultivars (Big Top, O'Henry, and Andross). Two different types of models were tested: Critical Date models (CD), which consider only the action of forcing temperatures accumulated from a fitted date, and Critical Chilling models (CC), which also take into account the action of chilling temperatures during the dormancy period. CC models were always at least a 40% more efficient than CD ones and the version which uses the Dynamic method to estimate chill (CCDY) had also the best accuracy with prediction errors generally around four days, so it was chosen as the best model. The validation process showed adequate accuracy and no significant predictive bias. The fitted values for CCDY parameters also had physiological sense, as the chilling values at which forcing heat began to accumulate were in accordance with the limit in which additional chilling is known to have little effect on heat requirements; and fitted threshold temperature for forcing heat accumulation was nearly the same considered to be the physiological base temperature for this species (4.7 °C). On the contrary, CD models fitted threshold temperatures for bud development inconsistent with peach physiology (between −6 and 0 °C) and were poorly adapted to predict phenological stages at a regional scale. All in all, the results showed that phenological evolution of peach cultivars can be simulated with good accuracy using simple sequential statistically fitted models that include chilling evaluated by Dynamic method and forcing heat accumulation evaluated by growing degree sums. These models could be used to estimate the phenological evolution of peach under a wide range of climatic situations, including the evaluation of the impact climate change scenarios may have on the phenology of the crop.