## 4 Discussion  
This study compares the performance of six phenological models in simulating the budbreak date of eight grapevine varieties across different European sites. We found that the model performances characterized by a Chilling-Forcing approach were generally higher compared to those obtained for Forcing approach when different starting dates were set (Figure 6 and Figure S17). Although model intercomparison studies on the estimation of budbreak date in grapevine have already been conducted by other authors [15; 29; 30], this study focused on evaluation of the models' reliability to reproduce budbreak in different environmental conditions, by considering the impact of temperature during the chilling and forcing accumulation period. In contrast to other works [27; 30; 57; 58], this model intercomparison considers the use of a long-term series of observed phenology, which include grapevine varieties with different precocity levels collected at various latitudes in three European countries (France, Luxembourg and Portugal, Figure 1 and Table 1). The extensive observed dataset coupled to the methodology applied in this study allowed to evaluate the influence of study site-temperature variability on the grapevine development. Moreover, the cross-validation scheme adopted in this model intercomparison experiment provided an exhaustive analysis of the bias and variance of the CF and F models while the comparison between two different types of parameterization (HP or AV) pointed out the issue to select a specific parameter set for model applications (Results section, Figure 5). This in-depth investigation of model fitting, validation and application was particularly useful for evaluating the influence of temperature during the dormancy period. In this context, the different model responses during budbreak predictions are related to the physiological requirements needed for dormancy fulfilment of the grapevine varieties. Differently from the CF models, which describe the entire dormancy period (endo- and eco-dormancy), the F models only describe the eco-dormancy period through forcing units accumulation, under the assumption that the chilling requirement was already satisfied [24]. Although the debate on which model type provides the highest reliability is still open [29; 30], our analysis shows that CF models, considering the influences of winter and spring temperatures on the accumulation of chilling and forcing units, may avoid the under- or over-estimation at the beginning of eco-dormancy, which occur when F models are set on fixed dates. In this study, we estimated a longer endo-dormancy for the vines from the Northern European sites (e.g., Gewurztraminer), which occurs later than the endo-dormancy of vines from the Southern European sites (e.g., Touriga Nacional), irrespective of the precocity level of the different varieties. In other terms, grapevine varieties such as Gewurztraminer (early budbreak) and Riesling (late budbreak) collected in Northern Europe showed a late endo-dormancy release (higher chilling requirement) with respect to Chardonnay (early budbreak) and Cabernet Sauvignon (late budbreak)mainly collected in Southern Europe (lower chilling requirement). Although this trend was examined in this study only from a modeling point of view without any physiological evidence on the dormancy period, our results suggested that plants cultivated at different latitudes might show distinct dormancy period lengths. The higher accuracy of CF compared to F models is likely linked to the ability of these models to estimate the endo-dormancy release and to better account for the effect of temperature in this phase, avoiding the use of a fixed a priori value for the beginning of the eco-dormancy period as in F models [27; 29].