Building new temperature indexes for a local understanding of grapevine physiology

Aim: Temperature corresponds to one of the main terroir factors influencing grapevine physiology, primarily evidenced by its impact on phenology. Numerous studies have aimed at expressing time with thermal indices such as growing degree days (GDD) and have thus enabled a better modelling of grapevine responses to temperature. However, some works have highlighted the need to adapt GDD to the considered pedo-climatic context and grape variety or to refine the time step at which temperature variables are computed. The present study aims to investigate the hypothesis that grapevine response to temperature depends on the production context, ie. plant material, pedo-climate, topography, orientation and cultural practices, and that thermal indices should then be locally adapted. 

Methods and Results: GDD with different base temperatures but also other indices based on other algebraic equations on daily average temperature were calculated starting from the bud break date and using data from weather stations located in the Bordeaux region (France), California (USA) and Israel. The dates of flowering and veraison were expressed according to each of these indices for three commercial blocks located near each weather station. For each block, the relative differences in the flowering and veraison dates were calculated for any couple of years and summed squared. The number of studied years considered ranged from fifteen to five depending on the blocks. The relative difference between two dates was computed as their difference in index-related degrees divided by the average index-related amount of degrees to reach veraison. The thermal index which minimizes the sum of the relative differences of flowering and veraison dates for all the years of the same block is considered to best illustrate the temperature local effect. As such, this local effect includes both grapevine physiological response to temperature and the difference between the weather station data and the conditions actually experienced by the vines.

Dates of flowering and veraison of all years coincide when expressed in a given thermal index for most of the blocks. The hypothesis whereby temperature is a predominant factor in grapevine phenology may thus be confirmed. Moreover, the thermal indices allowing such an adjustment are different between blocks of different locations, thus demonstrating that temperature effects on grapevine phenology are better captured when considered according to locally calibrated indices. 

Conclusion:

Temperature effects may be better captured by different thermal indices depending on the local context. 

Significance and Impact of the Study: In a precision viticulture context, a growing access to local and higher resolution weather data and grapevine observations enables models to be used locally. The present study therefore corresponds to a first attempt to highlight the importance of calibrating a local thermal index to improve the performance and operational relevance of any temperature-based model.