Climate change results in an increase in temperature which can be advantageous because it allows the ripening of the grapes but the grapes are too exposed, there is a risk of water stress. Garcia de Cortazar (2006) has shown that the effects on the behavior of vines in terms of yield and grape composition will certainly be very marked by climate change in the Mediterranean region. Yields will be affected downward, even though this is the French region where they are already among the lowest (Boutin, 2012). The question raised is: Should objective yields be lowered to limit the risk of water stress? Or, is climate change impacting yields?

The objective yield is the yield that the winegrower wishes to achieve or not to exceed if it is equal to a limit yield of appellation. This objective yield will define the winegrower's cultural choices, such as thinning. A study in Languedoc-Roussillon (France) (Payan, 2018) could not conclude to a significant and regular effect of the grape load on the water stress of the vine for the observed yields, both the differences in behavior between years and grape varieties are important. The results presented suggest that the effect of grape load on water stress is negligible. The grape load, and therefore the yield, was regulated by manual thinning before flowering or at the end of fruit set, with 4 bunches per vine for the “low load” modality and 14 bunches per vine for the “heavy load” modality. The yield measurements carried out at harvest for the “low load” modality vary from 30 to 50% of the “heavy load” reference. (80 hL / ha maximum theoretical for the highest yields)

Recent changes in yield, generally downward, can have several origins:

• The direct effects of climate change with greater evapotranspiration, reducing the size of berries.
• Changes in cultivation practices: change in the grape varieties for more qualitative grape varieties but with lower yields (according to Zebic (2011), the modification of the grape varieties in Hérault over the past 10 years can explain 8.6% of the drop in yield ), decrease in complantation, loss of the practice of pruning respectful of the feet which lead to diseases, nitrogen stress induced by a decrease in fertilization and / or grassing
• Development of wood diseases that ravage vineyards
• Change in the frequency of extreme events (these are no longer the same typical diseases that are found in some regions, eg mildew now in Languedoc).

Climate change is responsible for only a small part of the yield loss that can occur in the Mediterranean region. Indeed, other factors such as changes in practice are also responsible. Lowering objective yields does not seem necessary because the grape load has no effect on the water stress of the vine according to the study cited. On the other hand, depending on the time of water stress, it can have an impact on yield according to Levin et al (2020) as can be seen in the figure below. Their conclusion is that severe pre-veraison water deficits will reduce the size of berries during the current season, and the number of bunches per vine plant for subsequent seasons by affecting the differentiation of the primordia from inflorescences in the latent buds in training (Deloire, 2020).

Figure 1: The timing of water deficits dictates the exact effect on yield in the current and following season.

It can be concluded that the objective yield (expressed by the grape load) has a negligible impact on water stress and therefore does not need to be reduced. However, this target yield may be altered by water stress caused by climate change.

Author: Maddy Tintinger (INRAE)

References :

Boutin F., Payan J.C., 2012. Viticulture et changement climatique : adaptation de la conduite du vignoble méditerranéen. Innovations Agronomiques, INRA, 25, pp.193-203. ⟨hal-02647254⟩ (https://www6.inrae.fr/ciag/content/download/3826/36242/file/Vol25-14-Boutin.pdf)

Deloire, A., Jaillard, B., Pellegrino, A., Metay, A., Pantin, F., Romieu, C., Ojeda, H., Abbal, P., Carbonneau, A., Torregrosa, L., 2020. Traité de la vigne.

Levin, A.D., Deloire, A., Gambetta, G.A., 2020. Does water deficit negatively impact wine grape yield over the long term? Technical Reviews. https://doi.org/10.20870/IVES-TR.2020.4029

Payan, J.-C., Salançon, E., 2018. Effet de la charge en raisin sur la contrainte hydrique de la vigne. IFV - Institut Français de la Vigne et du Vin. http://www.vignevin-occitanie.com/wp-content/uploads/2018/08/ContrainteHydrique_JCPayan.pdf