The concept of growing degree days (GDD)
Vineyards are particularly sensitive to climate and especially temperature. That is why phenological vine data has been used to reconstruct temperature series over long past periods (Le Roy Ladurie, 1971; Chuine et al., 2004). For example, Garcia de Cortazar Atauri et al. (2005) created a predictive model of budbreak based on temperature.
Many temperature-related bioclimatic indices have been developed for viticulture (Huglin and Schneider, 1998). Winkler, in 1935 originally introduced the growing degree day concept in order to define the climate of the different viticultural regions in California. Growing degree days (GDD) are calculated from April to October in the North Hemisphere and from October to March in the South Hemisphere: GDD = ∑Tmdaily – 10 with Tmdaily (daily mean temperature) > 10 °C (50°F). This index, also named Winkler index, is widely studied because correlations between this index and the different phenological stages are statistically significant. Indeed, phenological stages can be predicted using the growing degree days since every variety of vine require its own amount of degree days to bud break, bloom or ripen. Figure 1 shows the average amount of degree days over the phenological season for different regions around the world.
Figure 1. Winkler Index classes (adapted from Vaudour (2003) and Tonietto & Carbonneau (2003))
Extreme temperatures effects on grapevines
Climate and especially temperature can either have a positive or negative impact on vines. While the amount of sunshine and rainfall will have an impact on grapes composition (sugar, acidity…), extreme temperatures can also cause irreversible damages to grapevines.
Frost can be extremely damaging for grapevines especially during the budbreak period. Indeed, spring is an intermediate season with alternation between cold and hot events. The main risk is to have a mild weather during the end of winter which will drive to an early budbreak. In these conditions, buds would be more exposed to a potential late frost event. Thereby, even if it sounds contradictory, global warming could also mean more late frost events in the future.
The spatial variability of frost can be very contrasted according to the topography of the vineyard. During nights with clear skies, radiative conditions are going to create inversions of temperatures with cold air being stuck at the bottom of the slopes and the milder air staying at higher elevations (figure 2). It means the consequences on vines can be really different in one single vineyard with some blocks entirely frozen and others staying intact.
Figure 2. Thermal profile of the lower atmosphere and the ground during a radiative event (adapted from De Parcevaux et Huber (2007))
Extreme heat can also be very harmful to grapevines especially when these heatwaves last for several days and occur during the ripening period. The climatic optimum for grapevines is between 25 and 35°C (77 and 95° F). Over 35°C, grape skin is exposed to burn marks and over 40°C (104°F) to scorching which is detrimental to grapes ripening (figure 3). Besides, the synthesis of anthocyanins, sugar, and polyphenols is stopped. The flavonoids synthesis, which leads to the tannins synthesis is also affected (Matus et al, 2006). During the ripening period, grapes are subjected to important chemical transformations and as a consequence are very sensitive to extremely hot temperatures. That is why Langellier (2003) explain that grapevines suffer more from thermal stress than water stress.
Figure 3. wilting vine leaves (A) burned grapes (B, C, D) following the heatwave of 2009 in South-East Australia (Webb et al., 2009)
If you are interested in mapping the differences of temperature within your blocks, have a look at TerraClima solution to identify areas with similar temperature regimes.