Effect of vineyard-scale climate variability on Pinot noir phenolic composition

he sensitivity of agricultural crops to climate change is a major area for climate impact studies. The relationship between climate and three key phenolic compounds in grape skins important to premium wine quality (anthocyanins, tannins, and total phenolics) has not been well-studied. Here we conducted a three-year field study to collect and analyze berry samples from Pinot noir vineyards in the Carneros and Sonoma Valley American Viticultural Areas of Californiaʹs North Coast wine country, and correlate phenolic measurements with climate statistics derived from hourly temperature measures at each vineyard site. We used several statistical approaches to identify key phenologically-based periods influencing phenolic concentration at maturity, including classification and regression trees, factor screening, principal component analysis, and pairwise correlations. The results from these statistical models showed that cool conditions following harvest the year before maturity, warm temperatures from budburst to bloom, and cool temperatures from bloom to veraison (the onset of ripening) were positively correlated with concentrations of all three classes of phenolics, although not all trends were statistically significant. Anthocyanins were positively and significantly correlated with temperatures between 16 and 22 °C from veraison to harvest. Tannins were significantly increased by warm nights preceding budburst and warm days from budburst to bloom. We measured relatively high levels of light interception (35% of incident photosynthetically active radiation), and we found that increased light interception was significantly correlated with lower levels of all three classes of phenolic compounds in this study. For the Pinot noir sites in this study, warm temperatures from budburst to bloom appear to increase phenolic concentrations, which is likely beneficial for wine quality. However, warmer periods during the preceding fall and summer during ripening appear to offset these effects. Given projections for greater summer warming in California with climate change, the overall impact of climate change on winegrowing is likely to be negative.