Daily climatic data were obtained from several sources to calculate growing degree days (GDD) for multiple sites in southwest Michigan, which contains the Lake Michigan Shore American Viticultural Area. The data were examined for spatial and temporal (1950 to 2011) patterns and trends over the region in order to better quantify the role of Michigan climate on juice grape production. The occurrence and severity of frost and freezing temperatures were also considered in this study, as subfreezing temperatures in late spring and early fall can have severe impacts on the region’s juice grape production and fruit quality at harvest. Michigan’s cool-cold climate has warmed in recent decades, particularly since 1980, with an average increase over the region of more than 3.7 GDD (base 10°C) per year. Southwestern Michigan was also found to have higher seasonal temperature variability when compared with Napa Valley (California). Since 1980, the season-to-season variability in Michigan has increased at a more rapid pace. The impacts of the increasing GDD have been positive for fruit quality, with a strong positive correlation between seasonal GDD and fruit maturation, indexed as total soluble solids (Brix). The growing season has also increased by 28 days in length since 1971. However, despite warmer temperatures, the number of days of potential frost and their seasonal variability in southwestern Michigan have remain unchanged, which continues to pose a risk for grapegrowers in the region. While it has become warmer in Michigan, and the spring warm-up is typically arriving earlier in the year, the number of days with damaging frost still has a profound impact on overall climate-related risk for grape production.
- ©2014 by the American Society for Enology and Viticulture