More info can be found at Bunting et al (2021). "Vitis vinifera Production in Michigan: Factors and Trends Driving Cultivation Patterns". Frontiers in Plant Science.
Introduction
In this study, we studied the suitability of MI vinifera across the state in two ways. First, through an extensive literature review, the key drivers and commonly noted trends guiding vinifera production were identified. Second, through trend analysis of the key drivers of suitability, the study investigated how such variables are changing, significantly, over space and time thus driving the observed expansion in vinifera production in the state.
Study Results
Trend analysis on important viticultural variables highlights the pattern trajectory of such critical variables both spatially and temporally across the state. The literature review indicates that, while MI is a cool climate for grape production, the variables impacting productivity, phenology, harvest, and overall grape quality are similar to those impacting other grape-growing regions around the world including, commonly, GDDs. Trend analysis revealed that these variables have evolved, sometimes significantly to influence the observed patterns in vinifera production. For instance, positive trends were observed in accumulated GDDs, particularly in 1990–2019 and 1983–2019 (see figure below). An increase of 5–10 GDDs per year was possible for most of the areas in the southern Lower Peninsula.
The figure above shows the magnitude of change in the annual numbers of growing degree days (GDDs; accumulated days per year) for different study periods (at α = 0.10 level). Locations of persistently changed GDDs are shown. Highest proportions of changed area were found in 1990–2019 and 1983–2019. However, greatest changes in GDDs were observed in 1983–1994 (− change) and 2000–2019 (+ change). For reference, typical GDDs in Lower Peninsula MI range from 750 to 2040 accumulated days per year.
The above figure shows locations of persistently changed precipitation (mm per year) for key growth periods (A) and the general growing season (B). Highest proportions of changed area during key growth periods were found in 1983–2019 and 1983–2012. Greatest changes in the variable were observed mainly in 1983–1994 (+ change). For reference, precipitation during critical growth periods ranges from 0 to 360 mm (Table 1). During the general growing season, negative changes in precipitation were mostly observed in 1983–2002 and 1983–2012. Total growing season precipitation typically ranges from 245 to 1,000+ mm per year.