Changes in Winter Temperatures, Chilling Hours, and Projected Impacts to Tree Fruit in the Midwest United States

Climate change in the Midwest has significantly impacted tree fruit production, which is sensitive to changes in both growing season and winter climate conditions. Winter in the Midwest has warmed faster than any other season over the past several decades, and models are confident in projections of continued winter warming. The impact of winter warming in the Midwest has been an increase in the accumulation of winter chill hours, and one of the implications of this trend is potential earlier dormancy break in fruit trees in the Midwest. Despite the significant trend of earlier last spring freeze events in the Midwest, milder winters could result in an enhanced risk of spring freeze for popular apple and peach cultivars grown in the region. A commonly prescribed adaptation strategy for climate change in the Midwest is using lower-chill requirement cultivars of fruit; however, the apparent trend of milder winters could increase the risk of significant spring freeze damage. Therefore, to better link recommendations of adaptation strategies for Midwest fruit growers with observations and expectations of changing winter climate conditions, we use a combination of observations and model projections to estimate chill hour accumulation and spring freeze risk in apple and peach crops in the Midwest through the 21st Century.

We find winter chill hours have increased significantly across the northern Midwest in the past 50-70 years, with little to no change in the southern Midwest. Projections indicate continuation of these trends, with a large potential for dormancy break that is 2 to 3 weeks earlier in the northern Midwest by mid-century, relative to historical. Despite the earlier dormancy break, spring freeze risk is also projected to decrease substantially in the northern Midwest due to increasing spring temperatures. The historical trends and projections of changes in winter chilling hours and spring freeze risk are much more uncertain, but still show a projected earlier dormancy break and bloom date into the latter half of the 21st Century. Overall, we find winter warming has impacted chilling hours and spring freeze risk in the Midwest, but projections indicate low-chill apple and peach cultivars may not be an effective adaptation strategy moving forward in the region.