Both mean temperature and daily temperature variance affect freeze risk in apples. Freeze damage to blossoms was assessed using a sequential model. In the model, once the chilling requirement was reached, growing degree days were accumulated and phenological stages determined based on growing degree thresholds derived from historical phenological observations. Critical temperatures for each stage were obtained from the literature and used to identify the occurrence freeze injury based on minimum temperature occurrence. In New York, temperature variance was the dominant climatological factor controlling freeze risk. A small <5% increase in variance counteracted mean temperature increases of up to 5.5 °C leading to increased freeze risk despite warming temperatures. In other apple-growing regions in the northwestern and southeastern United States, changes in freeze risk were dominated by changes in mean temperature. This demonstrates that in some regions the risk of freeze injury under future climate conditions may be more sensitive to changes in temperature variance. Variance is currently not well simulated by climate models. Because freeze risk also increases when the chill requirement is reduced, adaptation decisions to transition to lower chill requirement cultivars may be ill-advised in northern climates similar to New York as even the highest chill requirements were satisfied under the conditions with the greatest warming. This was not the case in other regions where the adoption of lower chill requirement cultivars may be warranted.
This work was supported by NOAA Contract EA133E07CN0090, the New York State Agricultural Experiment Station and the New York State Energy Research and Development Authority.
We appreciate the guidance and motivation we received from our colleagues Greg Peck, Alan Lakso, David Wolfe, and Jonathan Comstock. Phenological data were supplied by David Kain and Peter Jentsch.
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