We mapped and investigated observed historic and projected future changes in winter chill in California, quantified with two different chilling models (Chilling Hours, Dynamic Model).
For cultivars with chilling requirements above 1000 Chilling Hours, such as apples, cherries and pears, very few locations with safe chilling levels were found to exist today, and our modeling results project that virtually none will exist by mid century.
At present, chilling requirements are only available in Chilling Hours for most species and cultivars, providing the only basis for projections of future ranges of tree crops (Fig. 3).
When using the Chilling Hours Model to quantify winter chill, chilling losses were most severe in the SSacV, which lost 51% (B1 scenario), 61% (A1B) and 67% (A2) compared to 1950 winter chill.
Currently, the orchard industry is poorly prepared for such a transition, since chilling requirements are only available for the Chilling Hours Model, whose accuracy is likely to decrease with further temperature increases [8], [14].
The most common chilling model used in the state is the Chilling Hours Model [sometimes referred to as Weinberger Model; 17,18].
When using the Chilling Hours Model to calculate winter chill, safe winter chill was about 130 Chilling Hours lower than mean winter chill on average over all locations and scenarios analyzed (shown for Davis in Fig. 2).
The Chilling Hours Model consistently projected greater losses than the Dynamic Model, confirming its greater sensitivity to rising temperatures [8 ]
Research on chilling models in many subtropical regions has indicated that the Chilling Hours Model is not very precise in this climatic zone [12] [16].
The high sensitivity of the commonly used Chilling Hours Model to climate change [8] complicates the estimation of future ranges for tree crops.
