Assessing frost damages using dynamic models in walnut trees: exposure rather than vulnerability controls frost risks

Frost damages develop when exposure overtakes frost vulnerability. Frost risk assessment therefore needs dynamic simulation of frost hardiness using temperature and photoperiod in interaction with developmental stage. Two models, including or not the effect of photoperiod, were calibrated using five years of frost hardiness monitoring (2007-2012), in two locations (low and high elevation) for three walnut genotypes with contrasted phenology and maximum hardiness (Juglans regia cv Franquette, J. regiaxnigra Early' and Late'). The photothermal model predicted more accurate values for all genotypes (efficiency=0.879; Root Mean Standard Error Predicted (RMSEP)=2.55 degrees C) than the thermal model (efficiency=0.801; RMSEP=3.24 degrees C). Predicted frost damages were strongly correlated to minimum temperature of the freezing events (=-0.983) rather than actual frost hardiness (=-0.515), or ratio of phenological stage completion (=0.336). Higher frost risks are consequently predicted during winter, at high elevation, whereas spring is only risky at low elevation in early genotypes exhibiting faster dehardening rate. However, early frost damages, although of lower value, may negatively affect fruit production the subsequent year (R-2=0.381, P=0.057). These results highlight the interacting pattern between frost exposure and vulnerability at different scales and the necessity of intra-organ studies to understand the time course of frost vulnerability in flower buds along the winter. To predict critical periods facing frost damages, two dynamic integrated models of frost hardiness were adapted using five years of monitoring in three different genotypes of walnut trees and two locations. Frost hardiness was more accurately predicted when using both temperature and photoperiod as input variables. Higher frost damages (i.e. frost risks) were more closely related to temperature of the freezing events (i.e. frost exposure) rather than to frost hardiness or phenological stage (i.e. frost vulnerability). Even though frost risks are higher in winter at high elevation (exposure constrained) and spring at low elevation, especially in early genotypes (exposure and vulnerability constrained), fruit yields observed the following year were correlated to autumn damages.