Impact of Climate Change on Winter Chilling Trend for Deciduous Fruit Trees (Case Study: Hamadan)

Higher temperature as the result of climate change are likely to affect horticultural production. Deciduous fruit trees need winter chilling to break winter dormancy. Climate plays an important role in the successful production of deciduous fruit. Winter dormancy is one of the key factors of the annual cycle of deciduous fruit and nut trees along with the following breaking of the dormant state. This state is maintained through the winter period each year to protect against damaging cold temperatures. To be released from dormancy, trees require exposure to a predetermined quantity of cold temperatures in a process known as winter chilling or vernalization. Insufficient chilling can lead to sporadic and light bud break, poor fruit development, small fruit size and uneven ripening times. The main objective of this study is to investigate climate change effect on the winter chilling requirement (WCR) in Hamadan.
This research was performed based on the General Circulation Models (BCM2, HADCM3,GFCM2 and IPCM4) and different emission scenarios (A2, B1, A1B), as recommended by the Forth Report of the IPCC. The output of the GCMs was downscaled by LARS-WG model. The hourly weather data were generated as the inputs of three different Chilling Requirement Models (CRMs), and the winter chilling trend of deciduous fruit trees were predicted for Hamadan. The projected daily temperature time series were then converted into hourly temperatures. The projected hourly temperature data were run through each of the three chill models for all four GCMs in different scenarios.
Three chill models [the 0.0–7.2°C (CH), the Utah (UT), and the Utah Positive (UTPos) models] were used to investigate changes in chill accumulation in Hamadan, according to localized temperature change related to increases in global average temperatures. In addition, the winter chilling requirement time series were divided into two periods: baseline and future time. Historical daily minimum and maximum data from 1980 to 2010 were used from the Hamadan airport synoptic station. Future time horizon splittedinto early (2011-2030) and late (2031-2050) periods. For evaluating the long-term future changes in the chilling requirement, we used parametric and non-parametric tests.
The model results showed a decreasing WCR trend during the recent decade. In general, the outputs of downscaled climate models predicted a decreasing WCR trend for the study site. For the time horizon of 2031-2050, this dramatic reduction in the WCR varied rom 25 percent to 40 percent. Future chill profiles differentiated between the WCR models as demonstrated through Hamadan global average temperature, causing a small decline in accumulated chill unit, with further warming causing greater decreases. This decrease in the UT models can be due to the negative effect of high temperature during this period. The study result which showed the WCR mean during early time horizon 2011-2030, was not significant but further time horizon 2031-2050 had a very significant change, as compared to the baseline. The aim of this study was to assess changes in the WCR rather than completing a model skill analysis. Through using previous climate model performance studies a justification of the addition of GCMs was described. Such defenses for model selection are recommended in all climate change impact studies. Test of model output in other scenarios and different GCMs showed an insignificant versatility between them.
This research represents a significant update to the previous climate impact analysis of chill in cold semi-arid climate of Hamadan. It also highlights that sensitivity studies as a useful method for impact assessments. The severity and rate of decline of winter chilling requirement, depends on which chill model was used. The general trend showed decreasing of the winter chilling requirement against the winter temperature trend. Therefore, in the context of global warming, the earlier flowering dates of many deciduous tree species is likely leads to increased risk of damage during the late spring frost. For future fruit farm management, decisions can be implemented with deliberation of the likely changes in the winter chilling requirement reported here. There might be some adaptation, at least to some degree, being essential for most production areas in Hamadan and other similar climate conditions within the next 40 years. Reduction in winter chilling, prevents breaking winter dormancy, which finally may lead to serious damage to deciduous fruits.