Trend analysis of phenological stages length and chilling requirements of apple tree (Case study: Karaj station)

Determination of chilling and heating requirements is an important process for selecting fruit trees specifically adapted to climatic regions. In this study, the thermal thresholds of different phenological stages of apple tree (Red Delicious variety) were determined for further analysis. Long-term hourly and daily temperature data from 1985 to 2014 of Karaj station were collected and used for running three chilling requirement models namely CH, UTAH and CP. The results showed that the chilling requirement of this late variety is not satisfied. A significant decreasing trend in accumulated chilling units was observed. Application of growing degree days (GDD) model confirmed that the required heat units are maintained without limitation. The findings revealed that there exists a significant decreasing trend in occurrence date of phenological stages which might be due to higher temperature during the season. These conditions (earlier onset of vulnerable reproductive phases) increase the risk of frost and chilling damage. In most phenological stages of the apple tree, the mean air temperature showed an increasing trend. According to the results, late varieties are not suitable for current climatic conditions of Karaj climate and should be replaced with early ones. Further agroclimatic studies are recommended for more scrutiny across the country considering the sequences of global warming.