Simulation study of past survival trends in over-wintering of wheat in Iran

The snow-cover on soil, the factor that protects wheat from the biting cold, has generally declined; on the other hand, the surface air temperature has warmed. It is not clear whether these changes decrease winter-killing of wheat or favor it in Iran. Therefore, this study was mainly aimed to determine the past trend in frequency of winter-kill events (Y) for wheat with -4 oC frost tolerance, using the framework of acclimation/de-acclimation of wheat to temperature. The rate of change in Y was estimated over phases I (frost tolerance increases with decreasing temperature; hardening), II (it shows plateau state for a period with coldest temperature), and III (it decreases with warming; de-hardening) of wheat response to temperature. The input data for model was 39 to 44 years daily weather data for five locations as representative of major agricultural zones in Iran. The results indicated that the surface air temperature has warmed; consequently, the value of snow-falling has been decreasing and changes in acclimation/de-acclimation of wheat over the phases found to be happened in slowed manner. Over phase I, the frequency of winter-killing has remained constant just for Shiraz; for other four locations, the value of frequency has decreased (0.9 to 3.8 times decade-1). Over phase II, the decline varied between 0.8 to 4.7 times decade-1 across all five locations. Over other phase, the frequency has shown downwardly trend (1.3 to 3.7 times decade-1) just for three locations. Generally, it can be said that the over-wintering survival has been improving during past decades.