Climatology and trends in temperature-based agroclimatic indices over western Anatolia, Turkiye

Temperature has a control on growth, development and yield of crops. Extreme high and low temperatures can be a limiting factor on crops. Extreme temperatures have accompanied global average temperature increase. Investigating the changes in indices derived using daily minimum and maximum temperatures may provide information about how extreme temperatures related to crop production have changed under global warming. In this sense, it is aimed in this study to investigate the climatology and monotonic temporal trends of a number of temperature-based agroclimatic indices across western Anatolia, Turkiye. The indices included number of frost days (NFD), first fall/winter frost (FFF), last winter/spring frost (LSF), frost period (FP), accumulated frost (AF), plant heat stress day (PHSD), plant heat stress accumulation (PHSA), growing season start (GSS), growing season end (GSE), growing season length (GSL) and growing degree-day (GDD). Significance of trends was assessed using Mann-Kendall test, and trend rates were quantified by Sen's slope estimator. The results firstly showed that southern low-elevation stations have the lowest frequency, intensity and duration of frost as a result of rarely occurring frost events. These stations are also characterized by the highest GDD, the earliest GSS, the latest GSE and hence the longest GSL, even with never-ending growing seasons in many years. They are also exposed to the highest heat stress in terms of frequency and intensity. On the other hand, northern high-elevation stations have completely opposite character for all indices in comparison to southern low-elevation stations. Trend analysis revealed that frequency of frost events had decreasing trends (i.e., decreased NFD), accompanied by later onset of FFF, earlier onset of LSF and hence shortened FP and decreased AF. As for growing season indices, in general, GSS occurred earlier, GSE occurred later and GSL extended. Growing degree-day and heat stress indices had substantial increasing trends, implying that, over the course of study period, the study area had more thermal accumulation and exposed to more heat stress that may hinder crop development.

Automated summary

By investigating temperature-based agroclimatic indices in western Anatolia, Turkey, it was found that southern low-elevation stations have the lowest frequency, intensity, and duration of frost events, while northern high-elevation stations have the opposite characteristics. Trend analysis revealed decreasing trends in frost events, earlier start of growing season, later end of growing season, longer growing season length, and increasing trends in growing degree-day and heat stress indices.