Trends of Growing Degree-Days in Turkey

Abstract

Variations in the surface air temperatures show an associationwith the atmospheric CO2 under the hypothesis that warmertemperatures promote increases in plant growth in mid-northernlatitudes. Historical daily temperature records at seventy-fourweather stations (around 60-yr) in Turkey are used in orderto investigate this hypothesis. Separate calculations and analyses are undertaken for monthly and seasonal growing degree-days (GDD). The sequential version of Mann-Kendall rankstatistic is applied to demonstrate existence of any possible non-linear trends in accumulated GDDs above 5 °C over the months and seasons. Spatially coherent and statisticallysignificant trends of GDD are shown in some parts of Turkey. The statistically significant trends are negative mostly in summer and autumn seasons over the coastal areas of Turkey.These results strongly agree with the previous studies where weak cooling trends are observed in maximum temperatures overTurkey. They suggest the significant decreases in GDD may lead to long-term decreases in vegetation growth mostly overthe coastal regions of Turkey.