Growth periods variation of summer maize and winter wheat and their corre lations with hydrothermal conditions in recent years in China

Crop phenology is influenced by both climatic and agronomic conditions, especially temperature and precipitation, which directly affect tillage schedule. A climatic phenomenon of warming hiatus has been noted since 2000. This phenomenon differs with the popular views of global warming. From the perspective of crop phenology, numerous studies have been conducted to assess the changes in growth periods of summer maize and winter wheat in the past decades. A relatively clear linkage of phenology and climatic conditions has been confirmed. As necessary climatic variables for crop growth, both temperature and precipitation influence the processes of crop growth, including the time of occurrence of the main phenology and length of the growth period. However, the relationship between phenology and climatic condition in recent years has been hindered by the lack of specific analysis that corresponds this relationship with the warming hiatus. For this specific period therefore, analyzing inter-annual changes of main phenological events and exploring the relationship between phenology and temperature as well as precipitation have become necessary for assessing current and future impacts of climatic conditions on crop growth and food security. Using observed phenological and meteorological data for 2000-2013, this study focused on analyzing of phenological characteristics and variations in growth periods of summer maize and winter wheat. Corresponding matches were built among hydrothermal conditions during the growth periods of the two crops and then the relationships between the length of growth period and hydrothermal conditions were analyzed. Spatial analysis method was used to find the proximal meteorological stations to phenological stations. Also, classical statistic was used to analyze the trends in phenology of the two crops and the relationships between phenology and hydrothermal conditions. The results showed delays in the main phenological events of summer maize and winter wheat. About 64% of the observed data stations showed that maturity of summer maize had delayed and the proportion of station with delayed trends in winter wheat was 78%. For the period covered in the study, the growth periods of summer maize and winter wheat were more sensitive to temperature and precipitation changes. About 88% of the stations used for summer maize and 64% of the stations used for winter wheat showed negative correlation between growth period length and temperature. While 71% of the stations used for summer maize and 77% of the stations used for winter wheat showed positive correlation between growth period length and precipitation. The reason for these tends was attributed to climate change. Different from the general understanding of global warming, average temperature during the growth period of summer maize did not show an obvious increase or decrease in trend, but precipitation obviously increased in most of the stations. The proportion of the stations with increasing trends in precipitation was more than 68%, while both cold and dry trends were observed during the whole growth period of winter wheat. The proportion of the stations with decreasing trend in temperature or precipitation all was more than 60%. In addition, analysis of the crop rotation stations revealed that annual climate data could be used in place of climate data for the growth periods to analyze for phenology and growth of rotational crops.