The Spatiotemporal Shift Of The Northern Limit Of Winter Wheat In Northern China And Its Influence Mechanism

The research on the northern limit of winter wheat is of great scientific value and practical significance for ensuring regional food security and coping with climate change.Under the background of global climate change,the potential northern limit of winter wheat have significantly expanded northward,providing a wide range of potential planting areas for winter wheat.However,the actual area of winter wheat planting in the northern China has decreased significantly.A gap may exist between the potential northern limit of winter wheat and the actual northern limit of winter wheat in China.It is particularly important to clarify the changes of the northern limit of winter wheat and its influence mechanism.This study took the climate-sensitive and ecologically fragile northern winter wheat planting area as the research area.The time series MODIS remote sensing data was used as the main source to extract the actual northern limit of winter wheat in the four periods of 2001,2007,2014 and 2019 and clarify the temporal and spatial evolution characteristics of actual northern limit of winter wheat.Combined with the data of climatic factors and human activity factors,the geographic detector model was used to quantitatively analyze the influence mechanism of the formation and change of the actual northern limit of winter wheat.The main research contents and conclusions are as follows:(1)Remote sensing mapping of winter wheat and the analysis of its temporal and spatial changes.The winter wheat mapping based on MODIS time series data can better reflect the spatial distribution of winter wheat planting area in different periods in northern China.In the past 20 years,the dynamic degree change of winter wheat planting area was mainly significant reduction.The reduction area was about 57.61×10~3 km~2 and the reduction region was mainly in the transitional region of winter wheat planting.For the dynamic degree,the area of winter wheat planting increased by about 23.53×10~3km~2 and the areas had better resource conditions and higher human activity intervention degree.(2)The dynamic change and comparative analysis of the northern limit of winter wheat.From2001 to 2019,the actual northern limit of winter wheat moved southward by 37 km in the latitude direction.The potential northern limit of winter wheat moved northward by about 16 km in the latitude direction.Compared with the potential northern limit of winter wheat,the actual northern limit of winter wheat moved to the south by 103 km in the latitude direction,and there were about 33×10~3 km~2 potential planting areas of winter wheat.There was a gap between the actual northern limit of winter wheat and the potential northern limit of winter wheat.The reason for this gap was that the actual northern limit of winter wheat moved southward under the interacting effects of human activities and climate change,while the potential northern limit of winter wheat moved northward due to climate change.(3)Quantitative analysis for the influence of agro-climate factors on the northern limit of winter wheat.The average temperature of the coldest month,the annual extreme minimum temperature,the negative accumulated temperature in the overwintering period and the accumulated temperature before overwintering were the important agro-climate factors that affected the formation of the actual northern limit of winter wheat,and their influence coefficients were all greater than 0.50.However,there was information overlap between these factors.The average temperature of the coldest month with relatively large influence can be used as the explanatory factor.The threshold values of winter wheat suitable for planting were the average temperature of the coldest month-7.8?,the negative accumulated temperature in the overwintering period-583?,the annual extreme minimum temperature-22?,the accumulated temperature before overwintering 418?,the accumulated temperature in the growing period1721?,the precipitation in the growing period 207mm and the precipitation in the critical period 113mm.The interaction between the accumulated temperature before overwintering,the accumulated temperature during the growing period,the average temperature of the coldest month and other factors had a high explanatory power for the formation of the actual northern limit of winter wheat.(4)Quantitative analysis for the influence of human activity factors on the northern limit of winter wheat.The total power of agricultural machinery,rural electricity consumption,effective irrigation area and the amount of agricultural fertilizer application were the main human activity factors that affected the change of the actual northern limit of winter wheat.The interaction between rural electricity consumption,effective irrigation area,the total power of agricultural machinery and other human activity factors was relatively strong.The actual northern limit of winter wheat has shifted southward,which was contrary to the previous conclusion that the potential northern limit of winter wheat has shifted northward.The average temperature of the coldest month,the negative accumulated temperature during the overwintering period,the accumulated temperature before the overwintering,the annual extreme minimum temperature,and the effective irrigation area,the total power of agricultural machinery,the rural electricity consumption were the key factors that affected the formation and change of the actual northern limit of winter wheat.The effect of human activity factors on the southward movement of the actual northern limit of winter wheat was mainly positive.