Monitoring the dynamics of winter wheat planting areas in the North China Plain using dynamic-threshold decision tree classification

Winter wheat is one of the most crucial food crops in the world. North China Plain has also been one of the largest planting regions in China. It is vital to accurately monitor the spatiotemporal pattern of winter wheat planting area, in order to predict the grain yield for the national food security. Many previous studies have concentrated on the planting area of winter wheat over only a few years. It is still lacking in dynamic monitoring in the long term. The recently released products of large-scale winter wheat often suffer from coarse spatial resolution or limited temporal coverage. Furthermore, there are significant disparities in the areas, spatial patterns, and dynamics of winter wheat planting areas, as indicated by various remote sensing products. Therefore, it is very necessary to explore the spatial and temporal evolution of the winter wheat planting area. Taking the North China Plain as the research area, this study aims to develop the dynamic-thresholding decision tree classification, according to the thematic maps and the phenological characteristics of winter wheat. Crop phenology was also characterized under diverse climate conditions and years. The static thresholds were then reduced to dynamically calculate the relative phenological changes in the greening and browning periods per year. Land use maps were utilized to identify the potential training samples for the subsequent classification. A field test was finally carried out to monitor the dynamics of winter wheat planting areas in the North China Plain from 2003 to 2022. The results show that: 1) high accuracy was achieved in extracting the winter wheat planting areas, with a multi-year mean overall accuracy of 93.44% and strong alignment with statistical data. Notably, the accurate delineation was realized in more fragments, such as Beijing, Tianjin, and southern Henan, compared with the rest products. 2) The winter wheat planting area overall increased by 23% over the past 20 years. Moreover, there was a great variation in the space and time of winter wheat planting areas at a grid scale of 5 km×5 km. A consistent decrease was found in some regions, including the west part of Henan Province and the central-west part of Hebei Province. The continuous increase was in the areas like the eastern part of Shandong Province and the central-eastern part of Hebei Province. The planting area increased significantly after increasing in the remaining regions. 3) The continuously-planting areas of winter wheat only accounted for 5% of the total planting area (defined as the land with winter wheat planting in one or more years) in the study period. Planting times of less than 10 years were observed in 55% of the total, indicating a potential widespread occurrence of cropland fallow and abandonment. 4) Winter wheat planting areas also remained relatively stable in the central and southern parts of Hebei Province, the western part of Shandong Province, and the central-northern part of Henan Province. Conversely, frequent changes were found in the Beijing-Tianjin-Hebei urban clusters and the mountainous areas. Therefore, a novel approach was introduced to monitor the long-term planting areas of winter wheat on a large scale. The findings can provide a strong reference to better understand the spatiotemporal evolution of winter wheat planting areas in the North China Plain.