Spatiotemporal evolution of rapeseed phenological periods in the Middle and Lower Reaches of the Yangtze River from 1981 to 2024

To elucidate the response mechanisms of rapeseed phenological stages to geographical environments and climate change in the Middle and Lower Reaches of the Yangtze River Plain, this study systematically analyzed the spatiotemporal patterns of key phenological stages and their durations using observational data from 50 meteorological stations spanning 1981 to 2024. Methods employed included kernel density estimation, linear trend analysis, and correlation analysis. The results revealed that: (1) Spatially, the relationship between rapeseed phenology and geographical factors exhibited a distinct stage-dependent shift. During the vegetative growth stage (sowing to five-leaf), phenological timing advanced with increasing latitude (“high-latitude & early-sowing”), showing significant negative correlations with latitude (r = -0.200 to -0.285), primarily reflecting agronomic strategies to avoid winter frost. In contrast, during the reproductive growth stage (flowering to maturity), the pattern reversed to “high-latitude & late-flowering,” with strong positive correlations with latitude (r = 0.665 to 0.731), suggesting that photoperiod and accumulated heat became dominant controlling factors. (2) Temporally, a coordinated pattern of “delayed early stages but advanced later stages” and “shortened initial phases but prolonged subsequent phases” was observed. From 1981 to 2024, over half of the sites (50%-58%) exhibited delays in vegetative stages, while reproductive stages tended to advance (64%-88%), with significantly earlier flowering observed at 88% of the stations. Phase duration shortened from sowing to flowering at 56%-68% of the sites, but lengthened after flowering—especially from green-ripening to maturity—at 78% of the sites, resulting in an overall shortened growth cycle at 64% of locations. (3) This phenological evolution is closely linked to the seasonal asymmetry of climate change in the region, characterized by slower autumn warming and more rapid spring warming. These findings offer new insights into the geographical adaptation mechanisms of rapeseed phenology and provide valuable guidance for developing region-specific adaptive strategies for rapeseed production under global climate change.