Characterizing Spatiotemporal Patterns of Winter Wheat Phenology from 1981 to 2016 in North China by Improving Phenology Estimation

Phenology provides important information for wheat growth management and the estimation of yield quality. The relative threshold method has been widely used to retrieve phenological metrics from remotely sensed data owing its simplicity. However, thresholds vary substantially among locations, hampering us effectively detecting spatial temporal variations in winter phenology. In this study, we developed a calibrated based on ground observations. Compared with traditional method, our can minimize bias uncertainty caused by unreasonable determining dates. On basis, seven key dates three periods were estimated long-term series (1981–2016) Normalized Difference Vegetation Index North China (106°18′–122°41′E, 28°59′–39°57′N). Results show that pre-wintering (i.e., emergence tillering) occurred December south mid- late- October north, while post-wintering green-up onset, jointing, heading, milky stage, maturity) exhibited opposite pattern, is, January May February June north. Consequently, vegetative period increased 49 days 77 reproductive decreased 51 29 days. At regional scale, all predominantly advanced, most pronounced advancement being onset (–0.10 days/year, p > 0.1), (–0.09 jointing (–0.08 0.1). at scale extended 0.03 (p 0.1) 0.09 < 0.001) respectively. general, later events tended advance higher temperature, earlier emergence, tillering, jointing) showed weak correlation suggesting might be mainly affected ones more responsive warming. These findings provide critical reference improving under ongoing climate