Altitude, Phenology, and Cotton Yield in Arid Oases: Quantifying Their Interactive Relationships

Climate change induces cotton phenological changes, but the impact of these changes on yield and the regulatory role of altitude in the phenology-yield relationship remains unclear. Major Chinese cotton-growing regions (e.g., Xinjiang) are in arid and semi-arid areas with fragile ecosystems, making it crucial to clarify the phenology-yield correlation for ensuring regional cotton production security. Using long-term data (1981-2023) from 35 cotton monitoring stations in Xinjiang's arid oases, we analyzed key phenological variations, quantified phenology's impact on yield, and examined altitude's effects on phenology. The results showed that the dates of four key cotton phenology-sowing (Sow), emergence (Eme), squaring (Squ), and flowering (Flo)-exhibited an advancing trend at a rate of 0.037-0.050 days year^-1. In contrast, the dates of boll opening (Bol) and maturity (Mat) showed a delaying trend, with the delay rate ranging from 0.015 to 0.037 days year^-1. Most phenological stage durations changed slightly: Sow-Eme, Squ-Flo, Bol-Mat, and vegetative growth period (VGP) shortened, while Eme-Squ, Flo-Bol, reproductive growth period (RGP), and whole growth period (WGP) lengthened. Lint yield increased by 24.061 kg ha^-1 year^-1. A one-day delay in the occurrence dates of any of the five cotton phenological stages-Sow, Eme, Squ, Flo, or Bol-was associated with a yield reduction ranging from 0.895 to 9.780 kg ha^-1. In contrast, a one-day delay in the Mat led to a yield increase of 0.7876 kg ha^-1. Additionally, the extension of three growth periods (Sow-Eme, Squ-Flo, and VGP) resulted in a yield decline, while the prolongation of four other periods (Eme-Squ, Bol-Mat, RGP, and WGP) contributed to a yield increase. The most critical finding is that altitude has a significant association with cotton phenology and its yield response: every 100 m increase in elevation, cotton phenological dates were delayed, the durations of different growth stages were altered, yield was reduced by 0.250 kg ha^-1, and low-altitude areas exhibited more pronounced spatial heterogeneity in phenology and yield. However, this regulatory effect did not reach a significant level (p > 0.05), and the correlation between altitude and yield variability tended to be stronger in high-altitude areas than in low-altitude areas. This elevation-induced phenological shift is a key mediator of yield changes-elevational temperature variations are significantly associated with the duration of critical growth stages (e.g., the lengthening of reproductive growth period in low-altitude areas and shortening in high-altitude areas), which may indirectly affect dry matter accumulation and final yield formation. Corresponding policies for different altitudes should be formulated to offset the negative effects of phenological changes, providing scientific support for securing cotton production in arid oases.