Impacts of Extreme Climate Events on Subtropical Upland Crops: A 20-Year Case Study in the Hilly Area of Southwest China

Understanding how climate extremes affect crop growth in humid–subtropical hilly regions is essential for climate-smart agriculture, yet phenology-resolved evidence remains limited. We combined 20 ETCCDI extreme climate indices (1960–2024) with field records of wheat and maize production (2005–2024) from the hilly area of southwest China, and quantified climate–crop linkages using Mantel tests and generalized additive models; persistence and prospective tendencies were evaluated using Hurst (H) and Mann–Kendall statistics. Warming extremes intensified, with significant increases in TXx (0.22 °C decade−1), SU25 (2.48 days decade−1), and DTR (0.47 °C decade−1), while TNx and TNn declined and frost days increased; most precipitation intensity indices showed no significant trends except CDD, which increased by 1.73 days decade−1. Seasonally, warm extremes and CDD strengthened during the maize season, whereas climatic conditions during the wheat season were comparatively more favorable. Climate impacts on crop growth were stage-dependent, typically lagging by 1–2 months: wheat biomass was positively associated with TXx/TNx (strongest near heading), whereas maize production was more sensitive to temperature extremes (negative) and precipitation frequency indices; CDD significantly affected both crops. These findings suggest that compound heat–drought risks for maize could increase under the persistence and trend signals observed in the historical record, while modest warming may benefit wheat but cold extremes could remain a constraint for management.