Heat as a Located Constraint on Grain Production in a Humid Basin: Implications for Climate-Adaptive Land Management

In humid agricultural regions, where heat and water deficit are decoupled, temperature acts on crops in two opposed ways: moderate warmth advances development, whereas extreme heat damages it, and land management must distinguish them. This study examined the temperature response of grain production in the Sichuan Basin, southwestern China, and translated it into priorities for climate-adaptive land management. Growing degree days (GDDs) and extreme degree days above 30 °C (EDD30s) were built from gridded daily mean temperature; the response was estimated for 183 county-level land units over 2014–2024 with two-way fixed-effects models and full controls. July and August constitute the critical growth period; the findings are fourfold. (1) Over this window, 100 additional GDDs raised grain yield by 2.70% and 100 additional EDD30s lowered it by 3.24%. (2) Damage deepened as the threshold rose, from 2.52% at 30 °C to 10.23% at 34 °C per 100 degree-days, while coarse indices carried no signal. (3) The constraint centred on July, persisted under spatial inference at all bandwidths, and was not moderated by irrigation. (4) A total of 67 of the 177 estimation counties combined above-median exposure with above-median output, producing 69.6% of basin grain. The results point to a direct thermal channel rather than water deficit, supporting adjustments to cropping calendars and cultivar choice rather than irrigation expansion in the eastern plain.