Abstract:

Objective: This study aims to reveal the responses of dominant afforestation tree species in the semi-arid northern region of China to the northward shift of the 400 mm isoline by constructing an afforestation optimization framework that integrates temperature thresholds, root stratification, and risk zoning, thereby providing scientific support for ecological restoration projects under the influence of climate change. Method: A kernel normalized difference vegetation index (kNDVI) time series was constructed based on MOD13Q1 v6.1 data (2000–2024). The Theil-Sen slope was combined with Mann-Kendall (MK) test to quantify the northward migration characteristics of the 400 mm isoline. Random forest and partial dependence plot analyses were employed to extract the optimal photosynthetic temperature windows (21.2–22.3 ℃) of the dominant afforestation tree species of Pinus tabuliformis, Pinus sylvestris var. mongolica, and Caragana korshinskii in semi-arid northern China. The attribution of layered soil moisture content and cross-correlation function were used to calculate the lag response time of the dominant tree species to precipitation and their temperature sensitivity, which were then integrated with the precipitation-belt migration rates to construct the forestry risk index (RFI). Result: 1) The centroid of the 400 mm isoline significantly shifted northward at a rate of 3.89 km·a^–1, resulting in a 119.7% expansion of candidate suitable areas for P. tabuliformis, a 27.5% expansion for P. sylvestris var. mongolica, and an 8.1% contraction for C. korshinskii. 2) The optimal temperature zones for the kNDVI index of the dominant tree species were 23.1–23.8 ℃ for P. tabuliformis, 19.4–19.5 ℃ for P. sylvestris var. mongolica, and 23.0–23.6 ℃ for C. korshinskii. Combined with partial dependence curve analysis, it was found that when temperatures exceeded 25 ℃, the kNDVI index declined by ?11.03%, ?10.54%, and ?13.55%, respectively. 3) The spatial partitioning accuracy of RFI reached 89%, identifying high-risk areas, moderate-risk areas, and low-risk areas accounting for 12.3%, 27.5%, and 60.2% of the total study area, respectively. Subsequently, differentiated afforestation strategies and recommendations were proposed for different risk zones. Conclusion: This study has clarified the sensitivity of P. tabuliformis and P. sylvestris var. mongolica to high temperatures and soil moisture in 0–1 m layer, as well as the drought-resistance characteristics of C. korshinskii conferred by its deep root system. A differentiated regional afforestation strategy is constructed based on the “temperature threshold-root stratification-risk zoning” framework, providing scientific basis and practical pathways for ecological restoration project construction and forest regeneration in the semi-arid northern region of China.

Key words: precipitation belt shift, root stratification, temperature threshold, forestry risk index (RFI), dominant tree species