Background The Zhongtiao Mountain forest region serves as a critical ecological barrier in North China, where increasingly severe soil erosion causes significant threats to regional ecological security. As the primary agent for soil and water conservation, forest ecosystems' structural characteristics and ecological functions directly determine their water retention capacity. However, the response mechanisms between forest quality and soil erosion remain poorly understood. The study aimed to establish a scientifically rigorous and practical stand condition evaluation system to provide quantitative benchmarks and technical support for health diagnosis and functional enhancement of ecological public welfare forests.

Methods This study focused on the ecological public welfare forests in Zhongtiao Mountain, employing an integrated evaluation approach combining structural equation modeling (SEM) and the unit circle method based on systematic ecology theory and sustainable forest management principles. A multi-scale and multi-dimensional systematic evaluation was conducted on seven representative stand types in the region, including Quercus mongolica forests, Pinus tabuliformis forests, and Pinus tabuliformis-Quercus mongolica mixed forests.

Results 1) In the SEM, stand structure (weight=0.3986), stand composition (0.3399), and stand vitality (0.2615) were identified as the three key factors influencing stand conditions. Among these, diameter-class index, Simpson's diversity index, and health index emerged as the most representative indicators for each factor respectively. 2) The unit circle method evaluation revealed that mixed forests (e.g., P. tabuliformis-Q. mongolica stands with a score of 0.454) demonstrated significantly superior soil and water conservation functions compared to pure stands (e.g., P. tabuliformis forest scoring 0.253). This advantage primarily stemed from their broader niche width and higher resource utilization efficiency. 3) Comparative analysis showed that SEM was more suitable for examining relationships between stand structure and diversity characteristics, while the unit circle method proved particularly advantageous when evaluating stands with substantial compositional differences. These findings may provide quantitative evidence supporting the ecological benefits of mixed forests and offer methodological guidance for stand evaluation based on specific research objectives.

Conclusions Through quantitative analysis, this study has elucidated the response mechanisms between forest structural features and soil-water conservation functions, confirming the ecological superiority of mixed forests for erosion control. The findings provide a scientific foundation for sustainable forest management in ecologically vulnerable regions.