Objective Cunninghamia lanceolate, a fast-growing timber tree species in subtropical regions of China has good ecological and economic benefits but is highly fire-prone. Understanding the short-term effects of different fire severities on understory diversity and soil properties is critical for predicting post-fire ecosystem trajectories and informing science-based restoration strategies.

Method Three months post-fire, twelve 20 × 30 m plots were established in a Guangxi C. lanceolata forest across four fire severity classes: unburned (control), low, moderate, and high severity (n=3 per class). The species, number, height and coverage of shrub and herbs vegetation in Cunninghamia lanceolata plantation were investigated, and the species diversity index (Simpson index, Shannon-Wiener index, Pielou index, Margalef index) was calculated. Soil samples were collected from two layers (0~10 cm and 10~20 cm) using the five-point sampling method. Soil physical properties (soil moisture content and bulk density) and chemical properties (pH, organic matter, total nitrogen, total phosphorus, total kalium, available phosphorus, and available kalium) were measured. One-way ANOVA was used to analyze the significant differences in understory vegetation diversity and soil physicochemical properties under different fire severities. Redundancy analysis (RDA) was applied to explore the relationships between species diversity indices and soil physicochemical properties.

Result (1) ANOVA confirmed significant fire effects (p<0.05): species richness followed moderate>high>low>unburned; shrub diversity increased with severity, while herb diversity peaked under moderate fire (Simpson=0.85, Shannon=2.12, Pielou=0.90, Margalef=2.36). (2) Different fire severities significantly influenced the physicochemical properties of soil at different depths (p<0.05).0~10 cm Soil Layer: In moderate-severity plots, soil moisture content, bulk density, and organic matter content decreased significantly by 28.97%, 20.22%, and 43.31%, respectively, compared to unburned plots (p<0.05).10~20 cm Soil Layer: In moderate-severity plots, soil moisture content, bulk density, organic matter, total nitrogen, and total phosphorus decreased by 26.50%, 12.20%, 37.34%, 22.56%, and 27.38%, respectively, compared to control plots. In low-and moderate-severity plots, total nitrogen content was significantly higher in the 0~10 cm layer than in the 10~20 cm layer (p<0.05). (3) After fire disturbance, soil organic matter, total nitrogen, and total kalium in the 10~20 cm layer were identified as key factors influencing the differences in understory vegetation species diversity.

Conclusion Fire severity exerted significant short-term effects (p<0.05) on understory diversity and depth-specific soil properties in C. lanceolata forests. Diversity patterns were strongly linked to soil characteristics, with organic matter, total N, and total K explaining cross-plot variations. Targeted management of these soil factors offers potential pathways for enhancing post-fire vegetation recovery.