Assessing water conservation capacity in Cunninghamia lanceolata and broad-leaved mixed forests with different mixing proportions

To clarify the differences in water conservation capacity of mixed forests stands of Cunninghamia lanceolata mixed with the broad-leaved species Schima superba and Sassafras tzumu at different mixing proportions, a comparative study was conducted in the Yong’an State-owned Forest Farm, Fujian Province. Three mixed forests stand types (5 Cunninghamia lanceolata ×1 Schima superba ×4 Sassafras tzumu, 5 Cunninghamia lanceolata ×2 Schima superba ×3 Sassafras tzumu, 5 Cunninghamia lanceolata ×3 Schima superba ×2 Sassafras tzumu) and a pure Cunninghamia lanceolata stand (CK) were examined. Using stratified cutting, quadrat harvest, and cutting ring methods, the water-holding capacities of the canopy layer, understory vegetation layer, litter layer, and 0−60 cm soil layer were measured to compare the total water-holding capacities among stand types. The results showed that the water-holding capacities of different layers varied significantly among the mixed stands. Soil served as the main water reservoir, accounting for more than 98% of the total water-holding capacity. The canopy, understory, and litter layers of the mixed stands generally exhibited higher water-holding capacities than those of the pure Cunninghamia lanceolata stand. The total water-holding capacity of the stands followed the order: 5 Cunninghamia lanceolata ×3 Schima superba ×2 Sassafras tzumu (3231.89 t·hm⁻²)>5 Cunninghamia lanceolata ×2 Schima superba ×3 Sassafras tzumu (3186.36 t·hm⁻²)>pure Cunninghamia lanceolata forest (3024.22 t·hm⁻²)>5 Cunninghamia lanceolata ×1 Schima superba ×4 Sassafras tzumu (2935.40 t·hm⁻²).Correlation analysis indicated that the water conservation function of the stands was significantly and positively correlated with total aboveground biomass (P <0.05), while correlations with mean tree height and mean diameter at breast height were relatively weak. This suggests that biomass is an important structural factor influencing the water-holding capacity of mixed forests. Overall, in Cunninghamia lanceolata × Schima superba × Sassafras tzumu mixed forest, moderately increasing the proportion of Schima superba and maintaining a balanced conifer–broadleaf composition can enhance water regulation and soil water retention capacity. Among the tested combinations, the 5 Cunninghamia lanceolata ×3 Schima superba ×2 Sassafras tzumu mixture exhibited the best water conservation performance.