Growth dominance (GD) is an important indicator of structure—function coupling that describes how trees of different sizes contribute to stand growth. It plays a key role in understanding stand competition structure, resource-use patterns, and density regulation mechanisms in plantations. Although planting density strongly drives structural differentiation and competitive hierarchy formation, the dynamic coupling among density, structural heterogeneity, and GD remains insufficiently quantified. In this study, we used long-term density-control experiments of Chinese fir (Cunninghamia lanceolata) across Fujian, Jiangxi, Guangxi, and Sichuan provinces, and fitted generalized additive mixed models (GAMMs) incorporating plot-level random effects and autoregressive model of order 1 (AR (1)) temporal structures to analyze the temporal dynamics of GD under different planting densities. We quantified key competitive time characteristics—transition point of GD change (t0), peak rate of GD change (t*), and the structural–functional relationship between GD and diameter inequality (Gini index (GI)). The results showed that (1) stand GI exhibited a "decline–rise" pattern with age. The minimum GI (AgeGI-min) occurring at 8–10 years and was strongly negatively correlated with the natural logarithm of planting density (N0) (R2 = 0.86, P < 0.05), indicating that higher planting densities led to earlier structural convergence; (2) GD shifted from negative to positive with stand development and was significantly affected by age, planting density, and their interaction. The turning point of GD (t0) was strongly negatively correlated with lnN0, suggesting earlier transition to large-tree–dominated growth under higher densities; (3) GD and GI showed a significant "inverted U-shaped" relationship, with maximum GD occurring at intermediate levels of structural heterogeneity; (4) the optimal structure (GI*), at which GD was maximized, increased monotonically with planting density, indicating that stands at higher planting densities were characterized by higher levels of structural differentiation when GD reaches its maximum, reflecting stronger competition and enhanced size differentiation; (5) mean GI and mean GD were highly positively correlated with lnN0 (r = 0.997 and 0.900), whereas t0 showed strong negative correlations (r = −0.943 and −0.804), reinforcing that planting density regulates GD formation by altering structural development and the timing of competitive shift. Overall, planting density shapes the trajectories of diameter-structure evolution and GD formation, thereby modifying size-dependent growth allocation within stands. This study reveals a density-driven GI–GD co-evolution mechanism in Chinese fir plantations and provides scientific guidance for density management, timing of first thinning, and structure-oriented plantation management.
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To study the effects of the close-to-nature model of Cunninghamia lanceolata and Phoebe bournei on the physical and chemical properties of forest soil, and to provide a scientific basis for the transformation and sustainable management of pure Chinese fir forests.
Three close-to-nature transformation modes of Cunninghamia lanceolata and Phoebe bournei (T1, Cunninghamia lanceolata∶Phoebe bournei=3∶7; T2, Cunninghamia lanceolata∶Phoebe bournei=2∶3; T3, Cunninghamia lanceolata∶Phoebe bournei=1∶1 and one pure Cunninghamia lanceolata forest control (CK) were set up, with 9 sample plots in each mode. Soil samples were taken from 0-20, 20-40 and 40-60 cm at three different slopes: upper, middle and lower. The ring knife method was used to determine the physical properties of the forest land under different modes, including soil bulk density, moisture content, maximum soil water holding capacity, capillary water holding capacity, minimum soil water holding capacity, total porosity, capillary porosity, non-capillary porosity, etc. The chemical properties of the soil, including total nitrogen, total phosphorus, total potassium, hydrolyzable nitrogen, available potassium, effective phosphorus, organic matter and pH value, were determined according to the methods specified in relevant industry standards.
The soil bulk density and mass water content differed significantly between different modes. The bulk density of the T1 was the smallest in the three soil layers, and the bulk density of the CK was the largest. The mass water content of the three soil layers was: T1 > T3 > T2 > CK. In different modes and soil layers, except for the non-capillary pore CK of the 40-60 cm soil layer in the forest land interplanted with Phoebe bournei, which was greater than that of T2 and T3, the rest were mixed forests greater than the pure control forests. Interplanting Phoebe bournei can increase the pH value of forest soil. Under different close-to-nature transformations of Phoebe bournei, except for the total phosphorus content of the 20-40 cm soil layer, there were significant differences in the effects on the total nitrogen and total potassium content of the forest soil and the total phosphorus content of 0-20 and 40-60 cm. The total nitrogen and hydrolyzable nitrogen content of the soil were CK > T1 > T2 > T3 in the three soil layers. The total phosphorus and available potassium content of the soil were T1 > T2 > T3 > CK in the three soil layers. The total potassium content of the soil was T1 > CK > T2 > T3 in the three soil layers. The organic matter content of CK was the highest in all the different modes, and there was no significant difference in the organic matter content among the three soil layers of T1, T2 and T3. The physical property factors of forest soil were generally greatly affected by the content of organic matter, total nitrogen, hydrolyzable nitrogen, total potassium, available potassium and total phosphorus.
The close-to-nature transformation model of Cunninghamia lanceolata and Phoebe bournei can improve the physical properties and nutrient conditions of forest soil, especially T1 (3 Cunninghamia lanceolata and 7 Phoebe bournei), which has good promotion value.
Close-to-nature silviculture of Chinese fir plantations mixed with Phoebe bournei is the widely silviculture mode in China. Exploring the effects of close-to-nature silviculture mixed with P. bournei in Chinese fir plantations on forest growth and timber assortment output of Chinese fir provides insight into selecting suitable silviculture mode for large-size timber production.
Based on the Chinese fir forests mixed with P. bournei in Chenzhou Xishan Forest Farm, Hunan, four treatments were set in the study, including three close-to-nature silviculture modes mixing with P. bournei and one control mode. The diameter at breast height, tree height, crown width, tree volume and stand volume of Chinese fir as well as the timber assortment output were analyzed and compared under different modes.
Compared with the control mode, other than the stand volume, the mean diameter at breast height, mean tree height, mean crown width and mean tree volume of the three close-to-nature silviculture modes were all greater than those of the control mode. There were significant differences between diameter, tree height, crown width, tree volume and stand volume (P<0.01). Moreover, the timber assortment output rate of Chinese fir was also significantly different (P < 0.01). The large-size timber output and rate of mode T1 were the largest, followed by mode T2, T3, and CK, and the output and rate of large-size timber of Chinese fir decreased with the increase of the proportion of Chinese fir. The large-diameter timber output of Chinese fir was positively correlated with mean diameter, dominant height of stand and mean crown width, but negatively correlated with the retention density of Chinese fir.
Compared with pure Chinese fir stand, the close-to-nature silviculture modes can significantly promote the growth of Chinese fir. Close-to-nature silviculture mode mixed with P. bournei in Chinese fir forests (T1: the ratio of Chinese fir and P. bournei is 3∶7) is conducive to cultivating large-size timber of Chinese fir. A lower retention density of Chinese fir is helpful for optimizing the timber assortment structure and cultivating large-size /super-large-size woods of Chinese fir.
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