Abstract:

Objective: This study aims to investigate the regulatory effects of different close-to-nature management models on intraspecific and interspecific competition in Cunninghamia lanceolata plantation, clarify the variation patterns of total competition indices with diameter at breast height, and identify the optimal management model, so as to provide scientific basis and technical support for cultivating stable, highly efficient, and sustainable uneven-aged, multi-storied C. lanceolata plantations. Method: In Guangxi State-Owned Qipo Forest Farm, C. lanceolata pure plantations were managed using a close-to-nature approach combining thinning and enrichment planting. The plantations were subjected six treatments: D1 and D2 (strip mixture model, with thinning strip widths of 12 m and 15 m); J1 and J2 (intimate mixture model, with thinning intensities of 60% and 70%); T1 and T2 (group mixture model, with thinning group diameters of 12 m and 16 m), with a separate control (CK). The enrichment species were Dalbergia cochinchinensis, Albizia odoratissima, Toona ciliata, Castanopsis hystrix, and Garcinia paucinervis. A plot-based tree-by-tree survey was conducted on the stands after five years of management to obtain tree growth data and positional information. Voronoi diagrams were used to delineate the competitive neighborhood for each target C. lanceolata. Hegyi single-tree competition model was used to analyze the composition of major competitive tree species and their species competition indices (CI_s) under each management regime, and calculate the intraspecific competition index (CIA_intra), interspecific competition index (CIA_inter), and total competition index (CIA_total) for the target trees. Additionally, multiple functional models were fitted to characterize the relationship between these competition indices (CIA_total) and the DBH of the target trees. Result: 1) The diameter class structure of the arbor layer in the strip mixture model exhibited a reverse J-shaped distribution, while that in the group mixture and intimate mixture models showed a bimodal distribution, and the CK presented a unimodal distribution. All models had a wider diameter range than the CK, filling the gaps in the regeneration layer (2–4 cm). They also significantly increased the abundance of small-diameter (4–12 cm) and medium-to-large-diameter (20–32 cm) trees. 2) The main competitor species across all management models were still C. lanceolata (CI_s=40.97) and Pinus massoniana (CI_s=12.36). However, the planted precious species, including T. ciliata (CI_s=7.31), D. cochinchinensis (CI_s=6.17), A. odoratissima (CI_s=5.96), and C. hystrix (CI_s=5.37), had gradually entered the succession layer and begun to exhibit certain competitive ability. 3) The close-to-nature management model was able to effectively regulate stand competition intensity. Specifically, strip mixture and intimate mixture (D1, D2, J1, J2) models significantly reduced the intraspecific competition index of C. lanceolata, and the magnitude of this reduction increased with increasing thinning strip width or thinning intensity. In contrast, group mixture (T1, T2) significantly increased the interspecific competition index of C. lanceolata, with a greater increase observed for larger thinning gap diameters. The total competition index across management models ranked as T2 > T1 > CK > D1 > J1 > D2 > J2, with only the J2 model (70% intensity uniform thinning + enrichment planting) showing a significant difference from the CK. The uniform mixture models (J1, J2) was mainly dominated by interspecific competition. In contrast, for the strip and group mixture models, interspecific competition became dominant only at the greater thinning strip width (D2) or the larger thinning group diameter (T2). 4) The relationship between the total competition index and target tree DBH varied with management models. Under the uniform mixture models, this relationship followed a power function, with total competition index decreasing as DBH increased, and the decreasing trend leveled off when the DBH reached 35 cm, indicating that the relationship between competition and DBH tended to stabilize. Under strip mixture model, group mixture model, and CK, the relationship followed a quadratic polynomial function, with competition index first decreasing and then increasing as DBH increased. The total competition index began to rise when the DBH reached 44 cm, 30 cm, and 28 cm, respectively, suggesting that stand competition intensified at these stages, and thus thinning should be implemented before the turning point of recovery. Conclusion: The close-to-nature management can effectively alleviate the intraspecific competition pressure in C. lanceolata plantation and promote interspecific competition between the replanted precious tree species and C. lanceolata, thereby optimizing the stand competition pattern. Among all models, the uniform mixture model performs the best and can stabilize the long-term competitive dynamics of the stand, making it the recommended approach for the close-to-nature transformation of pure C. lanceolata plantation in this region.

Key words: intraspecific competition, interspecific competition, diameter class structure, close-to-nature management, uneven-aged multi-storied mixed forest, Cunninghamia lanceolata plantation, high-value tree species, southern Guangxi region