Objective Neighborhood competition evaluation is an important basis for target-tree selection, density regulation, and silvicultural tending in plantations, while competitor-tree selection methods and competition-index construction can directly affect the evaluation of competition intensity. To identify suitable methods for evaluating neighborhood competition in Platycladus orientalis and Pinus tabuliformis plantations in Beijing mountainous areas, this study compared the applicability of the four-neighbor method and the gradually increasing radius method, evaluated the performance of different competition indices and tree attributes in characterizing neighborhood competition, and analyzed the neighborhood competition characteristics of the two species, with the aim of providing a basis for target-tree competition identification, neighborhood regulation, and tending thinning in plantations.

Method Based on tree-mapped survey data from 15 sample plots of pure Platycladus orientalis and Pinus tabuliformis plantations in the Ming Tombs area of Beijing, trees with DBH ≥ 4 cm were defined as subject trees, and edge correction was conducted using the plot translation method. Competitor trees were identified using the four-neighbor method and the gradually increasing radius method, respectively; for the gradually increasing radius method, segmented linear fitting was used to determine the optimal neighborhood ranges at the species and diameter-class levels, and paired t-tests were used to compare the two species. Based on the Hegyi competition index, seven types of neighborhood competition indices were constructed using DBH, tree height, crown width, and their combinations. The suitability of the two competitor-tree selection methods was evaluated by comparing among-plot differences in competition indices, as well as the correlations and fitting performance between competition indices and tree attributes. Under the selected method, the performance of different competition indices and tree attributes in characterizing neighborhood competition was further evaluated, and neighborhood competition characteristics were compared between the two species.

Result (1) The overall optimal neighborhood ranges of Platycladus orientalis and Pinus tabuliformis were similar. For both species, the optimal neighborhood range was larger for small-diameter trees and generally decreased and tended to stabilize with increasing DBH. (2) Comparisons among plots within the same compartment showed that the four-neighbor method detected fewer significant differences in competition intensity than the gradually increasing radius method. The overall analysis across all sample plots showed that competition indices derived from the four-neighbor method generally had stronger correlations and higher goodness of fit with DBH, tree height, and crown width of subject trees. These results indicated that the four-neighbor method was more suitable as the main basis for the subsequent evaluation of neighborhood competition indices and tree attributes. (3) Among the seven types of neighborhood competition indices based on the four-neighbor method, CI-DHW generally showed good correlations and fitting performance with multiple tree attributes, indicating that CI-DHW, which integrates three tree attributes, provided a better overall characterization of neighborhood competition intensity. (4) Under the four-neighbor method, DBH generally showed stronger associations with neighborhood competition indices than tree height and crown width, indicating its potential as a basic indicator of neighborhood competition, with crown width of Platycladus orientalis and tree height of Pinus tabuliformis as auxiliary judging factors.

Conclusion Under the studied stand conditions, the most suitable competitor-tree selection method, basic indicator, and neighborhood competition index for evaluating neighborhood competition intensity in Platycladus orientalis and Pinus tabuliformis plantations were the four-neighbor method, DBH, and CI-DHW, respectively. These results can provide a basis for target-tree competition identification, neighborhood regulation, and related tending practices.