A Novel Model to evaluate Spatial Structure in Thinned Conifer- Broadleaved Mixed Natural Forests

Abstract

This study aimed to establish a management strategy for forest structures in Xiaoxing'an Mountains, China. We investigated the non-spatial structure factors affecting the spatial structure units of conifer-broadleaved mixed natural secondary forests via thinning and further quantified the spatial structure characteristics. Six forest plots (100 m×100 m) of similar stand conditions located in the Xiaoxing'an Mountains were used for the study. The thinning intensities were 10%, 15%, 20%, 25%, 30%, and 35% for plots A-F respectively. The structure units were constructed using Voronoi diagrams in ArcGIS and constrained by non-spatial structure indexes. Seven stand spatial structure parameters were selected, and each was quantified by the optimal distance model (TOPSIS and combination weight methods), which was directly used to evaluate and compare the spatial structure disparity of the structural units and reflect the degree of the spatial structure of tending and thinning. The effects of crown width and crown length on the spatial structure unit of a stand were respectively higher than that of diameter at breast height and tree height. Nine possible values for the number of trees adjacent to a central tree in the spatial structure unit were obtained via weighted Voronoi diagrams, and the distribution frequency of 3–8 adjacent trees accounted for 90–96%. The spatial structure parameters derived from weighted Voronoi diagrams were analyzed using the optimal distance model. The mingling intensity and levels of competition in the tended and thinned plots differed from the control. The spatial structure evaluation index of natural mixed forests was B (0.488) > C (0.487) > E (0.480) > D (0.479) > A (0.475) > Control (0.442) > F (0.433). Plot B had a higher proportion of trees at the medium open level (41.18%), with medium and upper stand variation (72.94%), and higher levels of competition (55.29%) than plot F. This indicated that the 35% interval caused a less than ideal stand spatial structure. For the unique properties of mixed coniferous natural forests, crown length had a greater influence on structure units than tree height. Thus, the spatial structure evaluation index based on the optimal distance model offers a new way of identifying the ideal spatial structure and its quantitative expression on conifer-broadleaved mixed natural secondary forests. The findings presented herein provide a basis for optimizing spatial structure management measures.