Objective  The ecological significance of 4-variate distribution of stand spatial structure parameters was deeply analyzed and a comprehensive evaluation index of stand spatial structure diversity based on the relationship between adjacent trees was constructed to provide a theoretical basis for formulating targeted forest structure adjustment strategies.

  Method  Based on the concept of biodiversity, taking the structural unit which was composed by reference tree and its nearest four adjacent trees as the object, organically integrating the 4-variate distribution of structural parameters of uniform angle index, mingling, neighborhood comparison and crowding, as well as the number of structural unit trees and forest layers, the genetic absolute distance formula and natural logarithm were used to express the evenness and richness of structural unit types, respectively, and the stand spatial structure diversity index was constructed. The validity of index was verified by long-term positioning monitoring sample plot data.

  Result  Using the stand spatial structure diversity index (D_FS) proposed in this study to measure the spatial structure diversity of forest types in different climatic zones or different origins, it was showed that the D_FS values of Quercus aliena var. acutiserrata natural forest (0.854) and broadleaved Pinus koraiensis forest (0.852) were almost the same, indicating that the two forest stands had similar spatial structural diversity. The 4-variate distribution types of Platycladus orientalis plantation were higher than other natural forest, however, its spatial structural diversity (D_FS = 0.382) was the lowest in the three stand types, mainly due to its lower diversity in both vertical (D_FSv = 0.369) and horizontal (D_FSh = 0.562) structure than the two natural forest stands. The average number of tree species in the structural units was higher in natural forest than in plantation, with 4.23 in Quercus aliena var. acutiserrata forest, 4.09 in Pinus koraiensis forest, and 1.98 in Platycladus orientalis plantation. The number of tree species in a structural unit fully demonstrated species richness of the structural unit.

  Conclusion  The integration of the three numbers including 4-variate distribution of stand spatial structure parameters, the number of tree species and number of forest layers in structure unit lay the foundation for constructing a valid spatial structure diversity index for forest stands. The stand spatial structural diversity index (D_FS), based on the concept of biodiversity, is not only a feasible quantitative expression of the 4-variate distribution of structural parameters, but also is an appropriate interpretation of the ecological significance of the 4-variate distribution of structural parameters, and a comprehensive scientific evaluation of the spatial structural diversity of the stand. The index is able to measure the difference of stand spatial structure diversity of different stand types.