Spatial Structural Characteristics of Different DBH Groups in Robinia pseudoacacia Plantation in the Old Course of Yellow River

Each tree in the 100 m × 100 m permanent sample plot of the Robinia pseudoacacia plantation in the Old Course of Yellow River was located and surveyed. The mingling( M_i ), dominance( U_i ), uniform angle index( W_i ) and crowding( C_i ) were calculated to describe the heterogeneity of spatial structure for different diameter at breast height(DBH) class trees. The revised forest spatial structure index(FSSI) and forest spatial structure distance(FSSD), and structure-based competition index(SCI) were used to describe the structural characteristics of the whole stand. The results showed that there were about 60.0% of the individual trees randomly distributed or associated with the same species, and more than 55.0% trees were randomly distributed or mixed at zero degrees in different DBH group; 47.1% of the super-large trees were very sparse, and occupied the smallest proportion of absolute inferior trees. While 57.7% of the small-diameter trees were very dense trees, and occupied the largest proportion of sub-dominant trees. The spatial structure of large trees was the best(FSSI=0.062, FSSD=1.041), and that of super-large trees was the worst. All the trees of different DBH groups were in weak competition state, but the competition intensity of small diameter trees was stronger. Therefore, the spatial structure of different DBH groups for the R. pseudoacacia plantation in the Old Course of Yellow River was poor, being urgent to adjust and optimize forest structure so as to accurately improve forest productivity, and to help the ecological protection and high-quality development of the Yellow River Basin.