Background In the hilly and gully loess region of northern Shaanxi, due to the complicated terrain and insufficient precipitation, the low survival rate of afforestation and serious individual differentiation of some stands have emerged since the implementation of the project of Returning Farmland to Forest. Traditional afforestation planning and design adopted regular plant and row spacing for planting, and there was a big gap between stand state and ideal structure of near-natural stands. The influence of stand space structure on the overall growth status of stands was ignored in the process of afforestation.

Methods In order to analyze the spatial structure of soil and water conservation forest in the loess area of northern Shaanxi, three plots (each 40 m×40 m) were selected for 3 types of forest (Armeniaca sibirica, Ulmus pumila, Populus hopeiensis forests), with the same site conditions. Taking the southwest corner of the plot as the origin, the spatial coordinates of each tree were obtained. The spatial distribution pattern, degree of differentiation and vertical structure distribution were analyzed. The mingling degree, uniform angle index, neighborhood comparison, open degree, forest layer index, and competition index were selected as the evaluation indexes, and the weight analysis was carried out by AHP, and the evaluation function was finally obtained.

Results 1) The mean value of all uniform angle exceeded 0.517, and stand distribution patterns were clustered type. The tree size differentiation was as follows: U. pumila > A. sibirica > P. hopeiensis. 2) The mean value of forest layer index was < 0.5, indicating that the forest structure was simple and vertical space utilization was low. 3) The competition indexes were all around 0.2, and they were in the transition state of weak competition or weak competition to moderate competition. According to the comprehensive score of plots, the mean value of all kinds of plots was low, and most of them belonged to grade 4 plots. From the perspective of the forest structure, U. pumila was superior to P. hopeiensis, and A.sibirica was the worst.

Conculsions The spatial structure of soil and water conservation forest stands in the loess area of northern Shaanxi has the problems of single species and unreasonable distribution, thus we should take corresponding measures to solve the problem. The priority of tending is determined according to the evaluation results of stand spatial structure. Strengthening the mixture and stand density is the major measures, and supplemented by the adjustment of individual trees and improving forest stability and health through proper tending.