Objective This study aimed to elucidate the non-spatial and spatial structural characteristics of different types of degraded coniferous forests in cold-temperate zone, thereby providing a theoretical basis for the structural and functional restoration of degraded coniferous forests in this region.

Method In the Genhe forest region of the Greater Khingan Mountains, 5 sample plots were established in degraded near-mature coniferous forests, as well as in stands subjected to fire and logging disturbance. Within each plot, trees, shrubs, and herbaceous plants were investigated to assess species diversity across different vegetation layers. Stand spatial structure analysis methods were employed to examine the univariate and bivariate distribution of the stand spatial structure parameters (uniform angle index, mingling, crowding, and neighborhood comparison) and structural diversity.

Result 1) In the degraded near-mature coniferous forest plot, tree density was relatively low, with a high proportion of dead individuals. Stands affected by fire disturbance exhibited a unimodal (bell-shaped) diameter distribution, characterized by a deficiency of small- and medium-sized trees. In the Logging-degraded coniferous and broad-leaved mixed forest plot, the proportion of Larix gmelinii was less than 40of the tree layer, which consisted of Larix gmelinii and Betula platyphylla. Shrub diversity was higher in stands with less disturbance, while herbaceous plant diversity was greater in stands with more severe disturbance. 2) Randomly distributed trees (W_i = 0.5) overwhelmingly dominated all sample plots, accounting for more than 55% in each plot, while evenly distributed trees (W_i = 0 or W_i = 0.25) represented the samllest proportion. The mean uniform angle index for the degraded near-mature coniferous forest plot, the fire-degraded coniferous forest plot, and the logging-degraded coniferous and broad-leaved mixed forest plot were 0.514, 0.494, and 0.508, respectively, indicating a random distribution. While the fire severely degraded forest plot and the fire-degraded coniferous and broad-leaved mixed forest plot had mean uniform angle index values of 0.563 and 0.540, respectively, reflecting a cluster distribution. Mingling (M_i) was generally low across all plots, with zero-degree mingling (M_i = 0) being dominant. The average neighborhood comparison of Larix gmelinii in all plots was less than 0.5, indicating its dominant position. Trees with neighbors exhibiting both random distribution (W_i = 0.5) and zero-degree mingling (M_i = 0) accounted for the largest proportion in each plot. 3) Structural diversity varied markedly among forest types. The logging-degraded coniferous and broad-leaved mixed forest plot exhibited the highest structural diversity (2.033), whereas the fire severely degraded forest plot showed extremely low structural diversity (0.248).

Conclusion Fire disturbance has led to a shortage of small-diameter trees within forest stands, hindered natural regeneration, and resulted in an unstable diameter distribution. Both fire and logging disturbances have altered the spatial distribution patterns of stands to some extent and reduced structural diversity. Therefore, for fire-disturbed stands, artificial interventions can be implemented to overcome regeneration barriers and facilitate natural regeneration. For structurally degraded stands, structured-based forest management techniques can be applied to optimize spatial structure, enhance structural diversity, and thereby promote the structural repair and reconstruction of degraded coniferous forests in cold-temperate zone.