To provide theoretical and technical support for the sustainable management and soil fertility maintenance of larch plantations.

Larix olgensis plantations with basically the same site conditions were selected. In 2018, selective cutting operations with different intensities (25%, 35% and 45%) were carried out according to the stock volume, and non-selective cutting was set as the control. Subsequently, three fixed sample plots were set up in the stands with different selective cutting intensities. In August 2023, after five years of selective cutting operation, the coverage thickness, standing stock, soil physical and chemical properties of litters were measured in each plot. Through single factor analysis of variance, Mantel analysis and structural equation modeling, the changes of litter cover thickness, standing stock and soil physical and chemical properties under different selective cutting intensities and their relationships were studied.

After 5 years of selective cutting, the litter cover thickness decreased by 17.4%, 30.3%, and 31.0% at 25%, 35% and 45% selective cutting intensities, respectively, compared to the control stand. The undecomposed litter layer thickness decreased by 32.7% and 36.1% at 35% and 45% selective cutting intensities, respectively, compared to the control stand. The semi-decomposed litter layer thickness decreased by 25.7%,44.2%, and 43.4% at 25%,35%, and 45% selective cutting intensities, respectively, compared to the control stand. High-intensity selective cutting (45%) significantly reduced soil bulk density in the 0-10 cm layer by 17.7% compared to the control. Moderate-intensity selective cutting (35% and 25%) also reduced soil bulk density in the 20-40 cm layer, with reductions of 15.0% and 11.0% respectively. Soil moisture content increased by 40.8% (0-10 cm), 21.8% (10-20 cm), and 18.7% (20-40 cm) after high-intensity selective cutting compared to the control. The maximum soil compaction degree in 0-40 cm soil layer was 785.0 kPa in control forest,724.4 kPa in 25% selective cutting forest, 525.6 kPa in 45% selective cutting forest and 503.9 kPa in 35% selective cutting forest. Under the moderate selective cutting (35%), the soil pH, total nitrogen, total phosphorus, organic carbon and hydrolyzable nitrogen in the 0-10 cm soil layer increased by 5.5%, 21.7%, 39.8%, 45.4% and 51.3% respectively compared with the control. Under the high intensity (45%) selective cutting, the contents of available potassium and available phosphorus in the 0-10 cm soil layer increased by 30.0% and 60.4% respectively compared with the control. The results of structural equation modeling showed that selective cutting had a significant positive effect on soil moisture content and soil chemical properties, and a significant negative effect on soil bulk density, litter cover and litter stock, and also had a significant negative effect on soil chemical properties through changing litter cover and litter stock.

Selective cutting management can significantly reduce the accumulation of litter on the surface of forest land, improve the microenvironment in the stand, and be beneficial to the decomposition of litter such as larch needles on the surface. After selective cutting with medium and high intensity (35%, 45%), it can accelerate the decomposition of litter, effectively promote the circulation of soil nutrients, and play an important role in maintaining soil fertility and preventing soil acidification.

Taking Larix olgensis plantation as the research object, a set of evaluation methods of stand spatial structure optimization was constructed, and based on this, a stand thinning model was constructed to provide a scientific thinning scheme for forest tending management.

After standardized calculation, the forest survey factors such as height, DBH and crown width were weighted to Delaunay triangulation network and the stand spatial competition unit was determined. The six forest spatial structure parameters of sample plot uniform angle index, dominance, crowding, stand open degree, competition index and mingling degree were calculated, and the entropy weight method was used to give the weight of each parameter. Combined with the idea of multiplication and division, the multi-objective programming was carried out, and then the stand spatial evaluation index (SEE) and stand spatial evaluation level (SET) were calculated. Based on SEE, a stand average thinning model was constructed (This model could select the trees with poor spatial structure in the stand and make them more evenly distributed in the stand) and applied to the actual plot. In Dongsheng Forest Farm of Lushuihe Forestry Bureau, 20 square plots of Larix mixed forest with an area of 0.06 hm² were established. The changes of spatial structure parameters, SEE, SET and the canopy projection map of 6 stands before and after thinning were analyzed to verify the rationality of the model.

SEE was applied to the survey stand, and it was found that the mean SEE of the 20 plots was between 0.138 and 0.216, and SET was distributed at level 1 and level 2. Among them, 85% of the plots belonged to level 1, and 15% of the plots belonged to level 2. According to the definition of SET, the stand quality could be judged to be poor, which needed to be optimized. Taking the sample plot z17 with the lowest rating index as an example, after the application of the average thinning model, it was found that the spatial structure parameters of each stand developed in the direction of optimization, and the SET of the sample plot was upgraded from level 1 to level 2.

Based on the weighted Delaunay triangulation, the stand average thinning model constructed by combining the multiplication and division method and the entropy weight method can improve the defects of similar models, optimize the spatial structure of the stand, and has important reference significance for the construction of stand growth and harvest prediction model.