Response of Understory Vegetation, Litter and Soil Characteristics to Stand Density in Picea Secondary Forests

Taking the 80-year-old Picea secondary forest in Pangquangou Nature Reserve of Shanxi Province as the research object, the understory vegetation diversity, litter and soil water conservation characteristics of the Picea secondary forest with different stand densities were analyzed by using the methods of variance analysis and multiple comparison through sample survey, sample collection and laboratory immersion treatment. The results showed that the smaller the stand density, the richer the species of shrub layer in the study area. The relationship between species diversity of herb layer and stand density was not obvious. The Euphorbia fortunei and Boehmeria nivea were the dominant species in the shrub layer, while Carex was the dominant species in the herbaceous layer. In general, the undergrowth vegetation species in the study area were relatively scarce. The litter thickness decreased with the increase of stand density, and there was no significant difference among 3 stands. At medium density, the biomass, maximum water holding capacity and effective storage capacity of litter were the largest, which were 36.56 t/hm², 81.50 t/hm² and 31.58 t/hm² respectively. The rule of soil bulk density was low density > high density > medium density, that is, the average soil bulk density of medium density stands is the smallest, 0.89 g/cm³. The total porosity, capillary porosity and non capillary porosity were low density < medium density < high density. When the total water holding capacity and total effective water holding capacity of soil layer of medium density forest were the largest, 3945.61 t/hm² and 364.00 t/hm² respectively. In conclusion, considering that the study area is located in the Loess Plateau, and the water conservation function of the forest in this area is more important, it is suggested that the reasonable stand density of 80-year-old Picea secondary forest in Pangquangou Nature Reserve of Shanxi Province should be controlled at about 600–700 plants/hm².