Response Of Growth And Regeneration Of Stands And Understory Species Diversity In Quercus Mongolica Secondary Forest To Stand Density

Six Mongolian oak sample plots with an area of 0.1 hm²were thinned in 2018,and two years after thinning,the changes in the growth of Mongolian oak trees（tree height,diameter at breast height,crown width,stem growth,individual wood volume growth）,the species composition of understorey vegetation and soil moisture and temperature conditions were investigated.The study explored three different densities after thinning,i.e.low(600plants-hm^-2),medium(720 plants-hm^-2)and high(900 plants-hm^-2).Effects on stand growth,understorey seedling regeneration and understorey vegetation diversity.It provides a reference and basis for the appropriate density of the stand based on dry-type management.The main findings are as follows.（1）The mean diameter at breast height and the mean height of trees at different densities did not differ significantly in the second year after thinning,but in the third year after thinning there was a significant difference in the diameter at breast height and the height of trees at different densities,with the mean diameter at breast height of 19.0 cm in the low density stand of 600 trees-hm^-2being significantly larger than the high density stand of 900 trees-hm^-2.In the high-density stand of 900 trees-hm^-2,the trees grew mainly taller due to competition,with a mean height of 14.4 m.The symmetry index of the canopy was significantly higher in the low-density stands than in the high-density stands,with a maximum value of 76.8%,and the most uniform growth of the canopy,while the symmetry index was lowest in the high-density stands,and the canopy growth was skewed.Density changes after thinning had a significant effect on the accumulation of tree wood volume,with the maximum individual wood volume of trees at low densities being 0.11 m³.In the short term after thinning,there was no significant difference in the rate of increase in individual wood volume between the three densities,but as the year of thinning gradually increased,the rate of increase in wood volume of trees at high densities slowed down.Significant differences in the growth of tree diameter at breast height occurred in the third year after thinning,with almost the same growth at breast height of 0.91 cm and 0.87 cm for low and medium density trees,respectively,and a decrease in the growth at breast height from0.55 cm in 2019-2020 to 0.50 cm in 2020-2021 for high density stands.（2）The field water holding capacity and capillary water holding capacity were the lowest at low densities,135.8 g-kg^-1and 167.3 g-kg^-1respectively,with the least amount of water available for plant use.At a medium density of 720 plants-hm^-2,the maximum water content in the soil was 7.91%.There was no significant difference in the water storage capacity of the soils in the three densities.At the low stand density of 600 plants-hm^-2the soil was more susceptible to temperature changes and warmed and cooled more quickly.The daily variation pattern of soil temperature was basically the same at all three stand densities,with soil temperature reaching its highest at 15:00 and lowest at 7:00.However,as density decreased,soil temperature was more strongly influenced by changes in temperature,with the highest soil temperature at 14.2°C at 15:00 and the lowest soil temperature at 10.7°C at 7:00 at lower densities.（3）The plant species in the herbaceous layer of the understorey vegetation were higher than those in the shrub layer at all three stand densities.A total of 70 species of understorey vegetation were surveyed,belonging to 41 families and 67 genera.Of these,24 species were found in 15 families and 22 genera in the shrub layer,while 47 species belonged to 27families and 45 genera in the herb layer.The most shrub species in the medium density were18 species.The herbaceous species have a maximum of 33 species at low densities.The understorey vegetation composition was similar in different density stands,with no significant differences in species diversity indices.In Mongolian oak pure stands,the importance value of Mongolian oak seedlings in different density stands ranged from 0.22 to0.46,with 0.23,,in absolute dominance.The composition of the dominant species of the understorey vegetation was similar in different density stands,with the dominant species in the shrub layer being Mongolian oak and hoodia.The dominant species in the herbaceous layer were mountain rosemary and three-leaved commissures.There is more overlap between the plant species in the shrub layer and the herb layer,but the dominant species are obvious,and the importance value of the dominant species is much greater than that of the other species.（4）Mongolian oak seedling regeneration was best at medium densities.The highest number of seedlings renewed at medium density was 324.The number of seedlings renewed at each height class was greatest at medium density.Only seedlings of seedling height class I were present at medium density.The number of Mongolian oak seedlings decreases with increasing height class,with the highest number of renewals in class III,192,285 and 139 at the three densities respectively.This far exceeded the number of seedlings in the other height classes.In terms of overall seedling growth,there were no significant differences in seedling height between the different stand densities.The ground diameter of the renewed seedlings at medium density was the largest and significantly higher than the other densities.The difference in ground diameter between seedlings of the same height class was found to be non-significant at 0.21 cm for seedlings of height class III at medium and high densities,while the ground diameter of seedlings of height class II was the largest at 0.48 cm at medium density.seedlings of height class I were only found at medium density.The mean ground diameter was 0.64 cm.The growth of seedlings during the growing season showed that the renewed seedlings at low densities had a significant competitive effect with other understorey plants during the growing season.The survival rate was the lowest at 50%.Seedlings at low densities grew mainly tall during the growing season,with the largest average tall growth of 1.73 cm.The seedling height to diameter ratio did not vary significantly and the quality of the seedlings was poor.At medium densities the radial growth of regenerating seedlings was rapid,with the largest increase in ground diameter of 0.47 cm,and the largest change in height to diameter ratio,with a significant improvement in seedling quality.The effect of stand density on seedlings was mainly in the allocation of light resources.The effect on seedling access to water resources was not significant.In summary,within the scope of this experiment,the retention of a density of 720trees-hm^-2in 40a-growth Mongolian oak natural secondary forests resulted in good growth,more uniform distribution of understorey vegetation,and significantly better seedling regeneration and growth than other densities.Therefore,the medium density(720 plants-hm^-2)is the most suitable for the natural regeneration of young Mongolian oak forests in Liaodong,and contributes to the sustainable development of Mongolian oak secondary forests in Liaodong mountainous area.