The Impacts Of Tending And Thinning On The Growth And Soil Physiochemical Properties Of The Mixed Forests Of Pinus Massoniana And Scheme Superba

Mixed plantation of Pinus massoniana and Schima superba,both of which are important tree species for afforestation in southern China,can improve the stand structure,boost the soil fertility,prevent and reduce forest fires as well as forest pests.Over the past 10 years a large number of mixed forests of Pinus massoniana and Schima superba have been cultivated in the southern Hunan region,but their initial planting density is generally so high that after canopy closure it gives rise to the dysplasia of vegetation layer in the forest area,thus hindering tree growth,causing low biodiversity,and incomplete ecosystems there.Hence,there is an urgent need for the quality improvement and transformation of these forests through tending and thinning.Thankfully,many scholars in China have already conducted research on the tending and thinning of man-made pure forests and its technical system is relatively mature.However,limited research has been carried out on mixed forests.Drawing on an 8-year-old mixed forest of Pinus massoniana and Schima superba as the research subject,with four treatments(CK:0%,T1:20%,T2:30%,T3:40%)set up based on the number of thinning trees,and each treatment repeated three times to observe the impact of thinning on tree growth,understory vegetation,forest hydrology,soil physicochemical properties,and other indicators,through the principal component analysis for comprehensive evaluation,this essay obtains the research results below:(1)There exists a positive correlation between the annual growth of diameter at the breast height of Pinus massoniana and Schima superba and the intensity of thinning,with significant differences among different treatments.The impacts of tending and thinning on the height growth of Pinus massoniana trees are significant,while it has little effect on the annual growth of Schima superba trees.The diameter distribution of mixed forests of Pinus massoniana and Schima superba shows a right shift trend with the increase of thinning intensity;the average annual growth of individual timber volume of two tree species is positively correlated with thinning intensity,while the stock volume is negatively correlated with the thinning intensity.(2)The number of understory vegetation species in the mixed forest of Pinus massoniana and Schima superba increases with the increase of thinning intensity,and so does the proportion of shrubs.The increase in vegetation importance values for the four treatments is T3>T2>T1>CK,and the importance values for herbs and ferns are greater than those for shrubs.Tending and thinning has a significant promoting effect on the Margalef index,Shannon wiener index,and Simpson index of understory vegetation in the mixed forest of Pinus massoniana and Schima superba.(3)Tending and thinning can reduce the soil bulk density of the mixed forest of Pinus massoniana and Schima superba.After the 4-year thinning,the annual reduction values of soil bulk density in the topsoil layers(0-20 cm)and core soil layers(20-40 cm)are T3>T2>T1>CK,with significant differences observed between each thinning treatment and the control.Tending and thinning can increase soil porosity.After the 4-year thinning,the annual increase values of capillary porosity,non capillary porosity,and total porosity are generally T3>T2>T1>CK,with the topsoil layer being greater than the core soil layer.After the 4-year thinning,the annual increase in water storage capacity of the topsoil layer is T3>T2>T1>CK,and the soil water storage effect of each treatment is better in the soil layer than in the topsoil layer.Tending and thinning reduces the probability of water loss and soil erosion,and significant differences are observed in soil water purification rate,runoff amount,wash-away amount,and runoff coefficient between thinning treatments and the control.(4)The soil chemistry properties of Pinus massoniana and Schima superba mixed forest are significantly affected by tending and thinning.After 4 years of thinning,the annual increase values of seven chemical indicators in the soil,including hydrolyzed nitrogen,available phosphorus,available potassium,total nitrogen,total phosphorus,total potassium,and pH value,are all higher in all thinning treatments than in the control,with significant differences observed between them.(5)The results of principal component analysis shows that the comprehensive evaluation scores of each treatment are T3>T1>T2>CK,and T3 treatment is the optimal thinning,that is,retaining about 102 plants/mu(around 61 Pinus massoniana trees and 41 Schima superba trees)in the 8-year-old mixed forest of Pinus massoniana and Schima superba after thinning,which can more significantly promote the growth of the Pinus superba mixed forest and improve its growth environmental factors,thus enhancing the multi-functional benefits of the forest.If ecological benefits are to be emphasized,it would be better to adopt T1 treatment retaining 136 plants or so/mu(about 82 Pinus massoniana trees and 54 Schima superba trees),which in turn greatly boosts the ecological benefits of the mixed forest of Pinus superba and Schima superba.In summary,tending and thinning,especially intense thinning,can promote the growth of the mixed forest of Pinus massoniana and Schima superba,facilitate the cultivation of large diameter Pinus massoniana and Schima superba,improve soil physicochemical properties,increase understory vegetation diversity,gain better hydrological benefits,promote forest quality,and achieve multiple functions and sustainable management of the forest.The research results provide practical experience and theoretical support for the scientific management of mixed Pinus massoniana and Schima superba,meanwhile making up for the shortcomings in the research on tending and thinning of mixed Pinus massoniana and Schima superba.