The Effects Of Phosphorus Addition And Litter Cover On Seedling Growth Of Constructive Tree Species In The Pine-oak Mixed Forests Of Qinling Mountains,China

The pine and oak mixed forests community is the most widely distributed,typical and important forest type in Qinling Mountains.It is mainly distributed between 1300 m and 1700 m altitude and its area accounts for about 60% of the natural forests in the Qinling Mountains.Quercus aliena var.acutiserrata,Pinus tabuliformis and Pinus armandii are the main tree species in the pine and oak mixed forest.Forest regeneration is the basis for maintaining the sustainable development of forest ecosystems.Under natural conditions,there are some limitations to regeneration.In order to understand whether nutrient supply is a limiting factor for forest regeneration,this paper takes three constructive species seedlings as the research object and conducts different experiments and studies.Two-factor crossover experiment of mixed ratio of litter cover and different concentration of phosphorus addition was carried out to explore the influence of phosphorus addition and litter cover on the growth and development of the dominant species seedlings in the pine and oak mixed forest of Qinling Mountains.(1)The aboveground and underground resource allocation strategies of the seedlings of Quercus aliena var.acutiserrata,Pinus tabuliformis and Pinus armandii under different concentrations of phosphate fertilizer and different mixing ratios of litter cover are different.The increase in the concentration of phosphate fertilizer increased the height of the seedlings of Quercus aliena var.acutiserrata by 74% and the root length of the seedlings of Pinus tabuliformis by 121.5%,and the root length of the seedlings of Pinus armandii by 16%.The increase in the proportion of oak leaf in litter cover increased the seedling height of the seedlings of Quercus aliena var.acutiserrata by 31.7% and root length of the seedlings of Quercus aliena var.acutiserrata by 20.4% but suppressing the root length of Pinus armandii by 47%;under the interaction of phosphorus addition and cover litter,it increased the seedling height by 30.5%,17.5%,9.03% respectively and the root length by 25.4%,171.5%,108.2% among the three seedlings.(2)The soluble sugar content of the seedlings of Quercus aliena var.acutiserrata,Pinus tabuliformis and Pinus armandii under different concentrations of phosphate fertilizer and different mixing ratios covered with litter are quite different.The increase in the concentration of phosphate fertilizer increased the soluble sugar content of the seedlings of Quercus aliena var.acutiserrata by 16.6% and the soluble sugar content of Pinus armandii by 9.8%;with the proportion of oak leaves in the litter increasing,the soluble sugar content of seedlings of Quercus aliena var.acutiserrata and Pinus armandii were increased by 21.1%,5.5%respectively.Under same mixing ratio of litter cover,the soluble sugar content of Quercus aliena var.acutiserrata and Pinus armandii seedlings showed a trend of first decreasing and then increasing with the increase of phosphate fertilizer concentration,and the final growth rate was 9.2% and 14.7%.(3)The nutrient content characteristics of Quercus aliena var.acutiserrata,Pinus tabuliformis and Pinus armandii seedling leaves under different concentration of phosphate fertilizer and different mixing ratio of litter cover are different.The increased concentration of phosphate fertilizer increased the carbon content by 6.3% of Pinus armandii,the nitrogen content by 74.3% of Quercus aliena var.acutiserrata and the phosphorus content of the three seedling leaves,and reduced the leaf nitrogen content of Pinus tabuliformis by 14%,the leaf nitrogen content of Pinus armandii by 9.8%;with the proportion of oak leaves in the litter increasing,the carbon content of Pinus armandii.was increased by 5.7%,and the leaf nitrogen content of Quercus aliena var.acutiserrata was increased by 25% and the nitrogen content of Pinus armandii by 6.8%,which reduced the phosphorus content of Pinus armandii by 8.8%.The increase in the concentration of phosphate fertilizer added under the treatment of mulching with the same mixed ratio and litter increased the nitrogen content of Quercus aliena var.acutiserrata,Pinus tabuliformis and Pinus armandii by 38.4%,4.9%,17.5% respectively and phosphorus content of Quercus aliena var.acutiserrata,Pinus tabuliformis and Pinus armandii by 500%,80.9%,80% respectively,in the meanwhile,reduced the carbon content of Quercus aliena var.acutiserrata by 14% and the carbon content of Pinus tabuliformis by 4.8%.(4)Phosphate fertilizer addition significantly reduced the ratio of nitrogen to phosphorus(N:P)of the seedling leaves of Quercus aliena var.acutiserrata,Pinus tabuliformis and Pinus armandii by 63.7%,56.7%,46% respectively;the cover of litter increased the ratio of nitrogen to phosphorus of Pinus armandii by 54.8%,reduced the leaf nitrogen and phosphorus ratio of Pinus armandii.by 14.8%;under the treatment of mulching with the same mixed ratio and litter,the addition of phosphorus fertilizer reduced the leaf nitrogen and phosphorus ratio of the seedlings of Quercus aliena var.acutiserrata,Pinus tabuliformis and Pinus armandii by 78.4%,41.9%,55.9%,so the addition of phosphate fertilizer is beneficial to alleviate the local phosphorus restriction in Qinling Mountains,and the phosphorus restriction is changed to nitrogen restriction.