Effects Of Planting Densities On Nutrient Cycling Of Populus Tomentosa Stand

China imports hundreds of million cubic meters of timber annually,and the development of fast-growing and high-yield timber forest is an important guarantee to meet the demand of timber.That China has 8.5 million hectares of poplar forest make it is ranking the first in the world,while the productivity is relatively low.Poulous tomentosa as a native tree specices in north China,has been well known for long growth age and new excellent clones have been successfully bred.From 2015 to 2017,and 2019,this research was carried out on the fast-growing and high-yield P.tomentosa plantation established in the year of 2007 to test potential of newly bred clones at Weixian country,which belongs to the 11^th and 12^th five-year experiment research program.First,the growth and stand volumes of 4supior P.tomentosa clones?S86,BT17,B331,1316?under 7 planting densities?D22:2×2 m,D23:2×3 m,D33:3×3 m,D43:4×3 m,D44:4×4 m,D45:4×5 m and D46:4×6 m[line spacing×row spacing]?were examined.Then the characterics of fine root,leaf,soil nutrients and leaf litter decomposition of P.tomentosa S86 clone stand were studied by the method of root order,soil drilling,and mesh bag in-situ decomposition,separately,under three typical planting densities?D22,D43,D45?.The study of the growth and nutrient cycling of different clones under the manipulate of planting density may provide the scientific basis for the integration and optimization of the comprehensive and intensive management of poplar fast-growing and high-yield forest.The main results as follows:?1?Planting density and clone significantly affected the stand mean height,DBH,volumes?stem and stand?.The mean height,DBH and stem volume was highest at D46,while the stand volume was highest at D23;B331 and S86 clone performed better than BT17,1316.?2?The biomass,morphology,and nutrient content of fine root of P.tomentosa S86 clone varied among planting densities,and the above-mentioned indicators was significant difference between 1-3order and>3 order fine root.Under planting density D22,1-3 order fine-root biomass,length,diameter,surface and total nitrogen content were higher,while that was higher in the>3 order under density D43and D45.?3?The quantitative and qualitative relationships between soil nutrient contents and fine root indicators?biomass,form and nutrient content?were analyzed,and the correlationships were affected by different soil nutrient factors and fine root orders.?4?Planting density also had significant effect on leaf traits.The morphology and nutrient content were smaller at palnting density of D22,while specific leaf mass and stoichiometry were higher.Correlation analysis showed that there were correlations among leaf morphology,nutrient content and stoichiometry.?5?The soil nutrients of P.tomentosa clone S86 stand were mainly concentrated in 0?20 cm top layer,which accounting for 69%?79%of the total.Planting density had a significant effect on soil nutrient contnets?except available potassium?.?6?The mean residue rate of leaf litter biomass was 77%,the mean residue rates of C,N and P were 26%,56%and 31%seperately.The biomass and nutrient residual rates of D22 were significantly higher than that of D43 and D45,while the C:N ratio was in descending order D22>D43>D45.Research indicated that planting density has significant effects on the growth and nutrient cycling process of P.tomentosa plantation.At the current stand age,stand under planting density 2×2 m,showed clear indicators of limitation on growing resource of individual trees,intensified competition between fine roots,decreased soil nutrient content,and slow decomposition of leaf litter,which may impede the growth and timber production of the stand,and thinning or irrigation,and fertilization should be carried out timely.