Effect Of Stand Density On Biomass Production And Nutrient Cycling For Quercus Acutissima Fuelwood

Stand density directly affects the distribution of light,heat,water and other nutrient resources in forest ecosystem,and hence restricts the biomass,productivity and nutrient cycling of plant community.In past,most plantations gave preference to intensive management for high stem densities.However,this management practice neglected the maintenance of productivity and nutrient sustainability over a long-term basis.In the current study,we analyzed the variations of tree biomass production,along with nutrient traits across stand densities(HD:5000,MD:2500,and LD:1700 stems ha^-1)of 10-year-old Quercus acutissima plantations in subtropical China.The specific objectives were as follows:to explicit the variation of nutrient traits with stand densities,to determine the effects of stand density on nutrient cycle,to reveal the nutrient supply-demand relationships in Q.acutissima fuelwood,and to select the optimal stand density.The results were as follow:?1?Leaf length?L?and specific leaf area?SLA?were higher in either HD or LD stands,and leaf length-width ratio was largest in HD stands.Specific root length?SRL?increased with the decreasing stand density.The height under canopy was larger in either HD or LD stands,and tree height?H?,the diameter at breast height?DBH?,canopy diameter?CD?and CD-H ratio rose with decreasing stand density.Tree biomass was highest in LD stands.Total stand biomass was 83411.0±2293.0,46824.8±2593.3,67676.7±2717.8 kg ha^-1in HD,MD and LD stands,and was higher in either HD or LD stands.Wood biomass was 57319.9±15999.9,32488.0±3647.5,48047.4±3272.3 kg ha^-1,and was highest in HD stands and second in LD stands and lowest in MD stands.Wood biomass was higher than leaf biomass in HD and MD stands,while was similar in LD stands.?2?Root and stem N:P decreased with the decreasing stand density,as well as leaf N:P was larger in HD stands than that in LD stands.Soil N and P concentrations were higher in LD stands,while soil N:P was lower in LD stands.Leaf and root P concentrations were positively correlated with soil P concentration.Leaf,stem and root N:P were positively correlated with soil N:P.P resorption efficiency?PRE?was higher in HD and MD stands than in LD stands.Total N accumulation was 406.3,184.2 and 302.2 kg ha^-1,and total P accumulation was 14.4,8.8 and 17.2 kg ha^-1in HD,MD and LD stands,respectively.Wood N and P contents in HD stands were similar to LD stands.Both leaf N and P contents were significantly greater in LD stands than those in HD and MD stands.?3?Annual litter biomass was 653.5±112.4,828.3±43.6,2763.8±163.0 kg ha^-1a^-1in HD,MD and LD stands,respectively,and was largest in LD stands.Litter P concentration was highest,while litter C:P and N:P was lowest in LD stands.Cellulose concentration rose with the decreasing stand density,and C:Lignin was higher in HD stands than that in LD stands.Litter weight loss?A?and decomposition rate?k?were highest in LD stands.The period of litter decomposes by 50%(t_0.5)was 2.4,2.8 and 1.6 years,and by 95%(t_0.95)was 10.3,12.0 and 6.8years.Litter C,N and P release rate?EC,EN and EP?were highest in LD stands.Both A and k were affected by litter C concentration,soil temperature,air moisture and light intensity.EP was affected by litter P concentration,C:P,N:P,Cellulose,Lignin:Cellulose,C:Cellulose,soli temperature,air temperature and moisture.?4?Soil P concentration was affected by litter weight loss,decomposition rate,C,N and P release rate.The SMA slope of relationships between soil P concentration and C release rate was 6.05,which was significantly higher than other SMA slopes.Tree P concentration was affected by soil,leaf,stem and root P concentrations,and the SMA slopes were 0.40,1.21,1.06and 0.91,respectively.Tree N:P was affected by soil,leaf,stem and root N:P,and the SMA slopes were 0.59,2.70,1.13 and 0.81,respectively.H,DBH and tree biomass were affected by soil and tree P concentrations,and tree N:P.The SMA slopes of relationships between H,DBH,tree biomass and tree P concentration,N:P were 1.23,-1.27,1.09,-1.12,2.68 and-2.76.These values were statistically higher than the SMA slopes of relationships between H,DBH,tree biomass and soil P concentration,N:P.Summing up,stand biomass was higher in either HD or LD stands.In terms of productivity,both HD(5000 stems ha^-1)and LD(1700 stems ha^-1)stands were the best choice for Q.acutissima fuelwood.P was the main limiting factor of tree growth and biomass.P loss in HD stands was similar to LD stands,but soil P concentration was highest in LD stands and lowest in HD stands,and hence inadequate soil nutrient supply might become a major limitation for subsequent tree production in HD stands over the long term.Reducing this soil nutrient depletion by increasing the nutrient return following harvesting has been critical for preserving soil nutrients over longer time period.Annual litter biomass and litter decomposition were highest in LD stands,thus,there were sufficient P supply and fast nutrient turnover rate in LD stands.Soil P stoichiometry was influenced litter decomposition and annual biomass production,and tree P stoichiometry was affected soil and organ P stoichiometry,and both of which affected tree size and biomass.Overall,our findings showed that more open forest stands(e.g.1700 stems ha^-1)were a preferential selection if one considered not only biomass productivity,but also nutrient sustainability.