| The development of Eucalyptus plantation has a great impact on the national security in ecology and supply of lumber,which attracts much attention from the academe and society.Three kinds of rotation(short term of 7a,medium term of 13a,long term of 21a)of E.grandis×E.urophylla plantations in DongMen Forest Farm are selected in this study to compare and investigate the biomass,productivity,carbon storage,forest stand and economic benefit by means of wild survey and indoor analysis.The aim is to reveal the characteristics of biomass and economic benefits for different age of stand of E.grandis×E.urophylla plantations.The results of the study are as follows:1.There exists significant discrepency among stand biomass for E.grandis×E.urophylla plantations with different age of stand.The forest stand biomass of 7a is 133.54±5.62 t/hm,while those of 13a and 21a are 335.16±33.57t/hm2 and 383.20±38.03 t/hm2,respectively.This shows that the stand biomass increases with increasing age of stand.The increasing magnitude from short term to medium one is 150.98%,and is much more significant than that from medium term to long term,the later is only 14.33%.2.The distribution of the biomass of each component is similar for tree layers of E.grandis×E.urophylla plantations with different age of stand.The proportions of different brand biomass can be listed in decending order as stem>root>bark>branch>leaf.The proportion of stem biomass increases at first,and then decrease with increasing age of stand.On the contrary,the proportions of branch biomass and root biomass,respectively,decrease at first,and then increase with increasing age of stand.On the other hand,the proportion of bark biomass keeps increasing while that of the leaf biomass keeps decreasing inversely with increasing rotation.3.The biomass of undergrowth vegetation increase with increasing age of stand of E.grandis×E.urophylla plantations.The stand biomass for short,medium and long term is 14.97 t/hm2,23.97 t/hm2 and 24.53 t/hm2,respectively.It is obviously that the undergrowth vegetation layer biomass increase slowly for medium and long term.The shrub layer biomass accounts for 54.34 77.76%of undergrowth biomass,while the herb layer biomass accounts only for 22.24-45.66%.4.The productivity of tree layer changes with stand age of E.grandis×E.urophylla plantations.The stand productivity of 7a,13a and 21a is 19.08±0.80 t/hm2·a,25.78±2.58t/hm2·a and 18.251.81 t/hm2·a,respectively.This shows that the productivity of E.grandis×E.urophylla plantations increases at early age and then decreases with the increasing age of stand,which achieves maximum for medium.5.The total carbon storage in the ecosystem of the E.grandis ×E.urophylla plantations of 7a,13a and 21a is 165.81±5.39 t·C/hm2,265.53×19.59 t·C/hm2 and 266.71±16.59 t·C/hm2?respectively.With the increasing rotation,the total carbon storage of tree layer increases,however the carbon storage in soil decreases.Those in tree layer and soil mass account for 39.38%and 60.62%,respectively for E.grandis×E.urophylla plantations with short term,61.28%and 38.72%,respectively for those with medium term,then 63.10%and 36.90%respectively for those with long term,carbon storage changes with age is decreasing.6.The stand volume increase significantly with the increasing rotation.They are 144.95 m3/hm2,346.97 m3/hm2 and 553.69 m3/hm2 respectively,for different rotation of 7a,12a and 21a.The economic benefit also keeps increase,which is 17,239.86 for plantations with 7a.it reaches highest of with 12a value(which is 30,297.61 ?/hm2).But the net present value will increase up to 30,297.61 yuan/hm2 near 13a.7.From the perspective of biological productivity and economic benefit,it can be concluded that 13a of the rotation is good for the E.grandis×E.urOphylla plantation in Guangxi DongMen Forest Farm. |
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