| Natural secondary forest which are valuable resources has a large ecological benefits and potencial production. As long as scientific management and rational use, forest ecosystems will be able to form a good system, material wealth will be increasing, ecological protection benefits will enhance. Especially, it will be important to maintain the local ecological environment in less-tree or plantation areas.Yuanling county is the most county in Hunan province, in its forest area, the natural secondary forest has been preserved well. To study them has important practical signigicance to study the succession of forest communities and the suitable habitat. In this study, physical and chemical properties of soil, patterns of stand biomass and productivity, nutrient biological cycle process in Sassafras tsumu, Liguidambar formosana, Quercus fabri, Quercus acutissima and Cunninghamia lanceolata Natural secondary forest ecosystems were investigated in Yuanling county, the results showed as following:1. The soils in the studied forests were acidic with a mean pH scale of 4.7-5.50. Soil water content averaged 22.75-26.25% and bulk density was 0.89-1.01g·cm-3. The concentration of total nitrogen (N), phosphate (P), potassium (K), Calcium (Ca) and Magnesium (Mg) in soils was 1.11-2.10g·kg-1,0.24-0.44 g·kg-1,12.43-15.50g·kg-1,0.92-1.98g·kg-1 and 1.21-5.39g·kg-1, respectively. The concentrations of micro- nutrients in soils in the study sites ranged as:Fe:10160.33-10555.73 mg.kg"1, Mn: 381.22-798.30 mg.kg-1, Zn:48.01-62.52mg.kg-1, Cu:29.34-65.12 mg·kg-1.2. Species diversity in tree layer2.1 Sassafras tzumu and Castanea henryi mixed community:It was found that 20 species plants belonged to 11 families and 19 genera, the important value ranged from 0.811 to 44.891. the important value of Sassafras tzumu and Castanea henryi which were the dominant species in the community were 44.891 and 20.788, respectively, the richness indexes was 20, the Shannon-Wiener diversity index, Simpson diversity index, Pielon evenness index and ecological dominance index were 2.666,0.911, 0.890,0.004, respectively.2.2 Sassafras tzumu and Liquidambar formosana mixed community:It was found that 13 species plants belonged to 9 families and 11 genera, the important value ranged from 1.116 to 42.370. the important value of Liquidambar formosana and Sassafras tzumu which were the dominant species in the community were 42.370 and 30.217, respectively, the richness indexes was 13, the Shannon-Wiener diversity index, Simpson diversity index, Pielon evenness index and ecological dominance index were 2.126, 0.847,0.829,0.004, respectively.2.3 Sassafras tzumu and Cunninghamia lanceolata mixed community: the important value ranged from 0.408 to 61.433. the important value of Cunninghamia lanceolata, Sassafras tzumu, Lindera communis and Alnus cremastogyne which were the dominant species in the community were 61.433,13.535,5.248 and 4.628, respectively, the richness indexes was 18, the Shannon-Wiener diversity index, Simpson diversity index, Pielon evenness index and ecological dominance index were 1.639,0.623,0.547, 0.044, respectively.In succession stage, the Sassafras tzumu and Cunninghamia lanceo-lata mixed community was in the mixed coniferous and broad-leave tree succession stage which dominated by coniferous trees in a subtropical forest. Sassafras tzumu and Castanea henryi mixed community and Liquid-ambar formosana and Sassafras tzumu mixed community was in the succession stage which dominated by light-requiring broad-leave trees in a subtropical forest.3 Biomass and productivity of natural secondary forest3.1 Among the three Sassafras tzumu mixed forest types, biomass of an individual tree was ranged from 185.5 to 828.9 kg in the Sassafras tzu- mu species. Biomass was highest in stem organ, accounting for 50% of the biomass of the total tree. Biomass was in an order:stem> branch> root> bark> leaves.Stand biomass of Sassafras tzumu and Quercus acutissima mixed stand was 297.65t·hm-2, of which overstorey was 258.90 t·hm-2, understorey was 16.21 t·hm-2, shrub layer was 3.64 t·hm-2, dead floor layer was 18.90 t·hm-2. Stand productivity was 16.65 t·hm-2·a-1, of which overstorey was 9.91 t·hm-2·a-1, understorey was 2.70 t·hm-2·a-1, shrub layer was 3.64 t·hm-2·a-1, dead floor layer was 0.40 t·m-2·a-1.Stand biomass of Sassafras tzumu and Liquidambar formosana mixed stand was 249.06 t·hm-2, of which overstorey was 215.55 t·hm-2, understorey was 21.36 t·hm-2, shrub layer was 0.18 t·hm-2, dead floor layer was 11.97 t·hm-2. Stand productivity was 11.26 t·hm-2·a-1, of which overstorey was 7.79 t·hm-2·a-1, understorey was 3.05 t·hm-2·a-1, shrub layer was 0.18 t·hm-2·a-1, dead floor layer was 0.24 t·hm-2·a-1.Stand biomass of Sassafras tzumu and Cunninghamia lanceolata mixed stand was 147.07 t·hm-2, of which overstorey was 117.03 t·hm-2, understorey was 13.43 t·hm-2, shrub layer was 0.15 t·hm-2, dead floor layer was 16.46 t·hm-2. Stand productivity was 10.16 t·hm-2a-1, of which overstorey was 6.79 t·hm-2·a-1, understorey was 2.69 t·hm-2·a-1, shrub layer was 0.15 t·hm-2·a-1, dead floor layer was 0.53 t·hm-2·a-1.The results indicated that the three Sassafras tzumu mixed forest types were relative steady state, the habitat conditions were beneficial to the growth of the Sassafras tzumu and other plants.3.2 Among the Quercus fabri mixed stand, the total biomass of overstorey in the forests was 112.65 t·hm-2, of which Quercus fabri was 36.30 t·hm-2, Sassafras tzumu was 34.76 t·hm-2, Cunninghamia lanceolata was 19.04 t·hm-2, the shrub layer 19.39 t·hm-2, the herbivorous layer was 3.64 t·hm-2, the dead floor layer was 15.09 t·hm-2. The net productivity of overstorey in the forests was 8.78 t·hm-2·a-1, of which Sassafras tzumu was 2.916 t·hm-2·a-1, Quercus fabri was 2.857 t·hm-2·a-1, Cunninghamia lan-ceolatawas 0.828 t·hm-2·a-1. The coniferous and broad-leave mixed stand was dominant by Deciduous broad leaf trees. The net productivity of leaves was 3.888 t·hm-2·a-1, accounting for 44.30% of the net stand productivity. It was important to mantian woodland productivity.3.3 Among the mixed stand which was mainly with Sassafras tzumu and Cunninghamia lanceolata specieses, the stand biomass was 144.68 t·hm-2, of which overstorey was 114.52 t·hm-2, accounting for 79.16%, shrub layer was 12.52 t·hm-2, accounting for 8.65%, herbivorous layer was 0.23 t·hm-2, accounting for 0.16%, dead floor layer was 17.41 t·hm-2, accounting for 12.03%. the stand was mainly with Sassafras tzumu and Cunning-hamia lanceolata community which has grown for 28 years, and it was fast-growing and shallow-rooted community.4. Nutrient concentration and biocycle of natural secondary forest4.1 Mean concentration of macronutrient elements in the Sassafras tzumuspecies was:N was 6.39 g·kg-1, P 0.36 g·kg-1, K 4.37 g·kg-1, Ca 4.18 g·kg-1 and Mg 1.71 g·kg-1.Mean concentration of macronutrient elements in the Quercus acuti-ssima species was:N was 4.92 g·kg-1, P 0.28 g·kg-1, K 4.05 g·kg-1, Ca 6.02 g·kg-1 and Mg 1.00 g·kg-1.Mean concentration of macronutrient elements in the Liquidambar formosana species was:N was 4.09 g·kg-1, P 0.24 g·kg-1, K 4.31 g·kg-1, Ca 11.31 g·kg-1 and Mg 1.55 g·kg-1.Mean concentration of macronutrient elements in the Quercus fabri species was:N was 1.32-18.87 g·kg-1, P 0.11-0.99 g·kg-1, K 1.34-6.79 g·kg-1, Ca 3.29-22.48 g·kg-1 and Mg 0.25-1.86 g·kg-1.Mean concentration of macronutrient elements in the Cunninghamia lanceolata species was:N was 0.68-10.47 g·kg-1, P 0.03-0.53 g·kg-1, K 0.39-2.21 g·kg-1, Ca 0.77-13.45 g·kg-1 and Mg 0.11-2.65 g·kg-1.4.2 Sassafras tzumu and Liquidambar formosana mixed stand:the annual uptake amount was 714.57 kg·hm-2·a-1, Annual retention amount was 116.15 kg·hm-2·a-1, annual return amount was 598.42 kg·hm-2·a-1, cycling rate coefficient (return/uptake) was 0.84.Sassafras tzumu and Quercus acutissima mixed stand:the annual uptake amount was 491.42 kg·hm-2·a-1, Annual retention amount was 114.18 kg·hm-2·a-1, annual return amount was 377.30 kg·hm·a-1, cycling rate coefficient (return/uptake) was 0.77.Sassafras tzumu and Quercus fabri mixed stand:the annual uptake amount was 178.59 kg·hm-2·a-1, Annual retention amount was 53.88 kg·hm-2·a-1, annual return amount was 124.71 kg·hm-2·a-1, cycling rate coefficient (return/uptake) was 0.70.The results suggested that the natural secondary mix-forest ecosystems were characterized by fast return rate and high cycling intensity in terms of nutrient biological cycle. These features provide benefits for the forest ecosystems in maintaining site fertility.
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