A Study On Ecological-sustainability On Different Rehabilitated Forest Models In Degraded Red Soil Region

The red soil region in Jiangxi became one of the most seriously eroded regions in China because of its ecological fragility determined by geological and climatic characters and severe human disturbance . The red soil ecosystem degradation indicated by severe soil erosion and runoff was also a serious problem and formed constraints to sustainable development in this region. For more than a decade of years,the forest rehabilitation and restoration in degraded red soil has been commonly concerned. Several forest and eco-environment construction projects were put into practice in red soil regions and also got great success. But some ecological sustainablity problems of those forest ecosystems were to be studied. How about the ecological sustainablity state of those rehabitated and restorated forests? How to make the quantitative and comprehensive evaluation ? Answers to these questions could be important to the selection of models and measures to rehabilish forests. In 1991, for vegetation restoration on different types of barren hills in Jiangxi province , the experimental forests were estabilished with an area of more than 640 hm2. According to the principles of restoration ecology and eco-economics , the experimental plots were located in serious degraded red soil region of Taihe county and intensively eroded red soil region of Yiyang county. Some coniferous and broaddleaved tree species have been studied including Pinus massoniana, P.elliottii , P.serotina Michx, P.taeda, Schima superba, Liquidambar fomosana, Eucalyptus camaldulensis. Different planting densities and distribution of mixed types were investigated . Lespedeza spp.and Gardenia macrophylla were planted as understory shrubs. 9 kinds of forest models have been studied,including pure Pinus rigida var. serotina forest with high planting density, Pinus rigida var. serotina forest with middle planting density, pure Pinus elliottii forest ,pure Eucalyptus camaldulensis forest,pure Schima superba forest,mixed forest of Liquidambar formosana and Pinus elliottii , pure Liquidambar formosana forest, mixed forest of Liquidambar formosana and Pinups massoniana ,and Pinus elliottii forest interplanted with Lespedeza spp. According to the research on the understory biomass , soil nutrients ,soil aggregation and microbe characteristics , the author tried to find out forest rehabilition process and its mechanism in the degraded red soil. Based on the comprehensive evaluation of ecological sustainablility on different forest models, some optimum models could be references for forest restoration in similar degraded ecosystem. Furthermore,some suggestions were given for the sustained management and health of forest ecosystem. The main research results were described as follows: (1). There existed a great change in quantity and species composition of understory plants after ten years of forest restoration in seriously degraded red soil region. The biodiversity and understory biomass of different rehabilitated models had some differences . It was indicated that there was a linear regression relationship between the understory biomass and degree of stand closure, volume weight of soil, capillary moisture capacity and aggregation degree. The understory biomass decreased with the increase of degree of stand closure and volume weight of soil . But it increased with increase of capillary moisture capacity and aggregation degree. (2).There were obvious differences among different rehabilitated forest models in soil microbical quantities,biomass and vertical distribution. According to the evaluation results of optimum comprehensive model, the value of soil microbial characteristics was ordered as follows: pure Schima superba forest > mixed forest of Pinus massoniana and Liquidambar formosana > pure Liquidambar formosana forest > mixed forest of Pinus elliottii and Liquidambar formosana > pure Eucalyptus camaldulensis forest > pure Pinus rigida var. serotina forest with a low planting density> pure Pinus elliottii forest > pure Pinus rigida var. serotina forest with a high planting density. (3). According to the order of soil enzyme activity of rehabilitated forests by PCA, the enzyme activity of pure Liquidambar formosana forest and mixed forest of Liquidambar formosana and conifers were the highest; that of pure conifer forests were lowest, and those of other models were medium. All of 5 kinds of enzyme activities declined with the increase of soil depth ,but there were obvious differences among different enzymes and among different rehabilitated forests . The soil of pure broadleaved forests had the greatest variance of sucrase enzyme activity along different soil depth; urease enzymes activity had the highest variance in different seasons with an average value 62.32% ,and proteinase enzymes activity had the smallest variance with an average of 13.04%. (4). Fine-roots play an important role in soil fertility restoration of degraded ecosystem. The vertical distribution of fineroots presented obvious layer pattern . Usually ,the fine-roots came to decrease with the increase of soil depth . Most of fineroots of rehabilitated forests distributed in the depth of 0-20 cm, which accounted for more than 75 percent of tatol fine-roots in the depth of 0-40 cm of soil. The regression analysis indicated that soil organic matter content, total nitrogen, available nitrogen, available potassium and PH value presented obvious linear correlationship with fineroot biomass. Both soil capillary porosity and total porosity had certainlinear correlationship with fineroot biomass. The effect of fineroot on soil chemical characteristics was greater than that on soil physical characteristics. Fine-roots could obviously increase water-stablized big aggregates with a diameter of more than 0.25 mm and 5 mm, and improved the stability of soil structure. Fine-root biomass presented close correlationship with soil microbical quantitie,bacteria quantitie,actinomycetes quantitie,fungi quantitie, soil respiration intensity and enzyme activity. (5). The restoration degree of soil chemical and physical characteristics of different rehabilitated forest models was different because of various understory species composition and pattern . Those of soil oganic matter content ,total nutigeon and available nitrogen of pure broadleaved forests and mixed forest were much better than those of other models. Total potassium and total phosphorus of different models presented no great difference. The volume weight of the soil decreased obviously after rehabilitating forests. The capillary moisture capacity also increased notably. The rehabilitated forests could give certain effects on the soil microelements with biggest one on available Mn ,but smallest effect on available B (6). Different models of rehabilitated forests of executed obvious effects on the soil aggregate structure in the depth of 0-40cm of soil .The soil aggregate structure of pure broadleaved forest is the best, that of mixed forests of coniferous and broadleaved tree species came second, and that of pure coniferous forest was the worst . The rehabilitated forests took some effects on the soil pore state. The soil pore state of mixed forest of Liquidambar Formosana and Pinus Elliottii was the best, and that of pure Pinus Elliottii forest was the worst.The order of the stability of soil structure of rehabilitated forests was: pure Schima superba forest > pure Liquidambar formosana forest > Liquidambar formosana and P .massoniana mixed forest in inter > Liquidambar formosana and P. elliottii mixed forest in line> pure P. rigigida var. serotina forest with middle planting density > pure P. elliottii forest> pure P. rigida var. serotina forest with high planting density. Pure broadleaved forest models were good ones which could improve the soil structure after 10 years of rehabilitation in the severely degraded red soil . (7). According to the choosen evaluation indexes of ecological sustainability, the auother has evaluated ecological sustainability of different rehabilitated forest models. The results indicated that the ecological sustainability of rehabilitated forests in the degraded red soil of Taihe was higher than that in the extremely degraded red soil of Yiyang. In Taihe experiment plots, the eco-sustainablity of pure broadleaved forest models was the best, then was that of mixed forests and that of all 3 pure conifer forest models was the worst. (8). In Taihe experiment plots, soil structure state, chemical and physical characteritics of some rehabilitated forests were not as good as that of control plot(CK) which was restorated by natural vegetation . The possible reason was that the soil of CK was not disturbed by sitepreparation and afforestation activities, so herbaceous helophyte could restore naturally when human was controlled .There were lots of fine-roots and high organic matter in the depth of 0-40 cm of the soil.It was shown that the degraded red soil in CK plot had certain ability for natural restoration. (9) Forest rehabilitation and restoration is a long and successive process. This study comparied mostly soil characteristics and ecosystem function among different rehabilitated forest models on early, got a preliminary conclusion and tendency of change. In order to probing mechanism of soil fertilizer restoration ,structure and function of rehabilitated forest ,changes and succession of vegetation composition and countermeasure for maintaining health of ecosystem , it is necessary that set up stationary plots for locating study.