Growth Caracteristics Of Herbaceous Vegetation Under Different Abandoned Years And Models In The Loess Hilly Region

Reforestation is the mainly ecological measure for comprehensive management of soilerosion and water loss in the loess hilly region. The percentage of forestry and grass coveragehas increased significantly after decades of development, and the environment has improvedas well. The research area was Zhifanggou of Ansai county, and the slope cropland was as thecontrol, the different abandoned years (0a、3a、10a、18a、28a、37a) of abandoned land anddifferent abandoned models at37a (forest of Caragana korshinskii, forest of Robiniapseudoacacia, forest of Pinus tabulaeformis, mingled forest of Pinus tabulaeformis+Amorpha fruticosa) were studied. The research mainly included the following index: soilphysicochemical property, herbaceous vegetation coverage, above-ground biomass, litter, fineroot biomass of herbaceous vegetation, root length density, root-top ratio. This could providea basis for analyzing the vegetation changing characteristics and choosing the optimizedmodel of abandoned land in the process of vegetation restoration in the loess hilly region. Themain conclussion are as follows:The soil under different abandoned years at each site was silty loam. The soil buckdensity increased firstly and then decreased. Soil total porosity, capillary porosity, soiladhesion stress, and water-stable aggregate have showed a trend of increasing. Abandonedyears had significant impact on the soil buck density at20-40cm, soil total porosity andcapillary porosity at0-60cm, and soil adhesion stress at0-40cm. The soil under differentabandoned years at each site was alkaline. The total organic carbon, total N and available K ofthe soil increased while the available P undulately decreased with the prolongation ofabandoned years. And the abandoned years had significant impact on each chemical indicatorof the soil.The species richness index, diversity index, evenness index, and root length density atthe same soil layer showed the trend of increasing-decreasing-increasing. The vegetationcoverage and biomass aboveground manifested as37a＞28a＞18a＞3a＞10a＞0a. The rootbiomass and root length density of each site have dereased as the soil depth increased. Theroot biomass at same soil layer of different sites manifested as28a＞37a＞18a＞3a＞10a＞0a.The value of root cutting coefficient of each site implied that the roots distributedintensively in the soil surface at the early conversion stage, while distributed in the subsoil atthe later conversion stage. On the whole, the results showed that the vegetation coverage and root biomass dramaticly improved as the abandoned years prolonged, and the effects ofconserving water and soil of vegetation would play effectively.The soil under different abandoned models at each site was also silty loam. The influenceof abandoned model on soil bulk density didn’t reach significant level while it significantlyaffected the total porosity and adhesion stress. The content of water-stable aggregate ofmingled forest was higher than that of other forest. The soil under different abandoned modelsat each site was also alkaline, the influence of abandoned models on each chemical index ofsoil reached significant level except the pH at the soil layer of0-20cm and organic carbon at40-60cm.The species diversity of abandoned land was the highest at different abandoned models,while the forest of Caragana korshinskii was the lowest. The vegetation coverage, biomassaboveground of abandoned land were also the highest, while the forest of Pinus tabulaeformiswas the lowest. There were no significant differences among different vegetation coverages,but the biomass aboveground had significant differences. Both the litter amount and rootbiomass of mingled forest of Pinus tabulaeformis+Amorpha fruticosa were the highest,while the forest of Robinia pseudoacacia were the lowest. There were no significantdifferences among litter amount, and root biomass had significant differences. Both root-topratio, root length density of mingled forest of Pinus tabulaeformis+Amorpha fruticosa werealso the highest, while the abandoned land was the lowest, and there were significantdiffernces between them. The conserving water and soil effects of herbaceous vegetation ofdifferent abandoned models were different. Overall, the mingled forest had better effects thansingle forest.