| Dynamics of the species diversity indexes of undergrowth, fine root characteristics and soil organic carbon were studied in Caragana korshinskii plantations at different stages, the results show:1. In the quadrats in Caragana korshinskii community from 10a to 50a, there are 47 higher plants which belong to 16 family and 37 genus, the species number of Poaceae, Asteraceae, Leguminosae and Rosaceae is 64.6% of the tatal, and species composition is complicated gradually; the main undergrowth type is herbage, Stipa bungeana, Artemisia sacrorum and Artemisia giraldii are the dominant species in Caragana korshinskii community; similarity coefficient of plantations with two contiguous ages, species diversity indexes and the overall interspecific connectivity are fluctuated at different growth stages, in the community from 10a to 30a of shady slope, the three kinds of indexes increase, the community reaches a they reach a steady state, form 30a to 40a, the three kinds of indexes decrease, they reach a instablestate, from 40a to 50a, they reach a steady state again, the replacement process of sunny slope lags behind that of shady slope, the undergrowth develop in stablestate and instablestate alternately until formation of climax vegetation; the average of R, H′, E and D in shady slope are 1.16, 1.07, 0.93 and 1.10 times of that in sunny slope.2. Fine root biomass, root length density and specific root length of the community in sunny slope, root length density and specific root length in shady slope are fluctuated as the diversity at different growth stages except for biomass in shady slope; the plantations of shady slope have more efficiency than that of sunny slope in input of biomass; fine root biomass and root length density decrease as the soil layer increases, proportion of fine root biomass and root length dengsity of 0~10cm soil layers in those of 0~100cm soil layers are: 20.9%~45.4% and 26.8%~48.3% , which are different significantly from other soil layers, specific root length is irregular on soil profile; the proportion of biomass and root length density of fine root with diameter <1mm in that of fine root with diameter <2mm is 67.4%~89.0% and 92.9%~99.6%, fine root with diameter <1mm plays an important role in the system; fine root biomass and root length have the closest relationship with total N and soil organic matter, then available K and available P, last total P, fine root biomass correlates significantly with root length density.3. The carbon sinks in growth process of Caragana korshinskii plantations, the SOCD of 50a both in shady and sunny slope are 1.49 and 1.88 times of that of 10a; in shady and sunny slope, the proportion of SOCD of 0~10cm soil layers in those of 0~100cm soil layers are 27.1%~39.0% and 30.0%~40.5%, SOCD in both shady and sunny slope distribute concentratedly in 0~20cm soil layer, so is LOC; A, AI, CPI and CPMI of Caragana korshinskii plantations generally increase in the growth stage, the soil quality is improved; LOC, NLOC and TOC of both slopes, CPI and CPMI of shady slope have the close relationship with soil fertility factors, could be used to monitor the change of soil quality.4. The values of species diversity indexes and fine root parameters of Caragana korshinskii plantations which are close to that of Sophora viciifolia forest are fluctuated at different growth stages, so, the community which is not stable develops in stablestate and instablestate alternately and this situation may be last a long time. But the development of Caragana korshinskii plantations could improve the soil quality.
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