In the hilly-gullied Loess Plateau, soil loss control was taken much attention because it’s serious soil erosion; and vegetation is the most efficient method to control soil erosion. However, vegetation restoration must be effected by the bad environment conditions. And the main propose of our study was to check the status of natural restoration vegetation in different sites, and the extent of soil erosion under different vegetation types. To do this, two typical watersheds, Zhifanggou and Danangou, were selected as study sites. We examined the correspondence between sites – natural vegetation – succession stage, analyzed the degree of succession of different natural vegetation, assessed the stability and health of natural vegetation, estimated the soil loss of different vegetation types, and then predicted vegetation and soil loss in the future. And the main results are shown as follows:
1) According to the Chinese vegetation classification system，there are grasslands, bush and forest in both Zhifanggou and Danangou watersheds. All the grasslands are natural, and the typical communities during the second succession of hilly-gullied Loss Plateau were easily found in the two watersheds. The formations of the early stage of succession, such as formation (For.) Artemisia scoparia, For. Heteropappus altaicus, For. Potentilla bifurca, For. Astragalus melitoloides, For. Stipa bungeana and so on, mainly distributed on the upper slope, and the formations of the late of succession, such as For. Bothriochloa ischcemum, For. Artemisia giraldil and For. Artemisia gmelinii, mainly distributed on the gully slope, and all For. B. ischcemum distributed on the south slopes. The shrub and forest in the two watershed are mainly artificial vegetation, but there are some natural Sophora viciifolia on the south gully slope in Zhifanggou, and some natural Cotoneaster multiflorus on the bottom of gully slope about 1100－1200 m in Zhifanggou watershed. 2) M. Godron test indicated that all the natural vegetation in Zhifanggou and Danangou watersheds are in an unstable state, and all they are progressing communities may account for it. Further investigation was done to check out the changes during the first and the second investigation, and to compare the stability of communities in different succession stage, and we found that the early stage For. A. scoparia developed most rapidly, but worst in stable; the middle stage For. S. bungeana, Lespedeza davurica developed slower and better in stable than For. A. scoparia; the later stage For. B. ischcemum, For. A. giraldil, For. A. gmelinii and shrubs developed more slowly, and species composition of these formations changed little, which suggests that they are more stable. In conclusion, the earlier formations developed faster, but less stable; the later formations developed slower, but more stable. And the species diversity and the Bray Curits index also showed a similar trend. 3) CVOR index model was used to evaluate the health status of vegetation in the different succession stages, and found that almost natural vegetation were unhealthy or in the alert state when For. C. multiflorus was used as the reference system, and most of the natural vegetation types were in the alert state or sub-healthy state when For. Syringa pekinensis－A. gmelinii was used as the reference system. However, shrubs were healthier than grasslands under each reference system, and the formations of the later stage were healthier than formations of the early stage.
4) Based on measured data, the soil erodibility values (K) were calculated and the results shown that the average K value of grasslands was 0.043 t•ha•h(ha-1•MJ-1•mm-1), and the average K value of shrub was 0.038 t•ha•h(ha-1•MJ-1•mm-1). According to algorithm of RULSE, the cover and management factor (C) was calculated, and the results show that the C value of croplands was 0.242, the average C value of grasslands was 0.197, the C value of For. A. scoparia, For. Artemisia gmelinii and For. Poa sphondylodes was approximately 0.22, the C value of later stage formations were less than 0.2, the C value of For. S. bungeana, L. davurica, For. B. ischcemum, For. A. giraldil and For. A. gmelinii were 0.196, 0.168, 0.175 and 0.179, respectively. And the C value of the For. S. viciifolia was 0.048, the C value of For. C. multiflorus was 0.086. 5) The soil erosion of different vegetation types estimated by Chinese Soil Loss Equation (CLSE) were much higher than observed, the estimated values were about 2-10 times of the observation values. The main reason why the estimate value was much higher was that the algorithm for S factor in CLSE was drown from slope of 5.14-28.8°, but 46.3% slope in Zhifanggou and 51.9% slope in Danangou had a gradient greater than 28.8°. In addition, the slope is a key factor of erosion intensity. As a result, CLSE was limited to estimate soil loss of watershed in the hilly-gullied Loess Plateau at some extent. 6) The soil erosion of different vegetation types estimated by Prediction Model of Water Erosion on Hillslopes (PMWEH) were more close to the measured value, the estimated values were about 1-4.5 times of the observation values. If the soil bio-crust was considered, the soil loss of different vegetation types estimated by PMWEH were about 1-4 times of the average observation values, and were about equal to the maximum observation values. Compared with CLSE, PMWEH was more suitable to estimate soil loss of watershed in hilly-gullied Loess Plateau.
7) After K values revised, the soil losses of different vegetation types in the study sites were estimated. And the results shown, the average erosion modulus of For. A. scoparia, For. A. gmelinii, For. P. sphondylodes and For. S. bungeana, L. davurica were 5863 t• (km-2•a-1), 7383 t• (km-2•a-1), 4483 t• (km-2•a-1) and 5352 t• (km-2•a-1), respectively, and the average erosion modulus of For. B. ischcemum, For. A. giraldil and For. A. gmelinii were 5094 t• (km-2•a-1), 5559 t• (km-2•a-1) and 5245 t• (km-2•a-1). The soil loss erosion intensity of artificial shrubs were moderate erosion, and the average erosion modulus of For. S. viciifolia was 1242 t• (km-2•a-1), average erosion modulus of For. C. multiflorus was 701 t• (km-2•a-1). Compared with the late-successional communities and early-successional communities, shrubs were more capable of reducing soil loss. 8) According to survey data, the probalility matix of different vegetation types during secondary succession was determined, and the vegetation types in the future were predicted. In 2060, 90% natural vegetation in the watershed would be shrubs, and the soil loss of Zhifanggou watershend and Danangou watershend would down to 1094 t•（km-2•a-1）and 1157 t•（km-2•a-1）, respectively. Key words: site; succession; communities; stability; health assessment; soil erosion; the hilly-gullied Loess Plateau