| The characteristics of structure and diversity of the plant community in the course of natural restoration was a scientific basis to construct the artificial vegetation in semi-arid loess area of northern Shaanxi, which was the typical fragile ecological region. From species’ composition and distribution, quantitative characteristics, plant diversity and interspecific relationship, the paper explored what was the interrelation between structure and function of the plant community, based on the vegetation survey in 54 sampling plots in this region, for Hegou watershed in Wuqi County of Shaanxi Province as an example. The results showed that:(1) The plant community was still mainly herbaceous and the zonal vegetation type was Artemisia sacrorum+Stipa bungeana communities in this watershed, which had been fenced for 15 years. Not only were there Populus simonii and Salix matsudana in gully bottom, but there were also three trees of P. simonii, Ulmus pumila and Ameniaca sibirica and six shrubs of Buddleja alternifolia, Prinsepia utilis, Spiraea trilobata, Caragana korshinskii, C. microphylla and Periploca sepium on some micro-topographies of slope aspect with favorable water condition, i.e., gully, collapse and platform. Landform was the main factor that affected the difference of species’ composition and the difference between gully bottom and other landforms was very clearly.(2) Only A. sacrorum、Lespedeza davurica、Potentilla acaulis、S. bungeana、A. giraldii、P. chinensis、Cleistogenes squarrosa and A. scoparia had the higher coverage and height. With the decline of landforms, the average height, coverage and biomass of herbaceous plant community gradually increased, but only its density slowly decreased. The order of the coverage on different slopes was successively semi-sunny slopes, semi-shady slope, shady slope and sunny slope, that of the height was shady slope, shady-shady slope, semi-sunny slope and sunny slope, and that of density and biomass were exactly the opposite to that of the height. With gradient increased, the coverage gradually decreased, the density firstly increased and then decreased, and the height and biomass both presented the "S" trend. The ratio of DW/FW fluctuated in the vicinity of 0.50.(3) The proper order of the diversity index was valley slope, top valley, gully slope and gully bottom on different landforms, and that was shady slope, semi-sunny slope, semi-shady slope and sunny slope on different slopes. Moreover, with the increase of gradient, the diversity indices were the "S" types. Only the Margalef index in the diversity indice and the coverage and the height in quantitative indice had relatively large correlation degree. With the increase of species richness, the coverage showed an increasing trend and the average height showed a downward trend.(4) Interspecific association indicated that the number of pairs of species having positive correlation was larger than that of negative correlation, but the proportion of significant and very significant pairs of species was very small. The positive and negative correlations between species-pairs were the indication of niche overlap and separation. The overall community association of variance ration was greater than 1 and the statistic W was more than X20.05 (N), indicating that the species in the total community were positively associated and this association was significant. Futher analysis with the law of M. Godron showed that the vegetation in loess area of natural restoration for 15 years, the distribution of most species is more independent, community is still in succession. |
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