| During vegetation restoration,great progresse has been made in loess hilly-gully region.As a pioneer species,Robinia pseudoacacia has been widely planted in the loess hilly-gully region,and played an important role in environmental improvement.However,due to the complex topographic change and environment heterogeneity in this area,it is difficult for R.pseudoacacia forest to achieve sustainable management objectives throughout the region.We still lack of the comprehensive assessment of“plant-soil-microbes” processes caused by Robinia pseudoacacia in understory ecosystem along environmental gradients.In view of this,with the stable natural ecosystem as a reference,we used the pairwise comparison method to investigate the effect of Robinia pseudoacacia on the understory plant community,plant and soil ecological stoichiometry,soil microbial community,microbial network characteristics along environmental gradient,which covers forest habitat,forest-steppe habitat and steppe habitat.We also investigate the find driving factors that influence formation of Robinia pseudoacacia artificial forest with low ecosystem functions.The main results are as follows:(1)The influence of Robinia pseudoacacia on understory community species composition and diversity changed with environment gradien.We found 10,501 plants,belongting to 110 species,92 genera,40 families,4 families(Compositae,Gramineae,Rosaceae and Violaceae)exsited in each region and all vegetation types.Compositae,Gramineae,Rosaceae,Legumes were the main families of the community.Robinia pseudoacacia increased the species richness of understory vegetation in forest and forest steppe,and decreased it in steppe habitat.Robinia pseudoacacia increased the number of Gramineae and Compositae,but decreased Cyperaceae.In forest habitat,the dominate species changed from the mesophyte Cyperaceae to the Gramineae with high tolerances to drought and barren environment,while Rosaceae with poor drought tolerance decreased after the introduction of Robinia pseudoacacia.In forest-steppe habitat and steppe habitat,Robinia pseudoacacia increased Asclepiadaceae and decreased Legume.In forest-steppe habitat the understory species changed from Compositae of annual herbs to Gramineae of perennial herbs,and increased Asclepiadaceae with better shade tolerance.In steppe habitat,the understory species changed from Gramineae and Compositae to Legumes,Gramineae and Compositae.The plant coverage and aboveground biomass was significantly lower than natural communities,and the total coverage decreased by 20%,while the aboveground biomass of forest area decreased by alomost 70 %.(2)The effects of Robinia pseudoacacia on the community structure and function of understory vegetation varied with environmental gradient:the variation of plant functional traits indicated that environmental gradient had certain screening effect on plant functional traits.In this study,the leaf nitrogen content and specific leaf area of Robinia pseudoacacia understory plant were significantly higher than those of native plant community,while the leaf tissue density was on the opposite.The results were consistent across the three vegetation zones.Compared with natural vegetation,leaf carbon content and leaf phosphorus content increased,while leaf carbon content in forest steppe and steppe decreased,but leaf phosphorus content had no significant change.The decrease of phosphorus may be due to the high consumption of phosphorus by rapid growth of Robinia pseudoacacia.The functional diversity of the understory community increased,indicating that the introduction of Robinia pseudoacacia had a positive effect on the improvement of the functional diversity of the understory plant community.(3)The effects of Robinia pseudoacacia on soil physical and chemical properties and ecological stoichiometry in three vegetation zones changed with environmental gradient:Soil organic carbon,soil total nitrogen content,soil N:P ratio and soil C:P ratio decreased in forest and steppe habitat,and the nitrogen limit was the main limiting factor,while the steppe area remained unclear.There were no significant effects of Robinia pseudoacacia on soil total phosphorus content,N:P ratio and p H value in three habitats.The ecological stoichiometric ratios of understory plant leaves were as follows: in forest habitat,forest steppe habitat and steppe habitat,the C:N ratio and C:P ratio of leaves decreased,while the N:P ratio of leaves increased in forest region and steppe region.Phosphorus was the main limiting factor in forest habitat and steppe habitat.After the introduction of Robinia pseudoacacia,forest steppe habitat leaves converted from phosphorus limit to nitrogen limit.Therefore,the soil nutrient content in relatively harsh habitat(steppe habitat)was significantly lower than that in better habitat(forest habitat).Therefore,R.pseudoacacia was not recommended as a species for large scale restoration in poor natural condition area because of its rapid growth and great demand for water and nutrients.(4)Robinia pseudoacacia changed the soil microbial structure and diversity and its effect changed with environmental gradient: The difference of microbial community structure between Robinia pseudoacacia and natural communities changed with environmental gradient.Fungal diversity increased significantly in forest area only,while the bacterial diversity increased in all habitats.And,there were significant differences in the co-occurrence network of microbial communities.The co-occurrence network relationship of fungal communities became loose and simple after the introduction of Robinia pseudoacacia in forest habitat,and intense and more complex in steppe habitat.The bacterial community was opposite,the structure of forest habitat was more complex,the steppe habitat was loose and the relationship was simplified.Environmental gradient can affect the main driving factors of microbial diversity.Robinia pseudoacacia directly affects the bacterial community composition and diversity in forest habitat,indirectly affects the fungal community composition and diversity through its effect on soil moisture and soil nutrients,and directly affects the fungal community composition and diversity in steppe habitat.Therefore,in vegetation restoration on the Loess Plateau,the changes of artificial vegetation on the structure and function of ecosystem,and soil and microbe should be considered,as well as the effect of different environmental gradients.This study showed that the effects of Robinia pseudoacacia on the ecosystem are siginificantly different along environmental gradient,and the ecological benefits of Robinia pseudoacacia on vegetation restoration in the Loess Plateau should be considered comprehensively. |
