Effects Of Robinia Pseudoacacin On Plant Community Function And Structure In Loess Hilly-Gully Region, China

Robinia pseudoacacia,with its strong adaptability,rapid growth rate,easy propagation,and resistance to drought and barrenness,was widely planted in the Loess Plateau as a pioneer tree species for vegetation restoration.It was distributed in different hydrothermal gradients,resulting in variance in species structure and system functions.However,the strong adaptability of Robinia pseudoacacia does not equal that it had good structure and ecological functions in all habitats.In some areas,sparse understory vegetation,single species dominated forest and the inefficient plantations due to poor growth made it difficult to perform the ideal ecosystem functions.In this study,the loess hilly-gully region hydrothermal gradient variation was used as a boundary condition,and the Robinia pseudoacacia was used as an exotic species.Through large-scale investigations and comparisons,the relationship between community diversity and systemic functional response of the understory communities of Robinia pseudoacacia forest was analyzed along environmental gradients,and the forming mechanism of the effective forest structure would be explored thus providing support for vegetation restoration under different environmental conditions.The results are as follows:(1)The environmental conditions of three vegetation zones are different.The differences in species composition and species diversity between the understory communities of Robinia pseudoacacia forest and the native plant communities are also significant.The final results of this study show that in general,in the steppe and forest steppe zone where water nutrient conditions is scarce,the number of species of the understory communities of Robinia pseudoacacia forest are less than that of the native plant communities,and most of them are herbs species.The species diversity of the understory communities of Robinia pseudoacacia forest are lower than that of the native plant communities.After the introduction into China,Robinia pseudoacacia occupied a large amount of ecosystem resource,limiting the growth and development of native plants,causing a decline in the species diversity of plant communities and changes in species composition.In the forest zone,the number of species in the understory communities of Robinia pseudoacacia forest was greater than that of the native plant communities.Most of them are shrubs.The undergrowth plants are less affected by Robinia pseudoacacia.The introduction of Robinia pseudoacacia increases the species diversity of understory communities of forest.(2)We selected functional traits in the paired understory communities of Robinia pseudoacacia forest and native plant communities in steppe zone,forest steppe zone,and forest zone,and analyzed the changes in structure and function of the understory communities of Robinia pseudoacacia forest and the native plant communities as well as effects of introduction of Robinia pseudoacacia on community structure and functions within the same vegetation zone.The results showed that the changes of leaf carbon,leaf nitrogen,leaf phosphorus,specific leaf area,leaf tissue density and other functional traits of the understory communities of Robinia pseudoacacia forest and the native plant communities showed consistent pattern with the vegetation zones.The leaf carbon,leaf nitrogen and specific leaf area of understory communities of Robinia pseudoacacia forest were significantly higher than that of native plant communities.However,the functional diversity indices(FRic,FEve,FDiv,FDis,and Rao)of the two communities differed with the vegetation zones.The introduction of Robinia pseudoacacia t increased the functional diversity of understory communities of forest,in forest zone while reduced the functional diversity of understory communities of forest in steppe zone.(3)From southeast to northwest,the hydrothermal conditions are gradually deteriorating,soil moisture and nutrients are gradually lower,and the biomass of the understory communities of Robinia pseudoacacia forest and the native plant communities gradually decreased.Under the same environmental gradient,the biomass of understory communities of Robinia pseudoacacia forest was lower than that of native plant communities,indicating that the introduction of Robinia pseudoacacia reduced the biomass of plant communities.Compared to steppe and forest-steppe zones,The forest zone has good water conditions,which benefits plant growth,resulting in a superior performance of understory communities of forest and high biomass in the south.(4)There is a significant positive correlations between species diversity and functional diversity,but functional diversity provided better explaination in changes in community productivity.This may be due to differences in calculation of two types of biodiversities.The species diversity is taxonomy-based,and cannot reflect the functional changes within a species.