Understory Plant Diversity And Its Influencing Mechanisms Of Typical Plantations In The Loess Region Of Western Shanxi

In order to control soil erosion,a large area of plantation has been built in the Loess Plateau,but there is still a lack of understanding of the development process and influence mechanism of shrub and herb plant communities in the plantation.Therefore,the typical plant community in Caijiachuan watershed was analyzed as the research object.The differences of understory plant diversity and species distribution and its environmental influence factors,as well as the potential contribution of soil seed bank to vegetation regeneration,were explored,thereby revealing the formation and influencing mechanism of understory plant diversity,and identifying the main potential driving factors and limiting factors of understory plant community.This could provide important references for the restoration and protection of species diversity and sustainable forest management.The main findings are as follows:(1)A total of 124 species of understory plants were found in all sample plots,including 38 species of shrub layer and 86 species of herb layer.In the shrub layer,Rosaceae had the widest distribution range and was the most dominant family in this area;while in the herb layer,Asteraceae and Poaceae had more herb species.The establishment of artificial forests on abandoned farmland has significantly improved the understory environment,thereby promoting the development of understory plant diversity.Robinia pseudoacacia forests and mixed forests may be more effective in promoting the restoration of understory vegetation than Pinus tabuliformis forests and Platycladus orientalis.Therefore,R.pseudoacacia species can be appropriately introduced into the P.tabuliformis forest or P.orientalis forest in this area.The species diversity and phylogenetic diversity of the shrub layer in the natural secondary forest were higher than those in the plantation,while the herb layer was the opposite.Therefore,the way of natural succession may be more conducive to enriching the plant diversity of the shrub layer in the forest community,while the way of plantation succession may be more beneficial to enrich the plant diversity of the herb layer.(2)Broad-leaved forests may be more effective than coniferous forests in improving soil properties.The soil properties after afforestation were closely related to the species diversity and phylogenetic diversity of understory vegetation.The effects of soil properties on the plant diversity of the herb layer were mainly concentrated in the topsoil(0-20cm),while the subsoil(20-50cm)had a greater impact on the plant diversity of the shrub layer.Soil water content had an important impact on species diversity and phylogenetic diversity of shrub and herb layers,and was a limiting factor affecting the restoration of understory plant diversity in this area.In addition,with the increase of soil total nitrogen,especially in the 0-10 cm soil layer,the phylogenetic structure of the herb layer community in the plantation changed from overdispersion to clustering.With the increase of soil total phosphorus(10-20cm),the phylogenetic structure of the shrub layer community tended to be overdispersion,while that of the herb layer tended to clustering.(3)Topographic factors had a significant impact on the understory plant community,but the topographic factors affecting the plant communities in the shrub layer and the herb layer were different.In the R.pseudoacacia forest,slope aspect and slope position were important factors affecting the species distribution of shrub layer and herb layer,while in the P.tabuliformis forest,slope and aspect were the most important topographic factors affecting shrub layer and herb layer community,respectively.From the shady slope to the sunny slope,the herb species richness in the R.pseudoacacia forest and the P.tabuliformis forest showed an increasing trend,but the highest value appeared in the semi-sunny slope instead of the sunny slope,and this trend was also found in the phylogenetic diversity of the herb layer in the R.pseudoacacia forest.In addition,environmental filtering induced by light led to the emergence of more closely related species in the herb layer community of the P.tabuliformis forest.The influence of slope position on the understory species diversity in the P.tabuliformis forest was not significant,but it had a significant impact on the understory species diversity in the R.pseudoacacia forest.With the increase of the slope,the species diversity of the herb layer in the R.pseudoacacia forest and the P.tabuliformis forest decreased significantly.Altitude had relatively little effect on the understory species diversity in this area.(4)A total of 66 species were identified from germinated seeds in soil seed banks.Asteraceae,Poaceae,Fabaceae,and Rosaceae made up the major components of soil seed banks,but Carex lancifolia(Cyperaceae)was the most widespread and seed-dense species.Afforestation can effectively promote the reconstruction of understory vegetation,thereby improving the species diversity of soil seed banks.The species richness of soil seed banks in artificial forests was significantly higher than that in natural restoration vegetation types,but the similarity of soil seed banks and understory vegetation species was lower for all vegetation types.The soil seed banks in R.pseudoacacia forests and mixed forests had higher species richness and seed density,so they may have greater potential for vegetation regeneration.In addition,afforestation can provide more suitable environmental conditions for seed germination and seedling formation,thereby increasing the vegetation renewal potential of soil seed banks.(5)Niche and neutral processes played an important role in the formation and maintenance of species diversity of the understory plant community in this area.The community assemblage of understory plants in all vegetation types was influenced by both niche and neutral processes,but the relative importance of these two processes varied by plant community type.In addition,with the improvement of environmental conditions,the limiting effect of deterministic processes such as environmental filtering on the assemblage of understory plant communities may gradually weaken.