Studies On The Overgrazing-induced Legacy Effects On Main Plants In Typical Grassland In North China

Legacy effect refers to the phenomenon that when the environmental disturbance is removed,its impact on every element,the relationship between elements,the structure and function of the ecosystem can still exist for a long time.The natural grassland ecosystem is the largest terrestrial ecosystem in our country,and it plays an important role in the process of China's economic prosperity,social stability,and ecological civilization.Grazing is the most important management method for grassland,and the grazing-induced legacies on plants may have important effects on the ecological processes,such as vegetation succession and productivity formation of natural grassland.However,the current domestic and foreign studies on plant legacy effects under grazing disturbances are still insufficient.Legacy effects are defined differently in various fields for different research purposes.In this study,the grazing-induced legacy effects on plants was defined as the phenomenon that the offspring of the same species in the long-term enclosed and free grazing plots showed significant phenotypic differences under the same growth environment.We take the “whether there is a legacy effect of grazing disturbance on typical grassland plants in northern my country — the induction way of plant legacy effect under grazing disturbance — The influence of plant legacy effects on natural grassland ecological process under the disturbance of grazing” as a technical route to carry out research.Since 90% of plants in the typical grassland in northern China are replaced by asexual propagation,this study adopts the test method of asexual propagator or plant seedling transplanting,and tries to keep the same initial state of asexual propagator or seedling in the two sample plots during transplanting.The main results are as follows:1.Leymus chinensis,Agropyron cristatum,carex,and Stipa grandis have obvious grazing legacy effects,and the plant legacy effects induced by long-term overgrazing are universal.All four plants showed that plant height dwarfed and individual plant biomass reduced,the legacy effect of grazing at the plant level is common to plant species.However,the effect has also variability among species.The number of ramet increases,the underground distribution decreases,and the total biomass remains unchanged in Leymus chinensis and Stipa grandis.However,Agropyron cristatum and Carex showed a decrease in the number of ramet,an increase in underground distribution,and a decrease in total biomass.The grazing-induced legacies on plants may be the change of epigenetic modification,while the vitality of asexual propagators may also play a role.The change of genotype composition may not explain the grazing-induced legacies on plants.The essential properties of the plant legacy effects caused by grazing need to be revealed directly by the development of molecular ecology of natural grassland,especially by the sequencing of plant genome of natural grassland.2.The redundant analysis results show that a single environmental factor can explain the phenotypic variation of Leymus chinensis in a homogeneous environment.The explanation rate of the variance of 0-10 cm soil porosity,soil capillary porosity,and soil bulk density is relatively large,VIP is 4.36%,4.15%,3.94%,respectively.The exsiting or not of Potentilla chinensis and Eragrostis,the relative abundance of Potentilla chinensis,the abundance of Eragrostis have reached a significant level in explaining the phenotypic variation of Leymus chinensis,and VIP is 3.54%,3.23%,3.23%,3.13%,respectively.Among the soil chemical properties,only 0-10 cm soil p H reaches a significant level,VIP is 2.82%.When multiple environmental factors are grouped for soil factors,vegetation factors,and material sources,the variance decomposition results show that the soil factors have the largest explanation rate,followed by vegetation factors,and the material sources are the smallest,and VIP is7.72%,6.47%,3.96%,respectively.When all factors are considered in the analysis,the explanatory rate of material source factor to phenotypic variation of Leymus chinensis was negative,and the phenotypic variation induced by grazing is explained by environmental factors.3.The confirmatory structural equation model analysis shows that grazing disturbance can directly or indirectly induce the legacy effect at the plant level.The legacy effect of natural height is mainly induced directly by grazing disturbance,and the direct and indirect effects are-0.313 and 0.010,respectively.The legacy effects of leaf number,stem biomass,and individual plant biomass are simultaneously directly and indirectly induced by grazing interference.The direct effects are-0.235,-0.447,-0.218,respectively and the indirect effects are-0.161,0.077,-0.064,respectively.The legacy effects of leaf width and leaf biomass were mainly indirectly induced by grazing interference.The direct effects were all 0,and the indirect effects were-0.4and-0.271,respectively.4.The plant legacy effect induced by grazing disturbance enhances the drought resistance of Leymus chinensis and Agropyron cristatum.Compared with Leymus chinensis,the drought resistance of Agropyron cristatum is more affected by the legacy effect of grazing;compared to ungrazed Leymus chinensis,Plant biomass and rhizome biomass of grazed Leymus chinensis are less affected by drought,and the number of ramet is more affected by drought;under the effect of grazing,the biomass of individual plant,number of ramet,and total biomass of Agropyron cristatum are all less affected by drought.The grazing legacy effect enhances the clipping resistance of Leymus chinensis,and has no significant impact on the adaptation of the Agropyron cristatum to the clipping treatment.Exploratory structural equation model analysis shows that the grazing legacy effect increases the near-surface distribution of the above-ground biomass by reducing the individual plant height and increasing the leaf angle of Leymus chinensis,thereby improving the spatial avoidance of Leymus chinensis;The aboveground and subsurface biomass distribution is not conducive to the avoidance of livestock feeding of Leymus chinensis,which may be a trade-off between spatial colonizing ability and grazing tolerance.5.The plant legacy effect induced by grazing disturbance changed the root interaction relationship between Leymus chinensis and Agropyron cristatum,which in turn changed its biodiversity effect under mixed species.Ungrazed Leymus chinensis and Agropyron cristatum roots interact with each other as an interfering competitive relationship,but the root interaction is not significant under the grazing legacy effect;the net biodiversity effect of the two changes from a negative effect to which there is no significant difference from 0.