| In recent years,the dynamics of woody plants under grazing disturbance is one of the hot issues in grassland ecology and grassland management.Ecologists focused on the dynamic change mechanism of woody plants and its impact on ecosystem functions and services;while grassland managers paid more attention on the ideal proportion and sustainable utilization of woody and herbaceous plants.Different plant populations or functional groups had various strategies to cope with grazing,which lead to changes of the relationship among plant populations or functional groups.In order to explore the response characteristics and principles of the relationship between shrubby and herbaceous plants under different stocking rates(no grazing,CK;light grazing,LG;moderate grazing,MG;heavy grazing,HG,respectively)in desert steppe,a long-term control experiment(2004-2018)of stocking rate was applied in a desert steppe.From 2016 to 2018,concentric circle method(radius 0.25 m,0.5 m,0.75 m and 1 m,respectively)was used to monitor individual quantitative characteristics of four main shrubs and semi-shrubs,including Artemisia frigida,Kochia prostrata,Caragana microphylla and Ceratoides latens,as well as the individual quantitative characteristics of plants and soil properties in adjacent areas(radius 1 m).According to the dynamic trend of shrub-herbaceous plants in time series(2004-2017),variance analysis was used to compare the difference of shrubs’ individual quantity characteristics,and the difference of main plant population density(Stipa breviflora,Cleistogenes songorica,A.frigida),functional groups density(perennial grasses,perennial forbs,shrubs and semi-shrubs,annuals and biennials),and community density in adjacent areas under different stocking rate and in different direction(east,south,west and north).The geostatistical method was used to analyze the heterogeneity of soil properties and the spatial distribution of population density of main shrubs(A.frigida,K.prostrata,C.microphylla and C.latens).The population density,functional group density,community density and community standing crop of main herbaceous populations(S.breviflora,C.songorica)were also analyzed.Canonical correlation was used to analyze the interactions between shrubs and their neighboring plant populations,functional groups and communities,as well as the relationship between shrubs and soil properties.This paper tried to reveal the tradc-off process of shrub-hcrbaccous on multiple organizational levels under biotic and abioticfactors interactions.The main results are as follows:(1)Stocking rate had a significant effect on species composition of plant communities in desert steppe.With the increase of grazing years and stocking rate,the dominant effect of herbaceous plants on the community was enhanced(the aboveground standing crop of MG and HG were higher than 90%from 2014 to 2017),meanwhile the functional group of perennial grasses represented by S.breviflora gradually occupied a dominant role in the community.Shrubs showed an opposite trend with the longer grazing time and heavier stocking rate,after 10 consecutive years of grazing(LG,MG,HG),the accumulated aboveground standing crop of shrubs tended to flatten.The individual quality characteristics of A.frigida,K.prostrata,C.microphylla and C.latens were generally decreased.(2)A.frigida and K.prostrata influenced the population density,functional group density and community characteristics in the range of 0.25 m in the nearest neighbor,while the effect of C.microphylla and C.latens on the population density,functional group density and community characteristics in the nearest neighbor was up to about 0.50 m.Shrubs and semi-shrubs had the orientation effect on the neighboring plants,but the influence of different shrub species on the direction existed differences.(3)There were distance effect,orientation effect,species effect and stocking rate effect of A.frigida,K.prostrata,C.microphylla and C.latens posed on soil nutrients in their adjacent areas.The distance effect was mainly reflected in the range of 0.50 m centering on shrubs,however the four shrubs showed different effects in orientation effect,species effect and stocking rate effect.These four shrubs effected on the soil environment(temperature,moisture,pH value)of 0.5 m in the immediate vicinity,and the closer to the shrubs,the effect was more obvious.Lowest soil temperature occurred in the north,the soil water content was the lowest in the vicinity of A.frigida and K.prostrata,and the soil water content was lower in the vicinity of C.microphylla and C.latens(about 0.2 m).(4)The shrubs-herbaceous interactions were multidimensional under the regulation of stocking rate.At the population level,A.frigida had a competitive relationship with S.breviflora,but it had an affinity relationship with C.songorica and the intraspecific competition was fierce.There was also a trade-off interactions between K.prostrata,C.microphylla and C.latens with the main populations(S.breviflora,C.songorica,A.frigida).At the level of function group,C.microphylla and C.latens competed with annuals and biennials.K.prostrata and perennial forbs were affiliative,whereas K.prostrata competed with shrubs and semi-shrubs,in addition it had a trade-off interactions with annuals and biennials.C.microphylla had a protective effect on perennial forbs and annuals and biennials,while it competed with shrubs and semi-shrubs.At the community level,the relationship between A,frigida and its community plants were competitive.K.prostrata and its community plants had the same trend and it was beneficial to increase the number of species in its vicinity.The increase of shrub height of C.microphylla and C.latens motivated the improvement of plant community’s height,density and species number.(5)The spatial distribution analysis showed that at the population level,with the increase of stocking rate,the spatial distribution of shrub populations became more patches.Scale and connectivity of these patches decreased,and the density reduced gradually.The spatial distribution of S.breviflora and C.songorica under different stocking rates showed good patch and connectivity.With the increase of stocking rate,the density of S.breviflora increased,while that of C.songorica increased first and then decreased.At the level of functional group,with the increase of stocking rate,the density of perennial grasses increased,while that of shubs and semi-shrubs,perennial forbs,annuals and biennials decreased.At the community level,with the increase of stocking rate,the community density first increased and then decreased,and the above-ground standing crop of community gradually decreased.The spatial heterogeneity of populations,functional groups and communities were mainly determined by structural factors whether grazing or not.(6)According to the order of organization level,the influence capacity of A.frigida and C.microphylla on the main plant populations,functional groups and community declined gradually,while the influence capacity of C.latens grewed up.The influence capacity of K.prostrata kept consistent.With the increase of distance,the shrubby effect disappeared,which replaced by the effect of stocking rate.Under the regulation of stocking rate,the relationship between shrubby and herbaceous plant populations,between shrubby and functional groups,between shrubby and communities became more complex.The three-layer relation of canonical correlation reflected the influences of A.frigida,K.prostrata and C.latens on main plant populations,functional groups and communities.The effects of stocking rate on plant populations and functional groups were multidimensional,while the effects on plant communities were consistent. |
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