Relative Effects Of Species Diversity And Functional Diversity On Temporal Stability Of Productivity In Alpine Meadows Under Perturbation Background

The functional stability of natural grassland ecosystems is influenced by many abiotic and biotic factors.In grassland ecosystem,livestock grazing and fertilizer application are the two most common disturbance factors affecting grassland ecosystems function,and can indirectly affect the stability of ecosystem function by changing biodiversity.However,there is no consensus on the direct impact of these factors and their intensity changes on the functional stability of grassland ecosystems,the indirect effects by changing attributes of biodiversity and the relative contribution of these diversity attributes to stability.Understanding the response of biodiversity to different disturbance factors and their intensity changes is necessary to predict the relationship between biodiversity,ecosystem function and stability caused by disturbance.Currently,many theoretical and experimental studies have confirmed that diversity is positively related to ecosystem functional stability,but most of them researched the effects of species diversity(SD)or functional group diversity(FGD)on ecosystem functional stability under relatively constant conditions.Therefore,these studies mainly focus on the stability changes caused by the change of the number of species or functional groups and their relative abundance,but rarely involve the consequences of the change of functional diversity(FD)of another component of biodiversity.Functional diversity encompasses not only the number and relative abundance of species,but also the differences in functional attributes among species,the size of functional trait space occupied by species and the regularity of distribution in that space.Because the changes of these attributes can reflect the functional complementarity and redundancy of co-existing species in community,it can have a more important effect on the change of ecosystem function and its stability.By combining species diversity with functional diversity,studying the relationship between diversity and ecosystem function and its stability under the influence of two common disturbance factors,grazing and fertilization,is of great significance to reveal the causes of changes in the functional stability of natural grassland ecosystem under the background of global change.This study was based on a long term field experiment of simulated grazing(including three levels of no clipping,moderate clipping and heavy clipping)and atmospheric nitrogen deposition(including two levels of no fertilization and fertilization)in alpine meadow of Qinghai-Tibetan Plateau.We measured and calculated a series of characteristics associated with structural and functional changes in communities,such as species richness(SR),Simpson's species index(D),functional richness(FRic),functional evenness(FEve),functional dispersion(FDis),community weighted means(CWM)of unidimensional single-trait(including CWMH of species plant height;CWMAGW of aboveground weight of individual plant;CWMCC of chlorophyll content of leaf;CWMLA of area of a leaf;CWMSLA of specific leaf area),aboveground net primary productivity(ANPP),and temporal stability of ANPP(characterized by the inverse of the coefficient of temporal variation of ANPP,ICV).We then analyzed the response of these measures to clipping,fertilization,year and their interactions,as well as the binary relationship between these measures,and further verified the driving factors of IC V change in each experimental treatment community by establishing a structural equation model(SEM),and determined the relative effects and pathways of these factors on ICV(i.e.,direct and indirect effects).The main findings are as follows:(1)Clipping had a significant positive effect on SR,and the significant positive effect on D only occurred without fertilization.On the contrary,fertilization had significant negative effects on both SR and D at any clipping level,and the negative effect was much larger than the positive effect of clipping.Both SR and D decreased with the year,and declined faster in the fertilized community,so the difference from the non-fertilized community became larger and larger.The results indicate that clipping can only alleviate but not prevent the decline in species diversity caused by fertilization.(2)Research on the response to functional diversity of multidimensional multiple traits showed that clipping and fertilization independently affected FRic and FEve,and the negative effect of fertilization on them was much greater than that of clipping.FDis was significantly reduced after clipping and fertilization,and the negative effect of fertilization was much greater than that of clipping,and the negative effect of clipping on FDis only occurred in fertilized communities.FRic,FEve and FDis showed obvious interannual fluctuations,and the difference in FEve and FDis between the non-fertilized and fertilized communities increased with year.This result shows that fertilization drives the reduction of functional diversity of multidimensional multiple traits more strongly.Although clipping can improve the function richness and function evenness,the positive effect is low,and it also reduces the function dispersion.Therefore,clipping also cannot prevent the functional diversity caused by fertilization from decreasing over time.(3)With the increase of clipping intensity,the community weighted means(CWM)of unidimensional single-trait CWMH,CWMAGW and CWMLA decreased,CWMCC increased,and CWMSLA did not change significantly.Fertilization caused a decrease in CWMSLA,and a significant increase in CWM for other traits.The impact of fertilization on CWMH and CWMSLA was greater than that of clipping,and the impact on CWMAGW;CWMCC and CWMLA was less than that of clipping.Therefore,the response of CWM to fertilization is relatively consistent,while the response to clipping is more diverse.The CWM showed different patterns with year,such as increasing type(CWMH and CWMSLA unimodal type(CWMAGW and CWMCC)or "M" type(CWMLA).These results indicate that under the influence of clipping and fertilization,the volume occupied by species tends to increase in the trait space of plant height and leaf chlorophyll content,while the volume occupied in the trait space of aboveground weight,leaf area and specific leaf area per plant tends to decrease.Although CWMH and CWMCC increased,the strong negative effect of fertilization led to the decrease of the volume occupied by species,distribution uniformity and the average distance among species in the niche space of multidimensional multiple traits,showing the trend of convergent assembly on multiple traits.(4)ANPP increased significantly after fertilization and clipping,and the effect of fertilization was greater than that of clipping.In contrast,ICV decreased significantly after clipping,and fertilization had no significant effect on it.No significant interaction effects of clipping and fertilization on ANPP and ICV was found.Therefore,in these two disturbances,the increase in ANPP was mainly caused by fertilization,but its temporal stability was controlled by clipping,which increases ANPP while clipping also reducing its temporal stability,while fertilization did not affect temporal stability.(5)Analysis of the binary relationship showed that in the composite community that combined all the experimental treatment levels and in the three clipping level communities on the non-fertilizing-fertilizing gradient,SD(including SR and D)was significantly negatively correlated with ANPP,and significantly positively correlated with FRic,FEve and FDis,indicating the nature of these relationships will not change due to differences in clipping intensity.However,our results also show that changes in fertilization levels changed or eliminated the above relationships,and nutrient enrichment may be a more important factor behind the nature of these relationships.Furthermore,the effect of diversity on ICV was also disturbances dependent.(6)The verification analysis based on SEM showed that clipping can directly reduce ICV,while fertilization can not in the composite community and any fertilization level community Furthermore,clipping can indirectly affect the ICV by changing the species diversity(SR and D)or and the CWMCC in these communities.Fertilization can only indirectly affect the ICV by changing species diversity(SR or D)and certain functional diversity(FRic,FDis or CWMCC)in the three clipping level communities.This further confirms that clipping not only has a direct effect on temporal stability,but also can indirectly affecte it by changing diversity factors,fertilization can actually indirectly affect temporal stability,which should be related to the stronger effect of fertilization on diversity and ANPP.(7)Ability to predict changes of ICV from species diversity and functional diversity depended on disturbance factors and their intensity changes.Species diversity was the only diversity factor to predict ICV in complex disturbance environments of composite communities,while clipping almost eliminated the influence of diversity factors and became a direct factor to predict ICV in non-fertilizing or fertilizing communities.Moreover,with the increase of clipping intensity,the relative influence of species diversity on ICV was weakened,and that of functional diversity of multidimensional multiple traits on ICV was enhanced,even functional dispersion(FDis)became the only predictor of ICV changes in heavy clipping communities.These results suggest that,as a source of functional diversity,species diversity plays an important role in determining the temporal stability of ecosystem productivity,but it is not a general diversity factor predicted productivity temporal stability,in the face of intense clipping disturbances more diverse functional attributes is more important for the temporal stability.In summary,clipping and fertilization,as the two most common disturbance factors in alpine meadow,had complex effects on species diversity,functional diversity,aboveground net primary productivity and its temporal stability of the community,and led to changes in the relationship between structural characteristics and functional attributes of community strongly depended on the differences in disturbance factors and their intensity.Alpine meadow is a typical nitrogen-limited ecosystem and is very sensitive to nitrogen addition.Therefore,fertilization strongly effected species diversity,functional diversity,aboveground net primary productivity,and determined the relationship between these characteristics.However,the productivity temporal stability of alpine meadow was mainly controlled by clipping disturbance,and it was also affected by the diversity change caused by clipping.Fertilization did not directly affect the temporal stability,but only indirectly by changing diversity.Therefore,the temporal stability of aboveground net primary productivity was also closely related to the change of diversity,but the ability of species diversity and functional diversity to predict the change of temporal stability depended on the difference in disturbance factor and its intensity.With the increase of clipping intensity,functional diversity of multidimensional multiple traits became the main diversity factor to predict the change of temporal stability.However,in the comprehensive environment resulting from clipping and fertilization disturbance,species diversity was still the only diversity factor to predict the change of temporal stability,reflecting the important role of species diversity as the basis of functional diversity in determining the temporal stability of ecosystem productivity.Due to the strong positive effect of fertilization on productivity,the strong negative effect on species diversity and functional diversity of multidimensional multiple traits,and the effect of clipping directly reduces productivity temporal stability,in the context of increasing nitrogen deposition in the future,therefore,the aboveground net primary productivity of alpine meadow will increase further,but its temporal stability will continue to decrease.Effective actions to reduce atmospheric nitrogen emissions and exogenous nitrogen input are the key to reverse this trend.