Effects Of Facilitation By Dasiphora Fruticosa On Plant Communities And Soil Nematode Communities In The Alpine Meadow

The role of plant-plant interactions in community assembly is hot spot in the research of community ecology.Recently,many studies have shown the importance of facilitation is strong as compare to negative interactions,particularly in the area with high altitude and low temperature.However,there were few studies as to facilitation on the Tibetan plateau,where facilitation likely play an important role in plant communities.Although,previous studies have shown that the facilitation by Dasiphora fruticosa might occur in the sub-alpine meadow,the lack of further studies limits our understanding of how facilitation affects above-and below-ground communities in either sub-alpine meadows and alpine meadows.We conducted experiments both in sub-alpine meadows and alpine meadows,in order to make up the research gap.Firstly,in a fenced sub-apline meadow,we used a removal procedure and shade cloth treatment to separate root from canopy effects and quantified the responses of understorey community.We conducted multivariate analyses to assess the relative contribution of root and canopy effects to community composition;Secondly,in a fenced alpine meadow,the above-and below-ground effects of the shrub in the absence or presence of the graminoids were manipulated to quantify direct and indirect responses of forb species,which were grouped with a PCA.For the forbs,six traits characterizing both plant stature and foliage traits were measured and a Redundancy Analysis was used to assess their patterns of direct and indirect responses to the dominant neighbours.Thirdly,we manipulated the above-and below-ground effects of the shrub,D.fruticosa,in the absence or presence of grasses and in a fenced and a grazed alpine meadow,in order to quantify direct and indirect responses of forb community biomass,richness and composition to the different parts of the shrub.We measured four important plant traits,height,lateral spread,specific leaf area(SLA)and leaf dry matter content(LDMC)of understory forb species in all treatments,calculated two metrics of functional diversity(trait range and community weighted mean)and assessed direct and indirect community trait responses to the different parts of the shrub.We used structural equation modeling(SEM)to explore how direct and indirect shrub interactions drove community structure via functional diversity.At last,in a fenced and a grazed alpine meadow,we explored how grazing by yak moderates the effect of the shrub D.fruticosa on nematode communities via changes in understory communities and edaphic properties.We found,(1)the facilitation by D.fruticosa on understory communities mainly resulted from shrub's root.Root and canopy effects strongly explained understorey species composition;(2)there were contrasting direct and indirect responses of forb species to the graminoids,and the above-and below-ground effects of the shrubs,separable into four species-groups.A significant positive indirect net effect of the shrub was observed at the community-level.Traits of forb species were more tightly related to direct than indirect responses to dominant neighbours,while plant stature had a primary influence over foliage traits,which was only very weakly related to indirect responses;(3)there were dominant indirect positive canopy effects of the shrub on the biomass,richness and composition of forbs,through reducing the negative effect of herbivory,whatever the presence of grasses.For functional diversity,there was a direct positive canopy effect on height and SLA in the absence of herbivores,whereas,in the presence of herbivores,shrubs had strong positive effects on these two traits,both with and without grasses.Lateral spread and LDMC had weak responses to the shrub.We found that height was by far the dominant trait mediating the direct and indirect effects of the shrub on forb biomass,richness and composition,but there were also weak mediating effects of SLA in the direct shrub effects on composition and richness;(4)in the ungrazed alpine meadow,D.fruticosa had no significant impact on the nematodes via plant interactions.By contrast,D.fruticosa indirectly increased nematode abundance through facilitating grass biomass,in the grazed alpine meadow.Our results shed new light on the role of plant facilitation in ecosystems,especially the one in the alpine meadow from Tibetan plateau.These findings not only highlight the importance of facilitation in community assembly,but also indicate the facilitation might moderate aboveground-belowground linkages.