Spatial Variation Of Typical Vegetation Trait And Potential Plant Community Distribution In Loess Hilly-Gully Region

The Loess Plateau is the key area for ecological protection and management in China.Since the implementation of the Grain for Green Project,vegetation cover in this area has been greatly increased and soil erosion has been effectively controlled.However,the large-scale vegetation restoration resulted in ecological problems such as excessive vegetation restoration,the mismatch between vegetation restoration and habitat,extensive soil dry layer and water shortage,which caused serious threats to the stability and sustainability of ecosystem functions.How to select suitable vegetation system to reform the unreasonable artificial vegetation is the key to transform vegetation construction"from quantity to quality",and achieve ecological protection and high-quality development on the Loess Plateau.Therefore,in this study,Yanhe River catchment in loess hilly region was taken as study area,which is divided into g steppe zone,forest-steppe zone and forest zone according to the geographical distribution of vegetation and changes in environmental geography.Taking the stable natural plant community as the reference,the Robinia pseudoacacia artificial plantation with the widest planting area was selected as the research object,plant functional traits research method was used to deeply analyze the response of Robinia pseudoacacia plantation and trait structure to environmental gradients,and reveal the formation process and mechanism of inefficient Robinia pseudoacacia plantation ecosystem.Taking the distribution of stable natural vegetation in different habitats as constraint condition,the plant community prediction method based on environmental filtering theory was used to simulate the species composition of potential communities in different habitats,which provided the basis for the transformation of Robinia pseudoacacia plantation.The main results were as follows:（1）There were significant differences in species diversity between natural vegetation and artificial vegetation in steppe zone,forest-steppe zone and forest zone.Species richness,Shannon-Wiener diversity,Simpson dominance index and Pielou evenness index of artificial vegetation were significantly lower than natural vegetation（p<0.05）.The high canopy closure and competition of Robinia pseudoacacia plantation caused species loss and species diversity reduction of the whole plant community,which threatened the stability and sustainable development of ecosystem functions.（2）There were significant differences in functional diversity between natural vegetation and artificial vegetation in steppe zone,forest-steppe zone and forest zone.Community weighted mean specific leaf area,leaf nitrogen（N）concentration,leaf phosphorus（P）concentration and root P concentration of artificial vegetation were significantly higher than natural vegetation in steppe zone and forest-steppe zone（p<0.05）.There were no significant differences in community weighted mean traits between artificial vegetation and natural vegetation in forest zone.The functional richness,functional evenness,functional dispersion and Rao`s quadratic entropy index of artificial vegetation were significantly lower than natural vegetation（p<0.05）.（3）The aboveground biomass of artificial vegetation in steppe zone(1384.25 g·m^-2)was significantly higher than natural vegetation(268.30 g·m^-2),and the soil water content was significantly lower than natural vegetation（p<0.05）.In order to maintain normal aboveground biomass,Robinia pseudoacacia plantations in steppe zone and forest-steppe zone consumed excessive soil water,which caused a significant decrease in soil water.There was no significant difference between the aboveground biomass of artificial vegetation(1490.13 g·m^-2)and natural vegetation in forest zone(1354.78 g·m^-2),thus soil water decline were not occurred.（4）There were different responses of natural vegetation and artificial vegetation to environmental variations.Principal component analysis of climate,topography and soil factors showed that average annual precipitation,average annual temperature and aspect were the three main environmental factors affecting the functional traits of plant communities.The community weighted mean specific leaf area,leaf N concentration,leaf N:P and root N:P of natural vegetation increased with increasing average annual precipitation and average annual temperature,while community weighted average leaf tissue density,leaf P concentration and root P concentration decreased with increasing average annual precipitation and average annual temperature.Community weighted mean specific leaf area,leaf N concentration,root tissue density,root N concentration showed significant relationships with aspect（p<0.05）.In artificial vegetation,only leaf tissue density,root tissue density,leaf N concentration,root N concentration and root N:P showed significant relationship with average annual temperature,and their correlations were weaker than natural vegetation.Variation partitioning analysis showed that the total explanatory of average annual precipitation,average annual temperature and aspect to community mean traits of natural vegetation（41.00%）was obviously higher than that of artificial vegetation（7.12%）.In artificial vegetation,community weighted mean traits exhibited better explanation to aboveground biomass variation than functional diversity index,thus the trait variation of dominant species in artificial vegetation played a major role in affecting ecosystem functions,which accorded with“Mass ratio hypothesis”.（5）The response of community-level traits of natural and artificial vegetation to environmental changes in Yanhe River catchment was mainly driven by intraspecific trait variation.The total explanation of average annual precipitation,average annual temperature and aspect to the intraspecific trait variation of natural vegetation（community weighted mean trait:20.29%;functional diversity:21.54%）was obviously higher than artificial vegetation（community weighted mean trait:17.35%;functional diversity:8.43%）.The environmental factors exhibited lower explanatory to the intraspecific trait variation of artificial vegetation but it was higher in natural vegetation,which further demonstrated that appropriate species selection according to environmental gradient in natural vegetation species pool would improve the success rate of vegetation restoration and reconstruction.（6）The Trait-space model accurately predicted the distribution of dominant species in stable natural plant communities along different environmental gradients based on environmental filtering（R²=0.59）.By analyzing the determination coefficient of average annual precipitation and average annual temperature on each functional trait variation and the influence of trait number on the predictive ability of the model,the functional traits added to the model were finally determined to be specific leaf area,leaf N concentration and root N concentration.Based on the relationship between the functional traits of stable natural plant communities and environmental factors,the potential community composition matching environmental conditions was simulated in the three vegetation zones.The prediction results showed that the relative abundance of the potential community species composition in steppe zone was in the order of Artemisia gmelinii,Bothriochloa ischcemum,Artemisia giraldii,Stipa bungeana,Lespedeza davurica and Stipa grandis.The potential community species composition in forest-steppe zone was Carex lanceolata,Stipa bungeana,Vitex negundo,Bothriochloa ischaemum,Artemisia giraldii,Artemisia gmelinii,Lespedeza davurica and Sophora davidii.The potential community species composition in forest zone was Quercus wutaishanica,Sophora davidii,Carex lanceolata,Artemisia gmelinii,Artemisia giraldii,Lespedeza davurica and Vitex negundo.Select suitable vegetation system to reform the unreasonable artificial vegetation is the key to transform vegetation construction on the Loess Plateau.From the perspective of trait structure,the results indicated that the artificial vegetation in Yanhe River catchment did not possess similar resource acquisition characteristics and environmental adaptability to natural vegetation.The traits distribution characteristics of artificial vegetation deviated from the distribution of natural vegetation traits filtered by environmental condition,especially in the steppe zone and forest-steppe zone.Trait structure of natural vegetation was more sensitive in response to environmental changes and the environmental filtering effect was stronger,while the artificial vegetation showed unconspicuous environmental filtering effect,indicating the unreasonable species selection and configuration along environmental gradient changes during artificial plantation.The prediction of potential community species composition through environmental filtering provided a reliable theoretical basis for reasonable species selection and vegetation restoration on the Loess Plateau.