| The process of plant community assembly was influenced by combined action of multiple factors, such as environmental factors, organisms interactions and disturbance. The environment filtering based on niche assembly was generally considered as the dominant process in the hilly Loess Plateau because of the diverse habitat conditions in this region. The community assembly rule based on traits was that the process of environmental factors filtered species according to their functional traits showed a multi-scale effect due to different roles of environmental factors at different scales. Therefore, studying the influence of environmental factors to the distribution pattern of plant community traits at various scales is of great significance to contribution to the trait-based assembly theory. Due to topographic changes and habitat fragmentation in Loess hilly region, vegetation presented a mosaic pattern of grass communities and forest at watershed scale. Specific to different erosion environment,studying the multi-scale filtering effect of environment to traits and the mechanism of plant community assembly and species coexistence are important issues for vegetation restoration in this region. This article selected 57 sample sites along with the climate, topography and edaphic gradients in Yanhe River basin. We measured leaf thickness(LT), specific leaf area(SLA), leaf tissue density(LTD), concentrations of leaf carbon, nitrogen, phosphorus(LCC, LNC, LPC) of the dominant species and main accompanying species in each community, then analyzed the relationships between environmental factors and these traits at different spatial scales, i.e.,vegetation zones, hillslopes and sample sites, and how the scaling-relations among traits changed along with environmental gradients in order to select the variation driving factors, reveal the species assemble rules and the underlying driving forces of plant ecological strategies change. The main results are as follows:1) The variation of community traits at different scalesThe degree of variation of leaf traits at various spatial scales degree showed differences. At the watershed scale, in addition to the leaf phosphorus content, the other five traits showed very significant differences(P<0.001) among 9 hillslopes within the scope of the watershed. At the regional scale, the differences of all traits among three vegetation zones achieved significant difference at 0.05 level. At the hillslopes, attribute values of traits in different hillslopes in the same vegetation zone had strong similarities and showed no significant difference. This indicated that it exist obvious scale effect and spatial autocorrelation in plant community leaf traits in the basin.2) Variation of relationships among traits along environmental gradientsThere were widespread correlations among plant community leaf traits, and in accordance with species level. Some trait-pairs had relatively stable relations, such as LNC-SLA, LPC-SLA, LNC-LPC and LNC-LTD, etc., their slopes or intercepts of regression relationship did not vary with the change of the main environmental gradient, only shift along common slope of Y axis. The slopes and intercepts of some other trait-pair relations will be occurred significant difference along with environmental gradients, i.e., the scaling-dependent relationships of LTD-SLA and LPC-SLA were stronger in lower precipitation area, while LTD-LT and LNC-LPC were strengthened over mean annual rainfall. Aspect has a significant influence on all trait-pair relations. With the increase of soil total nitrogen content, under the same conditions the SLA, LTD and LNC increased, LT and LPC decreased, the relationship between leaf nitrogen and phosphorus content was stronger. Overall, rainfall, aspect and soil nutrient were the dominant factors that drving the traits relationships occurred variation.3) Driving factors of traits variation at different scalesThe results of redundancy analysis showed that aspect caused the greatest impact on traits, the rest were, in order, soil organic matter, total nitrogen, hydrothermal factor, total phosphorus, elevation and positions. The multilevel modeling analysis results indicated that rainfall was the dominant factors determining the trait variations at the regional scale(hillslopes), whereas aspect, soil water content and organic matter were main factors driving the variations at the site scale.4) Relative contribution of different types of environment variablesPartial RDA results showed that the relative percentages of variation that can be explained by hrdrothermic factor alone, microhabitat factors alone and covariation between the two parts were, in order, 4.8%, 41.2% and 4.2%. The results of multilevel modeling analysis showed that the variance ratios of LT, LTD,LCC and LNC interpretated by rainfall were reached up to 36.08%, 10.91%, 57.25% and 36.08% respectively. The variance of LT, SLA, LTD, LCC and LNC explained by microhabitat factors(i.e., soil factors and aspect) were 43.60%, 81.17%, 75.39%, 7.28% and 46.43% respectively. In this study, the multilevel modeling was superior to RDA on explaining the variance of community traits in Yanhe River basin.5) Multi-scale filtering effects of environment on community traitsAt the regional scale, traits of species in same vegetation zone had similarities and a certain distribution range under the action of habitat filtering, resulting in the clustered dispersion of community traits. In the study of small scales, the differences of microhabitat factors caused habitat heterogeneity, could increase niche spaces for species, strengthen the regulating and controlling effects of environment filtering on traits, further lead to uniform distribution of community traits. |
PDF Full Text Request