| Functional diversity (FD) refers to the value of functional traits of organisms, the range and relative abundance in community. Functional diversity includes both changes in richness, composition, eveness of species and interspecific divergence of traits. With the development of ecology, an increasing number of evidences indicated that functional diversity play more direct role on ecosystem functioning than species diversity (SD), and is also the key feature for ecosystem functional characters and service. In recent years, the key point of the research on relationship between biodiversity and ecosystem functioning had shifted to functional diversity, rather than be confined to species diversity. Functional diversity was widely used for assessment of ecosystem process. As species diversity is the foundation of functional diversity, however, the change of species diversity (e.g. species loss or invasion) of a community would have a significant influence on functional diversity. Therefore, studies on the relationships and/or the changing patterns between species diversity and functional diversity have an important value to reveal the species coexistence mechanisms of community and to reflect the ecosystem functional effect of biodiversity. There is an increasing body of work demonstrating the patterns. Yet it is still unclear what relationships between them is, or how it should be influenced by various perturbations.As species diversity is the foundation of functional diversity and the pattern should remain positive correlation between them. When the environment is harsh at regional scale, species diversity and functional diversity present positive correlation along the grazing gradient, while present saturation increase or s-shaped curve when environment is suitable. At the local community scale, because of the change of species diversity is limited, species diversity and functional diversity present positive correlation usually, but the slope of the relationship is changed with the disturbance intensity, in other words, the relationship and pattern between species diversity and functional diversity depend on the environment quality and disturbance intensity. By reason of environment quality and disturbance intensity will affect the available resources and interspecific relation of community, according to the plant community assembly theory, we can predict that the stronger of disturbance, the weaker of interspecific competition, functional diversity reduced as a result of small niche differentiation, the slope of the relationship between species diversity and functional diversity will decrease in local community. and that the weaker of disturbance, the stronger of interspecific competition, a larger niche differentiation among species will lead to increase functional diversity and decrease the slope, the pattern of slope along the disturbance intensity should be slopel? slopem> slopeh (the subscript of slope l, m and h disturbance indicates no disturbance, moderate disturbance, and heavy disturbance, respectively). To examine the rationality of speculation above, we conducted the experiment on alpine Kobresia humilis meadow with clipping (unclipping, stubbled 3cm and 1cm) and fertilizing (13.8 g·m-2·a-1 urea+ 3.3 g·m-2·a-1 ammonium phosphate) treatments from 2007 to 2013. We used Simpson index and Rao index to measure species diversity and functional diversity of each sample plot, respectively. We studied the relationship between species diversity and functional diversity in six treatment combinations in the time series and investigated the potential influence of clipping and fertilizing on the relationship.The alpine meadow ecosystem of Qinghai-Tibet Plateau is a region where responds to global change sensitively. Both grazing and fertilizing are the most commonly disturbance variables in the alpine meadow. When the two variables to be working at the same time, the changes in species diversity, functional diversity and the relationship between them are ought to depend on the balanced effect of clipping (reduce intensity of competition) and fertilizing (increase intensity of competition). Six plant traits (namely growth form, economic group, life cycle, main reproduction modes, plant inclination, and plant height) were chosed and applied to calculate FD. We used firstly RMANOVA to examine the effects of clipping and fertilizing on SD and FD and use Mauchly's Test of Sphericity to decide whether there is annual autocorrelation or not in SD and FD. Then ANCOVA was used to examine the effects of clipping and fertilizing on the slope and intercept of regression equations between SD and FD. Finally, regression analysis was used to examine the changing pattern of SD and FD in six treatment communities.On the basis of community assembly theory, our study is aimed at testing proposed prediction above from three aspects as follows:Speculation ?:the interspecific trait-convergence assembly patterns (TCAP) drived by clipping and the trait-divergence assembly patterns (TDAP) determined by fertilizing should reduce and increase the functional diversity, respectively. Speculation ?:with the increasing of clipping intensity, the stronger interspecific trait-convergence assembly patterns will result in the reduction of functional diversity, decrease the slpoe of regression equations between SD and FD. Speculation ?:fertilizing increase the intensity of interspecific competition, will reduce the species diversity, increase functional diversity. Hence, the stronger trait-divergence assembly patterns after fertilizing will increase the slpoe of the regression equations between SD and FD. The results show as fellows:(1) SD and FD were improved by increasing clipping intensity significantly (p<0.001). But fertilization caused reduction of SD significantly (p<0.001) and had a weak influence on FD (i.e. marginal significance, p=0.065).(2) The discrepancy of SD was significant (p<0.05) among years for all of six kinds of treatment combinations and reduced year to year. In the same clipping treatment communities, the decline of SD began to show earlier in fertilizing communities than in no fertilizing communities.(3) In no clipping-fertilizing community, FD represented no significant difference among years (p>0.05), but increased significantly with years in the other communities, such as no clipping-no fertilizing, moderate clipping-no fertilizing, heavy clipping-no fertilizing, moderate clipping-fertilizing, and heavy clipping-fertilizing.(4) In all kinds of treatment combinations of clipping and fertilizing, the relationship of SD and FD presented two patterns including linear positive correlation in the unclipped and moderate clipped community(n=108, p<0.05) and no correlation in the heavy clipped community (n=108, p>0.05). The slope values of SD(x)-FD(y) relationship displayed a model of slopel? slopeh alopeh, along clipping gradients (the subscript of slope l, m, and h indicated no clipping, moderate clipping, and heavy clipping, respectively), and the values of slope was less than 1. This indicated that the change of SD-FD relationship was independent in heavy clipping community and the change rate of FD was less than SD in other communities.(5) No significant interactive effects of clipping and fertilization on SD and FD were found(p>0.05). Because of the slope values of SD(x)-FD(y) relationship also displayed a model of slopel?slopem>slopeh in no fertilizing, which indicated that fertilization cannot affect the pattern of relationship of SD-FD.(6) Although fertilization cannot affect the pattern of relationship of SD-FD, but fertilization increased the intercept of regression equations of SD-FD significantly, whereas, clipping decreased the slope of it.(7) With increasing the clipping intensity, the slope of SD-FD relationship decreased gradually, the patterns of SD-FD relationship shift from linear positive correlation to no correlation. Clipping disturbance should play a leading role in changing the pattern of SD-FD relationship and their slope model.As such, our study confirmed the Speculation ? partially based on the community assembly theory, and confirmed the Speculation ?, but the Speculation ? failed. As viewed from the theory, clipping and fertilizing determined the interspecific trait-convergence assembly patterns (TCAP) and trait-divergence assembly patterns (TDAP) should reduce and increase the functional diversity, respectively. Our results suggested that clipping disturbance may not induce inevitably the trait-convergence assembly, it is entirely possible to induce the trait-divergence assembly, and that the strong interspecific competition resulting from fertilizing cannot significantly intensify the trait-divergence assembly. This indicated that even if the single disturbance factor which drives the TCAP and/or TDAP may have different influences on functional diversity, which inconsistent with the speculation based on community assembly theory. Compared with fertilizing, clipping disturbance played a leading role on determining the pattern and slope of relationship between species diversity and functional diversity in this meadow community. Further explanation for the positive or negative effects of multiple disturbance factors on the basic components composed functional diversity, such as species richness, composition, evenness and interspecific functional traits divergence, will be great importance to forecast the potential influence of changing functional diversity on the alpine meadow ecosystem functioning under complex environment. |
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