| The relationship between species diversity (SD) and functional diversity (FD)(SD-FD relationship) has been a hot issue in ecology. SD-FD relationship studies has long been in-depth, but research in different scales and different ecosystem types are not consistent in conclusion. The present study shows that SD-FD relationship may appear to be a positive correlation, negative correlation, no correlation or S-shaped curve. However, few studies can explain the intrinsic reasons of inconsistent of SD-FD relationship. Rao index is the most common index that characterized functional diversity, we found that species evenness and trait dissimilarity among species (dij), two basic components of functional diversity, were positively correlated with functional diversity, but they tend to be the opposite change, so which one is responsible for functional diversity and then decides SD-FD relationship compared with the other one is yet unclear. Determination of SD-FD relationship will help us to understand that why species diversity and functional diversity cannot be equal, and that their role to ecosystem functioning are not the same. Ecosystem functioning is mainly determined by plant functional attributes, therefore the effect of functional diversity would be greater than species diversity on affecting ecosystem functioning, but it still lacks empirical research. In this study, a field manipulation experiment was conducted in alpine meadow at the Haibei Research Station of the Chinese Academy of Sciences from2007to2012. The experiment used a split-plot design with clipping treatment in the whole plot using three clipping levels (stubbled1cm,3cm and unclipped). Subplots were treated with fertilizer (fertilizing urea12.75g·m-2·a-1+ammonium phosphate3.06g·m-2·a-1during2007-2012and unfertilized). We selected14traits from33common species of communities in experimental treatment to measure functional diversity and dij. Through the control experiments, General linear model univariate ANO VA were used to compare dij, species evenness, species diversity and functional diversity differences in experimental treatments, redundancy analysis, curve fitting and binary linear regression were used to study the relationship between SD and FD, species evenness and dij, and studied the effect of dij and species evenness on the SD-FD relationship and the effect of SD and FD on the ecosystem functioning. The results showed that: (1) Dissimilarity of different traits among species with a multiple response modes in clipping and fertilizing treatments. Clipping caused growth form, specific leaf area, dry weight per plant, leaf area per plant, leaf dry weight per plant and plant height convergence among species (p<0.05), and dissimilarity of leaf shape among species was biggest in medium degree of clipping (p<0.05). Fertilizing caused specific leaf area, dry weight per plant, leaf area per plant, leaf dry weight per plant and plant height divergence among species (p<0.05). Clipping and fertilizing had no effect on dissimilarity of reproduction modes, life cycle, plant inclination and leaf margin among species (p>0.05), these traits on community assemble process may not be important. But clipping decreased the average dissimilarity of traits among species (p<0.05), caused traits convergence on the whole. The fertilizing has the opposite effect with clipping, but increased the average dissimilarity of traits among species only to a certain degree (p<0.1).(2) Clipping disturbance and fertilizing on species evenness, species diversity and functional diversity has exactly the opposite effect, showing increased effect of clipping (p<0.05) and decreased effect of fertilizing (p<0.1). Clipping caused communities become more homogeneous distribution of species, and fertilizing has the opposite effect, causing uneven changes in species distribution. But whether in the clipping and fertilizing treatment, changes in species diversity caused by synchronous changes in species richness and evenness, changes in functional diversity caused by opposite changes in species evenness and dy.(3) In the clipping and fertilizing gradients, redundancy analysis (RDA) showed that functional diversity was negative correlated with dy, but positive correlated with species evenness, SD-FD relationship was positively. Species evenness was positive correlated with dy. The first ordination axis respectively explained73%(ordered by dissimilarity of14traits among species) and76.6%(ordered by dissimilarity of single traits among species) variation of SD and FD. In the two order conditions, due to the species evenness was greater than dy in the vertical projection of the length of the first ordination axis; therefore the effect of species evenness is greater on affecting SD-FD relationship when compared with dy.(4) Curve fitting showed that species evenness deceleration decreased with dy (n=108, p<0.05), when dy decreases, functional diversity still increased because of species evenness increased rapidly, that is to say, the rate of changes in species evenness is faster than the rate of changes in dythat still increased FD, species evenness is a driving factor of variation of functional diversity and positive correlation SD-FD relationship. The results agreed to community structural changes caused by overgrazing or clipping in alpine meadow, because of competitive release effect inferior broad-leaved forbs gradually replaced quality grasses and sedges cause community degradation, although species diversity and functional diversity was increased at this stage, but the quality and feeding value of grass has declined significantly.(5) Curve fitting showed that FD deceleration increased with SD increasing (n=108, p<0.05). It indicated that there existed a certain degree of functional redundancy in community, and functional redundancy increased with species diversity increasing. Functional diversity will no longer increase when species diversity H’(Shannon-Wiener index) reached4.45, and at this stage dy is further reduced because of niche differentiation in order to balance the increase of functional diversity caused by species evenness. But in this study, it hasn’t reached the limit H’value which caused no correlation between species diversity and functional diversity. Despite dy and dissimilarity of growth form, specific leaf area, dry weight per plant, leaf area per plant, leaf dry weight per plant and plant height among species negatively correlated with functional diversity, but it not enough to offset species evenness increases caused functional diversity increases. Increase in species richness and evenness will further enhance the functional diversity of communities.(6) Curve fitting showed aboveground net primary productivity (ANPP) was negative correlated with SD as well as FD (n=108, p<0.05). Although species diversity H’is calculated by all species of quadrats, and functional diversity is calculated by33common species, but the results showed that the effect of FD is greater on affecting ANPP when compared with SD. Binary linear regression showed that the effect of SD could be excluded when considering the joint contributions of SD and FD of ANPP, the results confirmed the conclusions of ecosystem productivity functioning mainly determined by species attributes rather than the effect of species richness.In summary, functional diversity is correlated positively with species evenness; the effect of species evenness is greater on affecting functional diversity when compared with dij;species evenness is a driving factor of variation of functional diversity and positive correlation SD-FD relationship in alpine meadow grazing ecosystem. Although the effect of SD could be excluded when considering the joint contributions of SD and FD of ANPP, but we cannot completely do not consider the effects of species diversity, because accordingly lost of functional traits or decrease of the dimensionality of trait space from a system as species diversity declined (or species lost) will reduce the functional diversity of community. And only considering the effect of species diversity on ANPP, species diversity still has a certain contribution. It suggested a complementary relationship between species diversity and functional diversity, therefore the effect of FD is greater on affecting ANPP when compared with SD and the effects of species diversity and functional diversity should not be neglected when the relationship between biodiversity and ecosystem functioning was considered. |
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