Effects Of Plant Species Diversity On Ecosystem Functioning In Artificial Grassland

The effects of species diversity on ecosystem functioning is one of the most important problems in ecological research. A number of observational, theoretical and experimental works lead to the conclusion that local plant species diversity can have important effect on ecosystem functioning. However, little is known about the ecological mechanisms behind these effects of species diversity. To investigate the relationship between plant species diversity and ecosystem functioning, mainly including productivity, invisibility and soil water use efficiency, in artificial grassland and underlying mechanisms control it in relative short-term, we established experimental communities containing monocultures and different diversity gradient using two different types of plant species--nine cultivated annual species and ten perennial species. The results indicated that:(1) Individual species differed significantly in their productivity, showing that they had significantly differences in growth rate, resource use ability, and functional traits. Two communities showed different patterns of diversity - productivity relationships. In annual communities, the relationship between species diversity and community productivity was unimodal. While in perennial communities, community productivity was positively correlated to species diversity. Otherwise, species diversity and species composition both had significantly effects on ecosystem productivity in two different communities.(2) The relationships between species diversity and complementarity are complex in two communities. In annual communities, the analysis of four complementary indices indicated that complementarity do exist in some mixtures, but there were no significant linear relationship between species diversity and complementary effect, and this suggested that the interspecies interactions are represented not only by complementary resource use, but also by other interaction, such as competitive effect. In perennial communities, however, complementary effects existed almost in all mixtures, and there are significantly linear correlations between species diversity and complementarity. Otherwise, the magnitudes of complementary resource use represented by four indices are different. This suggests that complementary resource use may operate mechanistically in driving the diversity effect in perennial communities. These results demonstrated that multiple measures should be used in order to approach proper conclusions in diversity-ecosystem functioning research.(3) By using "additive partitioning" method introduced by Loreau and Hector, the net effects of diversity generated by a combination of sampling effects and complementary effects can be separated, and their individual contributions can be estimated also. Sampling effect and complementary effect may simultaneously operate on how species diversity enhances ecosystem functioning, but these two components responded to increasing species diversity differently in two communities. In annual communities, the relationship between complementary effect and species diversity are unimodal, suggesting that as the increasing of species diversity, the increase of community productivity was not only caused by complementarity, but also by competition interactions. This led to the results that community productivity is not necessarily increased with increasing species diversity in annual communities. However, the sampling effect showed a positive correlation to species diversity, and this indicated that the species with particular traits, such as higher productivity and competitive ability, are likely to dominate community and have large impacts on ecosystem functioning in annual communities. In contrast with annual communities, however, complementary effects are positively correlated to species diversity, while sampling effects showed no relationships to species diversity. This provides another evidence that complementary resource use between species drives the positive relationships between diversity and productivity in perennial communities.(4) In annual communities, the effects of three components of diversity-plant species diversity, plant functional group diversity and plant functional diversity, on community productivity and soil water content all showed similar trend, but the importance they contributed are different. Multiple regression analyses showed that as the increase of functional group diversity and functional diversity, community productivity increased significantly, while species diversity showed no significant effect on productivity. Species diversity, functional group diversity and functional diversity all showed negative correlations to soil water content, but only functional diversity showed significant effects. These results suggested that, compared with species number, functional differences among species and the range of functional traits carried by plants species are the basis of biodiversity effects on ecosystem functioning. Thus, we can draw a conclusion that in our short-term experiment, these diversity effects of increasing functional group diversity or functional diversity were likely because species differing greatly in size, life form, phenology and capacity to capture and use efficiently resources in diverse communities can realize complementary resource use in the temporal, spatial, and biological ways.(5) On average, annual species performed better in productivity and invasion resistance than perennial species. A positive diversity-productivity relationship and negative diversity-invasibility and productivity-invasibility relationships emerged in two different communities. However, the mechanisms underlying are different in two communities. In the annual communities, the observed positive diversity-productivity and negative diversity - invasibility relationships are linked by the sampling effect. In the perennial communities, however, the mechanism responsible for these observed relationships are the complementary effect. These results demonstrated that identical mechanisms drive both diversity - productivity and diversity - invasibility relationships, and that because species in different communities (here, annual and perennial) may differ in their life history, biological and physiological traits, and may have different resource use style, and different interactions among them, mechanisms responsible for the same biological processes are likely different. Therefore, in the study of relationship between biodiversity and ecosystem functioning, cautious must be paid when extrapolates conclusions from one ecosystem to others.