Abstract:

A number of observational, theoretical and experimental studies have indicated that local plant species diversity can have positive effects on ecosystem productivity; however, little is known about the ecological mechanisms that regulate this relationship. To investigate the relationship between plant species diversity and ecosystem productivity and the effect of resource complementarity on productivity, we established experimental communities containing different levels of diversity using nine cultivated annual plant species. To address questions of complementary resource use, we planted each species in monocultures as well as in different polycultures, which allowed us to evaluate complementary effects. Complementarity was assessed by using four analytical methods. The first two methods were tested for an absolute increase in productivity with increasing diversity. The hypothesis was that a polyculture would outperform the most productive monoculture of a component species, defined as overyielding effect 1 (OV₁), and a polyculture would perform better than the average yield of monocultures of the component species, defined as overyielding effect 2 (OV₂). The third method measured the relative yield of the polyculture, Relative Yield Totals (RYTs), and the last method measured the D value, the proportional deviation of the productivity of a polyculture from its expected value. The results indicated that, to a certain extent, species diversity showed a positive effect on community productivity. The relationship between species richness and community productivity could be represented by a quadratic equation y = -98.449x² + 1 039.2x - 42.407 (R² = 0.423). Large differences in productivity were found among treatments with similar levels of diversity indicating that species composition had an important impact on community productivity. Calculation of a complementarity index indicated that about 40% of the polycultures outperformed its most productive component monoculture, more than 95% of the polycultures performed better than the average yield of its component monocultures, and more than 50% of the polycultures had significant RYTs > 1 and D > 0. These results suggest that resource complementarity was partly responsible for the positive effect of species diversity on productivity. Complementarity, however, was not significantly related to species diversity. Four analytic methods were used for estimating the net outcome of complementary effects and the different levels of ecological interaction in a community. Each method had its advantages and disadvantages. Therefore, in assessing how complementarity influences ecosystem productivity, different methods should be integrated.

Key words: Plant species diversity, Productivity, Resource complementarity, Overyielding, Mechanism, Species composition