A reexamination of the relationships among phenological complementarity, species diversity, and ecosystem function

Interest in the effects of species diversity on ecosystem function has been rapidly increasing, and many studies have concluded that increasing species diversity improves ecosystem function. Theories predict that phenological complementarity, the asynchrony of species resource use and growth, may explain how species diversity enhances ecosystem function but it remains largely untested. In this paper, we tried to determine: (i) whether, in a species-rich natural assemblage, increasing species diversity enhances ecosystem function by increasing phenological complementarity, and (ii) how phonological complementarity was related to the scales of alpha-diversity. Over a three-year period, we conducted the experiment in 55 permanent plots with a relatively homogeneous environment in natural alpine grasslands, and estimated percent cover of each plant species present in each plot three times in each growing season. Species diversity in each permanent plot was quantified in two ways, the total number of species, and the evenness of species abundance. Phonological complementarity was quantified as the negative logarithm of the variance ratio. Ecosystem function was measured as total annual cover (i.e. the sum of maximum cover values recorded for each species in a growing season). Statistical analysis of the relationships between total annual cover, species diversity, and phenological complementarity found that total annual cover increased with species diversity, and phonological complementarity was responsible for the positive effects of species diversity on total annual cover. phonological complementarity, however, was not significantly related to species diversity. These results indicate that phenological complementarity was critical to the maintenance of total annual cover in our studied alpine plant communities. However, considering different mechanisms of species coexistence (e.g. local similarity), phenological complementarity is not necessarily linearly correlated with the scales of alpha-diversity in species-rich plant assemblages. In addition, other mechanisms, for example, the sampling effect for phonological complementarity, may simultaneously act on how phonological complementarity enhances ecosystem function. Therefore, greater understanding of how diversity and phenological complementarity influence ecosystem function will require more extensive experiments that control the effects of composition.