Phenological complementarity, species diversity, and ecosystem function

Increasing species diversity frequently enhances ecosystem function. Phenological complementarity, the asynchrony of species resource use and growth, may explain how species diversity influences ecosystem function but remains largely untested. We used an early successional plant community containing species with a variety of phenologies to test whether increasing species diversity enhances ecosystem function by increasing phenological complementarity. Over a two-year period, we increased environmental heterogeneity within an abandoned held with variation in disturbance, soil nutrients, water, light availability, and disturbance in 160 permanent plots, and measured percent cover of each plant species three times in each growing season. We did not manipulate species composition directly, and thus diversity and complementarity in each plot were the result of pre-existing conditions and responses of individuals to experimental treatments. Species diversity was measured in two ways, as the total number of species per plot and as the evenness of species abundances. Phenological complementarity was measured as the negative logarithm of the variance ratio. We tested whether the number of plant species per plot, species evenness, and their phenological complementarity in the first year predicted total annual cover in the second year. Total annual cover increased only moderately with number of species and evenness, consistent with studies that randomize species composition among replicate plots. Any effect that species number or evenness had on total annual cover, however, was not due to phenological complementarity. Rather, diversity was unrelated to phenological complementarity. These results indicate that naturally occurring variation in species diversity had little effect on whether phenological complementarity can enhance ecosystem function.