Effects of plant species diversity on individual plant performance

Positive effects of higher plant species and functional diversity on community productivity have been reported from several model experiments. However, the underlying effects of diversity on individual plants have received less attention. In spring 2005, we investigated the effect of plant species and functional diversity on individual plant performance in the Jena-Experiment, Germany. Among 82 large 1-60 species plots (20x20m) composed of species representing the functional groups grasses, legumes, and tall and small herbs, productivity was higher in more species-rich plots. We took measures of plant performance (maximum plant height, number of inflorescences, dry mass) for 4-6 individuals of all established species in all plots. In more diverse communities, both in terms of species and functional-group richness, individual plants grew taller, weighed less and had fewer inflorescence. These overall responses differed between species of different functional groups. Grasses showed the strongest positive relationship between individual plant height and plant species richness, while legumes were the only species weighting more in more species-rich communities. Overall, the general pattern of increased height at reduced dry mass suggests increased competition for light in more diverse communities. Thus, in communities of higher plant diversity, plant allocation appears ruled by a trade-off between investment in plant growth and reproduction on the one hand and in competitive ability on the other, depending on the functional type of the plant species. Our results demonstrate how measuring individual responses can be helpful for finding mechanisms underlying the relationship between community diversity and ecosystem functioning.