The influence of arbuscular mycorrhizae on the relationship between plant diversity and productivity

Ecological theory predicts a positive and asymptotic relationship between p lant diversity and ecosystem productivity based on the ability of more dive rse plant communities to use limiting resources more fully. This is support ed by recent empirical evidence. Additionally, in natural ecosystems, plant productivity is often a function of the presence and composition of mycorr hizal associations. Yet, the effect of mycorrhizal fungi on the relationshi p between plant diversity and productivity has not been investigated. We pr edict that in the presence of AMF, productivity will saturate at lower leve ls of species richness because AMF increase the ability of plant species to utilize nutrient resources. In this study we manipulated old-field plant s pecies richness in the presence and absence of two species of AMF. We found that in the absence of AMF, the relationship between plant species richnes s and productivity is positive and linear. However, in the presence of AMF, the relationship is positive but asymptotic, even though the maximum plant biomass was significantly different between the two AMF treatments. This i s consistent with the hypothesis that AMF increase the redundancy of plant species in the productivity of plant communities, and indicates that these symbionts must be considered in future investigations of plant biodiversity and ecosystem function.