Soil fungal pathogens and the relationship between plant diversity and productivity

Ecology Letters (2011) 14: 36–41 One robust result from many small‐scale experiments has been that plant community productivity often increases with increasing plant diversity. Most frequently, resource‐based or competitive interactions are thought to drive this positive diversity–productivity relationship. Here, we ask whether suppression of plant productivity by soil fungal pathogens might also drive a positive diversity–productivity relationship. We created plant assemblages that varied in diversity and crossed this with a ± soil fungicide treatment. In control (non‐fungicide treated) assemblages there was a strong positive relationship between plant diversity and above‐ground plant biomass. However, in fungicide‐treated assemblages this relationship disappeared. This occurred because fungicide increased plant production by an average of 141% at the lower ends of diversity but boosted production by an average of only 33% at the higher ends of diversity, essentially flattening the diversity–productivity curve. These results suggest that soil pathogens might be a heretofore unappreciated driver of diversity–productivity relationships.