Long-term dynamics of plant species richness due to fertilization are explained by dominance of one functional group

Nutrient deposition is viewed as a global threat to plant biodiversity because nutrient addition can quickly decrease plant species richness.  However, fertilization took 14 years to significantly decrease plant species richness in our long-term fertilization experiment in successional old fields in southwest Michigan.  We sought to examine why a lag occurred in the effects of fertilization on species richness, and we examined whether increases in the dominance of one functional group—tall species with long-distance (runner) clonality—would decrease the abundance of all other species, and thereby decrease species richness.  Although tall runner biomass initially increased due to fertilization, the biomass of this functional group dramatically declined six years into the experiment.  Tall runner biomass did not recover until 14 years into the experiment, at which point both species richness and non-tall runner biomass declined.  Over 86% of the species found throughout this entire experiment were non-tall runner, and our results suggest the effects of fertilization on species richness are mediated by increases in tall runner biomass at the expense of all other functional group.