Negative effects of fertilization on grassland species richness are stronger when tall clonal species are present

Fertilization of grasslands commonly leads to species loss and a dramatic shift in species composition, but the mechanisms underlying this pattern remain unclear. One oft-stated hypothesis is that fertilization increases competition for light and/or reduces spatial heterogeneity of resources, thereby reducing the niche dimensionality for species to coexist. Here, we summarize results from two long-term field experiments showing that the negative effects of fertilization on grassland species richness are greater in the presence of species with a spreading clonal growth form, particularly species that are tall (hereafter ‘tall clonals’). The interactive effect of fertilization and the presence of clonal species on the loss of species richness is associated with: (1) a pronounced increase in the relative abundance of tall clonals following fertilization, and (2) a greater reduction in mean light levels and reduced spatial heterogeneity in light when tall clonal species are present. We hypothesize that tall clonals gain a competitive advantage following fertilization as a result of two important traits related to clonality: (1) the ability to translocate resources between ramets in high to those in low resource patches and (2) the ability to spread vegetatively, especially under conditions where low-light levels limit recruitment from seeds. We suggest that efforts to mitigate the negative effects of nutrient enrichment on grassland diversity may be enhanced by controlling dominance by tall clonal grasses and forbs, and that these results have important implications for management efforts to maintain and enhance plant diversity in grasslands.