Fertilization decreases plant biodiversity even when light is not limiting

Many researchers hypothesize that plant richness declines at high soil fertility (and high productivity) due to light limitation. We tested this hypothesis in an old-field by independently manipulating fertilization and light levels via shade cloth (decreased light), vegetation tie-backs (increased light) and vegetation clipping (increased light). Droughts occurred during two of the four years of the study, and we found that higher light levels were generally associated with decreased plant richness in drought years but increased plant richness in wet years. Most importantly, fertilization decreased richness whether light availability limited richness (wet years) or did not limit richness (drought years), and the effects of fertilization and light manipulation treatments were additive. These results suggest that effects of fertilization on plant richness are at least partly independent of light levels and that competition for resources other than light plays a substantial role in the decline of plant richness after fertilization.