How do plant traits interact to determine fertilization effects on plant diversity in grasslands?

Background/Question/Methods

Nutrient pollution and fertilization are ubiquitous worldwide and lead to declines in plant species richness. However, the role of particular plant functional traits in determining which species benefit and which species are harmed in response to fertilization remains an open question.  In a 10-year field experiment, we examined how the traits of clonality and plant height affected plant community responses to fertilization. We hypothesized that fertilization would lead to greater species loss when tall, broadly-spreading clonal species were present, and that low-stature species were more likely to be excluded by fertilization in the presence of tall clonal species. We tested this hypothesis in a field experiment where we manipulated fertilization in a restored grassland community with and without a variety of clonal species. We measured changes in plant community composition, dominance, and diversity and evaluated species response to fertilization in relation to several traits related to plant growth form.

Results/Conclusions

Fertilization decreased species richness significantly more in the presence than absence of clonal species. In support of our hypothesis, we also found that plant height was a significant predictor of species loss due to fertilization in the presence of clonal species - low-stature plants were more likely to disappear. However, when clonal species were excluded from the community, plant height was not a significant predictor of species loss under fertilization. These results demonstrate that both clonality and plant height are important functional traits that determine plant community responses to fertilization. Moreover, measurements of soil nutrients and light suggest that competition for light was a major factor driving species loss in the presence of clonals. Light levels were reduced to a greater extent by fertilization in the presence of clonals, whereas soil nitrogen levels were actually higher in the presence of clonals. We suggest the ability of tall, clonal species to suppress light uniformly across microsites and to be able to recruit under reduced light conditions is an important component of their strong, negative impact on plant diversity under high fertility conditions. Control of tall, clonal species may therefore benefit the restoration and conservation of plant diversity under high soil fertility.