Long-term nutrient additions cause divergence in tallgrass prairie plant communities

Background/Question/Methods

Nitrogen (N) and phosphorous (P) deposition due to pollution and land-use change are dramatically altering biogeochemical cycles. These altered nutrient inputs affect plant communities in many different biomes generally increasing dominance and reducing diversity; however, most conclusions are based on mean responses to treatments while few studies examine community dispersion or the variable community responses to a single treatment. In a nine-year nutrient addition experiment in tallgrass prairie, we addressed the variability in community responses to N and P additions and how disturbance can impact that variability. At the Konza Prairie LTER in east-central Kansas, USA, two levels of N and four levels of P were added in a full factorial design to 5 m² plots on a watershed burned every other year. Nutrients were added at the beginning of each growing season, and plant species composition and productivity were measured each year. Mean differences in community composition and productivity were seen in response to N, P, and their interaction. However, means indicate response direction and magnitude yet mask response predictability. In this study we focus on community divergence as a tool to examine the predictability of community and ecosystem responses to environmental changes.  

Results/Conclusions

Little change in species composition occurred initially, but after four years of nutrient additions two distinct communities emerged based on N treatment. P level also interacted with N to cause community change. Mean shifts in the plant community were also accompanied by changes in community dispersion or community variability within a treatment. In particular, dispersion increased after four years of nutrient additions with the high N+P treatment having the most variable community composition. P alone did not affect community dispersion in any year, while N alone and in combination with each of the three P levels differentially affected dispersion. Burning had the opposite affect of nutrient additions, decreasing dispersion and causing community convergence. In tallgrass prairies fire shifts the competitive hierarchy favoring the dominant C₄ grass species and causing community trajectories to converge. However, the mitigating effect of fire decreased with each additional year of nutrient additions. Chronic nutrient additions and periodic disturbance interacted to affect levels of community divergence with the highest levels of community dispersion as the result of high N+P and no fire. Predicting community dynamics associated with impending global change requires not only evaluating mean responses but also understanding the variability.