The effects of nutrient addition on small-scale spatial dynamics in a mixed prairie

Background/Question/Methods

While it is accepted that environmental heterogeneity is an important factor in determining plant diversity, there have been few studies that examine this heterogeneity at small scales. Established theory on plant communities predicts that an increase in productivity will lead to a decrease in diversity. A key component of this prediction is that by supplementing the nutrients available to the community, small-scale heterogeneities in resource availability are erased, and the potential niche space is reduced.  In this study, we examined the effects of nutrient addition on the spatial relationships between species in a mixed prairie community near Ogallala, NE.  Using 10cm x 10cm grid squares, we mapped the fine-scale position of all plants in a series of 5m x 5m plots: Six experimental plots exposed to long term nutrient addition (5 year supplementation of nitrogen, potassium, and phosphorus) and six plots with control treatments. Using marked pair-correlation analysis, a formal measurement of neighboring plant densities, we examined how nutrient addition impacted inter- and intra-specific associations at different spatial scales, ultimately affecting species spatial relationships.  To further assess changes in community-spatial relationships, we calculated mingling indices, a measure of diversity at different scales, for the different treatments.

Results/Conclusions

We found that nutrient addition significantly altered the spatial relationships between species.  Although it was difficult to disentangle competitive interactions from environmental heterogeneities, it was evident that nutrient addition plots had higher levels of intra-specific aggregation. The pair-correlation functions of nutrient addition plots showed an increase in the radius of competitive neighborhoods of approximately 20%.  Nutrient addition plots also had reduced mingling indices of approximately 15%. Taken together, these results indicate that nutrient addition tends to strengthen inter-specific competitive effects.  This is widely compatible with the established, non-spatial theory.