Nitrogen (N) additions have decreased species richness (S) in hardwood forest herbaceous layers, yet the functional mechanisms for these decreases have not been explicitly evaluated. We tested two hypothesized mechanisms, random species loss (RSL) and non-random species loss (NRSL), in the hardwood forest herbaceous layer of a long-term, plot-scale, fertilization experiment in the central Appalachian Mountains, USA. Using a random thinning algorithm, we simulated changes in species densities under RSL and compared the simulated densities to the observed densities among N-fertilized (+N), N-fertilized and limed (+N+L), and reference (REF) plots in regenerating forest stands. We found a lower S in the +N treatment across all survey years and determined that the reduction in S was a function of NRSL. Furthermore, non-random effects were observed in certain species, as they occurred at densities that were either higher or lower than expected due to RSL. Differential advantages were also observed among species between +N and +N+L treatments, suggesting that species responded to either the fertilization or acidification effects of N, though no consistent pattern emerged. Species nitrophily status was not a useful trait for predicting specific species losses, but was a significant factor when averaged across all treatments and sampling years. Our results provide strong evidence that declines in S in the forest herbaceous layer under N fertilization are due largely to NRSL and not simply a function of species rarity.
Bibliographical noteFunding Information:
We thank Mark Burnham, Chris Cassidy, Jim McGraw, and the long-term support of the USDA Forest Service in establishing and maintaining research activities at the Fernow Experimental Forest. Funding for this research came from the USDA Forest Service and the National Science Foundation from their Long-Term Research in Environmental Biology program (Grant Nos. DEB-0417678 and DEB-1019522). M. B. Adams conceived, designed, and performed the experiment. C. A. Walter analyzed the data. C. A. Walter and W. T. Peterjohn wrote the manuscript; M. B. Adams and F. S. Gilliam provided analytical and editorial advice.
© 2017 by the Ecological Society of America
- competitive exclusion
- nitrogen deposition
- species diversity
- temperate forest