Moving beyond growth form: Predicting species responses to nutrient addition

Background/Question/Methods A number of experiments have been conducted in grasslands, tundra, and marshes to determine how these communities will respond to predicted global increases in nitrogen and other limiting factors.  These studies have consistently found that above ground production increases and species richness declines in response to fertilization. The generality of this response regardless of initial productivity, challenges our ability to determine what species traits may be consistently favored by increased nitrogen deposition and soil fertility. Species with a clonal growth form are common in these communities, and several studies have identified the presence of species with clonal growth forms as important in determining the decline in species richness in high fertility sites. Using a dataset of long-term fertilization experiments conducted in herbaceous dominated plant communities across North America, we tested how species differences in height and clonal growth form together predict responses to fertilization. Previous analyses of these data found that neither height nor clonality alone accounted for species loss in response to fertilization. Combining these two traits allowed us to evaluate how a composite trait related to species abundance under enhanced fertility and productivity.

Results/Conclusions  We found that across multiple communities, species with a tall, rhizomatous or runner growth form responded positively to increased nitrogen while those with a tall, clumped or basal growth form were more likely to decrease in relative abundance. Interestingly, although these communities generally had several species with each growth form, typically only one species within each height/clonal growth form group was the significant responder regardless of species richness of that group. The ability of a particular species to compete for light both vertically, by being taller than other species, and horizontally, by having a greater capacity to expand across space more than other species, appears to confer a competitive advantage under enhanced fertility in many grassland communities. Classifying species on the basis of two traits that are simple to measure (height and clonal growth form) increased our ability to predict species response to fertilization better than with each trait alone.  We also explored if species relative abundance was important in determining if it came to dominate in response to increased soil resources and found that this varied across sites. In some sites, it took longer for a strong responder to emerge and reduce diversity in the fertilized sites.