An experimental evaluation of the roles of dispersal history, mycorrhizal fungi, and nitrogen on grassland assembly: Diversity, plant community composition, and functional community parameters

We have been conducting an experiment in community synthesis where we altered the dispersal history (by planting 9 functional mixtures of forbs and legumes to a background of six c3 and c4 graminoids to yield between 6 and 30 species planted from a pool of 40 species), and nested fungicide application (which reduce MF colonization by 40%) and nitrogen addition (as ammonium nitrate). After three growing seasons, species richness (measured at 246 m2) and species density (measured at 1 m2) were positively correlated and showed similar responses to treatments. Richness increased slightly with forb planting, but not with legume planting. Richness was weakly correlated with the number of species planted (r = .287), but species density was not (r = .136, p = .06). This establishes a significant, but weak effect of dispersal limitation. Richness declined (by about 30%) with both MF suppression and nitrogen addition. Plant community composition was also significantly affected by history, MF, and nitrogen. In general, target species were only abundant in control plots, while MF suppression led to dominance by Poa pratensis, and nitrogen addition often led to dominance by Agropyron repens. MF and nitrogen effects were largely orthogonal. Functional leaf traits averaged over the community (i.e. functional community parameters) were also affected by MF and nitrogen. Leaf dry matter content increased while SLA decreased in response to MF suppression and nitrogen addition. In the future, we plan on developing structural equation models to better understand the multiple dependency relationships.