Linkages between patterns of community assembly and interactions in an experimental grassland

In a three year old 4.5 ha experimental grassland, we investigated patterns of community assembly in terms of species checkerboards and in terms of trait dispersion. Both the addition of nitrogen and the suppression of mycorrhizal fungi (by about 40%) not only altered community composition, but also brought about novel patterns of community assembly. The control communities did not have significant checkerboard scores (p = .252), while communities with added nitrogen had marginally significant checkerboard scores (p = .030) and communities with MF suppression had highly significant checkerboard scores (p < .001). Within the experiment, we found that MF suppression increased competition intensity by two- to four-fold. These results establish a linkage between increased competition and the expression of community assembly rules. We also found significant alterations in trait-based assembly patterns. The mean Nearest-Neighbor Distance in leaf dry matter content doubled as a result of MF suppression, in accordance with theory. Although neither control or MF suppressed communities were significantly under-dispersed (p = .099, .660 respectively), increased competition intensity moved the communities away from under-dispersion. The range in leaf DMC was moderately smaller than expected by chance (p = .035) in control communities, but not in MF suppressed communities. MF suppression did not change the range in DMC and this may explain the loss of species richness in MF suppressed communities. Other traits will also be discussed.