| Limiting resources and species tradeoffs are hypothesized to be important determinants of plant community composition, yet resource competition theory has been relatively untested in terrestrial plant communities. In this thesis, I use both experimental and observational data from multiple research sites to explore the role of limiting resources in determining patterns of relative species abundance, diversity, and invasion in grassland systems: (1) Species with greater abundance in nitrogen-limited habitats in Minnesota and Kansas, USA were better nitrogen competitors but poorer colonizers as determined by their monoculture traits. We found that community assembly was non-neutral because the identities of rare and dominant species were consistently predicted by species traits across a variety of experimental manipulations. (2) I propose a novel Niche Dimension-Diversity hypothesis: each limiting resource added to a plant community should reduce or eliminate an axis for tradeoff-caused coexistence and thus reduce diversity. Plant diversity was a linear function of the number of limiting resources in observational plots and in an experiment in which we added various combinations of up to four limiting resources in a California grassland system, and in the long term Park Grass, UK experiment. These results provide a potential explanation for the widely observed but poorly understood decrease in diversity following experimentally increased productivity via resource addition. (3) Competition for limiting resources can influence the invasion of exotic species. In a California grassland experiment, dominance of exotic annual plants occurred not because they were superior competitors, as had been assumed. Rather, native perennial species were superior competitors for multiple resources and could invade exotic grassland plots across a range of experimental conditions. (4) Few field studies have tested the hypothesis that species trade off in their abilities to compete for multiple limiting resources. I examined the relationships among California grassland species abundances and the availability of N, P, K, Mg, and light. The most abundant species tended to occur in a different region of multiple resource space suggesting tradeoffs between nutrients and light. Results from resource addition and monoculture experiments supported the observational findings. |
