Does Pattern of soil resource heterogeneity determine plant community structure? An experimental investigation

1. Spatial and temporal pattern of nutrient delivery to individual plants and plant populations can affect growth and allocation of biomass to roots and shoots. We predicted that it would also affect attributes of plant community structure, including species composition, diversity, and partitioning of biomass between above- and below-ground parts. We tested these predictions experimentally by providing the same quantity of nutrients in five different patterns to sown plant communities grown under field conditions for 2 years. We used orthogonal contrasts to compare the effects on community structure of homogeneous vs. heterogeneous nutrient supply, supply of nutrients in patches at two different densities, and provision of nutrients in spatially predictable vs. unpredictable patches. We harvested above- and below-ground plant biomass, and measured species richness, diversity, and species population sizes. 2. Pattern of nutrient delivery significantly affected community biomass and below-ground : above-ground biomass partitioning. Treatments with the lowest density of nutrient-rich patches supported 44% more total biomass than homogeneous treatments, largely as a result of greater (71% more) below-ground biomass, which resulted in a 39% increase in community below-ground : above-ground biomass ratio. 3. Above-ground community composition was also affected by treatment. There were several instances in which biomass, percentage cover or population sizes of different species, were affected by treatment. There were few significant differences between communities in treatments with predictable and unpredictable nutrient patches. Treatment did not affect species richness or diversity, despite high power to detect small differences. 4. We conclude that the spatial and temporal pattern of nutrient supply strongly affects some important facets of plant community structure, but has less influence on others. Further understanding of community responses to the pattern of nutrient supply will require experiments testing the responses of individual species to heterogeneity with and without competitors present.