Interactions between seed source, planting arrangement, and soil treatment in determining plant size and root allocation in Phlox drummondii.

Two varieties of the annual plant Phlox drummondii were grown in pots in the greenhouse. Pots differed in the mixture of varieties, total number of plants, spatial clustering of plants, and soil treatment.The shoot biomass of centrally located target plants was significantly influenced by the varietal composition within a pot, and the pattern of this genetic effect varied with both plant density and soil treatment. Density and soil treatment interacted strongly, with the negative effect of neighbors on shoot growth being significantly greater when soil nutrient concentrations were lower.The varieties differed significantly in their relative allocation of biomass to roots. Relative root allocation was increased in response to both the presence of neighbors and a decrease in soil nutrient availability. These factors did not interact significantly in their effects.Although both plant size and relative allocation to roots were influenced by varietal composition, these genetic effects were small relative to the effects of density and soil treatment.The strong interactions between plant location, plant identity, and soil treatment suggest that relations between neighbors will be very complex in natural populations occupying microgeographically heterogeneous habitats. A more complete understanding of these interactions will be required before realistic dynamic models of natural populations can be constructed.