Effects of nutrient distribution pattern and aboveground competition on size of individuals in Ipomoea tricolor populations

We investigated whether the distribution pattern of soil nutrients interacted with aboveground competition to affect plant size in Ipomoea tricolor Cav. populations. Six plants per pot were grown in a factorial experiment with combinations of heterogeneous or homogeneous nutrient distribution pattern and presence or absence of aboveground competition. Plants were harvested and ranked by their aboveground biomasses. In analyses of plant sizes in all ranks simultaneously, mean plant size was significantly affected by nutrient distribution pattern, aboveground competition, and their interaction. In analyses of plant sizes of each rank, aboveground competition affected plant size, which was found in all ranks. Nutrient distribution pattern affected plant size in the higher ranks, but not in the lowest rank. Selective root placement into nutrient-rich patches under heterogeneous conditions was observed. Our results suggest that the magnitude of the effect of nutrient distribution pattern on plant size changed among ranks owing to changes in aboveground competition and size-dependent growth rate. Size-dependent growth rate could explain the significant effect of nutrient distribution pattern independent of the effect of aboveground competition. Nutrient distribution pattern would then interact with aboveground competition and consequently affect size structure in a population.