Fine root responses to temporal nutrient heterogeneity and competition in seedlings of two tree species with different rooting strategies.

There is little direct evidence for effects of soil heterogeneity and root plasticity on the competitive interactions among plants. In this study, we experimentally examined the impacts of temporal nutrient heterogeneity on root growth and interactions between two plant species with very different rooting strategies: (sweet gum), which shows high root plasticity in response to soil nutrient heterogeneity, and (loblolly pine), a species with less plastic roots. Seedlings of the two species were grown in sandboxes in inter- and intraspecific combinations. Nutrients were applied in a patch either in a stable (slow-release) or in a variable (pulse) manner. Plant aboveground biomass, fine root mass, root allocation between nutrient patch and outside the patch, and root vertical distribution were measured. grew more aboveground (40% and 27% in stable and variable nutrient treatment, respectively) and fine roots (41% and 8% in stable and variable nutrient treatment, respectively) when competing with than when competing with a conspecific individual, but the growth of was not changed by competition from . Temporal variation in patch nutrient level had little effect on the species' competitive interactions. The more flexible changed its vertical distribution of fine roots in response to competition from , growing more roots in deeper soil layers compared to its roots in conspecific competition, leading to niche differentiation between the species, while the fine root distribution of remained unchanged across all treatments. . showed greater flexibility in root growth by changing its root vertical distribution and occupying space of not occupied by . This flexibility gave an advantage in interspecific competition.