Plant Root Architectural Traits Mediate a Trade-Off Between Suppression and Tolerance of Competitors

Plants' competitive ability involves both suppressing the growth of neighbours (competitive effect) and resisting or tolerating their suppression (competitive response). Competition for below-ground resources must be related to the ability of plants to acquire these resources, which is mediated by roots and their morphology. However, the role of root architecture in the competitive ability of plants, and in the possible trade-offs among growth potential, competitive suppression and competition tolerance involved, has not been extensively studied. We used a functional-structural root model coupled with an evolutionary algorithm to simulate the evolution of root architectures in five scenarios with different plant densities. We asked (1) does selection under different intraspecific competition scenarios result in different root architectures? and (2) do differences in these architectures result in differences in growth potential and competitive ability, that is, competitive effect and response? Our results indicate that as the number of neighbours increases, selection on traits such as branching angles, gravitropism and branching probability results in root architectures that are deeper and sparser, resulting in lower shoot biomass. We also found a difference in competitive ability among architectures, with a trade-off between resistance to competition on one hand, and competitive effect and maximum productivity (maximum shoot biomass) on the other: there is not a globally optimal strategy. Our findings have implications for management of invasive species, improvement of crop yield and the study of species co-existence.