Corrections for rooting volume and plant size reveal negative effects of neighbour presence on root allocation in pea

Plants are able to detect the presence of their neighbours belowground. The associated root responses may affect plant performance, plant-plant interactions and community dynamics, but the extent and direction of these responses is heavily debated. Some studies suggest that plants will over-proliferate roots in response to neighbours at the expense of reproduction, which was framed as a “tragedy of the commons”. Others proposed an “ideal free distribution” hypothesis, stating that plants produce roots simply as a function of the amount of available nutrients. However, experimental evidence for either hypothesis that is unbiased by confounding effects of rooting volume and plant size in their experimental setups is still lacking. We grew split-root pea plants in the presence or absence of a belowground neighbour at a range of rooting volumes, while providing equal amounts of nutrients per plant. Path analyses were used to disentangle the direct effects of neighbour presence on allocation...