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

1. Plants are able to detect the presence of their neighbours below-ground. 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. 2. 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 propose 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 set-ups is still lacking. 3. We grew split-root pea plants in the presence or absence of a below-ground 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 patterns from the confounding effects of rooting volume and plant size. 4. Within the chosen range of rooting volumes, the presence of a below-ground neighbour generally reduced plant root mass by 21% and total mass by 9%. A doubling of rooting volume generally increased plant root mass by 22% and total mass by 11%. Pod mass was only directly and positively correlated with vegetative mass. 5. The presence of a below-ground neighbour induced less root allocation but more pod allocation, whereas increased rooting volume caused a reduction in reproductive allocation. A large part of these effects, however, was indirectly mediated through the influence on plant total mass. 6. Synthesis. Not considering the effects of rooting volume and plant size may lead to misinterpretations of plant growth strategies in response to neighbours. Accounting for these factors, we found pea allocating less mass to roots in the presence of a below-ground neighbour. The obtained results can help to reconcile the various responses to below-ground neighbours as they are published in the literature.