Belowground plant competition: Uncoupling root response strategies of peas

Belowground plant competition was shown to induce varying responses, from increases to decreases in root biomass allocation or in directional root placement. Such inconsistencies could result from the fact that root allocation and directional growth were seldom studied together, even though they might represent different strategies. Moreover, variations in belowground responses might be due to different size hierarchies between plants, but this hypothesis was not studied previously. In a greenhouse rhizobox experiment, we examined the way both root allocation and directional root placement of Pisum sativum are affected by the size and density of Festuca glauca neighbors, and by nutrient distribution. We found that root allocation of P. sativum increased with the density and size of F. glauca. In contrast, directional root placement was unaffected by neighbor size and either increased toward or away from neighbors when nutrients were patchily or uniformly distributed, respectively. These results demonstrate that directional root placement under competition is contingent on the distribution of soil resources. Interestingly, our results suggest that root allocation and directional placement might be uncoupled strategies that simultaneously provide stress tolerance and spatial responsiveness to neighbors, thus highlighting the importance of measuring both when studying belowground plant competition.