Water depletion can generate contrasting root allocation responses to belowground competition

Plants competing belowground may produce extra roots, fewer roots, or no detectable change compared to plants growing alone. This inconsistency is often attributed to plants altering their root allocation in response to diverse cues, including neighbor detection and resource depletion by neighbors, but isolating these cues experimentally is challenging. Here, we hypothesize that water depletion alone can explain the range of root allocation strategies reported in the literature. We present this hypothesis as a water-explicit optimization model of root allocation that predicts a non-monotonic response. The model identified a critical depletion rate at which allocation shifted from increasing to decreasing with depletion. We tested this prediction using artificially rooted pots that imposed controlled water depletion while excluding living neighbors and their cues. A continuous artificial depletion gradient revealed the predicted hump-shaped pattern. These results reframe root overproliferation and underproliferation as positions along a single depletion-response curve.