Soil moisture influences the root characteristics of a herbaceous riparian plant along a regulated river

River ecosystems of regulated rivers are threatened by water extraction and flow regime alteration in the context of climate change and increasing human populations. Riparian plant root growth is important to sustain plant health and provide functions including bank stabilization. The root systems of riparian plants on regulated rivers may suffer from lower soil moisture due to lack of natural flow variability. This study aimed to evaluate how soil moisture influences the root system of a herbaceous riparian plant. Plants of Juncus amabilis were dug out along a soil moisture gradient, corresponding with positions close to or distant from the water margin and low or high relative bank elevation. Root depth, belowground space occupation, root mass fraction, and mean fractal dimension were used to evaluate root structural dynamics in relation to bank position and soil moisture. The ratio between root and aboveground dry weights of sampled plants was constant over the elevation range sampled. Plant root systems tended to grow deeper, occupy more belowground space, and have fewer branches as soil moisture declined. These findings indicate that lower soil moisture levels and reduced river flows may significantly influence herbaceous riparian plant growth and survival. Riparian plant health and function will likely be promoted by flow regimes that provide adequate and timely water delivery.