Riparian vegetation varies along hydrologic gradients, along which inundation and drought tend to be inversely correlated. Differentiating effects of inundation and drought on plant distributions is critical for predicting impacts of changes to baseflows and designing flow patterns to achieve vegetation objectives in regulated river systems. To this end, we conducted a greenhouse experiment where we decreased, increased, or maintained constant water levels experienced by a suite of riparian plant species. We related changes in new root growth and stomatal conductance under experimental conditions to species hydrologic niches in the field, specifically the median elevation at which they occur above the channel, along the regulated Colorado River in Grand Canyon. We found a significant negative relationship between root growth response to experimental inundation with increasing elevation above the channel in the field, and a negative response of stomatal conductance to inundation among the most xeric-adapted species. Drought responses were idiosyncratic with respect to hydrologic niche, and instead seemed to vary in relation to clonality and rooting depth. Several Salicaceae tree species that are uncommon along regulated rivers exhibited consistently negative responses to both drought and inundation relative to other species, which may explain their rarity. The results of this study suggest that riparian plant distributions along hydrologic gradients have been shaped primarily by current and past levels of inundation. However, future anticipated declines in the water table are likely to produce species-specific responses based on drought tolerance that may in part be predicted from the results of this experiment.

河岸植被随水文梯度而变化，洪水和干旱往往呈反比关系。区分洪水和干旱对植物分布的影响对于预测基流变化的影响和设计水流模式以实现受监管河流系统的植被目标至关重要。为此，我们进行了一项温室实验，减少、增加或维持一系列河岸植物物种经历的恒定水位。我们将实验条件下新根生长和气孔导度的变化与田间物种水文生态位相关联，特别是它们出现在大峡谷受管制的科罗拉多河沿线河道上方的中位海拔。我们发现，在最适应干旱的物种中，随着田间水道上方海拔的增加，根系生长对实验洪水的响应与气孔导度对洪水的负响应之间存在显着的负相关关系。干旱反应在水文生态位方面是特殊的，而且似乎在克隆性和根深方面有所不同。与其他物种相比，受管制河流沿岸不常见的几种杨柳科树种对干旱和洪水表现出持续的负面反应，这可能解释了它们的稀有性。这项研究的结果表明，沿水文梯度的河岸植物分布主要是由当前和过去的淹没水平决定的。然而，未来预期的地下水位下降可能会产生基于耐旱性的物种特异性反应，这可以部分地根据本实验的结果进行预测。