River regulation has altered the seasonal timing of flows in many rivers worldwide, impacting the survival and growth of riparian plants. In south-eastern Australia, demand for irrigation water in summer often results in high river flows during a season that would naturally experience low flows. Although unseasonal high summer flows are thought to significantly impact waterways, their effects on vegetation are poorly quantified. We investigated the responses of five grass species commonly occurring in riparian zones to different durations of submergence in summer. We experimentally tested the response of three exotic and two native grasses to four submergence treatments (4 weeks, 8 weeks, 2-week pulses and no submergence), and two levels of shading (no shading and 80% light reduction), over 8 weeks in summer and early autumn. All submergence treatments, including the 2-week pulse, resulted in the death of all plants of three species (Bromus catharticus, Dactylis glomerata and Rytidosperma caespitosum). Lolium perenne exhibited moderate survival rates in the shorter-duration unshaded submergence treatments, while Poa labillardierei largely survived all treatments. Similar responses across species were observed for plant height and biomass, although height generally increased while biomass growth was reduced by shading. These results show that even 2-week periods of summer submergence can reduce growth and cause the death of some riparian grasses. Although some species may survive longer submergence durations, impacts on other aspects of fitness, and ongoing effects of repeated unseasonal submergence, remain uncertain. Our study highlights that the impacts of unseasonal flows require further investigation and careful management.

河流调节改变了世界上许多河流的水流季节，影响了河岸植物的生存和生长。在澳大利亚东南部，夏季对灌溉水的需求通常会导致在自然会经历低流量的季节中河流流量高。尽管人们认为夏季非季节性的高流量会严重影响水道，但其对植被的影响尚无法量化。我们调查了夏季沿岸地区常见的5种草种对不同浸水持续时间的响应。我们通过实验测试了三种奇异草和两种原生草对四种浸水处理（4周，8周，2周脉冲和无浸水）以及在8周内的两种阴影水平（无阴影和80％减光）的响应在夏季和初秋。大扁雀麦，鸭茅和Rytidosperma caespitosum）。黑麦草在较短持续时间的无阴影浸没处理中表现出中等的存活率，而Poa labillardierei在所有治疗中都幸存下来。尽管植物的高度和生物量总体上观察到了相似的响应，但是通过遮荫，植物的高度和生物量通常会增加，而生物量的增长会减少。这些结果表明，即使是夏季浸水的2周时间，也会减缓生长并导致一些河岸草死亡。尽管某些物种可能会在更长的淹没时间中生存，但对适应性其他方面的影响以及反复的非季节性淹没的持续影响仍然不确定。我们的研究强调，非季节性流动的影响需要进一步调查和仔细管理。