How flood and low flow duration influences early instream vegetation community development: implications for restoration

Abstract Flow regime is a key driver of instream vegetation communities and is often altered through regulation or stormwater runoff. These altered flow regimes influence flooding and low flows, which are important for instream plant recruitment from the propagule bank. Using a mesocosm experiment, we investigated the effects of eight hydrologic treatments comprising two flood durations (1, 7 days) and three low flow durations (7, 14 or 21 days), or permanently moist or permanently flooded conditions, on early instream plant community development from the propagule bank and emergence of sown seeds from six instream species. The hydrologic treatments reflected conditions typical in southeast Australian lowland streams. Permanent flooding resulted in less recruitment overall and different seedling assemblages than the other treatments with similar patterns across water plant functional groups. More native species emerged than exotic species, with native species responding best to longer flood durations (seven days) and intermediate low flow durations (14 days). Our results indicate that low flow periods are critical for recruitment from the propagule bank, however, once emerged, instream species may be able to withstand a range of flow conditions. Provision of greater geomorphic complexity promoting shallow zones for colonisation and pioneer plant communities may promote instream vegetation.