Effects of Seasonal Inundation on Riparian Plant Growth and Reproduction

Timing and duration of flooding influence plant maintenance and reproduction, but the extent to which these factors affect different plant species and functional groups (terrestrial vs. riparian) remains unclear. Our study aimed to determine broad patterns of resilience of these functional groups to seasonal inundation. We conducted an experiment testing four riparian and four terrestrial plant species exposed to three inundation treatments (no inundation, 1 week, 4 weeks) in both spring and summer. In spring, long inundation often reduced the survival and growth of the terrestrial grasses, while riparian species showed high survival and stable growth across treatments. Flowering and seed production in both terrestrial and riparian species were often delayed or reduced by spring inundation. In summer, terrestrial grasses exhibited reduced survival and growth with inundation, while riparian species performed best with short inundation. Flowering in most species began earlier than the summer treatments, making the effects of summer inundation on flowering less clear. However, summer inundation promoted reproduction in the riparian species Juncus amabilis . Seed viability showed no clear patterns with treatments for most species in either season. Our findings suggest that spring inundation mimicking natural flows may support riparian plant communities by reducing terrestrial plant growth. Riparian species were also more resilient to summer inundation than terrestrial grasses, potentially benefiting from brief flooding. In regulated rivers, environmental flows in spring and short events in summer are likely to promote riparian over terrestrial plants.