Extreme flooding events on the Rhine and the survival and distribution of riparian plant species

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Abstract
Summary: Summer floods whose severity is affected by flooding duration, submergence depth and underwater light availability, have a large impact on the zonation of riparian plant species. We analysed the range and variability of these flooding components in the River Rhine and quantified their effects on the ability of Arrhenatherum elatius, Achillea millefolium, Rumex acetosa and Rumex crispus, to survive periods of submergence under experimental conditions. Survival characteristics were used to model species’ lower distribution boundaries for extreme and average floods and were compared with the current field distribution. Different light conditions were simulated by implementing three scenarios of suspended load. Extreme deep Rhine floods are characterized by very low median light transmission levels (i.e. below 0.5%). The largest survival responses in the experiment were observed at such low levels (0.4–3.5 µmol m−2 s−1). Strong effects of light were found in R. crispus and A. millefolium, but responses were weaker in A. elatius and R. acetosa. Submergence depth also affected survival, but not as strongly as light. For the flood intolerant species (A. millefolium and A. elatius) the average flood was predicted to have little effect on field distributions under normal light conditions. However, their actual field distributions in 2000 corresponded to the predicted lower boundaries in the extreme years. This suggests that extreme years determine the distributions of these species for many years. The suspended load scenarios significantly modified the predicted lower boundaries in both extreme and average years, implying that plant lower distribution limits may be significantly shifted upwards or downwards depending on the suspended load of the river system. The predicted lower boundaries of the intermediately tolerant R. acetosa and the highly tolerant R. crispus for both extreme and average years were below the actual field distribution in 2000. This suggests that their current distribution is only partly influenced by major flood disturbances and that other factors, either proximate or historical, may play a prominent role.