Is tolerance to summer flooding correlated with distribution patterns in river floodplains? A comparative study of 20 terrestrial grassland species

It is generally assumed that floods during the growing season have a strong impact on the distribution of grassland plant species in river floodplains but this proposition has never been tested. We examined the survival and growth responses of twenty species, originating from mid- and high-level floodplain grasslands along the River Rhine in the Netherlands, to total submergence for a maximum of two months in an outdoor flooding experiment. Plant survival and biomass reduction with flooding duration was determined as well as biomass recovery after de-submergence. Our results indicate that species survival is the most prominent factor correlated with species distribution in floodplain areas. Relatively flood tolerant species occurred mainly at low elevations along the floodplain while more flood sensitive species were restricted to high parts of the floodplain gradient. Biomass reduction rates during submergence were only marginally significantly correlated with species lower distribution boundaries along the flooding gradient. Biomass recovery rate was significantly correlated with species distribution patterns in the field only after 2 weeks of complete submergence, but not after 4 and 8 weeks. Our results suggest that the more flood tolerant species can have various ways to survive and recover from flooding, ranging from low rates of biomass loss and low recovery to relatively high rates of biomass loss and quick recovery. Our results are consistent with the notion that disturbance by floods during the growing season is an important determinant of species lower distribution boundaries in river floodplains. They also suggest that high survival under flooding may be achieved by different physiological mechanisms. Such mechanisms are discussed in this paper.