Can tree seedlings survive increased flood levels of rivers?

Flood tolerance and the avoidance of severe flood conditions in tree seedlings were examined experimentally to elucidate tree zonation along the Rhine and determine the effect of higher floods. In comparison with seedlings of Acer campestre from only incidentally flooded forest, seedlings of Fraxinus excelsior, Quercus robur and Ulmus minor from hardwood floodplain forest and Alnus glutinosa and Populus nigra from softwood floodplain forest showed a high tolerance to partial submergence. This partly correlated with morphogenetic adaptations, e.g. adventitious rooting and hypertrophy of stems and lenticels. Seedlings of all species were much more sensitive to total submergence than to partial submergence, especially when light transmission during flooding was strongly reduced. Seedlings of trees from softwood floodplain forest were able to endure longer periods of total submergence than those of hardwood floodplain forest. Because of their relative slow extension growth rates, seedlings of species from hardwood floodplain forest cannot avoid being totally submerged when flooded on lowlying sites. Therefore, the higher flood levels of the Rhine strongly reduce the likelihood of tree species from hardwood floodplain forests establishing on low-lying sites along this river.