CAN TREE SEEDLINGS SURVIVE INCREASED FLOOD LEVELS OF RIVERS

Flood tolerance and the avoidance of severe flood conditions in tree s eedlings were examined experimentally to elucidate tree zonation along the Rhine and determine the effect of higher floods. In comparison wi th seedlings of Acer campestre from only incidentally flooded forest, seedlings of Fraxinus excelsior, Quercus robur and Ulmus minor from ha rdwood floodplain forest and Alnus glutinosa and Populus nigra from so ftwood floodplain forest showed a high tolerance to partial submergenc e. This partly correlated with morphogenetic adaptations, e.g. adventi tious 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 s trongly reduced. Seedlings of trees from softwood floodplain forest we re able to endure longer periods of total submergence than those of ha rdwood floodplain forest. Because of their relative slow extension gro wth rates, seedlings of species from hardwood floodplain forest cannot avoid being totally submerged when flooded on low-lying sites. Theref ore, the higher flood levels of the Rhine strongly reduce the likeliho od of tree species from hardwood floodplain forests establishing on lo w-lying sites along this river.