Effects of irregular flooding on the establishment of tree species

The ability of tree seedlings to build up and maintain a high tolerance of total submergence in their first few years before severe flooding was examined, to elucidate tree zonation along the Rhine and to ascertain the impact of irregular high floods. Tolerance of total submergence was examined experimentally in relation to seedling age, time of the year and a previous period of total submergence. Seedlings of species characteristic of low-lying sites, such as Alnus glutinosa and Populus nigra, increased their tolerance of total submergence with age more and had a higher tolerance in their second or third year than species from higher sites, e.g. Fraxinus excelsior, Quercus robur and Ulmus minor. Degree of tolerance of total submergence was much less in summer than in spring. Species from highlying sites had low tolerance in summer. Tolerance of total submergence was only slightly dependent on a previous period of total submergence earlier in the year. The results indicate that irregular high floods in late spring and summer such as those that occur in the Rhine system strongly reduce the probability of tree seedlings establishing, especially those of species from hardwood floodplain forest.