Background/Question/Methods
The vegetation structure of floodplains is determined not only by physical disturbance but also by submergence stresses during flooding. However, the extent to which submergence stress influences the vegetation structure is not known. We investigated the response and tolerance of the seedlings of 2 Salicaceae species (Salix chaenomeloides and S. pierotii) and 3 Ulmaceae species (Celtis sinensis var. japonica, Aphananthe aspera and Ulmus parvifolia) to complete submergence and examined the effect of this condition on seedling growth. Salicaceae and Ulmaceae species are widely distributed in floodplains in the warm temperate regions of Japan. The former are mainly found in the lower areas of floodplains that experience frequent flooding, whereas the latter grow in the higher areas. The seedlings of these species were placed in a tank filled with tap water and submerged completely for 4 periods: 1, 2, 4 and 8 weeks.

Results/Conclusions
Based on the results of the experiment, it appears that the order of tolerance to complete submergence is related to the actual habitats of these species; that is, the highest tolerance was exhibited by S. chaenomeloides and S. pierotii, followed by C. sinensis var. japonica and U. parvifolia, and finally A. aspera. All the seedlings of S. chaenomeloides and S. pierotii retained their potential for shoot elongation and leaf production. The survival rate of the 1- to 4-week-submerged seedlings of C. sinensis var. japonica was more than 75%, and there was hardly any decrease in their biomass; however, all the 8-week-submerged seedlings died. Shoot elongation after the submergence treatment was considerably suppressed in the 8-week-submerged seedlings of U. parvifolia, with 50% of these seedlings dying. Although 80% of the 4-week-submerged seedlings survived, their biomass was statistically lower than that of the 1- and 2-week-submerged seedlings. The stress of submergence remarkably suppressed shoot elongation and biomass increase in the seedlings of A. aspera. Further, prolonged submergence noticeably decreased the survival rate of the A. aspera seedlings. These results may explain why these Ulmaceae species are rarely found in the lower areas of floodplains where flooding is frequent.