Response variations of Alnus subcordata (L.), Populus deltoides (Bartr. ex Marsh.), and Taxodium distichum (L.) seedlings to flooding stress.

Alnus subcordata is a native species distributed along bottomlands of Hyrcanian forests of northern Iran. In the last decade, this species along with the exotic species Populus deltoides and Taxodium distichum, has been widely used also for afforestation of bottomland areas. However, the relative flooding tolerance of these 3 species and their potential mechanisms for coping with flooding conditions are unknown to the present. Thus, in this study, variations in growth and morphophysiological responses to flooding of these species' seedlings were investigated during a 120-d outdoor experiment. Seedlings were subjected to 3 fixed treatments of 1) unflooded, 2) flooded to 3 cm in depth, and 3) flooded to 15 cm in depth, and their survival, growth, and some metabolic parameters were measured at the end of the experiment. Survival in seedlings of these 3 species was very high, but the root length, biomass accumulation, and chlorophyll content were reduced by flooding. Diameter growth in T. distichum increased with flooding, while it was negatively affected in the other 2 species. The leaf area, specific leaf area, and height growth were reduced in A. subcordata and P. deltoides by flooding, while no significant effect on these parameters was observed in T. distichum. Flooding also induced the formation of hypertrophied lenticels, and adventitious roots in all 3 species. But, proline concentrations of roots and leaves of all species did not vary with flooding. Generally, our results clearly indicated that the 3 species assayed in this experiment could be used for afforestation of riparian and floodplain regions of Hyrcanian forests, since survival percentages with 120 d of flooding were very high (＞ 90%).