Relaxation of microhabitat restriction through ontogeny of Itea virginica in cypress-tupelo swamps

摘要

Nearly all understory plant species in semi-permanently flooded cypress-tupelo swamp forests grow on microsites located above the water level, as opposed to growing on the flooded forest floor. Our field study indicates that one such species, Itea virginica (Saxifragaceae), has a strong association with elevated microsites that changes with life stage: adults are found significantly more often on the forest floor than seedlings, although not as frequently as expected for the amount of forest floor vs. elevated microsites present. High water level may inhibit seedling establishment, but not adult growth, thus suggesting a potential ontogenetic expansion of flooding tolerance. We conducted greenhouse flooding experiments on seedlings and adults of various sizes with three treatments: high flood (a percentage of aboveground biomass submerged), waterlogged (only root systems submerged), and control (no flooding). The effect of initial height above water and treatment on relative growth rate was significant for the seedlings: RGR decreased with increasing flood level; however, there was no significant effect for adults. Seedlings were more negatively affected by flooding than adults due primarily to their small size rather than any differences in ability to produce flood responses (i.e., enlarged lenticels or adventitious roots). Additionally, we conducted seed experiments investigating the effects of different substrates and durations of flooding on germination success; there were no significant effects of either factor on seed germination. This study increases our understanding of regeneration biology and can be valuable in planning restoration activities in cypress-tupelo swamps.