The effects of flooding regime on the rare Atlantic coastal plain species Hydrocoytle umbellata

Abstract

The hydrological regime on many lakes is changing through climate change, construction of dams, and land-use changes. We examined the effects of flooding regime on growth and development of the endangered semiaquatic species, Hydrocotyle umbellata L. (water pennywort), in a greenhouse experiment. Growth was twice as fast under dry, above-water (terrestrial) conditions as under flood conditions. Depth of flooding (15 or 30 cm) had little impact. Short-term floods at either the middle or end of the growing season were just as damaging as continuous flooding. The decrease in growth under flood conditions was largely due to a temporary reduction in unit leaf rate (growth per unit leaf area) rather than to a reduction in leaf area ratio (leaf area per unit mass). Flooded plants allocated more resources to the production of petioles at the expense of reproductive structures and to a lesser extent, belowground structures. They also produced fewer, larger leaves with more stomata on the upper than on the lower surface. These changes in structure were largely restricted to new leaves produced under flood conditions; most of the mature leaves produced in a terrestrial environment senesced upon flooding. Chemical analysis of tubers produced by plants grown under terrestrial conditions indicates that these structures function in carbohydrate storage. The likely impact of changes in hydrological regime on this endangered species is discussed in light of these findings.