Abstract
Low impact development (LID) design focused on minimizing runoff and improving effluent quality has led to an increase in LID features in built environments, such as rain gardens, bioswales, and constructed wetlands. These features require plants tolerant to various periods of flooding or saturated soils. Flooding can be chronic with freestanding water, chronic with long-term soil saturation but no freestanding water, or recurrent flooding with intermittent periods of drought. A system of screening using 12 replicated, floodable cells 1.2 m wide by 2.4 m long by 0.4 m deep equipped with liners and filled with 15 cm of sandy loam soil were engineered to mimic three types of flooding conditions and a well-drained control treatment. The well-drained control treatment was irrigated as needed to maintain ≥ –15 kPa soil moisture tension. Intermittent flooding consisted of inundating the cells weekly to the soil surface for 3 days and then allowing drainage. A bog or permanently saturated treatment was imposed by saturating the soil using float valves that maintained the subsurface water level even with the soil surface, but with no freestanding water. A chronically flooded treatment was maintained by use of float valves to achieve ∼7.6 cm of freestanding water above the soil surface. These cells were used to conduct two separate experiments with
Taxodium distichum
(L.) Rich., a species known to adapt to a wide range of flood conditions. Expt. 1 tested replicates of four different half-sib seedling populations and two cutting-propagated clonal selections for survivability, plant growth, and dry masses produced in the treatments. In general, seedling-propagated plants grew larger and survived at a greater percentage than cutting-propagated plants. In Expt. 2, replicates of three different genotypes were used as model taxa for testing genotypic responses. Even for this flood-adapted species, trunk diameter and root, shoot, and whole-plant biomasses were reduced in all three flood treatments relative to well-drained soil conditions. Survival was not statistically reduced by intermittent flooding, but was by chronic, boglike soil conditions and free surface-water flooding. Reduced oxygen concentrations were documented in flooded soils, particularly during the late summer. Soil temperatures in the control plots were ∼5 °C greater in late spring than those in all three flood treatments, whereas in late summer, the control and intermittent-flooded plots were similar and ∼2 °C warmer than the bog treatment, which was ∼2 °C warmer than the standing-water treatment. Soils were 9 to 13 °C warmer in late summer than in late spring.