The Response of Water Willow Justicia americana to Different Water Inundation and Desiccation Regimes

Abstract

American water willow Justicia americana has been planted in reservoirs to provide important littoral habitat for fish because of its ability to form dense stands, spread along shorelines, grow in water up to 1.2 m deep, and withstand harsh conditions. The response of water willow to periods of inundation or desiccation has not previously been quantified and is critical for evaluating its potential success in reservoirs. We tested the inundation response of plants at depths of 0.75, 1.50, and 2.25 m for 2, 4, 6, and 8 weeks. Response to desiccation was investigated using drying durations of 2, 4, 6, and 8 weeks. In addition, 2- and 4-week drying periods were tested separately in June, July, and August. Number of leaves, total height, and total dry weight were measured as indicators of plant condition. Condition rapidly declined after inundation for 4 weeks at all experimental depths and plants did not recover by the end of the experiment. A significant decrease in height and increase in leaf number was observed after 8 weeks of desiccation. Condition also declined from June to August during the second desiccation experiment. Overall, water willow appeared to be more resistant to desiccation than to inundation. A 5% overall mortality was observed for the desiccation trials versus a 69% overall mortality from the inundation trials. Even the shortest inundation duration in this study (2 weeks) resulted in mortality of 40% or more across all depth treatments and was probably due to light limitation. Our findings provide information that can be used to select candidate reservoirs for water willow establishment based on expected water-level fluctuations. Additionally, this information could be used to manage water levels in reservoirs where water willow currently provides important habitat for fish.