Effect of deep flooding on nutrients and non-structural carbohydrates of mature Typha domingensis and its post-flooding recovery

Vegetation health is a critical factor for the sustainable performance of constructed treatment wetlands. Biomass, growth, nitrogen (N), phosphorus (P), and total non-structural carbohydrates (TNC) of Typha domingensis (southern cattail) were examined in response to a six-week flooding period at three depths: 40 (control), 91, and 137 cm above the soil surface, and a subsequent four-week recovery period at a flooding depth of 40 cm. New shoot, root, rhizome, belowground and total biomass, relative growth rate (RGR), and belowground TNC storage of plants significantly decreased with increasing flooding depth. Plants maintained at a 137-cm flooding depth had higher leaf TNC concentration and storage than those growing in the 40- and 91-cm depth treatments. Shootbase total nitrogen (TN) and root total phosphorus (TP) concentrations differed significantly among the three flooding depth treatments, but flooding depth did not significantly affect leaf TN and TP concentrations or N/P ratio. At the end of the four-week recovery period, new shoot, leaf, rhizome, shootbase, total biomass, and RGR were lower in the 137-cm depth treatment than in both the 40- and 91-cm depth treatments, but there were no differences for these variables between the 40- and 91-cm depth treatments. Root biomass was lower in the 91- and 137-cm depth treatments than in the control treatment. Results indicate that impacts in terms of total biomass and RGR reduction, caused by the 91-cm flooding depth, were reversed during the recovery period, while the impacts of the 137-cm depth on biomass and RGR were not reversed. Root and belowground biomass reduction caused by both the 91- and 137-cm flooding treatments were not reversed during the recovery period. At the end of the recovery period, the difference in leaf TNC concentration among plants from the three flooding depth treatments was small, indicating that previously accumulated photosynthetic products in leaves may have been transported to roots, rhizomes, and shootbase for plant recovery. In contrast, the TNC concentration and storage in belowground organs of plants growing in the 91- and/or 137-cm depth treatments were lower than those of the control plants. Therefore, flooding to a 137-cm depth was detrimental to the growth, nutrient uptake, and photosynthate transport of T. domingensis. Establishing flooding depth and duration thresholds for management of T. domingensis-dominated marshes is necessary to minimize deep water impacts to vegetation and thereby maintain a sustainable treatment wetland. In the design of stormwater treatment wetlands in areas influenced by monsoonal climate, it is important to consider using a water regulation tool, such as an upstream reservoir, to store excessive stormwater runoff from the watershed and to avoid hydraulically overloading the wetlands during heavy rainfall events.