Evaluating the response of Sagittaria lancifolia to combined inundation and nitrogen addition using a marsh organ

Freshwater diversion may be implemented to restore coastal wetlands. With more freshwater coming into the estuarine systems, salinity decreases while nutrients and inundation increase. With salinity decreases, brackish marshes may transition to freshwater marshes or open water over time. However, research is generally lacking on how freshwater marsh plants can adapt to fluctuations of water levels, extended flooding, and elevated nutrients. Here we investigated the response of a dominant southeastern freshwater marsh species-Sagittaria lancifolia to inundation and nutrient addition. We built a marsh organ with six rows at different heights that underwent varying inundation durations. Each row had eight replicates with four randomly chosen for nitrogen fertilizer addition. We found that the end-of-season belowground biomass reached the maximum at around 85% of inundation while aboveground biomass increased with inundation, showing strong tolerance of inundation for this species. Fertilization did not impact biomass. However, the interaction between fertilization and percent inundation time was important in predicting leaf traits, including count, length, and width. Fertilized vegetation showed an increase of leaf count with inundation while control vegetation showed a decrease of leaf count with inundation. While both control and fertilized vegetation showed increase in leaf length and width with inundation, the increasing rate was greater in the fertilized vegetation, showing that fertilization helped leaves adapt to higher inundation more effectively. On the other hand, control vegetation showed a higher probability of flowering as the season continued, while fertilization reduced the likelihood that plants would produce flowers as the time went on, demonstrating the trade-offs that fertilization had on vegetation traits. This research will enhance our mechanistic understanding of how large-scale restoration activities that alter the biophysical environment could impact marsh vegetation.