Phragmites australis Reproductive Fitness, Productivity, and Morphology Along a Salinity Gradient in the Anahuac National Wildlife Refuge

Abstract

Anthropogenic changes to freshwater inflow into estuarine environments may have detrimental consequences, as changing salinity regimes may cause some species to proliferate at the expense of others. One such species of management concern is Phragmites australis (common reed) in the Anahuac National Wildlife Refuge (ANWR, Texas, USA). This grass can potentially invade lower salinity portions of estuaries following disturbances such as hydrological alteration or nutrient input. In the ANWR, the recent (2019) closure of a tidal inlet (Rollover Pass) as part of a hydrological restoration project resulted in decreased tidal input and lowered salinity in the ANWR. As a result, Phragmites management may become more challenging within the refuge. To assess the potential for Phragmites proliferation, a field survey was conducted at three sites along a salinity gradient within the ANWR. Soil salinity was on average three to four times higher at the two downstream sites (Site 2 and Site 3) than at the site furthest upstream (Site 1). Phragmites stem density and height were similar at all sites in the spring, but plants were 21-27% taller at the inland site in the fall (Site 1). However, plants at Site 1 (in the spring) and Site 2 (in the fall) contained 9% and 11% more chlorophyll a content in their leaves compared to Phragmites at the highest salinity site, respectively (Site 3). Phragmites inflorescences were 32.5% longer at the lowest salinity (16 ± 5.51 cm) in comparison to those at Site 2 (10.8 ± 1.54 cm) and 48% longer than those at Site 3 (8.25 ± 3.41 cm). Inflorescences at Site 1 had 35% more seeds (201 ± 181 cm) than those at Site 2 (68.8 ± 34.7 cm). Seeds at the lowest salinity site were the only seeds to germinate, but there was no significant difference in germination rate amongst the sites. Although there is some indication that Phragmites may have higher fitness at the lowest salinity site, stands of this species persisted at all three sites. In addition, soil salinities remained > 20 ppt at downstream sites for more than two years following the hydrological restoration project. These results suggest that the tidal input into this estuarine environment was adequate to maintain the salinity levels that may be needed to prevent the rapid near-term proliferation of Phragmites within the reserve.