Predicting effects of altered tropical storm regimes on recruitment of native and invasive species along the coastal transition ecosystem at Grand Bay, Mississippi, USA

Background/Question/Methods

Climate change-driven shifts in disturbance regimes may provide opportunities for the proliferation of non-native species. Coastal ecosystems along the northern Gulf of Mexico are predicted to experience intensification of tropical storms and sea level rise during this century. The coupling of these two factors should result in saltwater intrusion, thereby increasing salinity stress at locations that are further inland along the coastal transition. These environmental changes may create opportunities that bolster regeneration of aggressive exotics that invade readily in disturbed or extreme conditions. Triadica sebifera is a highly invasive species that is moderately salt-tolerant and thrives in many coastal plant communities throughout the southeastern US. We hypothesized that intensification of tropical storms and associated storm surge regimes will favor recruitment of Triadica compared to native species. We measured germination of invasive Triadica and native species, Baccharis halimifolia and Morella cerifera in growth chamber and greenhouse experiments. Percent germination of these species was compared across a range of salinities (0, 10, 20, and 30 ppt) in both experiments and different soil types as well in the greenhouse study. Soil substrates were collected from five habitats spanning the coastal transition at Grand Bay National Estuarine Research Reserve in coastal Mississippi.

Results/Conclusions

In the growth chamber, germination of all species significantly declined at high salinity levels.   In the greenhouse experiment, there was a significant interaction between salinity and soil type. In all soil types and for all species, germination was significantly reduced at 20 and 30 ppt; however, there were some species-specific differences. All species had reduced germination in soils from the most seaward locations, even at lower salinity treatments. Compared to other species, Triadica had higher germination at 30 ppt in soil from the most inland locations, indicating that storm surges may favor its initial establishment at inland sites. The seeds of Baccharis at 0 ppt hade the highest germination across all soil types, which suggests that this species can proliferate under less saline conditions typical of more inland habitat. These results suggest that salinity is a limiting factor influencing recruitment of invasive and native species and that Triadica appears to be capable of establishing under saline conditions. We predict that with sea level rise and intensification of tropical storms, native and invasive will have different responses to increasing salinity stress. The consequences of these differential responses are likely future reorganization and restructuring of coastal plant communities.