Response of native and invasive species to canopy openness and storm surge in different habitats in coastal Mississippi

Background/Question/Methods

Coastal ecosystems along the northern Gulf of Mexico are predicted to experience intensification of tropical storms and sea-level rise during this century. These disturbances usually result in significant changes in forest microclimate which may provide opportunities for the growth and regeneration of exotics that invade readily in disturbed and open habitats. Invasive Triadica sebifera, which is shade and moderately salt tolerant, has proliferated in many southeastern coastal plant communities. We hypothesized that canopy damage and storm surge associated with intense tropical storms would favor the persistence of Triadica compared to natives. Responses of invasive and native species to these disturbances are likely influenced by the interaction of prior site conditions, distance from the coast, and forest structure. We established a greenhouse experiment representing two forest types with contrasting field conditions: Maritime Pine Forest (MPF) and Mixed Hardwood Forest (MHF), common at Grand Bay National Estuarine Research Reserve in coastal Mississippi. We compared seedling mortality of Triadica and two co-occurring native species, Baccharis halimifolia and Morella cerifera, in response to shade and storm surge. Seedlings were planted in two light levels: 70% shade and no-shade for MPF and 90% and 30% shade for MHF and were exposed to no-surge or storm surge treatments. Shade levels represent before and after hurricane wind damage.

Results/Conclusions

Contingency table analysis result indicated that Baccharis mortality was independent of surge but was significantly impacted by shade (p < 0.05) in MPF. However, Morella mortality significantly increased with shading and storm surge (p < 0.05). Triadica mortality was not affected by shade, but mortality increased with storm surge. In MHF, Baccharis and Morella mortality was significantly affected by both increased shade and storm surge treatment (p < 0.05). In contrast, Triadica mortality was not affected by shade. However, its mortality was significantly affected by storm surge (p < 0.05). Overall, Baccharis and Triadica can withstand storm surge and shade. However, Morella may be impacted by both the factors. Although, community response to future environmental changes will be complex, we predict that with intensification of tropical storms, native and invasive species will exhibit differential mortality in response to post-disturbance conditions. These responses will ultimately lead to shifts in coastal plant assemblages.