Facilitation plays a key role in Marsh-Mangrove interactions along a salinity gradient in South Texas
Ziegler, Mikaela J.
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Climate change is resulting in fewer and less intense freezes which allows tropical mangroves, Avicennia germinans, to expand into saltmarshes in the Gulf of Mexico, causing a regime shift from herbaceous to woody plant dominance in many areas. Species interactions may affect the extent and rate of this regime shift. The Stress Gradient Hypothesis (SGH) predicts that at low stress plant-plant interactions will be competitive and shift to facilitative at high stress. Climate models show rising temperatures and changing precipitation patterns may lead to prolonged hypersaline conditions in some areas. Hypersalinity stress converts salt marsh dominance from S. alterniflora to succulent forb species or unvegetated tidal flats. Understanding how interactions between marsh species along stress gradients affect different life stages of A. germinans can provide more insight into which salt marsh habitats are vulnerable to encroachment. We utilized a transplant experiment and two observational field studies within different habitats to investigate A. germinans seedling survival, growth, and reproductive output under moderate and hypersaline conditions. Seedlings were observed at moderate and hypersaline sites in four habitats: A. germinans canopy cover, shoreline and inshore succulent marsh habitats, and tidal flats. Propagules were collected from moderate and hypersaline sites along the south Texas coast and transplanted within three habitats: A. germinans canopy cover, succulents, and tidal flats. Reproductive branches of mature trees were monitored for the development of propagules at moderate and hypersaline sites. By the predictions of the SGH, A. germinans canopy was competitive with seedlings under moderate salinity as succulents had higher seedling densities and mean heights, but canopy became facilitative at hypersalinity due to high seedling recruitment and growth. Propagules in moderate salinity had slightly lower survival rates than those in hypersalinity; propagule survivability was similar in succulent forbs and A. germinans canopy. Survival rates based on natal sites were marginally different, suggesting there was no local adaptations to hypersalinity. No difference was found in the quantity or size of propagules between both sites, while survival and growth conformed to SGH predictions, reproduction did not. Estuaries of moderate salinity with succulents or hypersalinity with mangroves are most vulnerable to mangrove encroachment.