Tropical mangrove species are expanding into temperate saltmarshes worldwide, representing a global, climate-driven transition. Mangroves are primarily limited by their lower temperature tolerances, but when this constraint is relieved, local factors, such as biotic interactions, could determine expansion rates. Along the north Florida coast, black mangrove, Avicennia germinans, is rapidly replacing native saltmarshes. Black mangrove expansion is influenced by biotic interactions with both live standing marsh cordgrass, Spartina alterniflora and with subsidies of dead Spartina wrack. Dead Spartina wrack and mangrove propagules both get stranded in marshes on high tides, and we were interested in how wrack-propagule co-occurrence affects propagule establishment and growth in the field. To examine effects of wrack on mangrove propagules, we performed two field experiments near the northern extent of the mangrove expansion to examine short-term establishment and long-term growth. We placed mangrove propagules in high marsh habitat into the following treatments: saltmarsh, saltmarsh with wrack, and in bare plots (trimmed saltmarsh) and measured propagule retention and establishment over the short term (15 days). We also planted mangrove propagules into the same treatments and measured propagule survival and growth over the long term (9 months).
Results/Conclusions:
In the short-term field experiment, saltmarsh wrack facilitated mangrove propagule establishment by trapping and stabilizing propagules and accelerating initial propagule rooting. However, in the long-term experiment, propagules in the saltmarsh wrack treatment had stunted growth and development, and fewer propagules were able to survive over time due to wrack smothering. Thus, saltmarsh wrack can facilitate initial propagule establishment, but can hinder propagule growth and survival over longer time periods. Together, these experiments suggest that the same resident species can have both positive and negative effects on the expansion rate of an expanding competitor, depending on life history stages.