Tropical mangroves are expanding into temperate saltmarshes worldwide, representing a global, climate-driven transition. Mangrove distributions are primarily limited by mangrove lower temperature tolerance, but when this constraint is relieved, local factors, such as biotic interactions, may determine the rate of mangrove expansion. Both mangrove and saltmarsh species are ecosystem engineers that create physical changes in their environment. These structural changes may also feedback to affect the engineer, and may play an important role in facilitating range expansion. We were interested in how engineering feedbacks between mangroves and saltmarsh interact to affect the rate of mangrove expansion into saltmarshes in north Florida. We performed two field experiments to examine how structural differences between black mangroves and native saltmarsh species affect mangrove propagule retention and survival. First, we examined propagule retention in fringing and interior transitional habitats by establishing plots of mangrove, saltmarsh, bare habitat, and a root structural mimic. Propagules were added to plots on both neap and spring tidal cycles and retention was observed with time across each treatment. Second, we observed propagule survival over time in the same habitats by planting propagules into plots of mangrove, saltmarsh, bare habitat, and a canopy structural mimic.
Results/Conclusions:
In our first experiment, we observed that propagule retention is dependent on both habitat and tidal cycle stage. More propagules were retained in interior versus fringe habitat, and while there was no difference in propagule retention between treatments on neap tides, significantly more propagules were retained in saltmarsh habitat relative to the other treatments during spring tides. These results may be explained by an interaction between tidal height and vegetation structure, and there may be some vegetation height threshold that determines propagule retention. In our second experiment, we observed that in both fringe and interior habitats, propagule survival was significantly lower in mangrove plots compared to the other treatments. Together, these experiments suggest that positive and negative engineering feedbacks between mangroves and marshes interact to affect mangrove propagule success in different habitat and environmental conditions.