Manifestations of climate change are changing the coastal landscape by altering physical conditions affecting the survival, distribution, and reproductive success of wetland vegetation. Such changes are evident along Turtle Creek, a tidal creek located along the Big Bend coast of Florida, where decline of forested freshwater islands and expansion of mangroves are simultaneously occurring due to climate change. Long-term field data show that forested freshwater islands, historically characteristic to the Big Bend coast, cannot withstand the combined effects of increased tidal flooding and extreme weather events. Concurrently, sea level rise and fewer freeze events in this region are promoting the northward and landward expansion of mangrove populations. This study investigates bottom-up and top-down influences on potential mangrove encroachment into regions formerly dominated by healthy forested freshwater islands. In an ongoing study, we are comparing mean response of caged and non-caged black mangrove propagules (Avicennia germinans) across four landscape positions (creek edge, marsh plain, forested freshwater island edge, and island interior) within and between three sites subject to different tidal flooding frequencies (healthy forest site receiving 1 week of flooding, intermediate forest site receiving 16 weeks of flooding, and a decadent forest site receiving 26 weeks of flooding per year).
Results/Conclusions
We found that propagule predation by purple marsh crab (Sesarma reticulatum) was 100% within 11 days in non-caged plots across all landscape treatments at all three sites. Predation in caged plots was reduced to 0-24% over 54 days across all plots, indicating that caging is effectively allowing us to control predation while comparing the effects of environmental conditions on propagule fate. To date, mean seedling establishment is twice as high at intermediate and decadent sites (mean=0.29) compared to the healthy site (mean=0.15) and establishment is lowest along the creek edge at intermediate and decadent sites (mean=0.18 and 0.04, respectively). Conversely, seedling establishment is much greater along the creek edge at the healthy site (mean=0.38) compared to nearly 0 at the other landscape positions. This study demonstrates that while climate change may promote bottom-up factors favoring mangrove encroachment in sites formally occupied by healthy forested freshwater islands, top-down factors, such as crab predation, may inhibit the expansion of black mangroves into new areas. These results can inform potential mangrove expansion patterns and demonstrate the likelihood of black mangroves replacing forested freshwater islands as the dominant wetland forest along the Big Bend coast.