Many coastal ecosystems along the south Atlantic are transitioning from tidal freshwater forested wetlands (TFFW) to marsh due to increasing tidal inundation and saltwater intrusion. In 2004, we established long-term research sites in GA, SC, and LA to understand how climate factors (temperature, precipitation, streamflow, sea-level rise, droughts, and hurricanes) interact to elevate soil salinities and flooding that collectively foster forest dieback and habitat conversion in TFFW of the Southeastern US. We have documented changes in forest structure and growth from 2004-2016 subject to a variety of flooding regimes. Historical documents and aerial images since 1949 have been used to track changes in vegetation through the years at one site. In addition, we have monitored surface elevation change using the rod surface elevation table-marker horizon technique in conjunction with feldspar markers. Shifts to alternate states occur through the degradation of resilience, which is an emergent property of a system that determines the persistence of interactions and is a measure of the system’s ability to absorb changes to biotic and abiotic parameters. Changes to external drivers can decrease resilience, increasing the likelihood of a shift to an alternative state. The state change may be due to a stochastic shock or a gradual change in environmental parameters. An example of a gradual shift is the continual increase in sea level that erodes the resilience of the freshwater forested state and ultimately results in the transition to oligohaline marsh.
Results/Conclusions
Community response to environmental change often occurs over a period of years, thus indicating the need for long-term studies. While litterfall estimates seem to be well defined with 3–5 years of data, stem growth across hydrological gradients in some areas are still not clear even with 12 years of data. Salinity, soil total nitrogen, flood duration, and flood frequency affect forest diameter increment, litterfall, and basal area the greatest, and in predictable ways. Even small concentrations of salinity (e.g., <2 g/L) can drastically decrease basal area increment growth rates and litterfall production. We determined that during this study period, TFFW in these sites are keeping pace with sea-level rise, in contrast to previous work using Cs-137 techniques from Atlantic coastal TFFW that conclude consistent surface elevation deficits for TFFW. In another area, 60 years of aerial photographic analysis found that wetland forest mortality and marsh expansion could be explained primarily (r2 = 0.95) by sea level rise and topographic gradient.