Coastal wetlands have experienced regime shifts through major changes to their foundational species. Salt marsh die-off precipitated by grazers has turned healthy marshes into salt barrens; cordgrasses in the genus Spartinahave invaded and altered coastal wetlands in temperate zones around the world and mangroves have increased in abundance at their poleward limits in recent decades, transforming salt marshes dominated by herbaceous plants to ecosystems dominated by woody plants. In the temperate-tropical ecotone where mangroves and saltmarshes meet, mangroves have increased in abundance since 1984. However, between 1942 and 1984, mangrove habitat expanded and contracted, indicating that mangrove expansion may not be solely attributable to recent climate change but may be an ongoing, dynamic process of expansion and contraction over the last few centuries. In this study, we investigate the drivers of regime shifts between mangrove and saltmarsh in the temperate-tropical ecotone. We combined historical records and imagery with field observations and previous remote sensing analysis to evaluate the hypothesis that large, infrequent disturbances, namely, freeze events and hurricanes, are the main drivers of this regime shift.
We found that in the absence of freeze events, mangroves slowly but eventually occupy the ecotone by short-distance dispersal as propagules short distances from the parent plant outcompete the saltmarsh. The speed of the regime shift will be increased by long-distance dispersal due to hurricanes, particularly by hurricanes in late September to early October that track south to north along the coast of Florida. Mangroves had rapid expansion following hurricanes in 1944 and 2004 and contraction following freezes in 1962 and 1977. During the 1980s, there were four severe freezes that prevented mangrove growth. Severe freezes and hurricanes are large, infrequent disturbances that shifted the foundational species in the mangrove-saltmarsh ecotone between 1942 and 2014. These disturbances likely caused similar regime shifts in earlier time periods, which is supported by historical accounts of mangrove presence in the ecotone and accounts of mangrove die-offs following freeze events. Under the current predictions for climate change, we expect a change to this natural oscillation due to warming temperatures and changes to hurricane frequency/intensity, both of which favor mangrove expansion.