Climate change is causing a poleward shift in the distribution, phenology, and abundance of many species and ecosystems around the world. Temperate and tropical plants and animals are migrating poleward in response to global warming, displacing native species, altering biodiversity patterns, and modifying ecosystem structure and function. These changes are especially visible along latitudinal gradients at the ecotone where the tropics transition into the temperate zone. The highest ‘velocity’ of climate-driven change is occurring in wetlands, affecting both saltmarshes and mangrove ecosystems. Herbaceous salt marshes dominate temperate coastal wetlands along eastern North America, currently occupying over 3500 miles of shoreline from northern Florida to Canada. Salt marsh species are not physiologically limited to a temperate climate whereas mangroves are limited to the tropics. The latitudinal limits of salt marsh plants are thought to be set by disturbance, herbivory, and competition, whereas the latitudinal limit of mangroves is set by their inability to tolerate freezing temperatures. Both mangroves and saltmarshes are extremely productive and highly valued both ecologically and economically. They protect and stabilize shorelines, serve as nurseries for commercially important fisheries, filter sediments and nutrients from upland runoff, sequester carbon in belowground peat accumulations.
Along the Atlantic coast of North America at the southern edge of their range, salt marshes in the temperate-tropical ecotone are steadily being displaced by a northerly expansion of tropical and subtropical mangrove trees that already dominate coastal wetlands in tropical regions below latitudes from 28°N to almost 28°S. Our research has documented that mangroves along the coast of Florida are expanding beyond their historical range limits where they are encroaching on and displacing salt marsh ecosystems. In just over the past couple of decades, coastal wetlands in this temperate-tropical ecotone have converted to mangrove forests, which has resulted in a major change in the structure of these ecosystems. The structure of coastal wetlands has shifted from herbaceous marshes, where almost all the aboveground biomass dies back to the ground on an annual basis, to tree-dominated mangrove forests where only ~20% of the aboveground biomass recycles annually. Similar observations around the world indicate that this a global phenomenon. However, it is not clear how this shift from saltmarsh to mangrove is affecting the functional properties of coastal wetlands.
The goal of this project is to investigate the causes and consequences of the current and future climate-driven displacement of temperate saltmarshes by tropical and subtropical mangroves. We are testing the hypothesis that climate change is facilitating the rapid encroachment of mangrove trees into herbaceous saltmarsh ecosystems, and that this invasion will dramatically alter the structural complexity, community composition, biotic interactions, nutrient cycling, carbon sequestration, water filtration, and stability of sensitive coastal environments.