Interactions between landscape disturbance and gradual environmental change: Plant community migration in response to fire and sea level rise

Plant community migration models for coastal wetlands assume that migration rates are a function of landscape slope and the rate of sea level rise. In the Albemarle-Pamlico estuarine system in coastal North Carolina, relative rates of sea level rise are between 30--40 cm per century and landscape slope is minimal. Thus, predicted rates of migration range between 60 meters to 15 km per year, which is far greater than observed. This thesis uses lidar elevation data, and evidence from land cover change modeling, field observation, and experiments, to develop a new model describing plant community response to sea level rise using multiple stable state theory. The Albemarle Peninsula is surrounded by a micro-tidal, oligohaline estuary, and frequent disturbance, primarily fire, is common in its forests and marshes. The interaction between chronic (sea level rise) and acute (fire) processes may create complex patterns in rates of vegetation change as ecosystem resiliency and stability change through time. To develop a multiple stable state model, I asked the following questions, (1) Is the transition from forest to marsh gradual or episodic? (2) Is the physical environment (hydroperiod and salinity) different along the gradient of marsh, dead forest and healthy forest? and, (3) Do dead forests, recently invaded by marsh species, represent permanent state changes in response to a changed physical environment or is interspecific competition preventing forest recovery? Land cover change modeling showed that since 1930, marsh invaded forest communities in both gradual migration events and in discrete patches. Historic fire ignition data were correlated with areas that experienced higher episodic conversion to marsh. The forest to marsh gradient was correlated primarily with hydroperiod and salinity, but groundwater salinity was generally low (<5 ppt). Planted pine seedlings survived in marsh under dead pine snags, and a greenhouse experiment demonstrated that pine seedlings can tolerate moderately brackish, drained soils. These data suggest that forest mortality adjacent to marshes may not always represent a permanent state change from forest to marsh, and that multiple stable states (regulated by disturbance) may exist as a forest transitions to marsh in response to sea level rise.