EFFECTS OF DISTURBANCES IN THE FACE OF SEA LEVEL RISE ON COASTAL WETLAND VEGETATION ALONG THE NORTHERN GULF OF MEXICO

Author

Camille L. Steenrod, Southern Illinois University CarbondaleFollow

Date of Award

8-1-2022

Degree Name

Master of Science

Department

Plant Biology

First Advisor

Gibson, David

Abstract

Natural and anthropogenic disturbances drive change in ecosystems, especially highly disturbed coastal systems, which are at the interface between the land and the sea and contain both aquatic and terrestrial ecosystems. This transitional zone is at the forefront of climate change. As sea level rises, disturbance regimes are expected to change. Simultaneously, the frequency and intensity of extreme storm events, such as hurricanes, may increase, along with increases in fire intensity and severity in certain regions. Historically, fire was a natural disturbance along the northern Gulf of Mexico where lightning frequency is high; however, today fire along the Gulf is often anthropogenic in origin (i.e., prescribed fire). As disturbance regimes change, the interaction between hurricanes and fire is likely to become increasingly prevalent, since increased production of dead debris from more intense hurricanes is likely to serve as additional fuel material for fires. Sea level rise may also act synergistically with the typical pulse disturbances coastal ecosystems face, including hurricanes and fire. This combination of acute and chronic stressors may prevent coastal ecosystems from recovering and returning to their pre-disturbance state if layered legacies of these events decrease ecosystem stability and resilience. The goal of this study was to investigate the effects of layered legacies of disturbances on community composition, species distributions, extent of coastal zones (e.g., salt marsh, fresh marsh, forest) and vegetation vigor in coastal communities over a 17-year period (2004 to 2021) in coastal Alabama to explore the resilience of coastal systems and their persistence in the face of sea level rise. A combination of ground-collected data from 2004, 2011 and 2021, and fine resolution satellite images taken every other year from 2006-2019 were analyzed. Disturbances altered community composition between 2004 and 2021, which coincided with expansion of salt marsh and fresh marsh species distributions at lower elevations, and declines in woody species in the scrub-shrub ecotone and forest at higher elevations. The scrub-shrub ecotone disappeared, and the forest began to deteriorate, while the extent of the fresh marsh increased. Additionally, vegetation vigor (as measured by the Normalized Difference Vegetation Index; NDVI) was calculated from moderate resolution Landsat images within one month prior to and following each extreme storm event from 2004-2020. NDVI decreased after some extreme storm events but increased after others, and there was an overall increase in NDVI over the last five years of the study period. This study was conducted at a critical time; coastal systems are facing an increasing amount of chronic stress from sea level rise, in addition to more immediate stress from pulse disturbances. Despite these stressors, coastal systems along the northern Gulf of Mexico appear to be more resilient than previously realized because upslope migration of species is evident. Extreme storm events and fires appear to contribute to, and even promote, the persistence of coastal wetlands in the face of sea level rise. However, persistence of coastal wetlands along the northern Gulf of Mexico coast may be prevented in areas dominated by upslope barriers to migration (i.e., current/future urban development and levees), such as in Louisiana.