Drought and Productivity Trends in Freshwater Flooded Forest Swamps in the North American Coastal Plain

Abstract

Climate change has resulted in an increase in drought conditions within the southeastern United States, impacting a variety of ecosystems and species, including the freshwater flooded forest swamps of the North American Coastal Plain (NACP). With the threat of drought leading to a decline in wetlands and shifts in seasonality patterns in the NACP, there is significant concern about the effects of a drying climate on the net primary productivity of bald cypress-tupelo swamps and the potential for ecosystem degradation or conversion. This study seeks to understand the relationship between the net primary productivity of wetlands dominated by bald cypress-tupelo swamps and drought conditions. We used Moderate Resolution Imaging Spectroradiometer (MODIS) imagery from 2000 to 2024, taken from the 500 largest flooded forest swamps in the southeastern USA using Google Earth Engine, to track the changes in productivity of these swamps over time. The trends in MODIS Fraction of Absorbed Photosynthetically Active Radiation (FPAR), Normalized Difference Vegetation Index (NDVI), and Enhanced Vegetation Index (EVI) were compared against three common drought indices using seasonal decomposition and linear regression along with local bivariate spatial associations. We found that while there were small global correlations between drought and productivity metrics, the impact of droughts on these swamps is highly spatially variable, with specific geographical regions displaying differing impacts of drought and precipitation levels on the vegetation productivity. Coastal and southern regions, including the states of Texas, Florida, and Louisiana, are experiencing a decrease in precipitation but have mixed productivity trends. Meanwhile, more inland and coastal northern areas display an increase in precipitation and wetness. Overall, our results suggest that cypress-tupelo swamp productivity varies based on a variety of factors, including, but not limited to, precipitation and drought levels, but some areas may be at risk of further degradation due to drought under future climate change. Further research is encouraged to fully understand vegetation productivity in relation to environmental conditions in individual cypress-tupelo swamps.