Do hurricanes or freezing events regulate the sustainability of subtropical mangroves on the Gulf of Mexico coast?

Abstract

Global warming has caused poleward mangrove expansion, but extreme climatic events have significantly impacted the mangroves along their boreal limits. This study aimed to determine which natural hazard is more important in limiting the sustainability and survival of subtropical black mangroves—hurricanes or freezing events? Satellite and drone images indicated Hurricanes Zeta and Ida (2020 – 2021) caused only minor damage to Avicennia trees (~5%, 6.32 ha) compared with the extensive mortality caused by the winter freeze of Dec/2017- Jan/2018 (~ 89%, 110 ha) at Port Fourchon, Louisiana. However, mangroves impacted by winter freezes had a faster recovery (~1 year), while the losses of mangrove areas by hurricanes are longer-lasting. This finding is novel and important because it implies that subtropical mangroves have low resistance but high resilience to winter freezes, while these forests present high resistance but low resilience to hurricanes. Overwash processes driven by hurricanes are the primary threat to mangroves at Port Fourchon due to the high rate of beach barrier retreat, which causes the burial of the back-barrier wetlands. In 2022 a new beach renourishment project is currently underway, resulting in shoreline progradation. This human intervention is essential to guarantee the stability of the beach barrier and slow down the increased losses of mangrove and salt marsh areas caused by sea level rise and extreme events. The mangrove at Port Fourchon is a microcosm of the general ecological balance of mangroves growing along their northern distribution limit. Thus, our findings apply to the coastal wetlands on the entire northern Gulf of Mexico.