Mangrove Livelihood and Extreme Weather Events

As climate change occurs, the frequency of freeze events will affect the presence and abundance of mangrove trees in Louisiana and neighboring states along the northern Gulf of Mexico coast. Black mangroves (Avicennia germinans) provide Louisiana with numerous ecosystem services, including habitat for wildlife, sediment stabilization, and carbon sequestration. However, these trees are highly sensitive to cold weather events. Fewer freeze events would allow for expansion of the black mangrove at the expense of salt marsh vegetation, while more frequent freeze events would result in the contraction or death of mangrove forests.

This project used historical temperature data to identify the frequency, duration, and intensity of cold-air outbreaks in and near the Louisiana coastline. Criteria for defining extreme events were based upon the expected mortality and presence thresholds of black mangrove trees, in particular minimum air temperatures in a range of -9 degrees C to -7 degrees C. This range represents a climate window where mangrove tree survival is at stake. Below the lower threshold, mangrove mortality is likely. Optimal mangrove growth and dominance is likely above the higher threshold.

This study examined cold air outbreaks from the present dating back into the late 1800s, using data from the NOAA National Climatic Data Center, which includes the U.S. Historical Climatology Network and the 19th Century Forts and Voluntary Observers Database. Temperatures below -9 degrees C were rare along the Louisiana coastline, but all stations indicated multiple days reaching this threshold in the past 130 years. Stations along or near the Gulf of Mexico experienced minimum temperatures on average 3 degrees C warmer compared to those just 50 km inland. Collectively, these results help explain the current distribution of mangrove forests in Louisiana and can be used to evaluate the potential for future mangrove forest range expansion in response to climate change.