Effects of the 2017–2018 winter freeze on the northern limit of the American mangroves, Mississippi River delta plain

Abstract

Global warming has led to a tropicalization of ecosystems, especially in the northern hemisphere, where freeze-sensitive plants, such as Avicennia germinans, are expanding northward, exposing tropical species to greater annual climate extremes. The effects of this driver on the boreal limits of mangroves need to be investigated for projecting the fate of mangroves in the face of global warming. Based on QuickBird, aerial/drone images, and historic air/seawater temperatures, this work analyzed the 2017–2018 winter freeze impacts on mangroves of Bay Champagne, Louisiana, USA. This winter freeze, marked by 14 days of subfreezing daily minimum temperatures, degraded ~90% (111 ha) of the studied mangroves. This macroclimatic setting caused defoliation and dry branches on Avicennia trees. Such degradation occurred mainly in mangroves established after 2004, where trees of low heights (1–1.5 m) and density between 1000 and 10,000 trees/ha were present mainly on topographically lower tidal flats (13–26 cm above mean sea-level). By contrast, healthy Avicennia trees were typically taller (1.5–2.2 m), growing at intermediate-density (4000–8000 trees/ha) stands, and occupying higher grounds (20–46 cm) in the inner part of mangrove areas. Canopy height and tree density modulated the winter freeze damages on Avicennia trees, as these factors attenuated the wind impacts along a microclimatic gradient. The microtopography of tidal flats may have also influenced the Avicennia degradation. The recovery of mangroves was rapid (~1 year) and regulated by a microclimatic and microtopographic gradient. Winter freeze impacts on mangroves will probably be attenuated as the increase in the stature and density of mangrove trees. These processes could contribute to the continued northward expansion of mangroves. However, recurring winter freezes affect mangrove productivity that is a critical factor for mangroves to keep pace with relative sea-level rise (SLR).