Karen L. Cummins | USGS Science

USGS2023

Temperature thresholds for leaf damage from two extreme freeze events (2018 and 2021) near the northern range limit of black mangroves (Avicennia germinans) in southeastern North America

Extreme winter temperatures govern the northern range limit of black mangroves ( Avicennia germinans ) in southeastern North America. There is a pressing need for studies that advance our understanding of how extreme cold temperature events affect mangroves near their range limits. However, such events are infrequent and challenging to study at regional scales. Here, we compared the damage to mangroves from extreme freeze events in 2018 and 2021, using local data from sites in USA (Florida, Louisiana, and Texas) and northeastern Mexico (Tamaulipas). In 2018, mangrove damage was concentrated in Louisiana and the upper Texas coast, where minimum temperatures ranged from -4 °C to -7 °C. In 2021, damage from a more severe freeze event was concentrated along the central to northern coasts of Texas, where minimum temperatures ranged from -4 °C to -10 °C. We used regional temperature and vegetation data from these events to quantify temperature thresholds for A. germinans leaf damage. Our results indicate that A. germinans leaf damage is likely to occur when temperatures are between -4 °C and -6 °C. These findings help refine temperature thresholds for A. germinans leaf damage and advance understanding of the effects of extreme freeze events on mangrove range expansion. This information is valuable for anticipating future range dynamics in a warming world.

Atlantic Ocean, Gulf of Mexico↗

USGS2023

Integrating remote sensing with ground-based observations to quantify the effects of an extreme freeze event on black mangroves (Avicennia germinans) at the landscape scale

Climate change is altering the frequency and intensity of extreme weather events. Quantifying ecosystem responses to extreme events at the landscape scale is critical for understanding and responding to climate-driven change but is constrained by limited data availability. Here, we integrated remote sensing with ground-based observations to quantify landscape-scale vegetation damage from an extreme climatic event. We used ground- and satellite-based black mangrove ( Avicennia germinans ) leaf damage data from the northern Gulf of Mexico (USA and Mexico) to examine the effects of an extreme freeze in a region where black mangroves are expanding their range. The February 2021 event produced coastal temperatures as low as − 10 °C in some areas, exceeding thresholds for A. germinans damage and mortality. We used Sentinel-2 surface reflectance data to assess vegetation greenness before and after the freeze, along with ground-based observations of A. germinans leaf damage. Our results show a negative, nonlinear threshold relationship between A. germinans leaf damage and minimum temperature, with a temperature threshold for leaf damage near − 6 °C. Satellite-based analyses indicate that, at the landscape scale, damage was particularly severe along the central Texas coast, where the freeze event affected > 2000 ha of A. germinans -dominated coastal wetlands. Our analyses highlight the value of pairing remotely sensed data with regional, ground-based observations for quantifying and extrapolating the effects of extreme freeze events on mangroves and other tropical, cold-sensitive plants. The results also demonstrate how extreme freeze events govern the expansion and contraction of mangroves near northern range limits in North America.

Gulf of Mexico↗