Frequency of extreme freeze events controls the distribution and structure of black mangroves (Avicennia germinans) near their northern range limit in coastal Louisiana

Abstract

Aim Climate change is expected to result in the tropicalization of coastal wetlands in the northern Gulf of Mexico, as warming winters allow tropical mangrove forests to expand their distribution poleward at the expense of temperate salt marshes. Data limitations near mangrove range limits have hindered understanding of the effects of winter temperature extremes on mangrove distribution and structure. Here, we investigated the influence of extreme freeze events on the abundance, height and coverage of black mangroves (Avicennia germinans) near their northern range limit in Louisiana. Location Coastal Louisiana, USA. Methods We quantified the relationships between the frequency of extreme freeze events and A. germinans abundance, height and coverage using: (a) mangrove observation points recorded via aerial surveys from a fixed‐wing aircraft; (b) 30 years of temperature data; and (c) mangrove mortality and leaf damage temperature thresholds. We used freeze frequency data and mangrove–climate relationships to evaluate and spatially depict the risk of A. germinans freeze damage across Louisiana. Results We identified strong negative relationships between the frequency of extreme freeze events and A. germinans abundance, height and coverage. Avicennia germinans is most abundant, tall and continuous along the south‐eastern outer coast of Louisiana, where the frequency of extreme freeze events is reduced (i.e., lower risk of mangrove freeze damage) by the buffering effects of comparatively warm Gulf of Mexico waters. Conversely, the risk of A. germinans freeze damage has historically been very high across Louisiana's Chenier Plain and within more inland wetlands in the Deltaic Plain. Main conclusions Our analyses advance understanding of how the frequency of extreme freeze events controls the distribution, height and coverage of A. germinans near its northern range limit. In addition to informing climate‐smart coastal restoration efforts, our findings can be used to better anticipate and prepare for the tropicalization of temperate wetlands due to climate change.

Climate extreme variables generated using monthly time‐series data improve predicted distributions of plant species

“…Extremes of low temperature (i.e. frost events) can cause dieback (Cavanaugh et al 2014, Matusick et al 2014) and limit the poleward distribution of plants (Cavanaugh et al 2014, 2015, Osland and Feher 2020, Osland et al 2020). Observed declines in frost events have facilitated the expansion of frost sensitive species to cooler climes (Cavanaugh et al 2014) while increases in hot and dry conditions are contributing to plant dieback at regional and global scales (Mitchell et al 2014, Allen et al 2015).…”

Replacement of oyster reefs by mangroves: Unexpected climate‐driven ecosystem shifts

“…With A . germinans now found in north Florida, Louisiana, and Texas (Osland, Day, & Michot, 2020) and intertidal reefs of C. virginica found in North America from Texas to Nova Scotia, it is extremely likely that A . germinans will soon be encountered moving further poleward on intertidal oyster reefs.…”

“…Worldwide, tropicalization of temperate ecosystems is occurring, with poleward movement of many species of flora and fauna, including cold‐tolerant marine mammals, reptiles, coastal fishes, terrestrial plants, and insects, currently underway (Osland et al, In review). As with mangroves, movement and distributions of these species appear to be dependent on extreme cold events (Cavanaugh et al, 2014; Osland, Day, & Michot, 2020; Osland et al, 2019). Climate‐change‐induced foundational species shifts are occurring globally in other coastal ecosystems as well.…”

“…Data from 1940 to 2017 were used to determine the number of days since an extreme freeze event prior to each image (described below). We focused on minimum temperature for analysis as multiple recent studies have pinpointed freeze events and not mean temperature to be the determinant in mangrove–salt marsh transitions (e.g., Cavanaugh et al, 2014, 2015; Osland, Day, Hall, et al, 2020; Osland, Day, & Michot, 2020). Because the study area includes both R. mangle and A .…”

Emerging Wetlands From River Diversions Can Sustain High Denitrification Rates in a Coastal Delta

“…increasing air temperature induced by climate change (Osland et al, 2020). Further, this deltaic plain is currently undergoing a historical wetland loss (i.e., 28-84 km 2 yr −1 ; Couvillion et al, 2016;Wang et al, 2014) caused by human impacts that continue altering the MR watershed hydrology and sediment transport (Elliton et al, 2020;Jafari et al, 2019).…”