Climate-driven regime shifts in a mangrove–salt marsh ecotone over the past 250 years

Abstract

Climate change is driving the tropicalization of temperate ecosystems by shifting the range edges of numerous species poleward. Over the past few decades, mangroves have rapidly displaced salt marshes near multiple poleward mangrove range limits, including in northeast Florida. It is uncertain whether such mangrove expansions are due to anthropogenic climate change or natural climate variability. We combined historical accounts from books, personal journals, scientific articles, logbooks, photographs, and maps with climate data to show that the current ecotone between mangroves and salt marshes in northeast Florida has shifted between mangrove and salt marsh dominance at least 6 times between the late 1700s and 2017 due to decadal-scale fluctuations in the frequency and intensity of extreme cold events. Model projections of daily minimum temperature from 2000 through 2100 indicate an increase in annual minimum temperature by 0.5 °C/decade. Thus, although recent mangrove range expansion should indeed be placed into a broader historical context of an oscillating system, climate projections suggest that the recent trend may represent a more permanent regime shift due to the effects of climate change.

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

“…Conversely, following extreme freeze events that kill or damage mangroves, the coverage of freeze‐tolerant salt marshes has expanded. Similar mangrove expansion and contraction cycles have also occurred in Florida (Cavanaugh et al., 2019; Stevens et al., 2006) and other mangrove range limits across eastern North America (Kennedy et al., 2016, 2017; Sherrod & McMillan, 1981, 1985). Since the last regionally relevant major freeze event occurred in December 1989, the mangrove–marsh ecotone has been expanding northward in Louisiana and across the region (e.g., in coastal Texas and both coasts of the Florida peninsula).…”

“…An historical analysis of records from a similar mangrove range limit along the Atlantic coast of Florida indicates that mangroves have also been present there for at least several centuries—Cavanaugh et al. (2019) identified reports of mangrove presence in north‐eastern Florida dating back to 1766 and 1788 by botanists William Bartram (Bartram & Harper, 1943) and André Michaux (Michaux & Sargent, 1889), respectively.…”

“…How long have mangroves been present in Louisiana? The recent ~30‐year expansion of mangroves since the last major, mortality‐inducing freeze event in 1989 happens to coincide with the availability of remotely sensed Landsat data; thus, there has been some debate regarding the novelty of the last 30 years of mangrove expansion in eastern North America (Armitage et al., 2015; Cavanaugh et al., 2014, 2019; Giri & Long, 2014, 2016; Osland, Day, et al, 2017; Saintilan et al., 2014). Historical literature accounts show that A. germinans has been present and documented in Louisiana for at least 300 years—dating back to the period of active European exploration and colonization (Table 1).…”

“…Scientists have used the term “tropicalization” to describe the transformation of temperate ecosystems by these poleward‐moving tropical species (Macy et al., 2019; Osland & Feher, 2020; Scheffel, Heck, & Johnson, 2018; Vergés et al., 2014; Yáñez‐Arancibia, Day, Twilley, & Day, 2014). In coastal wetland ecosystems, warming winter air temperatures are expected to allow tropical mangrove forests to move poleward, in some cases at the expense of temperate salt marsh ecosystems (Cavanaugh et al., 2019; Saintilan, Wilson, Rogers, Rajkaran, & Krauss, 2014). The ecological implications of these marsh‐to‐mangrove transformations are large (Guo et al., 2017; Kelleway et al., 2017), and ecologists working near mangrove range limits are increasingly challenged to better understand the drivers and implications of mangrove range expansion.…”

Foliar water uptake by coastal wetland plants: A novel water acquisition mechanism in arid and humid subtropical mangroves

“…Changes to non‐saline water availability will negatively affect plant productivity and persistence of many coastal wetland ecosystems (Lovelock, Feller, et al, 2017). The ecological implications from loss of mangrove habitat include shifts in the geographical range limit of species (Cavanaugh et al., 2019; Osland et al., 2017), changes in species composition within communities (Saintilan, Wilson, Rogers, Rajkaran, & Krauss, 2014), and changes in the structure and functioning of adjoining coastal ecosystems (Pittman, 2017; Sheaves, 2005). Thus, as the incidence of extreme weather events continues to rise, foliar water uptake may become an increasingly important mechanism for mangroves to maintain water balance during periods of water deficiency (Schreel & Steppe, 2020).…”