Mangrove expansion and contraction at a poleward range limit: climate extremes and land‐ocean temperature gradients

Abstract

Within the context of climate change, there is a pressing need to better understand the ecological implications of changes in the frequency and intensity of climate extremes. Along subtropical coasts, less frequent and warmer freeze events are expected to permit freeze-sensitive mangrove forests to expand poleward and displace freeze-tolerant salt marshes. Here, our aim was to better understand the drivers of poleward mangrove migration by quantifying spatiotemporal patterns in mangrove range expansion and contraction across land-ocean temperature gradients. Our work was conducted in a freeze-sensitive mangrove-marsh transition zone that spans a land-ocean temperature gradient in one of the world's most wetland-rich regions (Mississippi River Deltaic Plain; Louisiana, USA). We used historical air temperature data (1893-2014), alternative future climate scenarios, and coastal wetland coverage data (1978-2011) to investigate spatiotemporal fluctuations and climate-wetland linkages. Our analyses indicate that changes in mangrove coverage have been controlled primarily by extreme freeze events (i.e., air temperatures below a threshold zone of -6.3 to -7.6°C). We expect that in the past 121 yr, mangrove range expansion and contraction has occurred across land-ocean temperature gradients. Mangrove resistance, resilience, and dominance were all highest in areas closer to the ocean where temperature extremes were buffered by large expanses of water and saturated soil. Under climate change, these areas will likely serve as local hotspots for mangrove dispersal, growth, range expansion, and displacement of salt marsh. Collectively, our results show that the frequency and intensity of freeze events across land-ocean temperature gradients greatly influences spatiotemporal patterns of range expansion and contraction of freeze-sensitive mangroves. We expect that, along subtropical coasts, similar processes govern the distribution and abundance of other freeze-sensitive organisms. In broad terms, our findings can be used to better understand and anticipate the ecological effects of changing winter climate extremes, especially within the transition zone between tropical and temperate climates.

Temperature thresholds for black mangrove (Avicennia germinans) freeze damage, mortality and recovery in North America: Refining tipping points for range expansion in a warming climate

“…Longer‐term effects would likely occur at temperatures below −7°C, which is when we expect that ecosystem stability and mangrove aerial coverage would be affected by landscape‐scale mortality. These findings are supported by historical analyses of aerial imagery from the mangrove‐marsh ecotone near Port Fourchon, Louisiana, which indicate that temperatures near −10°C in the 1980s resulted in widespread mangrove mortality and landscape‐level reductions in mangrove area; however, freeze events with temperature above −6°C in the 1990s and 2000s produced no decreases in mangrove area (Osland, Day, et al, ).…”

“…Of the three common mangrove species in the region, A. germinans is the species that extends furthest north where it expands and contracts in response to the absence or presence of winter air temperature extremes, respectively (Giri & Long, ; Osland et al, ; Rodriguez, Feller, & Cavanaugh, ; Sherrod & McMillan, ). Within the past decade, several studies have demonstrated that winter temperature thresholds determine the northern range limit of A. germinans in eastern and central North America (Cavanaugh et al, , ; Osland, Day, et al, ; Osland, Enwright, Day, & Doyle, ; Osland, Feher, et al, ). Despite the use of divergent data sources, methods and mangrove response variables, the temperature thresholds identified in those studies span a range between −3 and −9°C (Table ).…”

Warming accelerates mangrove expansion and surface elevation gain in a subtropical wetland

“…Taken together, enhanced mangrove growth and cover and decline of salt marsh cover in response to warming shows that conversion of salt marsh to mangroves was accelerated in chronically warmed plots. However, expansion and retraction of mangrove coverage have occurred multiple times in the last century, corresponding with minimum annual threshold events (Osland et al, ). Our results indicated that temperature between freezing events may also impact the rates of expansion between minimum temperature threshold events.…”

Investigating Sub-tropical Community Resistance and Resilience to Climate Disturbance

“…Even in ecosystems where ECEs become less frequent or less intense, the absence of these disturbances can still have a pronounced effect on ecosystem structure. Extreme cold spells for instance, often control the poleward distribution of species (Osland et al 2013;Kreyling et al 2015;Osland et al 2016). If these events decrease in frequency or intensity, then we may expect that species distributions will likely rapidly shift in accordance with that change (Osland et al 2013;Osland et al 2016).…”

“…Extreme cold spells for instance, often control the poleward distribution of species (Osland et al 2013;Kreyling et al 2015;Osland et al 2016). If these events decrease in frequency or intensity, then we may expect that species distributions will likely rapidly shift in accordance with that change (Osland et al 2013;Osland et al 2016). Given the gravity of any future changes to the ECE regime, we must develop more mechanistic understandings of how ECEs influence ecological processes, in order to predict the functioning of ecological systems in the future.…”