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

Abstract

Climatic warming can change how coastal wetland plants grow, thus altering their capacity to build land and keep pace with rising seas. As freeze events decline with climate change, mangroves expand their range to higher latitudes and displace salt marsh vegetation. Warmer air temperatures will likely alter above‐ and below‐ground plant dynamics as this dramatic coastal wetland biome shift proceeds, which in turn may result in changes in ecosystem function such as sediment building. We used a large scale in situ warming experiment in a subtropical wetland to increase both marsh and mangrove ecosystem air temperatures. We assessed how 2 years of continuous warming influenced above‐ and below‐ground plant growth and surface elevation relative to sea level. We found that chronic warming doubled plant height and accelerated the expansion of mangrove into salt marsh vegetation, as indicated by a sixfold greater increase in mangrove cover in warmed plots compared to ambient temperature plots and a corresponding loss in salt marsh cover. Surface elevation gain, a measure of soil‐building capacity, increased due to warming over a 2‐year period and these changes in surface elevation were driven by increased mangrove root production in warmed plots. Synthesis. Our findings suggest that, in some coastal wetlands, warming can facilitate plant community changes from marsh to mangrove, with corresponding increases in growth that help coastal wetlands to keep pace with sea‐level rise.

Quantifying how changing mangrove cover affects ecosystem carbon storage in coastal wetlands

“…In order to maintain equilibrium with sea level (Nyman et al 2006, McKee et al 2007, Kirwan et al 2016, Morris et al 2016), many coastal wetlands must increase vertical soil elevation largely through belowground storage of OC (Breithaupt et al 2017). Studies comparing adjacent marsh and mangrove systems are divided on whether mangrove ecosystems support larger (Comeaux et al 2012, Bianchi et al 2013, Yando et al 2016, Coldren et al 2018 or similar (Perry and Mendelssohn 2009, Henry and Twilley 2013, Doughty et al 2016, McKee and Vervaeke 2017) OC stocks and storage rates compared to marshes. Studies comparing adjacent marsh and mangrove systems are divided on whether mangrove ecosystems support larger (Comeaux et al 2012, Bianchi et al 2013, Yando et al 2016, Coldren et al 2018 or similar (Perry and Mendelssohn 2009, Henry and Twilley 2013, Doughty et al 2016, McKee and Vervaeke 2017) OC stocks and storage rates compared to marshes.…”

“…A crucial unknown variable is how shifting dominant species identity and cover following mangrove expansion or mortality may alter OC storage. Studies comparing adjacent marsh and mangrove systems are divided on whether mangrove ecosystems support larger (Comeaux et al 2012, Bianchi et al 2013, Yando et al 2016, Coldren et al 2018 or similar (Perry and Mendelssohn 2009, Henry and Twilley 2013, Doughty et al 2016, McKee and Vervaeke 2017) OC stocks and storage rates compared to marshes. Furthermore, studying adjacent marsh and mangrove-dominated areas risks confounding the impacts of the vegetation itself with the abiotic conditions driving vegetation zonation.…”

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

“…Freeze‐tolerant salt marshes dominate areas that are too cold for mangrove forests, while mangroves dominate coastal reaches with mild winters. Hence, coastal wetlands in this region contain a dynamic mosaic of mangrove forest and salt marsh plants (Coldren, Langley, Feller, & Chapman, ; Langston, Kaplan, & Angelini, ; Macy et al, ; Perry & Mendelssohn, ; Weaver & Armitage, ).…”

Representing the function and sensitivity of coastal interfaces in Earth system models

“…Another visibly prominent shift in coastal ecology is the poleward migration of mangroves due to declining freeze frequencies and landward migration due to seawater intrusion 84 . Conversion from herbaceous-dominated to woody plantdominated wetlands greatly increases aboveground carbon uptake on the landscape scale 52,85 and can accelerate soil elevation gain 86 and long-term wood retention in channels and floodplain microtopography 85 , influencing long-term persistence of these ecosystems. Conversely, seawater intrusion into freshwater wetlands at the upstream edge of the coastal interface can cause vegetation death and accelerated soil carbon loss resulting in the collapse of the ground surface and a conversion of the plantdominated wetland to open brackish water 87 .…”