Extreme Precipitation and Flooding Contribute to Sudden Vegetation Dieback in a Coastal Salt Marsh

Abstract

Climate extremes are becoming more frequent with global climate change and have the potential to cause major ecological regime shifts. Along the northern Gulf of Mexico, a coastal wetland in Texas suffered sudden vegetation dieback following an extreme precipitation and flooding event associated with Hurricane Harvey in 2017. Historical salt marsh dieback events have been linked to climate extremes, such as extreme drought. However, to our knowledge, this is the first example of extreme precipitation and flooding leading to mass mortality of the salt marsh foundation species, Spartina alterniflora. Here, we investigated the relationships between baseline climate conditions, extreme climate conditions, and large-scale plant mortality to provide an indicator of ecosystem vulnerability to extreme precipitation events. We identified plant zonal boundaries along an elevation gradient with plant species tolerant of hypersaline conditions, including succulents and graminoids, at higher elevations, and flood-tolerant species, including S. alterniflora, at lower elevations. We quantified a flooding threshold for wetland collapse under baseline conditions characterized by incremental increases in flooding (i.e., sea level rise). We proposed that the sudden widespread dieback of S. alterniflora following Hurricane Harvey was the result of extreme precipitation and flooding that exceeded this threshold for S. alterniflora survival. Indeed, S. alterniflora dieback occurred at elevations above the wetland collapse threshold, illustrating a heightened vulnerability to flooding that could not be predicted from baseline climate conditions. Moreover, the spatial pattern of vegetation dieback indicated that underlying stressors may have also increased susceptibility to dieback in some S. alterniflora marshes.Collectively, our results highlight a new mechanism of sudden vegetation dieback in S. alterniflora marshes that is triggered by extreme precipitation and flooding. Furthermore, this work emphasizes the importance of considering interactions between multiple abiotic and biotic stressors that can lead to shifts in tolerance thresholds and incorporating climate extremes into climate vulnerability assessments to accurately characterize future climate threats.

A Model of the Spatiotemporal Dynamics of Soil Carbon Following Coastal Wetland Loss Applied to a Louisiana Salt Marsh in the Mississippi River Deltaic Plain

“…However, in many regions of the world, coastal wetlands are threatened by climate and land use change impacts, such as relative sea‐level rise and extreme climate events (Babcock et al., 2019; IPCC, 2021; Schuerch et al., 2018; Stagg et al., 2021) and barriers to inland migration (Borchert et al., 2018), putting decades and even centuries of stored carbon at risk (Baustian et al., 2021). For example, while carbon sequestration in U.S. coastal wetlands provides an overall net sink of 8.5–8.7 Mt CO 2 e y −1 , high rates of coastal wetland land loss in the Mississippi River Delta Plain, through processes like submergence, have resulted in significant emissions, weakening the coastal wetland sink (Crooks et al., 2018) and potentially exacerbating climate impacts.…”

Restoration and resilience to sea level rise of a salt marsh affected by dieback events

“…The drought‐induced mechanism of sudden marsh dieback has been associated with the interaction and cascading effects of multiple stressors, such as extreme porewater salinity, sediment acidification due to the oxidation of air‐exposed sulfur minerals into sulfuric acid, infection by fungal pathogens, and an exponential increase of periwinkle snail herbivory (Alber et al, 2008; Hughes et al, 2012; McKee et al, 2004; Silliman et al, 2005). More recently, extreme precipitation from catastrophic tropical cyclones was suggested to trigger marsh vegetation diebacks by either intrusion of salt water into brackish/freshwater marshes and/or by extending periods of tidal inundation, which in turn exceed the physiological tolerance of marsh plants (Marsh et al, 2016; Ramsey et al, 2012; Stagg et al, 2021). The co‐occurrence of vegetation loss with extreme drought and flooding events in our study provides substantial evidence that marsh vegetation dieback is attributed to extreme climatic events in the USA Southeast.…”

Restoration and resilience to sea level rise of a salt marsh affected by dieback events in Charleston, SC

“…Extreme drought events have been linked to sudden vegetation diebacks in a number of salt marshes in the USA Southeast and Gulf of Mexico over the last 25 years (Alber et al, 2008; Lindstedt et al, 2006; McKee et al, 2004). More recently, extreme precipitation from catastrophic tropical cyclones was suggested to trigger marsh vegetation diebacks by either intrusion of salt water into brackish/freshwater marshes and/ or by extending periods of tidal inundation which in exceed the physiological tolerance of marsh plants (Marsh et al, 2016; Ramsey et al, 2012; Stagg et al, 2021). Our results reinforce previous studies which attribute marsh vegetation dieback to extreme climatic events in the USA Southeast and Gulf of Mexico.…”

“…The drought-induced mechanism of sudden marsh dieback has been associated with the interaction and cascading effects of multiple stressors, such as extreme porewater salinity, sediment acidification due to the oxidation of air-exposed sulfur minerals into sulfuric acid, infection by fungal pathogens, and an exponential increase of periwinkle snail herbivory (Alber et al, 2008; Hughes et al, 2012; McKee et al, 2004; Silliman et al, 2005). More recently, extreme precipitation from catastrophic tropical cyclones was suggested to trigger marsh vegetation diebacks by either intrusion of salt water into brackish/freshwater marshes and/ or by extending periods of tidal inundation which in turn exceed the physiological tolerance of marsh plants (Marsh et al, 2016; Ramsey et al, 2012; Stagg et al, 2021). The co-occurrence of biannual vegetation loss with extreme drought and flooding events in our study provides substantial evidence that marsh vegetation dieback is attributed to extreme climatic events in the USA Southeast.…”