Effects of mangrove cover on coastal erosion during a hurricane in Texas, USA

Abstract

We tested the hypothesis that mangroves provide better coastal protection than salt marsh vegetation using 10 1,008-m 2 plots in which we manipulated mangrove cover from 0 to 100%. Hurricane Harvey passed over the plots in 2017. Data from erosion stakes indicated up to 26 cm of vertical and 970 cm of horizontal erosion over 70 months in the plot with 0% mangrove cover, but relatively little erosion in other plots. The hurricane did not increase erosion, and erosion decreased after the hurricane passed. Data from drone images indicated 196 m 2 of erosion in the 0% mangrove plot, relatively little erosion in other plots, and little ongoing erosion after the hurricane. Transects through the plots indicated that the levee (near the front of the plot) and the bank (the front edge of the plot) retreated up to 9 m as a continuous function of decreasing mangrove cover. Soil strength was greater in areas vegetated with mangroves than in areas vegetated by marsh plants, or nonvegetated areas, and increased as a function of plot-level mangrove cover. Mangroves prevented erosion better than marsh plants did, but this service was nonlinear, with low mangrove cover providing most of the benefits.

Buried hurricane legacies: increased nutrient limitation and decreased root biomass in coastal wetlands

“…However, in our study, mangrove cells inhibited surface sediment accretion post-hurricane. Results from other studies in these same wetlands found that mangrove cover enhanced shoreline retention by reducing vertical and fringe erosion, which is likely attributed to greater soil strength in mangrove cells (Armitage et al 2020, Pennings et al 2021. Coastal vegetation species differentially modify soil strength through organic detritus input, and greater root biomass and organic content in these mangrove cells (Charles et al 2020), and may lower soil bulk density and increase sediment stability (Feagin et al 2009).…”

“…Coastal vegetation species differentially modify soil strength through organic detritus input, and greater root biomass and organic content in these mangrove cells (Charles et al 2020), and may lower soil bulk density and increase sediment stability (Feagin et al 2009). Our research suggests that mangroves function as critical biophysical filters on multiple spatial scales despite storm-driven changes in sediment biogeochemistry, retaining accreted sediments along coastal fringe margins (Charles et al 2020) and reducing sediment even at low plot-scale mangrove cover (Pennings et al 2021). Increases in mangrove cover in coastal wetlands may therefore enhance physical shoreline retention and reduce storm sediment and wrack deposition (Armitage et al 2020, Pennings et al 2021.…”

“…We measured reduced nutrient, but not carbon concentrations, in accreted surface sediments and soils, and these persisted one year after the hurricane. We anticipated a high sulfide (δ 34 S) accumulation, as indicated from depleted δ 34 S content in accreted surface sediments and soils, where plant stress and sedimentation reduced soil conditions (Holmer and Hasler-Sheetal 2014), which we observed in marsh cells across plot-scale mangrove cover, especially in the eroded fringes of plots (Armitage et al 2020, Pennings et al 2021. We predicted lowest root biomass in treatments of Notes: Bolded values denote significance of P < 0.05 using α = 0.05.…”

“…Instead, storm surge sediment accretion was highest and most variable in marsh cells and decreased in mangrove cells with plot-scale mangrove cover (Table 1). Relative to the marsh cells, mangrove cells likely promoted autochthonous sediment retention and may have inhibited storm-driven sediment deposition and erosion (Armitage et al 2020, Charles et al 2020, Pennings et al 2021). Our observations suggest that mangroves trap debris at the very front of plots and of mangrove cells and that this likely inhibited the transport of allochthonous sediments into and through areas with high mangrove cover (Guo et al 2017).…”

“…Mangrove expansion into herbaceous marshes can alter above-and belowground soil organic and inorganic matter concentrations. Mangroves increase sediment retention and accretion, reduce erosion, attenuate waves, and increase carbon and nutrient storage (Friess et al 2011, Comeaux et al 2012, Kelleway et al 2017, Chen et al 2018, Charles et al 2020, Pennings et al 2021. Mangrove roots stabilize the soil substrate and increase elevation, and the prevention of vertical erosion and accumulation of slower-decaying organic matter increase soil carbon content in mangroves (Gedan et al 2011, Charles et al 2020.…”