Buried Hurricane Legacies: Increased Nutrient Limitation And Decreased Root Biomass In Coastal Wetlands

Plant identity and cover in coastal wetlands is changing in worldwide, and many subtropical salt marshes dominated by low-stature herbaceous species are becoming woody mangroves. Yet, how changes affect coastal soil biogeochemical processes and belowground biomass before and after storms is uncertain. We experimentally manipulated the percent mangrove cover (Avicennia germinans) in 3 x 3 m cells embedded in 10 plots (24 x 42 m) comprising a gradient of marsh (e.g., Spartina alterniflora, Batis maritima) and mangrove cover in Texas, USA. Hurricane Harvey made direct landfall over our site on 25 August 2017, providing a unique opportunity to test how plant composition mitigates hurricane effects on surface sediment accretion, soil chemistry (carbon, C; nitrogen, N; phosphorus, P; and sulfur, S), and root biomass. Data were collected before (2013 and 2016), one-month after (2017), and one-year after (2018) Hurricane Harvey crossed the area, allowing us to measure stocks before and after the hurricane. The accretion depth was higher in fringe compared with interior cells of plots, more variable in cells dominated by marsh than mangrove, and declined with increasing plot-scale mangrove cover. The concentrations of P and delta S-34 in storm-driven accreted surface sediments, and the concentrations of N, P, S, and delta S-34 in underlying soils (0-30 cm), decreased post-hurricane, whereas the C concentrations in both compartments were unchanged. Root biomass in both marsh and mangrove cells was reduced by 80% in 2017 compared with previous dates and remained reduced in 2018. Post-hurricane loss of root biomass in plots correlated with enhanced nutrient limitation. Total sulfide accumulation as indicated by delta S-34, increased nutrient limitation, and decreased root biomass of both marshes and mangroves after hurricanes may affect ecosystem function and increase vulnerability in coastal wetlands to subsequent disturbances. Understanding how changes in plant composition in coastal ecosystems affects responses to hurricane disturbances is needed to assess coastal vulnerability.