LB 9-74 - The Impacts of Hurricane Ian Sediment Deposition on Mangrove Forest Mortality, Leaf Area Index, and Leaf Physiology in Southwest Florida

Mangroves are literally at the frontline of climate change stress, impacted by both sea level rise and more intense hurricanes. In 2017, a 1-ha long-term monitoring plot was established within a fringe mangrove system in Imperial River Preserve in Bonita Springs, Florida with the intention of developing a baseline for understanding the impacts of sea level rise and hurricane disturbance events. Hurricane Ian impacted the site in September 2022, flooding the system with >3 m of storm surge. Qualitative and quantitative observations at this site indicated delayed and spatially dependent stress and mortality in the mangroves. The purpose of this study was to quantify the depth of sediment deposits, organic content of the upper portions of the soils, species-specific mortality, leaf area index, and leaf-level physiological responses along an apparent storm surge sediment deposition gradient. In March 2023, a 180 m transect was established, extending from the western to eastern edges of the peninsula, and covering the sediment deposition gradient at our site. Data were collected every 20 m along the transect. Sediment deposition on the western end of the transect was 9.4 cm deep and demonstrated a logistic-shaped decline to 0 cm of deposit 80 m into the forest. From 80 m to the eastern edge there was no evidence of sediment deposition from the hurricane. Linear regression analyses revealed organic content at 5 cm below the soil surface was negatively related to the sediment deposition depth (p=0.0003, r2=0.79) and there was no significant relationship between organic content 20 cm below the soil surface and sediment deposition depth (p=0.09, r2=0.24). Rhizophora mangle mortality was linearly related to the depth of sediment deposition (p=0.0001, r2=0.96). Laguncularia racemosa mortality demonstrated a strong exponential relationship with sediment deposition depth (100% mortality in areas with even the smallest amounts of deposition). Variations in LAI were closely related to L. racemosa mortality across the transect. Laguncularia racemosa had significantly lower quantum efficiency where sediment deposition occurred (p=0.02), while R. mangle was not significantly impacted (p=0.28). Both species demonstrated reductions in stomatal conductance where storm surge sediments were present (p < 0.001). The results from this study indicate both R. mangle and L. racemosa are sensitive to hurricane sediment deposition. The data also suggest the trees are experiencing water stress possibly linked to decreased oxygen diffusion to the root zone in areas were hurricane sediment deposition occurred. Further research is required to elucidate this mechanism.