Data accompanying "Sediment addition at the saltmarsh-mangrove ecotone shifts species dominance with implications for restoration"

Manuscript abstract Community responses to restoration are notoriously difficult to predict, and intensifying global pressures (e.g. sea level rise, species range shifts) amplify this challenge. Subtropical communities, which are already responding rapidly to multiple stressors, provide an ideal context for exploring restoration responses under evolving ecological conditions. To investigate how restorative sediment amendments that address sea level rise vulnerability influence alternative plant communities in the subtropics, we manipulated sediment thickness (+0cm, +15cm or +30cm) and sediment composition (1%-silt or 10%-silt) in plots initially dominated by smooth cordgrass (Spartina alterniflora) or black mangroves (Avicennia germinans) in a 26-month field experiment on Florida’s northeastern coast. We show that sediment thickness shapes ecological recovery timelines with both plant biomass and invertebrate abundance recovering faster from +15cm than +30cm additions, but saw little effect of sediment composition on outcomes within the 26-month period. Cordgrass and mangroves were not equally tolerant of burial: cordgrass biomass in sediment addition plots recovered to or doubled control-level biomass by the end of the study while mangrove biomass remained less than half that of reference levels post-burial. Resultantly, initial mangrove dominance was minimized or reversed by sediment addition. However, establishment of new mangrove propagules was 4-6 times higher in sediment addition plots than controls, suggesting that elevational increases could nullify initial mangrove biomass loss by facilitating the longer-term conversion to mangrove forest. Mobile invertebrates, especially burrowing crabs, recovered from burial by the end of the study, but sessile ribbed mussels failed to reestablish in sediment addition plots. Sediment addition alleviated salinity stress but had little effect on sulfide concentration between treatments. While plants and invertebrates returned, soil properties in sediment addition plots differed starkly from controls at the final timepoint, indicating that these remain altered well beyond 26 months but do not necessarily limit community recovery. Our results reveal that emerging ecological communities may tolerate and benefit from restoration similarly to better-studied temperate systems. However, we show that community composition in these habitats may shift and develop along different trajectories in response to restoration interventions with implications for managers targeting specific post-restoration outcomes. Methods In July 2022, we established an experiment at a saltmarsh in St. Augustine, Florida, USA (29.868361, -81.314222) that protects key coastal infrastructure, including a hospital and university, and was identified through regional restoration planning as a candidate location for large-scale TLP application. The climate in St. Augustine is subtropical, with temperatures over the last decade averaging 17oC in spring (January-April), 26oC in summer (May-August) and 21oC in fall (September-December), and total rainfall averaging 77mm, 128mm, and 124mm respectively per season (National Oceanic and Atmospheric Administration National Estuarine Research Reserve System [NOAA NERRS], 2025). The 18-ha study site is microtidal (tidal range=1.305m; NOAA, n.d.) and receives fresh and saltwater inputs (salinity=16-36 psu; NOAA NERRS, 2025). Most of the interior saltmarsh at this site has converted to mudflat and the remaining landward strip is actively retreating as edge vegetation is lost (Fig. 1a). Concurrently, shrub black mangroves are expanding laterally from higher elevations and replacing cordgrass, driving a transition from saltmarsh to mangrove forest at the site as typical of other ecotonal marshes in the region (Fig. 1b). Experimental design We established fifty experimental plots in a randomized block design within the degrading area of the marsh. Plots were established around either individual mangroves (“main trees”) where cordgrass was sparse, or continuous cordgrass with little or no mangrove cover (n=25 per species, n=5 per treatment; Fig. 1c), hereafter referred to as ‘initially mangrove’ or ‘initially cordgrass’ respectively. Plot boundaries were defined using 90-cm diameter PVC cylinders sunk 5cm into the sediment and cut to the aboveground height of the sediment thickness to be added (+15cm or +30cm). Control plots (+0cm) were designated by 5cm-tall rings set at the surface. Alternate sediments (“1%-silt” [99% sand, 1% silt, beige appearance] and “10%-silt” [90% sand, 10% silts, dark brown appearance]; Fig. S1 in Supporting Information) were sourced from a dredged material management area north of the project site, floated by boat into the marsh at high tide, and manually added to the top of all sediment addition plots. Sediment settling was accounted for by topping off plots with additional sediment the following day and again a month later as needed. Elevation was recorded at plot centers using a Real-Time Kinematic Global Navigation Satellite System (RTK GNSS) prior to sediment addition and a year later. All sediment addition plots were trenched to a depth of 30cm at the start of the experiment and again annually to sever lateral root connections. We monitored each plot before sediment was added in July and annually in the Fall thereafter for two years marking 2 months, 14 months and 26 months post-sediment burial. Plot photos were taken at each timepoint to document visual changes. Plant responses To estimate aboveground biomass, we recorded plant density and morphology data within 40×40cm quadrats placed at the center of each plot to later be used in allometric equations. For cordgrass biomass, we counted all stems and measured the height of 10 random stems within the quadrat. For mangrove biomass, mangroves within the quadrat were distinguished between life stages into saplings (unbranched) and adults (branched) and counted. We measured sapling and adult height and basal trunk diameter (BTD, recorded 2cm above the soil surface) and for adult trees also recorded the crown area using the diameter of the widest crown section (D1) and diameter of the perpendicular (D2) as inputs for the equation of an ellipse (crown area=[(D1)/2]×[(D2)/2]×π). Also included in mangrove biomass of each plot, we counted all pneumatophores and measured the height of 10 random pneumatophores within the quadrat. Pneumatophore density values were also used separately to evaluate the impact of burial on pneumatophores. Allometric equations to estimate biomass were developed using 131 cordgrass stems, 50 pneumatophores and 53 mangroves (26 adult, 27 sapling) that were collected from the study site, measured as described above, and dried at 60oC to a constant weight. Further details on allometric equation development can be found in Appendix S1. To evaluate mangrove propagule arrival and establishment as an indicator of mangrove secondary succession, we also counted mangrove propagules (young mangroves with the cotyledon present) and rooted saplings within the entire plot. Invertebrate responses Macro-invertebrate density was monitored within 25×25cm quadrats centered and placed before other samplings to avoid disturbing vegetation and associated invertebrates. We counted individual mangrove tree crabs (Aratus pisonii), marsh periwinkle snails (Littoraria irrorata), mangrove periwinkle snails (Littoraria angulifera), ribbed mussels (Geukensia demissa), and burrows of the following crab species using methods from Angelini et al. (2015) as a proxy for number of individuals: adult and juvenile fiddler crabs (Uca spp.), purple marsh crabs (Sesarma reticulatum) and mud crabs (Xanthidae). See Hansen et al. (2025) for the reliability of estimating abundance using burrow counts. Soil and hydrological responses To compare soil properties between sediment addition treatments and controls, we collected syringe cores (5cm depth, 2.7cm diameter) 10cm from the plot center (offset 90 degrees between timepoints) and dried them at 60oC to a constant weight to determine soil bulk density (dry weight in a known volume) and moisture (% water loss of sample). Samples were subsequently burned for 3 hours at 650oC to determine organic matter (OM) content. We also collected porewater samples at 15cm-depth using sippers (Rhizosphere Research Products, article 19.21.38) to identify potential abiotic drivers of stress and/or regrowth. From these samples, we measured salinity using a refractometer and sulfide concentration following methods developed by Cline (1969). Data description The data file contains all datatsets used in our manuscript, organized across nine excel sheets: (1) experiment layout, (2) RTK elevation, (3) vegetation counts, (4) vegetation measurements, (5) invertebrate counts, (6) soil properties, (7) sulfide content, (8) salinity, and (9) allometric equation development data. Detailed descriptions of each sheet and their associated variables are provided in an additional "README" sheet (10) in the file.