DOI: 10.5061/dryad.pc866t208
title: Strategic planting and nutrient amendments to accelerate the revegetation of rapidly retreating coastal dunes

description: The increasing frequency and severity of disturbances to coastal dune
 ecosystems necessitates the development and implementation of restoration
 strategies that rapidly accelerate reestablishment of vegetation, enhance
 dune accretion, and ultimately preserve dune ecosystem services. To assess
 how to rapidly re-establish vegetation to counter ecosystem losses, we
 conducted a manipulative field experiment on a created dune in Northeast
 Florida, USA to determine what combinations of planting density, outplant
 species composition, and nutrient addition maximize dune revegetation
 rate. After three months, we found that increased planting densities
 combined with nutrient addition elevated aboveground plant biomass by
 868-2,961%, while growth in sparsely planted and unfertilized treatments
 was negligible or negative. These thickly revegetated, high density +
 nutrient addition plots accreted 1-5 cm more sediment, demonstrating that
 this planting method can rapidly kickstart dune-building processes.
 Additionally, densely planted, fertilized plots containing bitter panicum
 (Panicum amarum) produced 1.7 times more biomass and accreted 1.5 times
 more sediment than plots containing sea oats (Uniola paniculata),
 suggesting that this species and planting scheme may be most effective for
 rapid dune building. These findings reveal that coupling nutrient addition
 with dense planting can trigger self-sustaining, reinforcing plant growth
 and dune building feedbacks within months, likely warranting the cost of
 additional transplants by enhancing the long-term success of dune
 restoration investments.
description: Our field experiment was conducted on recently restored dunes
 located within the Guana Tolomato Matanzas National Estuarine Research
 Reserve (GTM-NERR) along the Florida A1A corridor (30° 1’ 20.8164” N, 81°
 19’ 21.6732” W). The dune was constructed from May-June 2022 in a project
 led by St. Johns County using dredge sand sourced from nearshore deposits
 near the site (Figure 1). In late June 2022, we established 96 2.25
 m<sup>2</sup> plots along an elevation isocline on the crest
 of initially unvegetated restored foredunes. Plots were spaced 2 m apart
 to ensure independence and marked at the four corners with bamboo stakes
 inserted ~0.5 m deep into the sand. We used a 2 × 2 × 3
 factorial design with two levels of nutrient addition (ambient or
 fertilized), two levels of plant density (36 vs. 144 propagules per 2.25
 m<sup>2</sup>; hereafter referred to as low vs. high), and
 three levels of plant diversity (monotypic sea oats, monotypic bitter
 panicum, 50/50 mixed bitter panicum and sea oats) (12 total treatments, n
 = 8 replicates per treatment). The lower planting density treatment was
 consistent with standard planting densities used in conventional dune
 restoration designs, while we halved the plant spacing in the high-density
 treatment, resulting in densities four times the conventional amount
 (Rogers & Nash 2003). Sea oats and bitter panicum transplants were
 purchased from Green Seasons Nursery (Parish, Florida). Transplants were
 grown in 96 cell count planting trays containing potting soil inoculated
 with vesicular-arbuscular mycorrhizal (VAM) fungi (<em>Glomus
 sp.</em>). All transplants were initially of approximately the same
 height (45 cm) and had a 6.35 cm long root mass. From June 22-July
 1<sup>st</sup>, transplants were added to treatments. These
 planting dates fell within the optimal seasonal planting range for dune
 grasses in this region (April-October) (Williams 2007). Transplants were
 planted in 15 cm deep hand-dug holes, according to supplier guidelines,
 and nutrients were added to appropriate treatments in the form of
 slow-release fertilizer pellets during planting (Osmocote® 15-9-12, 5 g
 per propagule), consistent with Southeastern US dune restoration manual
 guidelines (Rogers & Nash 2003; Williams 2007, Miller <em>et
 al.</em> 2018; FDEP 2021). All transplants were planted in a
 regularly spaced grid. In treatments with mixed assemblages of bitter
 panicum and sea oats, the two species were planted in an alternating
 pattern. Plots were watered for five minutes each immediately following
 planting (~225 L per plot) and received water from ambient rainfall
 thereafter. Immediately following the establishment of our experimental
 plots, the area surrounding all plots was planted with sea oats in a ~40 ×
 40 cm grid pattern by a contractor overseen by St. Johns County staff.
 These plants were planted using power augers, a water-retention gel, and
 slow-release fertilizer pellets. <em>Aboveground
 biomass</em> Plots were monitored immediately
 following planting and each month thereafter for the 15-month duration of
 the experiment. During monitoring, we enumerated and measured the heights
 of all sea oat<em> </em>leaves and bitter panicum<em>
 </em>stems within a 0.16 m<sup>2</sup> quadrat placed at
 the center of each plot<em>.</em> We also determined the
 percent cover of each plant species. Plot aboveground biomass was
 estimated from plant height measurements using a regression of stem/leaf
 height vs. stem/leaf biomass determined from naturally established plants
 taken from around the experimental area. To evaluate
 whether bitter panicum and/or sea oats gained more aboveground biomass
 when in monoculture versus mixed assemblages, we compared each species’
 aboveground growth rates at the end of the first growing season. We
 calculated the change in aboveground biomass as the difference between the
 estimated final and initial aboveground biomass in each plot.
 <em>Belowground biomass</em>
 Belowground biomass was evaluated near the end of the first
 growing season on September 25th, 2022, and again at the end of the second
 growing season on September 26<sup>th</sup>, 2023, from four 3
 × 15 cm cores taken from the center of each plot. For each sample,
 macroscopic roots and rhizomes were separated, dried to a constant weight
 at 70°C, and weighed to the nearest 0.01 g.
 <em>Abiotic condition metrics</em>
 To explore whether there were self-reinforcing feedbacks between
 aboveground plant growth and abiotic stress, we quantified abiotic
 conditions in all treatments. In August 2022, one month following
 deployment, we quantified water loss from identical small kitchen sponges
 placed in each plot as a proxy for plot desiccation stress at the sediment
 surface (Silliman et al. 2011). Four plastic toothpicks were inserted into
 each sponge to create “legs” that elevated sponges 1 cm above the dune
 surface so they would not collect any sediment. We conducted this
 experiment on a clear day with minimal cloud cover, with temperatures
 ranging from 30.7-31.2°C, and wind speeds between 9.01-10.9 km/h.
 Immediately prior to deployment, the dry weight of each sponge was
 determined in the field using a battery-powered scale. Sponges were then
 wetted with fresh water and weighed again. A wet sponge was deployed to
 the center of each plot for 2.5 h. After this time, each sponge was
 collected and immediately weighed again. Percent water loss was determined
 by: [(initial wet weight−final wet weight)/(initial wet weight−dry
 weight)]*100. Following our desiccation stress
 experiment, we used an infrared thermometer to determine the difference
 between sediment surface temperature at the center of each plot and the
 temperature of adjacent unvegetated substrate. Additionally, we quantified
 the difference in photosynthetic photon flux (µmol
 m<sup>-2</sup> s<sup>-1</sup>) at the sediment
 surface within plot interiors and on adjacent bare sediment using a
 quantum meter (Spectrum Technologies model LQS-QM). We calculated the
 amount of light attenuated by vegetation in each plot as the percent
 difference in photosynthetic photon flux between plot interiors and
 adjacent bare substrate. Finally, we evaluated average sediment moisture
 content (%) at a 10 cm depth (n = 3 readings per plot) using a ΔT SM150
 soil moisture kit. <em>Sediment surface
 elevation</em> Near the end of the first growing
 season, Hurricane Ian made landfall in southwestern Florida on September
 28<sup>th</sup>, 2022, and subsequently passed to the south of
 our study site as a tropical storm. We used this event as an opportunity
 to assess potential differences in the functional capacity of vegetation
 in experimental plots to accrete sediment and thus initiate the vertical
 dune-building processes. We took elevation measurements
 of the dune plots using Unmanned Aerial System (UAS) Structure from Motion
 (SfM) almost one month after the hurricane on October
 21<sup>st</sup>, 2022, using a Parrot Anafi Ai. This dataset
 exhibited a relative accuracy of less than 2 cm. All experimental plots
 were georeferenced using Real-Time Kinematic Global Navigation Satellite
 System (RTK GNSS) and adjusted to the NGS CORS network, ensuring alignment
 with the established geodetic reference system, with baselines to the base
 station kept below 2 km. UAS SfM data were processed using Agisoft
 Metashape and were also aggressively filtered based on confidence values.
 Subsequently, datasets were further processed using ArcGIS Pro to generate
 Digital Elevation Models (DEMs). We compared sediment
 surface elevation in treatment plots to unmarked locations between
 treatment plots that had been restored by contractors using conventional
 methods (hereafter referred to as control plots). The dimensions of these
 plots were roughly equivalent to those of treatment plots and were
 truncated by 0.5 m from either the northern or southern edge. Treatment
 and control plot areas were subdivided using 3 × 3 grids and plot
 statistics (maximum, minimum, and mean elevations) extracted from each of
 the resulting nine sub-grids using the Geoprocessing tool Zonal
 Statistics. We then calculated the difference in mean plot elevation
 (calculated from the nine plot subdivisions) of each treatment plot and
 the control plot adject to its southern border.
 Following our initial evaluation of plot elevation, the site was
 impacted by a nor’easter (November 5th 2022) and Hurricane Nicole
 (November 10th, 2022). We assessed plot relative sediment surface
 elevation again using identical methods after one year on September
 26<sup>th</sup>, 2023 to determine if trends observed at the
 end of the first growing season continued during the second.
description: # Strategic planting and nutrient amendments to accelerate the
 revegetation of rapidly retreating coastal dunes
 [https://doi.org/10.5061/dryad.pc866t208](https://doi.org/10.5061/dryad.pc866t208) ## Description of the data and file structure This file is the primary dataset used for the various analyses described in our study, "Strategic planting and nutrient amendments to accelerate the revegetation of rapidly retreating coastal dunes." We conducted a manipulative field experiment on a created coastal dune in Northeast Florida, USA to determine what combinations of planting density, outplant species composition, and nutrient addition maximized coastal dune revegetation rate and sand accretion. ### Files and variables #### File: Morton\_et\_al.\_2024\_Rapid\_Dune\_Restoration\_data.xlsx **Description:** This file contains all data from our dune planting experiment. The Above- and belowground biomass tab contains data on plot plant biomass metrics, the "Abiotic metrics" tab contains data on plot abiotic metrics, and the "Elevation change" tab contains data on plot elevation change. Missing values are indicated by "NA." ##### Variables | **Above- and belowground biomass** | **This tab contains data on plot plant biomass metrics from our dune planting experiment.** | | :--------------------------------- | :--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- | | plot | Plot number | | year | Sampling year | | treatment | Plot treatment designation. "H" indicates high density planting, "L" indicates low density planting, "U" indicates sea oats "P" indicates bitter panicum, "P+U" indicates mixed plots, and "N" indicates fertilizer addition | | plants | Plant species assemblage (sea oats, bitter panicum, or mixed assemblage) | | nutrients | Nutrient addition (nutrients added or ambient nutrients) | | density | Planting density (high or low density) | | uniola.biomass.estimate.g.per.m2 | Plot sea oats aboveground biomass estimate (g per m2) | | panicum.biomass.estimate.g.per.m2 | Plot bitter panicum aboveground biomass estimate (g per m2) | | total.biomass.estimate.g.per.m2 | Plot total plant biomass estimate (g per m2) | | belowground.biomass.g.per.m2 | Plot belowground biomass (g per m2) | | | | | **Abiotic metrics** | **This tab contains data on plot abiotic metrics from our dune planting experiment.** | | plot | Plot number | | treatment | Plot treatment designation. "H" indicates high density planting, "L" indicates low density planting, "U" indicates sea oats "P" indicates bitter panicum, "P+U" indicates mixed plots, and "N" indicates fertilizer addition | | nutrients | Nutrient addition (nutrients added or ambient nutrients) | | plants | Plant species assemblage (sea oats, bitter panicum, or mixed assemblage) | | density | Planting density (high or low density) | | weight.change.g.per.hour | Water loss rate from wet sponges placed in each plot (g water per hour) | | temperature.difference.C | Difference in sediment surface temperature between plot interiors and nearby unvegetated areas (C) | | light.percent.dif | The % difference in photosynthetic photon flux (µmol m-2 s-1)  between plot interiors and adjacent unvegetated areas. | | avg.moisture | Moisture content (%) of plot sediment (average of three measurements) | | | | | **Elevation change** | **This tab contains data on plot elevation change from our dune planting experiment.** | | plot | Plot number | | block | Block number | | year | Sampling year | | treatment | Plot treatment designation. "H" indicates high density planting, "L" indicates low density planting, "U" indicates sea oats "P" indicates bitter panicum, "P+U" indicates mixed plots, and "N" indicates fertilizer addition | | nutrients | Nutrient addition (nutrients added or ambient nutrients) | | plants | Plant species assemblage (sea oats, bitter panicum, or mixed assemblage) | | density | Planting density (high or low density) | | elevation.change.m | Difference in mean plot elevation (m) of each treatment plot and the control plot adjacent to its southern border. | ## Code/software **Excel Files:** * **.xlsx/.xls**: Utilized for viewing data.