Interplay of Hydroperiod on Root Shear Strength for Coastal Wetlands

The evolution of coastal wetlands is a complex process which is difficult to forecast, made more complicated by the addition of changing climatic conditions. Here, long term ecological and geomorphological data are coupled to geotechnical measurements at a coastal wetland in North Inlet estuary, South Carolina. The coupled methodology is presented and discussed in context of understanding coastal wetland system evolution in a changing climate. Specifically, the root shear strength of Spartina alterniflora across a range of elevations was investigated using a cone penetrometer test. Elevation, shear strength, and biomass are shown to be critically interconnected. Root strength was shown to decrease with increased inundation time and decreased elevation (i.e., mudflats). Conversely, the data set illustrates the importance of maintaining key elevation ranges in relation to sea‐level to optimize wetland resilience.


Introduction
Coastal wetlands are highly complex ecosystems that are primarily influenced by flooding frequency and duration (hydroperiod) and salinity, and secondarily by temperature, rainfall, nutrient availability, oxygen levels, sediment type, and drainage (Ibáñez et al., 2012).For salt marshes like those in North Inlet, South Carolina, USA, the biomass productivity of Spartina alterniflora is parabolic with a peak value occurring at slightly less than mean sea level (Morris et al., 2002).At the two extremes of high and low elevation, productivity decreases because of hypersalinity and hypoxia, respectively, and the marsh macrophyte community is replaced by unvegetated tidal mud flats (Morris et al., 1990).An optimum marsh elevation for coastal wetland productivity exists, but it differs regionally as a function of tidal range, salinity, competition, among other factors (McKee & Patrick, 1988).For example, hydroperiod and vegetation productivity relationships have been documented in mesotidal estuaries in the USA, such as Plum Island, Massachusetts (Morris et al., 2013), Chesapeake Bay, Maryland (Kirwan & Guntenspergen, 2015), and Great Bay, New Hampshire (A.R. Payne et al., 2019).
Defining the hydroperiod and biomass productivity relationship provides a metric to predict how coastal wetlands will respond to natural and anthropogenic factors, such as rising sea levels, river diversions, hurricanes, nutrients, and restoration.For example, Snedden et al. (2015) found in Breton Sound, Louisiana, USA that the lower belowground biomass in S. alterniflora at the Caernarvon Diversion outfall area was a result of prolonged flooding from operation of the diversion, especially during the growing season.The passage of hurricanes often results in high rates of localized sediment deposition (Cadigan et al., 2021;Cadigan, Bekkaye, et al., 2022;Cahoon, 2003;Jafari et al., 2020).Baustian and Mendelssohn (2015) reported up to 12 cm of sediment deposition during the 2008 hurricane season in Louisiana, where S. alterniflora exhibited production increasing by a factor of three with deposition up to 9 cm.The application of a thin layer of dredged sediment to the marsh surface is a restoration tool to build elevation capital, decrease flooding duress, and increase long-term resilience to rising sea levels (Cahoon et al., 2020;Gailani et al., 2019;Morris et al., 2021;Passeri et al., 2015;A. J. Payne et al., 2021;Stagg & Mendelssohn, 2011).
The root shear strength of wetland vegetation directly controls the ability of wetlands to resist edge erosion from wind-generated waves (Cadigan, Jafari, Wang, et al., 2023;Chen et al., 2013;Marani et al., 2011;Reed, 2001), uprooting from tropical cyclone storm surge and waves (Howes et al., 2010;Morton & Barras, 2011), and collapse from excessive inundation (Cadigan, Jafari, et al., 2022;Chambers et al., 2019;DeLaune et al., 1994).It can also serve as a proxy of belowground biomass productivity to project restoration trajectories, from thin layer placement to freshwater diversions.This is important because measuring belowground biomass is time-consuming and operator dependent.Root shear strength generally increases with an increase in belowground biomass because roots add strength due to the tensile force required to break roots and rhizomes (Sasser et al., 2018).For instance, Day et al. (2011) quantified the root shear strengths of S. alterniflora at Bayou Chitigue and Old Oyster Bayou, Louisiana, USA to explain why the salt marsh at Old Oyster Bayou was weaker and converting to open water.They attributed this difference to lower marsh platform elevations of the sediment deprived Old Oyster Bayou, that is, lower sediment accretion rates, higher inundation periods, and accumulation of toxic higher sulfide.Howes et al. (2010) also quantified the root shear strengths of S. alterniflora in Breton Sound, Louisiana to explain the uprooting near the Caernarvon diversion outfall area during Hurricane Katrina in 2005.Thus, quantifying root shear strength can improve landscape-scale wetland morphodynamic models and adaptive management of restoration projects.However, the interplay of root shear strength across the tidal regime still remains unknown.As a result, the objective of this study is to quantify this relationship by conducting cone penetrometer tests (CPTs) in a tidally dominated salt marsh estuary along the Atlantic Ocean.

Site Description
The study in Figure 1a was located within the North Inlet estuary in South Carolina, USA, which is also the location of an on-going salt marsh primary production study that first began in 1984 (Morris, 2000;Morris et al., 2002Morris et al., , 2013)).This marsh has transgressed over an old beach ridge such that 0.25-0.40m of marsh sediment overlies well-sorted, tightly packed beach sands.This is especially evident at the Goat Island (GI) site which is proximal to the uplands (Gardner et al. (1992) and the marsh platform immediately overlies sand without a mudflat deposit.North Inlet is a tidally dominated salt marsh estuary with a watershed area of ∼75 km 2 , with minimal freshwater input (Morris et al., 2002).North Inlet drains a 32 km 2 estuary of which ∼82% is intertidal mudflat and salt marsh dominated by the grass S. alterniflora Loiselt, and the remaining ∼18% is open water.Figure 1b shows the co-located Surface-Elevation Tables (SETs) and CPTs, which include Debidue Creek (DDC), Oyster Landing Marsh, Old Man Creek (OMC), GI, Bly Creek, Sixty Bass Creek (B60), and South Town Creek (STC).These sites were selected because of the long-term SET measurements.
Figure 1c shows the hydroperiod or percent time flooded as a function of elevation normalized to the mean high water (MHW) and mean low water (MLW), and Figure 1d shows the aboveground and live belowground biomass productivity with hydroperiod.The live belowground biomass productivity was predicted using the Coastal Wetland Ecological Model (Morris et al., 2022) using empirical data from North Inlet estuary (Morris et al., 2002(Morris et al., , 2013)).The MHW and MLW were reported in Morris et al. (2013) as 70 cm NAVD88 and -70 cm NAVD88, respectively.Because North Inlet is a marsh predominantly flooded by astronomical tidal components, the percent time flooded (%F) in Figure 1c is determined approximately by the vertical position (relative elevation Z) of the marsh landscape between the MWH and MLW levels.Morris et al. (2013) approximate the %F or hydroperiod as a linear line bounded by MHW and MLW.When the percent time flooded is 100, the dimensionless depth (MHW Z)/(MHW-MLW) is unity, where the marsh elevation Z corresponds to MLW. Figure 1d presents the aboveground standing biomass from a marsh organ in North Inlet as a function of the dimensionless depth (MHW Z)/(MHW-MLW).The peak aboveground biomass of ∼1,300 g/m 2 occurs at 0.25 (i.e., 35 cm NAVD88), and the standing biomass corresponding to MHW and MLW are approximately 965 and 520 g/cm 2 , respectively.

Cone Penetrometer Testing
Conducting root shear strength measurements of wetland vegetation is complicated by the physical and hydraulic heterogeneity inherent to wetlands, including rhizome development, presence of shells and organics, burrowing crabs, and very low strengths (Blum & Roberts, 2012;Cadigan, Bekkaye, et al., 2022;Cadigan, Jafari, et al., 2022; Jafari, Harris, Cadigan, & Chen, 2019; Jafari, Harris, Cadigan, Sasser, et al., 2019).Because the root system and underlying organic-rich mudflat soils are particularly difficult to sample in an undisturbed manner, cone penetrometer testing were used to characterize the in situ stratigraphy and shear strength.Jafari, Harris, Cadigan, Sasser, et al. (2019) developed a methodology for wetlands to conduct rapid triplicate tests while limiting variability due to operator influence, measuring resistance with 0.5 cm resolution with depth, provide a high degree of repeatability, and overcome issues of shells and roots on smaller devices.The CPT used within this investigation consisted of a modified sleeve with 2.5 cm length perpendicular fins attached to the sleeve was used to increase the resolution of soil resistance throughout the vegetated root-mat that has proven successful in coastal wetlands in Louisiana, USA (Cadigan, Bekkaye, et al., 2022;Cadigan, Jafari, et al., 2022;Cadigan, Stagg, et al., 2022;Harris et al., 2020;Jafari, Harris, Cadigan, Sasser, et al., 2019), the Everglades in Florida, USA (Harris et al., 2023), and Avalon, NJ, USA (Harris et al., 2021).

Results
Shear strength profiles from the cone penetrometer at each site are found in Figure 2. CPT profiles from previous investigations typically indicate that the peak strengths are located in the upper 15 cm and decreased with depth, which corresponds to fewer live roots (Cadigan, Jafari, et al., 2022;Cahoon, 2003;Graham & Mendelssohn, 2014;Turner, 2011).The shear strength decreases asymptotically below the root zone until it approaches a constant shear strength with depth, where there is stable organic matter (Gardner et al., 1992).Moving from north  (Morris et al., 2002(Morris et al., , 2013)).
to south across North Inlet indicates that the depth of holocene sand is shallow at DDC and the thickness of the mudflat increases toward STC.Based on Gardner et al. (1992) the mudflat thickness is 110 and 250 cm at OMC and Sixty Bass, respectively, where the mudflat extends to depths greater than 4 m at STC.The CPT profiles were conducted to depths of 1 m.The shear strength of the organic-rich mudflat below the root zone ranges from 10 to 40 kPa, indicating that this deposit is consistently present across North Inlet.Shear strengths below 100 kPa typically exhibited a constant shear strength within the root zone, suggesting that a dense root mat is responsible for the sharp peak strength.Across the sites, the peak root shear strength increases from a baseline of mudflat toward higher elevation salt marsh.Moreover, at GI in Figure 2f, cone penetrometer soundings were conducted within a control and nutrient-enriched site within the high marsh elevation.At the control site, the average shear strength was 145 kPa compared to the nutrient enriched strength of 160 kPa.A comparison of these values suggests that the nutrient enriched plot is stronger than the control plot.
Peak root shear strength declines parabolically with percent flooding or increasing inundation (Figure 3a).Using a shear strength of 10 kPa as a baseline for mudflat (see gray diamond tests in Figures 2a and 2d), the root shear strength increases parabolically with decreasing percent time flooded.The peak shear strength approaches 150 kPa at a percent time flooded of 0.17, which corresponds to optimum aboveground biomass productivity in Figure 1d.The aboveground productivity and root shear strength (see green crosses) concomitantly increase in a parabolic relationship in Figure 3.The majority of root strength gain occurs in a narrow range of biomass productivity of 800 to 1,000 g/m 2 .

Discussion
Figure 3 indicates that root shear strength is indeed interlinked with the wetland hydroperiod and aboveground productivity.Because higher root: shoot ratios in S. Alterniflora are indicative of less favorable soil conditions (Nyman et al., 1995) it can be surmised that the root shear strength also dramatically decreases within a narrow range of belowground productivity.In fact, the decrease in root strength from 20 to 90 kPa in Figure 3 is significant and is best exemplified by the difficulty in walking on the marsh platform.It also raises the question to what extent belowground productivity is correlated to root shear strength.
Variation in root shear strength is linked to live root biomass in marshdominated wetlands (Cadigan et al., 2020;Cahoon, 2003;Cahoon et al., 2020;Coleman & Kirwan, 2019;Graham & Mendelssohn, 2014;Sasser et al., 2018;Silliman et al., 2019).In a study of 11 coastal marsh types in Louisiana, live belowground biomass explained the most variation in root shear strength across sites with soils ranging from organic to mineral (Sasser et al., 2018).Using the elevation gradient at North Inlet as a testbed, one explanation is that the decreasing live root biomass corresponds to a less connected network of rhizomes that create the resistance against the cone penetrometer during shearing (see trend of longer, denser root networks in Figure 4a).For low live root biomass, it is unlikely that sufficient interconnect roots exist to increase the macro root strength.A series of X-ray computed tomography images of cores at varying elevations could elucidate this hypothesis, similar to the method in Davey et al. (2011).

10.1029/2023GL106531
The asymptotic increase in root shear strength at peak live root biomass suggests a denser root system is not the only factor contributing to the CPT measured root system strength.Instead, it could also be attributed to increasing tensile strength of roots.Wetland plants growing under relatively submerged conditions often experience oxygen deficiencies at root tissues, which they counter by forming aerenchyma that enhances metabolic efficiency and facilitates internal oxygen transport (Armstrong, 1980;Armstrong et al., 1991;Jackson & Armstrong, 1999).For example, the amount of aerenchyma for S. alterniflora roots was 179% higher by area for flooded roots compared to drained roots (Maricle & Lee, 2002).Because the percentage of air space (i.e., porosity) in roots is significantly greater in flooded plants than drained plants (Končalová, 1990;Maricle & Lee, 2002;Smirnoff & Crawford, 1983), a logical progression in this analysis would involve taking the same roots and performing tensile strength tests, which should elucidate a weaker strength for higher percentage of aerenchyma (higher air space and porosity).As a result, tensile strength of individual wetland plant roots, which is highly correlated with root morphometrics (diameter, cross-sectional area, and volume) (Hollis & Turner, 2018), may be a key trait, along with soil factors, influencing root shear strength.
Figure 4b conceptualizes these hypotheses on the interplay of macro root system shear strength with the density, structure, and tensile strength of live roots.Beyond live root biomass, the root shear strength could be influenced by soil composition (sand, silt, and clays), porosity, organic content, and porewater chemistry such as nutrients (Day et al., 2011;Jafari, Harris, Cadigan, Sasser, et al., 2019;Teal et al., 2012).These factors are interrelated and thus result in significant interannual fluctuations in productivity from a landscape scale down to a local level (Mendelssohn & Morris, 2002), but precisely how they affect root shear strength is not fully understood.One interesting finding from this investigation was the nutrient enriched plot exhibiting higher strength as compared to the control plot (Figure 2f), which suggests that nutrient enrichment may not weaken the root shear strength and hence belowground biomass productivity, however more work is required to further investigate this (Wigand et al., 2018).
Marsh organs provide the flexibility to extend the experimental percent time flooded to capture productivity decreases due to hypersalinity and inundation.Thus, it is anticipated that the root shear strength concomitantly decreases with the aboveground standing biomass.The paucity of data below a root shear strength of 40 kPa also warrants further testing at lower marsh elevations.Additional shear strength measurements are needed at higher elevations to test this hypothesis, or a pilot-scale marsh organ experiment that facilitates CPT shear strength measurements.This root shear strength is also a function of regional geology as varying sediment characteristics control soil drainage and topography as this can influence inundation uniquely on ebb and flood tides.Finally, these root shear strength have been limited to predominantly limited to wetland grasses but can be expanded to woody vegetation (i.e., mangroves, swamps, coastal forests, etc.).

Implications
Existing numerical models of wetland evolution, of which there are many, simulate the spatial distribution of sediment fluxes and vegetation characteristics from simple empirical models that predict sedimentation patterns as a function of topographic variables (Temmerman et al., 2003) to physics-based models that simulate water and sediment flow paths on the basis of simplified hydrodynamic schemes (D'Alpaos et al., 2007;Rinaldo et al., 1999) or on the basis of a full hydrodynamic description of the feedbacks between tidal flow and vegetation (Temmerman et al., 2005).Connecting the various geomorphological and ecological factors in numerical models is particurlay critical in changing climate conditions in order to understand the physics behind marsh evolution not currently accurately encapsulated by numerical models (Cadigan, Jafari, Wang, et al., 2023;A. J. Payne et al., 2021;A. R. Payne et al., 2019;Saintilan et al., 2023).Indeed previous discussion around the interplay between vegetation, sediment composition in terms of percentage of fine-grained material, and marsh edge geometry notes that the lack of geotechnical knowledge on how vegetation stabilizes salt marshes severely limits the advancement of numerical models of these systems (Bendoni, 2015).Storms trigger wave attack of marsh boundaries and uprooting of the marsh platform (Bendoni et al., 2014(Bendoni et al., , 2015(Bendoni et al., , 2016(Bendoni et al., , 2019(Bendoni et al., , 2021;;Cadigan et al., 2021;Cadigan, Bekkaye, et al., 2022;Howes et al., 2010;Jafari et al., 2020;Priestas & Fagherazzi, 2011;Zhu et al., 2020), yet the inclusion of high quality root-soil and mudflat strength measurements are only now emerging (Cadigan, Bekkaye, et al., 2022;Cadigan, Jafari, et al., 2022;Harris et al., 2020Harris et al., , 2023;;Jafari, Harris, Cadigan, & Chen, 2019;Jafari, Harris, Cadigan, Sasser, et al., 2019).Integrating the components discussed here can help to facilitate large-scale mapping over space and time of the potential impacts to wetland evolution ranging from sea-level rise (SLR) to extreme events like hurricanes, droughts, and heavy rainfall events.

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Emerging satellite and other unmanned aerial technologies that are groundtruthed to CPT-measured shear strength (including of roots) can be cross-correlated to remotely sensed aboveground standing biomass to broadcast root strengths across large regions.More detailed geomorphic models with incorporated strengths that predict the marsh platform elevation will concomitantly result in better predictions of the expansion of the channel networks, wave scour, and lateral erosion of the marsh edge.
CPT to measure root and soil shear strength could also be of interest in terms of predicting the evolution of wetlands in rapidly changing coastal environments, where large-scale mapping and connection of these properties could be used to identify areas at high risk of collapse and release of greenhouse gases within the root system (Cadigan, Jafari, et al., 2022;Chambers et al., 2019;Valentine et al., 2023).This could facilitate new collaborative applications: (a) Mapping wetland root strength under future SLR and climate change scenarios (IPCC, 2023;Koffi et al., 2020;Saintilan et al., 2023;Schoolmaster et al., 2022;Williams & Erikson, 2021), (b) Developing long-term root shear strength trajectory metrics to determine the tipping point above the lower limit of productivity before the marsh collapses (Cadigan, Jafari, et al., 2022;Chambers et al., 2019;Morris et al., 2021), (c) Evaluating the efficacy of restoration interventions (thin layer placement of dredged material and river diversions) for restoring degraded marshes (Harris et al., 2021), and (d) Predicting wetland resistance toward hurricane-induced uprooting, wave-induced marsh retreat, and potential for compaction (Alizad et al., 2016a(Alizad et al., , 2016b(Alizad et al., , 2018;;Cahoon et al., 2020).

Conclusions
The root shear strength of wetland vegetation directly controls or could serve as a proxy for wetlands to withstand disturbances, such as edge erosion from wind-generated waves, uprooting from storm surge and waves, and collapse from excessive inundation.Thus, the focus of this study was to quantify root shear strengths of S. alterniflora in the North Inlet estuary, SC, USA, and understand how it is influenced by hydroperiod.Similar to the parabolic relationship of aboveground biomass developed from marsh organs in a tidally dominated salt marsh, the root shear strength increases with decreasing percent time flooded.When the root shear strength was compared to predicted live root biomass, it was found that increasing root density only partially explained increases in root shear strength, with confirmation needed from X-ray computed tomography scans.It is likely that root shear strength is also influenced by the tensile strength of live roots.Future studies are warranted to investigate the interplay of root shear strength in varying wetland geomorphic settings and incorporate these relationships into landscape models to predict wetland capacity to natural and anthropogenic disturbances.Program (CSAP), and USACE Engineering with Nature (EWN) program.This study would have never happened without the guidance and collaboration of Karim Alizad, now an assistant professor at K.N.Toosi University of Technology in Tehran, Iran.We gratefully acknowledge his exceptional work in this field.In addition, the authors would like to acknowledge Karen Sundberg from the Baruch Marine Field Laboratory for their assistance in the field and data collection and Amina Meselhe, now a graduate student at Oregon State University, for her conceptualization of figures.The views expressed are those of the authors and do not reflect the official policy or position of the US Army Corps of Engineers, Department of Defense, or the US Government.

Figure 1 .
Figure 1.(a) Project location along the U.S. east coast, (b) North Inlet estuary, South Carolina with study sites (blue circles), (c) Relationship of normalized marsh surface elevation to percent time flooded, and (d) aboveground biomass obtained from marsh organ data and live belowground biomass obtained from MEM simulations are correlated to percent time flooded(Morris et al., 2002(Morris et al., , 2013)).

Figure 2 .
Figure 2. Shear strength profiles quantified by the cone penetrometer tests at North Inlet, SC, USA in habitats across a range of elevation: (a) Debidue Creek, (b) Old Man Creek, (c) Sixty Bass Creek, (d) Bly Creek, (e) South Town Creek, and (f) Goat Island.The range of marsh elevations aggregate to three classifications, where low, mid, and high refer to red triangle, orange square, and blue circle, respectively.

Figure 3 .
Figure 3. Developed inter-relationships of percent time flooded (Figure1c) and aboveground standing biomass (Figure1d) with root shear strength.The range of marsh elevations aggregate to three classifications, where low, mid, and high refer to red triangle, orange square, and blue circle, respectively.

Figure 4 .
Figure 4. (a) Conceptual transect showing depth of water and level of above and belowground biomass from the mudflat to high-elevation marsh sites and (b) conceptual relationship between root properties (i.e., live root volume and tensile strength) and root shear strength.