Variable Effects of Experimental Sea-Level Rise Conditions and Invasive Species on California cordgrass

Sea-level rise (SLR) will produce unprecedented changes in tidal marsh systems that already cope with daily tidal perturbations, disturbances from storms, and salinity changes from droughts and runoff events. Additionally, negative impacts from non-native invasive species may alter marsh plants’ susceptibility to SLR stressors like inundation and salinity. Increasingly, tidal marsh communities must tolerate both changes in the physical environment from SLR and invasive species impacts. To assess the response of a threatened tidal marsh cordgrass (Spartina foliosa) to both stressors, we implemented a �eld experiment in San Francisco Bay, CA, USA, exposing cordgrass to a treatment that extended tidal inundation projected with SLR using a recently developed in situ method. At one of two �eld sites, we also enclosed the cordgrass with or without the invasive European green crab, Carcinus maenas. We found that cordgrass responded negatively to longer inundation, although these effects varied by site and year. In higher inundation treatments, cordgrass survival increased with increasing surface elevation of the plot. Cordgrass survival was lower in the presence of invasive crabs relative to controls. We did not �nd interacting effects of increased inundation and green crab presence on any response variables, which highlights the need to consider how latent or sequential effects of multiple stressors may affect ecosystems. This study demonstrates signi�cant biological responses to invasive species and inundation. Evaluating relative effects and timing of multiple stressors, especially those induced by climate change and invasive species, will help us to manage threatened ecological communities in a changing world.


Introduction
Sea-level rise is projected to produce unprecedented changes in tidal marshes that have experienced habitat degradation from land use change and other stressors (Dahl 1990).Tidal marshes already must cope with daily tidal perturbations as well as stochastic disturbances from large storms and salinity changes from droughts and runoff events.Sea-level rise is projected to increase salinity and inundation in estuaries (Cloern et al. 2011) and periods of hypoxia (Morris et al. 2002) likely increasing the edaphic stress experienced by ecological communities.Increased inundation associated with sea-level rise can negatively impact plant communities through increased levels of hydrogen sul de or other toxins that directly impact rhizomes and roots (Cronk & Fennessy 2001), and limit biomass, growth, and nutrient uptake (Mendelssohn andSeneca 1980, Koch andMendelssohn 1989).Tidal marsh plant tolerance to inundation, salinity stress from tidal uctuations, and competition often determine the range limits of plant species along an estuarine gradient (Bertness and Ellison 1987).How inundation affects tidal marsh community structure depends on the plant species' susceptibility to inundation stress and the rate of increased inundation from sea-level rise.
Other biotic interactions, like predation and facilitation through the amelioration of low oxygen or high salinity conditions (Zhang and Shao 2013), can also structure tidal marsh communities (Bertness 1985, Bertness andGrosholz 1985, Bruno andBertness 2001).
Non-native species invasions are increasingly common, and especially prevalent in coastal systems that experience heavy shipping tra c and human use.Non-native invasive species can decimate native coastal plant populations and impact nutrient cycling in coastal and other systems (Garbary et al. 2014, Gallardo et al. 2016).As such, negative impacts from non-native species may alter marsh plants' susceptibility to sea-level rise stressors like increased inundation and salinity.
In order to persist, marsh vegetation and the organisms that utilize it will likely need to withstand or adapt to both physical changes from sea-level rise and impacts from introduced species.Effects of multiple stressors can be additive, synergistic or antagonistic.On the east coast of the US, Crotty et al. (2017) found synergistic negative impacts of increased inundation and native crabs on a native foundational plant species.Increased inundation resulted in sediment softening that facilitated burrowing and grazing by crabs (Sesarma reticulatum) which decreased both above and belowground smooth cordgrass (Spartina alterni ora) biomass.Using projections from models assessing sea-level rise scenarios, they found that marshes previously impacted by die-off were projected to be more impacted by future sealevel rise.Since crabs will readily move from areas with harder sediment to adjacent habitat with softer sediment to forage (Crotty et al. 2017), marsh die-offs may increase exponentially with sea-level rise and without control of S. reticulatum.However, the sign and magnitude of crab impacts, both direct and indirect, on tidal marsh plants are context-dependent, ranging from severe negative associations, such as runaway herbivory, to positive associations like oxygenation of the sediment through bioturbation In this study on the west coast, USA, we experimentally increased inundation in situ in areas of tidal marsh cordgrass (Spartina foliosa), and additionally exposed these experimental areas to invasive crabs, Carcinus maenas.We gathered physical and biological data to understand how tidal marsh vegetation responds to these two stressors.We hypothesized that: 1) in the presence of increased inundation, cordgrass growth and survival, and redox potential, would decline, sul de would increase, and ammonium would increase due to lower redox potential inhibiting transformation to nitrate, 2) in the presence of invasive crabs, cordgrass growth and survival would decrease, and redox potential, and ammonium from crab excrement (Montague 1980), would increase, and 3) invasive crabs and inundation would interact to negatively affect redox potential and cordgrass growth and survival, despite any positive effects on redox potential from crab bioturbation.

Study Site
China Camp State Park, a component site of the San Francisco Bay National Estuarine Research Reserve (NERR), is an ancient and centennial marsh complex with extensive meadows of California cordgrass (Spartina foliosa) and little anthropogenic impact in comparison to surrounding marshes.The native S. foliosa is a low-elevation marsh "foundation" species in San Francisco Bay that serves as habitat and nesting ground for a range of species including endangered animals such as Ridgway's Rail (Rallus obsoletus) and Salt Marsh Harvest Mouse (Reithrodontomys raviventris).It is a focus of restoration throughout San Francisco Bay via efforts of California State Coastal Conservancy's Invasive Spartina Project.San Francisco Bay is a highly invaded estuary and the number of invasions are rapidly accelerating (Seebens et al. 2017).Spartina foliosa is also threatened by invasion of hybrid cordgrass (Spartina foliosa x Spartina alterni ora, Ayres et al. 2003).Spartina foliosa is relatively tolerant of inundation and predicted to increase at moderate rates of sea-level rise, but signi cantly decline at higher rates (Parker et al. 2011).This current study was conducted in Spartina foliosa meadows in the Bullhead Flat and Buckeye Point sites of China Camp State Park (Fig. 1).San Francisco Bay experiences mixed semi-diurnal tides and is projected to experience between 10 and 20 cm of sea-level rise by the year 2050 (Vitousek et al. 2017).Near China Camp State Park, predicted MLLW-MHHW tidal range is 1.80m (Gallinas Creek, NOAA tide station 9415052, http://tidesandcurrents.noaa.gov).An increase of 20 cm of sea-level is projected to result in nearly double the inundation time at current mean higher high water at a site near where this study took place (Janousek et al. 2016).

Green Crabs
In addition to sea-level rise, another potential stressor for S. foliosa is the non-native European green crab, Carcinus maenas, which was introduced to San Francisco Bay in the 1980s and has spread along the west coast, USA (Cohen 1998).In San Francisco Bay, these crabs are abundant mostly in low intertidal marsh areas, often co-occurring with S. foliosa, and can negatively impact the establishment of newly planted S. foliosa in San Francisco Bay (Gonzalez et al. 2023, in press).C. maenas consume a broad range of smaller invertebrates including native bivalves, native crabs and surface-feeding amphipods (Grosholz et al., 2000, Neira et al. 2006), and can out-compete native crab species for food (Cohen 1998).Its foraging also leads to poor survivorship of tidal at fauna by lowering sediment organic matter, redox potential and chlorophyll a (Neira et al., 2006).

Experimental Design
Small-scale mesocosm experiments exploring impacts of increased inundation (Spalding andHester 2007, Cherry et al. 2009) may not re ect natural conditions as well as in situ experiments.Previous eld experiments exploring impacts of inundation and other physical stress on marsh vegetation have used space for time approaches including marsh elevation gradients as inundation treatments and areas with naturally occurring poor drainage (Schile et al. 2011), but these are often confounded by other parameters that are coupled with elevation or lack of tidal ushing.'Marsh organs' are a useful method to quantify effects of tidal inundation on sediment characteristics, but previous studies were limited by the "bottle effect" of the organs (Schile et al. 2017) and the inability to assess changes to animal communities in the soil.We therefore crafted experimental 'marsh boxes' to manipulate inundation in situ (based on the design in Cherry et al. 2015), as described below, to explore a larger suite of biological and physical responses.
At Buckeye Point in spring of 2021, we installed eight marsh boxes (1m x 0.6m x 0.6m) in areas of S. foliosa that were inserted 10cm into the sediment to reduce lateral water drainage (Fig. 2).These plots were accompanied by eight unaltered control areas of S. foliosa for comparison.As boxes may impact sediment characteristics and ow around the plots, we also installed eight partial marsh boxes, which allowed water to ow out at the corners, to account for artefactual effects of the boxes.For manipulation control plots, we dug a thin strip around the plots to simulate disturbance from box or partial box installation and cut through any belowground root masses connected to the plot.Over winter 2021, boxes lled with sediment and/or were broken due to waves and heavy tidal ows, affecting the S. foliosa within and likely disrupting natural processes occurring in boxes.Therefore, in spring 2022, we reduced the number of replicates to six for all treatments and moved all boxes approximately one meter away from the previous box's location into an area of unaffected cordgrass.Sets of three inundation treatments were contained in blocks which were distributed across the experimental area, with a total of 48 plots and eight blocks in 2021 and 36 plots and six blocks in 2022.
In spring of 2022 we repeated our inundation treatments at an additional site, Bullhead Flat, and added a cage enclosure component (Fig. 2).We enclosed C. maenas in plots with established S. foliosa as well as cage controls (cages, no crabs) and open unmanipulated controls (no cage) (Fig. 2).The enclosures (0.5 m x 0.5 m) were constructed from 7 mm Vexar mesh into which two C. maenas were added (based on natural local C. maenas densities, J. Gonzalez, unpublished data).The cage controls used the same cages but with no crabs.Carcinus maenas individuals used in the experiment were acquired at nearby sites using Fukui collapsible crab traps (60 × 45 × 20 cm, 1.25-cm mesh).Two C. maenas were added in each cage in July to account for death or escape of crabs.Even with crab additions, C. maenas density in cages remained within the realm of natural abundances in San Francisco Bay, which can reach up to four crabs per trap on average (J.Gonzalez, unpublished data).
These three crab treatments were subject to a tidal inundation treatment that simulates the extended tidal inundation projected with sea-level rise, as at Buckeye Point.We installed six marsh boxes (2m x 1m x 0.4m) which each had two in-ow check valves to let water in, and no exit valve, so that the water slowly drained through the open bottom.We also used a partial box as a control, which contained shorter versions of all four walls (0.2m high) with openings at the four corners to allow water to ow more freely in and out.As a third treatment, we had areas without boxes experiencing the ambient inundation regime as unmanipulated controls.These three inundation treatments were established as 2 x 1 m plots separated by at least 2 m.The overall experimental design was set up as six blocks in a transect parallel to the bay-marsh edge to account for habitat heterogeneity common in tidal marshes, such as differences in surface topography and proximity to channels.Within each of the six blocks, the three inundation treatments were randomly assigned, and within each inundation treatment, the three cage treatments were randomly distributed (Fig. 2).
In the center of each of the 18 inundation treatments at Bullhead Flat and 48/36 inundation treatments at Buckeye Point, we took measurements of orthometric height (m NAVD88) using a Real-Time Kinematic (RTK) GPS GS07 GNSS receiver and CS20 LTE controller (Leica Geosystems), and corrected elevation data using benchmarks taken before and after sampling.Positions were received via the Leica California SmartNet RTK network.Orthometric heights of inundation treatment areas ranged from 0.80m to 0.98m at Bullhead Flat and 0.82m to 1.06m at Buckeye Point.

Quanti cation of Biological and Physical Responses
We evaluated water levels in each inundation treatment plot over the course of 8-10 days at each site and year using HOBO U-20L water level loggers (Onset Data Loggers, Cape Cod, MA).Water level in each plot was calculated from temperature and pressure data using the Barometric Compensation Assistant in HOBOware Pro (version 3.7.23)and corrected using local barometric pressure data (Gnoss Field Aiport, NOAA climatological station 00135, https://www.ncdc.noaa.gov/).We calculated average inundation time per plot by summing the number of minutes that water level was 2-4cm above the marsh surface in the plot to avoid effects of rapid data uctuations during shallower conditions (2cm in July 2021, 4cm in June & September 2022).These values then were converted to inundation hours per day.We gathered temperature and light data in two plots without boxes and four plots with boxes using light and temperature loggers (HOBO Pendant Temperature/Light Data Logger).
We monitored plots once a month for twelve weeks (June through August or September) in summer 2021 and 2022 at Buckeye Point, and summer 2022 at Bullhead Flat.During each sampling point we measured the number of total cordgrass stems and stand height as the average height of the ten tallest stems in each plot.Also monthly, we measured redox potential in each plot at 10cm below the sediment surface, targeting the area adjacent to the S. foliosa root system, using a portable Mettler-Toledo mV meter (Mettler Toledo Seven2Go pH/mV Meter).At the end of the experiment, we collected belowground biomass by taking a sediment core in each plot (5cm wide PVC corer to a depth of 25cm).We sieved cores to extract plant roots, dried roots at 60°C and weighed them.We also took porewater samples using porewater sippers inserted 10cm into the sediment during low tide (10cm long porous tubes [0.15µm], Rhizophere Research Products, Wageningen, The Netherlands) at the end of the experiment.Porewater samples were later analyzed for ammonium and nitrate (UC Davis Analytical Lab), and sul des were analyzed based on methods by Cline (1969).

Statistics
We performed statistical analyses using R programming software (version 4.0.3R Core Team 2023).We used linear mixed models and generalized linear mixed models ("lme4" package in R) including a random effect of block or block nested within year, to account for habitat heterogeneity in tidal marshes, and determined the appropriate distribution for each dataset using goodness-of-t statistics.We evaluated whether partial boxes signi cantly in uenced all responses, and if they were in uential, we included those data in the full model (Table S1).We evaluated statistically signi cant differences in inundation hours per day among categorical inundation determinations.We also evaluated effects of C. maenas presence vs. absence, inundation, and the interaction between those two on the following response variables: change in S. foliosa stem density as number of stems/m 2 and stand height over a 11-12-week period, as well as belowground biomass/m 2 , redox potential, and porewater ammonium, nitrate and sul de at the end of the experiment in August or September.We compared S. foliosa stand height, stem density, and belowground biomass during the 2022 season in control plots at both sites, with measurements taken within four days of each other.We evaluated differences in mean temperature/day (°C) and light intensity/day (lumens) among treatments using a generalized linear model.We used the "emmeans" package (estimated marginal means) to evaluate pairwise comparisons post hoc, and deemed differences as signi cant if p values were less than 0.05.

Bullhead Flat
We found moderate increases in inundation in experimental box treatments in 2022.Boxes were inundated for approximately 9% and 6% longer than ambient controls without boxes and partial boxes, respectively, although these were not signi cant increases (Fig. 3, 16.5 ± 1.6 hours/day vs. 15.2 ± 0.1 and 15.6 ± 0.3 hours/day, Z=-1.70, p = 0.207, and Z=-1.30, p = 0.396).Average temperature per day and light intensity per day did not vary between boxes and ambient treatment areas without boxes (Z=-0.90,p = 0.367 and Z=-0.585, p = 0.558).The apparent difference in baseline inundation at the Bullhead Flat and Buckeye Point as shown in Fig. 3 is likely due to differences in ambient water levels during the measurement periods in June and September.We compared water level data from the two sites across a ve-day period in June 2022 and found that the two sites were inundated for a similar amount of time (12.08 ± 0.32 s.e. vs. 12.14 ± 0.61 s.e.hours/day at Buckeye Point and Bullhead Flat, respectively).

Response of Redox Potential and Porewater
Ammonium, Nitrate, and Sul de

Buckeye Point
Spartina foliosa plots within box treatments with increased inundation gained 51% fewer stems than controls without boxes and 43% fewer stems than partial box controls over the course of the summer growing season in 2021 (Fig. 5, Z=-10.04,p < 0.001 and Z=-4.32, p < 0.001, respectively).In addition, average stand height gained over the course of the experiment was 14% lower in treatments with increased inundation relative to ambient controls and 13% lower relative to partial box treatments in 2021 (Z = 2.53, p = 0.031; Z = 3.06, p = 0.006, respectively).In 2022, S. foliosa plots gained 46% fewer stems in box treatments relative to ambient plots without boxes, and 22% fewer stems in partial boxes (Fig. 5, Z=-5.30, p < 0.001, Z=-2.85, p = 0.012, respectively).We found no treatment effect on change in stand height in 2022.We observed yearly variation in S. foliosa growth and survival that may have resulted in reduced treatment effects in 2022.Plots with boxes, without boxes and with partial boxes gained fewer stems over the course of the growing season in 2022 relative to 2021 (Fig. 5, Boxes with increased inundation: Z = 5.46, p < 0.001; Ambient inundation control: Z = 5.71, p < 0.001; Partial box controls: Z = 5.48, p < 0.001).Also, change in S. foliosa stand height within ambient control plots without boxes and partial box control plots in 2022 was signi cantly less than the year prior (Fig. 5, Z=-3.85, p < 0.001; Z=-4.06, p < 0.001).We also found that, in treatments with increased inundation only, in both years, cordgrass stem density at the end of the experiment increased with the orthometric height of the plot, signi cantly so in 2021 (Fig. 6, 2021: R 2 = 0.68, p = 0.012; 2022: R 2 = 0.61, p = 0.067).A similar pattern emerged when we compared change in cordgrass stem density over 12 weeks with orthometric height (2021: R 2 = 0.61, p = 0.022; 2022: R 2 = 0.29, p = 0.27).The amount of belowground biomass per plot in plots with increased inundation did not differ across inundation treatments in August 2022 (Χ 2 = 1.56, p = 0.459).

Discussion
Through our novel approach of manipulating both the presence of an invasive species and tidal inundation in situ, we found strong negative effects of increased inundation and green crabs (Carcinus maenas) on the aboveground growth of Spartina foliosa.
Marsh boxes increased inundation relative to partial box and ambient control treatments at both sites, but this increase was not signi cant in 2022 in Buckeye point or Bullhead Flat due to high variability in water retention among box treatments.Also important to note is that the inundation levels that these boxes captured are on the low end of what we might expect with sea-level rise (i.e., a doubling of inundation time, Janousek et al. 2016).Spartina populations are predicted to be unaffected, or even increase, with moderate rates of sea-level rise, yet decline with substantial increases as sea level continue to rise (Parker et al. 2011).This modeled prediction is supported by experimental work that found Spartina alterni ora stem density increases with moderate increases in inundation yet declines under extreme sea-level rise scenarios (Ober and Martin 2018).Signi cant impacts to stand height and belowground biomass may only occur with higher levels of inundation over prolonged periods.The responses we found in the present study may increase in magnitude or potentially change sign as sea-levels rise and inundation levels increase.
Despite the modest increase in inundation, and consistent with our hypothesis, we found that S. foliosa responded negatively to increased inundation, although these effects varied by year and site.At Buckeye Point, the increase in S. foliosa stem density and height over the course of the growing season in 2021 was signi cantly less in box treatments relative to ambient controls without boxes and/or partial boxes.Decreased S. foliosa stem density and stand height was likely driven by increased inundation and subsequent decreased oxygen and cascading changes in the sediment in 2021.Redox potential was lower in 2021 in plots where inundation was experimentally increased, and these low oxygen conditions may have contributed to higher levels of sul de and ammonium coupled with low concentrations of nitrate, as oxygen is needed to transform ammonium into nitrate and sul de is produced in anaerobic conditions.These redox potential, sul de, and ammonium responses are consistent with previous work that evaluated sediment chemistry in response to inundation (Koch & Mendelssohn 1989, Schile et al. 2017).High levels of sul de may have functioned to reduce growth and increase stem loss by negatively impacting roots and rhizomes (Cronk & Fennessy 2001, Koch & Mendelssohn 1989).However, there was generally less dense and shorter S. foliosa in Buckeye Point in 2022 relative to 2021 making signi cant treatment effects harder to capture.This could be due to environmental variability year to year affecting wave energy and sediment deposition, as previous studies found correlations between increased wave energy and reduced accretion and shorter and less dense S. foliosa (Swales et al. 2004).Fewer or more stressed S. foliosa could have resulted in decreased uptake of ammonium as well.
However, in Bullhead Flat, S. foliosa did not respond to increased levels of inundation.Increased inundation did not statistically affect redox potential, and nutrient concentrations remained consistent across inundation treatments.Differences in geomorphic context and the integrity of S. foliosa stands in each site may partly explain this difference.The S. foliosa in the experimental area in Bullhead Flat is taller and denser than in Buckeye Point (Fig. 7).A more robust S. foliosa population could be due to site characteristics that provide better conditions for growth and survival.Bullhead Flat is a protected cove with a long, gently sloping mud at that could facilitate wave attenuation from the bay, whereas Buckeye Point is more exposed.Sites with S. foliosa that vary in condition may respond differently to increased inundation from sea-level rise, and future studies could examine how the geomorphic setting and hydrodynamic exposure of sites affect the resilience of foundational vegetation to sea-level rise.
Notably, we found that cordgrass survival within plots exposed to increased inundation was largely dependent on the elevation of the plot; we observed higher stem density in higher elevation plots at the end of the experiment (Fig. 6) and less stem loss through time than in lower elevation plots.These data can be used to inform optimal planting locations to establish S. foliosa in restoration sites so that it may persist with sea-level rise (Fig. 8).This information can also be used to select sites for conservation purposes or sites to restore that have the highest likelihood of persisting with increasing levels of inundation.For example, given a ~ 6% increase in inundation time, managers may consider planting S. foliosa higher than ~ 0.97m to achieve full growth potential in a salt marsh with similar geomorphic characteristics as Buckeye Point.Also, we found that S. foliosa belowground biomass increases with increasing surface elevation at both sites.Turner et al. (2004) documented lower Spartina belowground biomass in impaired wetland sites relative healthy sites.This disparity suggests that the speci c position within the narrow elevation range where these plants exist may in uence their response to sea-level rise, and may have contributed to variability in plant responses in our inundation treatments.This also supports planting S. foliosa in higher elevations within their elevational distribution.Lower elevation plants, characterized by potentially lower belowground biomass, could face more severe impacts from rising sea level over time.
In addition, we found that cordgrass survival decreased in the presence of C. maenas.This nding is consistent with a previous cage study that found reduced S. foliosa stem density in the presence of C. maenas (Gonzalez et al. 2023, in press).Physical disturbance by C. maenas is a possible mechanism, as it disturbs estuarine vegetation in other systems by digging large pits and tearing eelgrass stems (Garbary et al. 2014).However, we did not nd interactive effects of inundation and C. maenas.Stressor interactions may be simultaneous, sequential, or latent, and it could be that there is a temporal decoupling of the most severe effects of these two factors in this speci c system that confounds interpretation of these results (Cheng et al. 2015).Carcinus maenas are less active in the winter and early spring in San Francisco Bay, and more active in the summer season as water and air temperatures warm.This is when their effects on S. foliosa are likely to be captured, as we found in this study.S. foliosa senesces in the winter and regrows from its rhizomal root system each spring through late summer.In the winter, fewer aboveground stems containing aerenchyma that funnel oxygen to their root system may reduce oxygen availability in the soil, leading to increased sul de production, which can in turn have deleterious effects on roots and rhizomes (Cronk & Fennessy 2001).It is possible that effects of increased inundation may impact S. foliosa's senescent root structure more severely than its aboveground growth through the summer growing season.This work focused on the summer growing season, and future studies should evaluate how increased inundation affects S. foliosa and other marsh vegetation over a longer timescale, spanning both the winter, and summer growing season.Negative effects of inundation on S. foliosa belowground biomass in the winter may in uence its growth in the summer, producing sequential multiple stressor effects rather than simultaneous.As such, and consistent with Cheng et al. (2015), we suggest that ecologically relevant local conditions are considered in empirical tests of multiple stressors across systems.

Conclusion
We found a negative response of a threatened foundational plant species to increased inundation and an invasive crab.We also found that these effects varied by site and year due to site-speci c factors and water retention dynamics.These ndings align with predictions of previous studies about S. foliosa populations responding to sea-level rise, indicating that more signi cant impacts may manifest with higher levels of inundation over prolonged periods.Notably, our research highlights the importance of considering surface elevation in planting locations and conservation planning to ensure S. foliosa survival in the face of sea-level rise.Additionally, we observed that C. maenas negatively in uenced cordgrass survival, but did not interact with inundation to affect S. foliosa.Sequential or latent interactions of these two stressors were not studied here but warrant further investigation.Our ndings emphasize the need to consider local ecological conditions when studying the impacts of multiple stressors across different ecosystems.This information can also be used to guide management decisions in coastal areas with green crabs.Crabs sometimes inhabit higher elevation areas, such as mud ats and cordgrass meadows, during high tide when these habitats are inundated, and retreat to lower elevation or subtidal zones at low tide to avoid terrestrial predators (Klein Breteler 1976, Hines and Ruiz 1995).As such, increased inundation of cordgrass may amplify its exposure to crabs.Planting cordgrass in higher elevations may additionally minimize its exposure to C. maenas and mitigate potential negative effects.Also, the quantitative responses associated with inundation time we found in this study can be used to parameterize models that predict future effects of sea-level rise along with invasive species, and provide guidance on priority areas for removal, conservation and/or restoration.Overall, this study contributes valuable insights into the complex dynamics of salt marsh ecosystems in the context of environmental changes and introduced species.

Declarations
We sincerely thank the editors and anonymous reviewers for their time spent reviewing this manuscript and for their thoughtful comments and suggestions.We would also like to thank scientists and staff with The experimental setup of one block at each site, depicting an aerial view of box and crab treatments.

(
Silliman and Bertness 2002, Alberti et al. 2007, Bertness and Coverdale 2013, Bertness et al. 2014, Bertness 1985).Additionally, many physical factors are in uenced by inundation in addition to sediment hardness which could in uence impacts and feedbacks within the community.Lastly, physical and ecological variables, such as tidal regime, and plant and animal species, differ across tidal marshes and may elicit different outcomes.

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