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Published in final edited form as: J Environ Manage. 2023 Dec 12;351:119657. doi: 10.1016/j.jenvman.2023.119657

This Land is Your Land, This Could Be Marsh Land: Property Parcel Characteristics of Marsh Migration Corridors in Rhode Island, USA

Erin Burman a, Nathaniel Merrill b, Kate Mulvaney b, Michael Bradley c, Cathleen Wigand b
PMCID: PMC11492132  NIHMSID: NIHMS2025761  PMID: 38086115

Abstract

Salt marshes, critical habitats offering many ecosystem services, are threatened by development, sea level rise (SLR) and other anthropogenic stressors that are projected to worsen. As seas rise, some salt marshes can migrate inland if there is adjacent, permeable, undeveloped land available. Facilitating marsh migration is necessary for coastal resilience efforts, but extensive coastal development can make finding suitable migration corridors challenging. This work seeks to characterize the changes in land use, ownership, and economic value at the property parcel level within projected 2050 marsh extent for the state of Rhode Island, USA. We find that most parcels currently containing salt marsh are publicly owned, whereas most adjacent parcels projected to contain new salt marsh in 2050 are privately owned. Additionally, parcels containing new marsh in 2050 have 47% higher per-hectare assessed values than parcels containing current marsh. We describe the locations and characteristics of parcels within migration corridors with the lowest per-hectare values that may be the most cost-effective for marsh conservation practitioners to protect. This study highlights the expanding land use types and landowner sets that will be involved in marsh conservation decisions, and the economic value of potential migration corridors where costly tradeoffs may be necessary to promote coastal resilience.

Keywords: Coastal squeeze, sea level rise, marsh migration, coastal resilience, climate adaptation, land ownership

Introduction

Sea level rise (SLR) is a worldwide concern affecting coastal habitats and communities. Rhode Island, located in southern New England, USA, faces accelerated sea level rise, with projections increasing at a rate much faster than the global average (Sallenger, Doran, & Howd 2012). Critical salt marsh habitat in the state, already reduced in area due to human development, is vulnerable to SLR. Future SLR projections forecast major losses for Rhode Island marshes through erosion and drowning in the absence of coastal adaptation efforts such as allowing for marsh migration (Raposa et al. 2017). Marsh migration can occur when rising seas inundate adjacent areas, allowing new marsh to grow in migration corridors1. For marshes to successfully migrate, they need permeable landward areas with conducive environmental conditions upon which to grow and spread.

In rural areas with less development, such as many parts of the Chesapeake Bay, migration has been projected to be constrained primarily by the topography of the landscape (Molino et al. 2022). Many of the initial areas targeted by coastal practitioners to facilitate migration are well-suited to facilitate new marsh, such as agricultural areas, open space, and forested areas (NASA 2021). However, in more urbanized areas, such as the northeast United States, human development is often limiting (Field et al. 2017). “Coastal squeeze,” originally coined by Doody (2004), refers to the extensive development of coastal areas coupled with the loss of marshes through erosion and sea level rise that limits the potential space for salt marsh to thrive and/or migrate (Torio and Chmura 2013). Coastal squeeze results in the loss of ecological and cultural ecosystem services, which can greatly impact local human communities (Smart et al. 2021). Past research on coastal squeeze primarily investigated the loss of coastal habitats due to hardened structures such as seawalls or riprap (Pontee 2013) but has expanded to also consider other impediments to marsh migration such as impervious surface, tourist accommodations, and invasive Phragmites (Luo et al. 2013; Smart et al. 2021). For marshes to be able to migrate on developed coasts, existing lands and properties in the migration corridors need to be managed differently, with managed retreat of some uses (Spidalieri 2020). Overcoming coastal squeeze is critical for marsh migration and is also necessary as a part of greater coastal resilience and climate adaptation planning.

Knowing what kind of properties are in marsh migration corridors helps inform planning for and prioritization of adaptation and conservation actions. While migration onto public lands may require land use change, migration onto private lands presents substantial logistical, financial, and legal challenges (Field et al. 2017; Spidalieri 2020). On public lands, facilitating landward marsh migration requires proactive decisions about land use change. These decisions range widely in cost and effort, from discouraging mowing in marsh migration corridors to the removal of impervious cover like parking lots that hinder establishment of marsh plants. In areas that are not currently publicly owned or protected, decisions about land acquisition or other legal protections will need consideration, and these actions also range widely in costs and effort (Spidalieri 2020). Land acquisition or protection for conservation and migration purposes takes extensive effort to overcome land use-planning challenges (Gerber and Rissman 2012). Such actions require working with landowners who may distrust coastal managers (Perry et al. 2020), lack financial incentives to cease current uses and allow for marsh migration, and/or do not recognize the value of their land for coastal protection in lieu of its current uses (Field et al. 2017). Marsh migration planning must consider these challenges when determining the appropriate site-level decisions.

Stakeholders in Rhode Island have begun to incorporate marsh migration potential into coastal resilience planning. Recently, a Rhode Island statewide assessment added migration potential as a component of salt marsh condition and vulnerability (Kutcher et al. 2022). This component estimates and characterizes landward marsh migration potential using a combination of remote sensing data and field observations of features (e.g., geomorphology, hydrology, vegetation) within a 61-m landward buffer from the marsh edge to coincide with the state management authority of 200 feet landward from coastal features such as marshes (CRMC 2023). Using these data, Kutcher et al. (2022) determined a migration potential score to help guide efforts to prioritize and plan for landward marsh migration on a statewide scale. In addition, the University of Rhode Island (URI) Environmental Data Center has created a parcel-level prioritization tool for marsh migration decision-making that incorporates potential migration scenarios, size of the parcel, presence of adjacent conservation, and soil types in identifying priority parcels (Bradley et al. 2022, URI 2023). This tool begins to incorporate social and economic information, such as parcel boundaries and some parcels’ official conservation status, into prioritization. This is critical, because although many existing marsh migration prioritization efforts focus on biogeophysical attributes, many of the barriers to migration will be overcoming social challenges (Mulvaney et al. 2022). Other migration planning efforts on the U.S. east coast provide a road map for considering social and ecological tradeoffs within migration corridors. For example, in the Chesapeake’s Blackwater National Wildlife Refuge’s Blackstone 2100 plan, tradeoffs were compared across road density, compatible land use, and the likelihood of corridor area development. The plan projected that corridors that could support the largest areas of new marsh also be adjacent to conservation lands to allow for continuation of management (Lerner et al. 2013).

Here, we present an analysis of landscape-scale social components of marsh migration for the state of Rhode Island, USA. We overlay marsh migration modeling with existing parcel-level data to investigate changes in land use and estimate some of the potential tradeoffs and costs in enabling marsh migration. The work presented here uses a similar spatial overlay as Bradley et al. (2022), but summarizes the parcel information across more social dimensions, like ownership and economic value. We summarize the status of this front line in dimensions relevant to conservation planning, highlighting the shift in ownership of suitable marsh area, the expanding land use type and landowner set that will be involved in decisions, and the economic value of the land where tradeoffs will need to be made between the built and natural environment.

Site Profile

Like much of the northeastern United States, Rhode Island’s coast is heavily urbanized, with population and housing densities among the highest in the nation (RIDOP 2006). There are high levels of residential, commercial and industrial development including primary or second homes, ports, public parks and beaches, marinas, and more. Much of this land development comes with heavy alterations to the natural ecological features, with the remaining marsh areas in patchy fragments throughout the state. Salt marshes in New England are generally small in area and often exist as narrow fringing systems due to the lack of a broad coastal plain (Roman et al. 2000). While small salt marshes dominate in New England, some marsh systems of notable size are associated with barrier islands or spit systems (Figure 1).

Figure 1. Urban development and salt marsh in Rhode Island.

Figure 1.

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Urban development in Rhode Island is concentrated in the Providence area at the top of Narragansett Bay. Small areas of salt marsh exist throughout the state’s coasts, in the Bay and in coastal lagoons (Photo credit: Jae-Young Son, Cathy Wigand.)

Rhode Island’s salt marshes are likely to be heavily impacted by sea level rise (Raposa et al. 2015). Using historic New England maps dating back to the late 1700s and early 1800s, Bromberg and Bertness (2005) estimated that the region’s coastal marshes had declined on average 37 percent, with the greatest state losses in Rhode Island (53%). Historically, salt marsh losses were attributed to salt marsh conversion to agricultural and urban lands (Roman, 2017; Bromberg and Bertness, 2005) and more recently, to marsh drowning due to accelerated sea level rise (Watson et al. 2017).

Efforts to protect existing salt marsh starts with legal protection through Rhode Island’s Coastal Resource Management Council, tasked with the management of lands within 200 feet (61 m) of coastal features, including salt marsh (CRMC 2023). State-level environmental planning and rules propagated by Rhode Island’s Department of Environmental Management, in coordination with local planning, zoning and ordinances, limit allowable disturbance and nearby development. Federal land management entities within Rhode Island, such as Sachuest Point and Trustom Pond National Wildlife Refuge, own and border salt marsh in the state. A network of land trusts in Rhode Island purchase property for conservation and leverage state and federal grant programs, like NRCS’s Wetland Reserve Easements (NRCS 2023), to perpetuate protection near existing salt marsh and plan for migration (Kutcher 2021).

Methods

To understand potential opportunities and barriers to marsh migration through a social lens, we overlaid results from an existing marsh migration model with commercially available parcel data. We summarized the parcel information in terms of land use (public, private, residential, commercial, etc.), ownership type and economic value, comparing the parcels with existing marsh to parcels projected to contain new salt marsh by 2050. Data were processed and analyzed using Python 3.7, ArcGIS Pro 2.8, and Microsoft Excel.

Data: Sea Level Affecting Marsh Model (SLAMM)

We used existing SLAMM projections (Bradley et al. 2022) to estimate potential marsh migration corridors in Rhode Island. The implementation of SLAMM used in this paper assumes 86 cm of SLR by 2050, corresponding to NOAA’s regional projections for Rhode Island assuming a high rate of sea level rise (Sweet et al. 2017). Other input parameters include elevation and landcover in the study area, tide data, wave fetch, and a sediment accretion rate. The landcover input in Bradley et al. defined marsh using National Wetlands Inventory (NWI) and NOAA habitat data defined in 2010 and 2012, respectively. For the purposes of this study, we refer to these initial conditions as “existing” or “current” marsh. We applied a modeling option that allowed projected marsh to migrate onto developed land to predict the maximum possible extent of migration areas should human development patterns change in the future. SLAMM outputs both a tabular summary and 1 meter resolution raster image layers of the current and projected future marsh area at the assumed localized level of sea level rise (NOAA 2021; Clough et al. 2016(a)). More information about SLAMM and this implementation can be found in the supplementary materials (Section S1).

Data: Parcels

To understand current land use at a state-landscape level, we purchased commercially available parcel-level data for all properties in the state of Rhode Island from Lightbox (2022). For our analysis, we needed parcels’ location and outline, land use type, and assessed value. The LightBox dataset provided all three of these attributes for 85% of parcels in Rhode Island.

Other data

We used an impervious surface raster layer to identify areas such as buildings, parking lots, and roads that might inhibit marsh migration (RIGIS et al. 2011). Additionally, we identified protected and conserved areas in Rhode Island using the Protected Areas Database of the United States (PAD-US). PAD-US areas are “dedicated to the preservation of biological diversity and to other natural, recreation and cultural uses, managed for these purposes through legal or other effective means” (USGS 2022).

Parcel data cleaning

To overlay the parcel data with SLAMM, all the parcel data had to be associated with a single unique boundary. Lightbox tax assessor data with a single set of coordinates (point data) rather than a boundary were dropped. Additionally, we deduplicated the data where multiple parcels with the same location ID and shape existed in the same place. This was most often due to multiple individually assessed condos on the same parcel. When deduplicating, we summed each duplicate’s tax assessed value and assigned it to the single remaining parcel using the “dissolve” tool with location ID as the dissolve field. Additionally, we characterized parcel ownership as public or private based on the parcel’s Standardized Land Use Code label. We further categorized private parcels as residential, commercial, agricultural, and other private. We considered parcel ownership as public if labeled “Government/Public Use” or “Exempt (Fully or Partial),” and classified all other parcels as private. Although “Exempt (Fully or Partial)” included parcels with both obviously public owners (e.g., some state parks) and potentially private owners (e.g., Audubon Society land), it was not feasible to consistently subdivide this category and all such parcels were considered publicly owned in this analysis. However, most private, tax-exempt land uses (e.g., nonprofits, religious institutions) had a Standardized Land Use Code of “Institutional” and were considered privately owned for this study.

Spatial overlay

We overlaid the current and projected future SLAMM raster layer outputs to identify areas of new marsh by 2050. Performing this overlay with the “union” tool allowed us to distinguish between existing marsh projected to be lost, persistent marsh that exists today and is also projected to exist by 2050, and entirely new marsh by 2050. For this study, we focused on current marsh (which includes current marsh projected to be lost as well as persistent marsh) in comparison to entirely new marsh by 2050. The SLAMM model predicted new marsh in the vicinity of current marsh as expected but also predicted small areas of new marsh that were not near any current salt marsh. Thus, for all analyses limited the new marsh to only those areas within a 61 meter buffer of current marsh, reflecting Rhode Island’s Coastal Resource Management Council’s jurisdiction around coastal features (CRMC 2023). This focuses our analysis on marsh migration and areas most likely to be marsh in the future, but at the cost of potentially ignoring completely new marsh areas.

Using ArcPy for Python, we overlaid the SLAMM outputs, the LightBox parcel data, the impervious surface layer, and the PAD-US layer. We then summarized characteristics of marsh and parcels in both current and projected marsh areas by 2050 using the “summarize within” or “union” tools in ArcPy, depending on the characteristic. For methods used to calculate specific summary statistics, refer to Table S2 in the supplementary materials.

Block Island, a sparsely-populated island approximately 14 km south of mainland Rhode Island, was included in analyses but is omitted in map figures in this paper.

Economic value of future marsh land

To summarize the economic value of the land where new salt marsh may exist in the future, we tabulated parcels’ tax assessed value. Tax assessed values are related to, but lower than, market values (Clapp and Giacotto 1992), and market values can be much more expensive than transactions for conservation easements (Field et al. 2017). Relative costs matter most in this analysis and parcel-level market values were not available at scale, so we used the tax assessed values and did not attempt to correct them to estimates of market price. We included only privately owned land in this assessment and dropped any non- or zero-assessed value parcels. Assessed values included the value of land and any buildings. We summarized results by total assessed value across marsh scenarios and by mean/median value per parcel and per hectare.

We sorted the parcels by their value per hectare to create two cost curves for the protection of new marsh area. To focus on the lower end of the cost curve where more economically efficient planning might be conducted, we censored the analysis where there was a cumulative sum of 2,000 hectares of new salt marsh, similar to Gardner and Johnston (2020). The marginal cost curve compares the cumulative area of new marsh from the marsh migration model to its corresponding marginal cost. The total cost curve plots the cumulative area of new marsh against the cumulative sum of the value of parcels. In addition, we created a two-way scatter plot of parcel value per hectare against the new marsh area those parcels might contain.

Results

The overlay of marsh migration projections and parcel data reveals differences in the characteristics of parcels overlapping with projected new marsh in 2050 compared to today (Table 1; Figure 2). By 2050, just 40% of new marsh will be on public versus 60% on private land, whereas today, most (59%) of the marsh is on public land versus 41% of marsh area on private land (Figure 3a). The 869 hectares of new marsh by 2050 are projected to exist on a greater number of smaller parcels (almost 7,900) than the 1,420 hectares of current marsh covering 4,720 parcels. Additionally, a smaller percentage of potential new marsh than current marsh is within PAD-US conserved parcels, whereas a larger percentage of potential new marsh than current marsh is currently used for residential and commercial purposes. The percent of marsh that is classified as impervious surface increases by two orders of magnitude to a projected 10 percent of the new marsh area by 2050 from 0.1 percent of current marsh.

Table 1. Characteristics of property parcels overlapping with current and projected future salt marsh in Rhode Island, USA.*.

Current marsh New marsh 2050 Rhode Island
Marsh extent (hectares) 1,420 869 --
Number of parcels 4,720 7,849 384,595
Median parcel area (hectares) 0.23 0.13 0.09
Number of privately owned parcels 3,194 5,853 339,929
Percent privately owned, by number of parcels 78% 85% 95%
Percent privately owned, by area 41% 60% 77%
Percent publicly owned, by area 59% 40% 23%
Percent residential, by area 17% 33% 49%
Percent commercial, by area 3% 6% 5%
Percent agricultural, by area 5% 5% 9%
Percent protected / conserved, by area 52% 36% 32%
Percent impervious surface, by area 0.1% 10% 13%
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*

For definitions of summary statistics listed here, refer to Table S2 in the supplementary materials. Some salt marsh (~10%, by area) does not overlap with any property parcel. These areas of marsh are omitted from summary statistics of overlap with parcels, but are included in summary statistics of overlap with PAD-US and impervious surface.

Figure 2. Changes in characteristics of parcels overlapping with marsh, current versus 2050.

Figure 2.

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Parcels projected to contain new salt marsh in 2050 are more numerous, average smaller in area, are more likely to be privately owned, and have a higher median per-hectare value than parcels containing current salt marsh.

Figure 3. Land ownership, assessed value, and marsh migration in Rhode Island.

Figure 3.

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A. Land ownership in Rhode Island (left), and an example of marsh projected to migrate from predominately-public to predominately-private land by 2050 (right). B. Assessed value per hectare in Rhode Island (left), and an example of marsh migrating from lower to higher assessed value parcels by 2050. Note that the city of West Greenwich in western interior of the state was missing from the parcel data.

New marsh is projected to overlap with parcels with higher per-area values (Table 2). Private parcels containing current marsh have a median assessed value of $1.5 million per hectare, while private parcels with projected new marsh by 2050 have a median assessed value of $2.2 million per hectare. There is a large range in assessed value for these private parcels with projected new marsh, ranging from $260 to $94 million per hectare, with a mean value of more than $750,000 and median value of over $400,000. The value of the land and built infrastructure near current and future marsh can inform priorities for marsh protection and planning for migration, highlighting where the costs of conservation are relatively lower or higher, as measured by the economic value of the land.

Table 2. Assessed values of property parcels overlapping with current and projected future salt marsh in Rhode Island, USA.

Assessed values only include privately-owned parcels.*

Current marsh New marsh 2050 Rhode Island
Total assessed value of all privately owned parcels $3.1 billion $4.5 billion $120 billion
Median assessed value of parcels $484,200 $440,100 $241,200
Mean assessed value of parcels $968,589 $767,911 $353,562
Median assessed value per hectare $1,457,804 $2,188,684 $2,525,741
Mean assessed value per hectare $2,522,169 $3,544,217 $3,476,930
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*

For definitions of summary statistics listed here, refer to Table S2 in the supplementary materials

By sorting private parcels by their per hectare value, we can identify the parcels, groups of parcels, and general areas that will likely be least expensive to protect for marsh migration. Figure 4 shows the marginal and total cost curves for areas of new marsh. For example, protecting 50 hectares of future marsh through purchasing the lowest assessed value parcels would require 141 separate purchases distributed throughout the state, with some clustering in Kent and Washington counties (Figure 5). Nearly all of these parcels (96%) are classified by the data provider as vacant land, with a median assessed value of $3,300 and a total assessed value of $5.9 million. These 141 parcels have a mean area of 4.5 hectares and a median area of 0.8 hectares. Efficient marsh migration conservation planning may need to be piecemeal and opportunistic, economically and spatially. A prioritization of this type based on economic value ignores contiguity goals in conservation; however, our methods of identification of future areas of salt marsh (defined as being projected within 61 m of existing marsh) address some of this dimension.

Figure 4. Marginal (top) and total cost curves (bottom) based on parcel assessed value for hectares of new marsh within the 2050 projected salt marsh area.

Figure 4.

Figure 4.

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Each dot represents a single parcel, sorted left to right by their per hectare values. The plots are censored at 2000 hectares of new marsh (x axis) to focus on the potentially more efficient choices for protection.

Figure 5. Privately owned parcels that contain a combined 50 hectares of projected new marsh by 2050.

Figure 5.

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These 141 parcels have the lowest assessed values per hectare, representing the likeliest least-expensive group of private parcels possible to protect to conserve for 50 hectares of new marsh. Points are translucent to show density.

With the shift to a larger number of parcels in projected marsh scenarios, the transaction costs of land protection will be a barrier and an important consideration in prioritizing scarce resources. Thus, acquiring fewer, larger parcels may be of greater interest to conservationists than an equivalent area of more, smaller parcels. Figure 6 shows the marginal costs of parcels scattered against the projected marsh area those parcels contain, with larger areas of marsh for a similar value potentially having greater conservation value. As an example, prioritizing parcels containing greater than one hectare of projected new marsh by 2050 with assessed values less than $1,000/hectare yields a priority list of seven parcels in Rhode Island. All seven are classified as vacant land. These relatively cheaper and larger marsh area parcels are scattered throughout the state, though four are in Newport County.

Figure 6. Parcels containing new marsh within the 2050 projected salt marsh scattered by the parcel’s cost per hectare and the number of hectares of marsh per parcel.

Figure 6.

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Each dot represents a single parcel. The total area of potential new marsh per parcel may be an important priority along with the cost, as protecting each parcel, or part of a parcel, results in transaction costs coordinating with landowners.

Establishing and protecting marsh migration corridors is one management approach to build coastal and salt marsh resiliency. Projections for marsh migration corridors in Rhode Island suggest marked transitions from current marsh on large public parcels with lower assessed values to future marsh on smaller, higher-value, privately owned parcels. The change is predictable because current marsh lands are usually illegal or undesirable to build upon and have persisted where protected on public properties, yet are often adjacent to areas that have become highly developed and valued due to their coastal location. As marsh migrates from protected, undeveloped areas towards developed, impervious areas with property owners who may not want marsh on their property, protecting these marsh migration corridors will present complex decisions requiring significant costs and efforts.

Our analysis reveals the extent of marsh migration onto a new set of property parcels at a landscape scale. The private parcels in migration corridors represent thousands of landowners who make a highly complex set of decisions about their properties based on their attitudes, values, and use of the site (Epanchin-Niell et al. 2022). Initial work such as Field et al. (2017) has begun to understand these behaviors and identify conservation practices that may be more acceptable to homeowners. However, actual land use change due to marsh migration is just beginning, so ultimate decision-making and outcomes remain unknown. Even on public property, where projected marsh migration has fewer barriers to allowing for marsh migration, it will not all be converted to marsh. Land is used by individuals and societies for a myriad of reasons that are important to understand, and marsh migration is just one piece of that broader picture.

It is a challenge for coastal resilience practitioners, governments, and other decision-makers to prioritize parcels that enable marsh migration for conservation (URI 2023). With limited budgets, decision-makers would prefer to minimize costs, but coastal properties are often expensive. These high values reflect people’s perceptions of risk, reward, and what can be done with the property in a regulatory context. Our cost curves exemplify one economic prioritization tool, assuming outright buyouts of parcels for sea level rise and marsh migration. While conservation will be expensive, our cost curves show that there is a range of possibilities for how to spend coastal management and conservation funds. Coastal managers might prefer to buy several larger plots rather than many smaller plots, because each plot purchase can take considerable time and effort to organize through landowner engagement and transaction costs. Additionally, much protection currently does and will continue to take place in the form of easements, not requiring the purchase of entire parcels. Our cost curves assume outright buyouts but can suggest relative costs of easements. However, though easements are popular in some conservation discussions, Field et al. (2017) found that conservation easements were the least-preferred option among various sea level rise conservation activities among coastal landowners who instead preferred future interest agreements, direct purchase, or restricted covenants. Stakeholder priorities, feasibility, and available funding, as well as the willingness to conserve a specific parcel or set of parcels may drive conservation decisions and many of the best practices for implementation remain uncertain. Consequently, the future realized costs of conservation will depend on the interaction of many factors such as the legal interpretation and enforcement of marsh protection, landowner choices, the relative costs of conservation easements, and the progression of the real estate market over decades.

Examples of coastal resilience planning from other states can provide roadmaps for conserving private properties in marsh migration corridors. In Maryland, the Department of Natural Resources has an ongoing, extensive effort to prioritize areas for conservation in response to impacts from climate change and sea level rise. As a part of this effort, they assessed the overlap of marsh migration projections and parcels to prioritize land acquisitions and conservation easements on properties that may gain marsh in the future (Papiez 2012). Other programs highlight the ambivalence of stakeholders about acquiring land that is losing value due to inundation. Areas that will be wet enough to support marsh are likely to experience flooding of residential or commercial infrastructure at that site and may have lower assessed values accordingly. These areas may not be prioritized in purchasing because buyers think the parcels and infrastructure should be retired. For example, Spidalieri (2020) described a program, GreenPrint, for the state of Maryland that identifies land projected to be submerged by 2050 so that the state does not purchase those lands. On the other hand, we have anecdotally noted lots of high-value residential development on marsh-adjacent properties in Rhode Island, suggesting that low-lying coastal properties are maintaining or even gaining value. The uncertainty of parcel values and how they will change in the future, limited availability of funds for land acquisition, and varying motivations of stakeholders makes purchasing and conservation decisions difficult, but preliminary data on parcels and their attributes can contribute to the identification and prioritization of different possible migration corridors.

Marsh migration will not always be the best solution to marsh loss. Where marsh migration is not ecologically, economically, or logistically feasible, natural resource managers have considered other adaptation and restoration actions to restore drowning or impaired marshes and increase resilience to climate change stressors. These adaptive remedies, applied singularly or in unison, include applying clean sandy sediments on the marsh to increase its elevation (Messaros et al. 2010; Davis et al. 2022; Raposa et al. 2022), installation of living shorelines to provide shoreline and marsh protection (Mulvaney et al. 2022), and installation of runnels (shallow ditches typically 0.15 – 0.3 m wide and in depth) to drain impacted areas of ponded water on the marsh platform to allow for revegetation (Besterman et al. 2022; Watson et al. 2022).

Methodologically, our study demonstrates the potential of commercial parcel data in characterizing the social dimensions of conservation and land use planning. Aggregated parcel-level data have historically been available only at an individual parcel, or at best, town-by-town level, when made publicly available at all. The increasing availability of commercially aggregated, parcel-level data for larger scales allows for better anticipation of the regional or state-level decisions that will be necessary to enable marsh migration and coastal adaptation planning in general. Benefits of such commercial parcel data include standardization and frequent updates. However, these data are constrained by the quality of data provided to commercial aggregators by individual municipalities. Nonetheless, these types of parcel data will be especially useful for resource managers in urbanized areas where extensive land development along the coast may be a barrier to migration, as described for coastal Rhode Island.

This study had some limitations, with uncertainties in data and models potentially affecting our results. Data layers may have errors in extent, as exemplified by our overlay indicating that 0.1% of current marsh was impervious. Additionally, the tax assessed valuation data used here is an imperfect approximation of actual purchasing costs. While tax assessed values are lower than actual market costs, is it also possible that coastal parcels are overvalued and will become cheaper in coming decades as they become inundated. Thus, it is unclear whether the value tabulations presented here are systematically too high or too low. Also, the parcel data were missing some coastal parcels, especially in the town of Narragansett, and it is unclear if the missing data were missing at random or in correlation with land use and assessed value. In addition, while our decision to combine duplicate parcel geometries and sum their assessed values appears to have improved the validity of parcel values, if parcels were duplicated erroneously they would now be overvalued. Finally, our use of a SLAMM implementation that did not protect development from marsh growth and assumed a high rate of sea level rise likely overestimates future marsh extent. However, as previously noted, we consider this an advantage in elucidating the maximum extent of possible future marsh area.

Conclusion

We identified property parcels overlapping with projected marsh migration corridors that represent the front line of migrating marsh. These areas will need to be actively managed for land use change to allow marsh to migrate landward and continue provisioning key ecosystem services. Most of these parcels are private properties—like homes, businesses, and more—that would be expensive to conserve, both in terms of effort and cost. Landowners and communities have existing priorities and uses for these parcels, and conservation stakeholders have limited funds, so promoting resilience will require skilled engagement, careful prioritization, and likely a variety of conservation strategies including outright purchasing, conservation easements, and more. As states and communities continue to plan for climate adaptation, considering these options need to be incorporated into decision making to optimize conservation spending and ecological protection.

Supplementary Material

Supplement1

Footnotes

1

We use the term “migration corridors” in this paper to refer to any land that is not currently marsh, but is projected to become marsh in the future in our modeling scenario.

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