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Between a bog and a hard place: a global review of climate change effects on coastal freshwater wetlands

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Coastal wetlands are significant components of the coastal landscape with important roles in ecosystem service provision and mitigation of climate change. They are also likely to be the system most impacted by climate change, feeling the effects of sea levels rise, temperature increases and rainfall regime changes. Climate change impacts on estuarine coastal wetlands (mangroves, saltmarsh) have been thoroughly investigated; however, the impacts on coastal freshwater wetlands (CFWs) are relatively unknown. To explore the current knowledge of the impacts of climate change on CFWs globally, we undertook a systematic quantitative literature review of peer-reviewed published literature. We found surprisingly little research (110 papers of an initial 678), the majority of which was conducted in the USA, focusing on the effects of sea level rise (SLR) on CFW vegetation or sediment accretion processes. From this research, we know that SLR will lead to reduced productivity, reduced regeneration, and increased mortality in CFW vegetation but little is known regarding the effects of other climate change drivers. Sediment accretion is also not sufficient to keep pace with SLR in many CFWs and again the effects of other climate drivers have not been investigated. The combination of unhealthy vegetation communities and minimal gain in vertical elevation can result in a transition towards a vegetation community of salt-tolerant species but more research is required to understand this process.
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REVIEW ARTICLE
Between a bog and a hard place: a global review
of climate change effects on coastal freshwater
wetlands
Rebekah Grieger
1,2
&Samantha J. Capon
1,2
&Wade L. Hadwen
1,2,3
&
Brendan Mackey
1,3
Received: 10 October 2019 / Accepted: 4 August 2020 /
#The Author(s) 2020
Abstract
Coastal wetlands are significant components of the coastal landscape with important roles
in ecosystem service provision and mitigation of climate change. They are also likely to
be the system most impacted by climate change, feeling the effects of sea levels rise,
temperature increases and rainfall regime changes. Climate change impacts on estuarine
coastal wetlands (mangroves, saltmarsh) have been thoroughly investigated; however, the
impacts on coastal freshwater wetlands (CFWs) are relatively unknown. To explore the
current knowledge of the impacts of climate change on CFWs globally, we undertook a
systematic quantitative literature review of peer-reviewed published literature. We found
surprisingly little research (110 papers of an initial 678), the majority of which was
conducted in the USA, focusing on the effects of sea level rise (SLR) on CFW vegetation
or sediment accretion processes. From this research, we know that SLR will lead to
reduced productivity, reduced regeneration, and increased mortality in CFW vegetation
but little is known regarding the effects of other climate change drivers. Sediment
accretion is also not sufficient to keep pace with SLR in many CFWs and again the
effects of other climate drivers have not been investigated. The combination of unhealthy
vegetation communities and minimal gain in vertical elevation can result in a transition
towards a vegetation community of salt-tolerant species but more research is required to
understand this process.
Keywords Coastal swamp .Sea level rise .Tidal freshwater wetland .Floodplain .Tidal marsh
https://doi.org/10.1007/s10584-020-02815-1
Electronic supplementary material The online version of this article (https://doi.org/10.1007/s10584-020-
02815-1) contains supplementary material, which is available to authorized users.
*Rebekah Grieger
rebekah.grieger@griffithuni.edu.au
1
Griffith University (School of Environment and Science), Nathan (QLD), Australia
2
Griffith University (Australian Rivers Institute), Nathan (QLD), Australia
3
Griffith University (Griffith Climate Change Response Program), Nathan (QLD), Australia
Published online: 15 August 2020
Climatic Change (2020) 163:161–179
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1 Introduction
Coastal wetlands, alongside alpine and arctic systems, and tropical coral reefs, are widely
considered to be ecosystems most threatened by climate change (IPCC 2014;Schuerchetal.
2018). Rising sea levels can cause shifts in wetland vegetation towards dominance by more
salt-tolerant species (e.g. mangroves), resulting in an inland migration of vegetation commu-
nities where an absence of physical barriers permits (Morris et al. 2002; Raabe and Stumpf
2016;Schuerchetal.2018). Changes in rainfall patterns are also expected, with longer dry
periods likely to further exacerbate impacts of saline intrusion (IPCC 2014). Additionally,
anticipated increases in the frequency and intensity of extreme climatic events (e.g. cyclones,
storm surges, and floods), may significantly, sometimes permanently, alter the structure and
processes of many coastal wetlands along with the ecosystem services these provide (IPCC
2014; Middleton 2016a;Middleton2016b). Furthermore, changing atmospheric gas concen-
trations will alter coastal wetland productivity, shifting systems from carbon sinks to sources of
emissions as rates of decomposition and methanogenesis outstrip the carbon storage capacity
of vegetation and soil biomass (Krauss et al. 2018). Given the significant effects of climate
change anticipated, there is an urgent need to understand the responses of coastal wetlands to
inform the development of effective adaptation strategies.
Most research investigating effects of climate change on coastal wetlands has focused on
saline ecosystems (i.e. mangrove and salt marshes), which support many critical ecosystem
services in coastal regions including habitats for fisheries, carbon sequestration, and storm
mitigation (Mitsch and Gosselink 2015). Coastal freshwater wetlands (CFWs) are similarly
valuable in terms of their provision of critical ecosystem services but have been relatively
understudied (Saintilan et al. 2018). Given the high vulnerability of CFWs to climate change,
as well as the concentrated anthropogenic pressures these wetlands typically face in the coastal
fringe, there is an urgent need to better understand the risks faced by these overlooked
ecosystems.
Here, we define CFWs as freshwater wetlands that exist on coastal plains which have the
potential to be impacted by saline intrusion, either by overland flows (e.g. storm surges and
king tides) or by rising saline groundwater (Grieger et al. 2019). Globally, CFWs are referred
to using various typologies which differ according to their local hydrological and vegetation
conditions. In North America, for example, CFWs are often known as tidal freshwater forested
wetlands (TFFWs) or tidal freshwater marshes (TFMs), which occur in the upper tidal reaches
of estuarine systems where water levels fluctuate with tides but salinity is generally less than
0.5 g/L (Cowardin 1979). TFFWs in the USA and tidal várzeas in Central and Southern
America generally exist in catchments with broad flat coastal plains with large tidal ranges and
high riverine discharge (Duberstein and Krauss 2016; Mitsch and Gosselink 2015;Odum
1988). Where river discharges are lower, coastal plains often support larger areas of estuarine
wetlands compared to CFWs which, in such situations, are typically more dependent on
rainfall and fresh groundwater than riverine freshwater or tidal flows (Rogers et al. 2017;
Saintilan et al. 2018).
Ecological functions of CFWs remain poorly understood even though their importance and
function has been a key research focus in North America and, increasingly, in Australia
(Conner et al. 2007). In the USA, CFWs are rrecognised as important habitat for a large
number of threatened and endangered species, many of which rely on wetlands for survival
(Baldwin et al. 2009). Similarly, in Australia, coastal wetlands along the east coast are known
to be important habitats for a wide range of protected fauna and flora (Traill et al. 2011),
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including many colonial and migratory waterbirds (Wilson et al. 2011). There is also growing
recognition of the many important ecosystem services provided by CFWs including nutrient
cycling (Ensign et al. 2008; Hopfensperger et al. 2009; Loomis and Craft 2010; Von Korff
et al. 2014), carbon sequestration (Krauss et al. 2018; Loomis and Craft 2010; Marin-Muniz
et al. 2014), and flood and storm mitigation (Doyle et al. 2007;Middleton2009).
In this review, we synthesise current knowledge of the effects of climate change on CFWs
globally and identify major knowledge gaps and research priorities. First, we provide a stocktake of
existing scientific literature on CFWs, describing the spatial and temporal distribution of published
studies and the wetland types, research focus and methods employed by these. Second, we explore
the observed responses of CFWs in these studies to key climate change drivers to detect trends in
reported observations. Third, we explore the findings of experimental studies that have examined
responses of CFWs to key climate change drivers. We also examine the application of models to
predict responses of CFWs to climate change. Finally, we provide a synthesis of key findings and
highlight major management and research priorities.
2 Methods
2.1 Search methodology
We conducted a systematic quantitative literature review to synthesise current knowledge of
climate change effects on CFWs. This rigorous, comprehensive method allows reproducibility
with clearly defined rules for inclusion and exclusion of literature (Pickering and Byrne 2014).
Searches of peer-reviewed literature in English were conducted in the online repositories
Thompson ISI Web of Science and Scopus in November 2017 and updated in February 2020
to capture all relevant research published before 2020. The following search terms were used
to capture literature relating to coastal freshwater wetlands; coastal wetland,coastal marsh,
coastal swamp,coastal forest,coastal floodplain wetland,tidal freshwater marsh,
tidal freshwater wetland,andtidal freshwater forest.Thespecifictermcoastal freshwater
wetlandwas not included in this list as there is not currently a clear definition of CFWs which
is used globally. Hence, a broader search was conducted to identify papers specifically relating
to CFWs as well as papers examining a range of wetlands including CFWs. A second set of
search terms, separated from the first by the AND operator, was used to identify literature
within the first set specifically relating to climate change; climate change,global warming,
and sea level rise.
Publications returned from searches were then screened for inclusion in a shortlist based on
the abstract. Papers were excluded if they were solely literature reviews or methodological
studies, did not relate to contemporary climate change (i.e. papers examining responses to SLR
in the geological past), or only concerned saline/estuarine systems (i.e. papers which included
brackish marshes along a gradient from fresh-to salt-water were included). Reference lists of
review papers were also searched so that any relevant literature not captured in previous
searches was also included in the database.
2.2 Data classification
The remaining short-listed papers were read in their entirety, with specific information entered
into an Excel database on (I) citation information, (II) study location, (III) study duration, (IV)
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wetland type, (V) methods used, (VI) attribute of CFWs examined, (VII) aspect of climate
change investigated, and (VIII) response of attribute to climate change (Supplementary
Material 1).
2.3 Data synthesis and visualisation
To identify the spatial distribution of global CFW research, the location of each
reviewed study was plotted on a world map and the percentage of publications from
each continent was calculated. To visualise publication output over time the number
of annual publications regarding key components of climate change (i.e. temperature
increase, extreme events, altered rainfall patterns) was calculated and plotted as a
stacked bar graph. Variation in research methodologies was investigated by calculating
the number of papers in each broad methodology category (observational, experimen-
tal, modelling) and identifying the most common methods used within these. To
identify which key aspects of climate change current research has concerned, the
sum of publications in each category was calculated and visualised this using an area
proportional Euler diagram (eulerr package in R; Larsson (2018)). Finally, the major
research findings of each aspect of climate change were also synthesised within each
broad methodology category.
3Results
Note: Short-listed papers will be referenced by numbers throughout the results section with the
full references included in supplementary material 2(S2).
3.1 Stocktake of research into climate change effects on CFWs
From an initial list of 678 papers, we identified 110 peer-reviewed original research
papers that fit the search criteria. Of these, the majority of studies were conducted in
the subtropics (78.4%), specifically in the USA (77.4%) where majority was from the
east coast (Fig. 1.), with only 7.2% from Europe, 5.4% from Asia and 4.5% each
from Australia, and South America (Fig. 1), and only one study (S2 61) conducted
across multiple continents.
Publication output was greatest between 2010 and 2019 inclusive (~82%), after the first
publication appeared in 1992 (Fig. 2.). Significantly, the greatest publication output occurred
in 2019 with 20 papers published. Most studies (~ 66%) reported on research conducted over
short time scales, i.e. < 1-year up to 3 years. Only 33% of papers investigated CFW responses
to climate change over longer time frames (310 years and > 10 years), mostly within the USA
(29 papers) but with some longer term studies from Australia (S2 10; 13), the Netherlands (S2
90; 91), Puerto Rico (S2 72; 108), and China (S2 93; 110). Papers presenting long-term data
are identified in the supplementary material by an asterisk (S2).
Research on CFW responses to climate change generally focuses on three attributes: (I) soil
processes, such as nutrient cycling and microbial dynamics (47 papers); (II) sediment accretion
and carbon sequestration (39 papers); and (III) vegetation distribution and function (73
papers). Much less research attention is given to fauna (S2 15; 18; 45; 85) and hydrological
responses to climate change (eight papers).
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3.1.1 Aspects of climate change
Five main aspects of climate change are examined in the short-listed studies: (1) SLR, (2)
altered rainfall regimes, (3) temperature changes, (4) extreme events, and (5) changes in
atmospheric greenhouse gases (CO2and CH4). Amongst papers only considering a single
aspect of climate change, effects of SLR on CFWs have received significantly greater research
attention (68 papers) compared to altered rainfall (six papers), extreme events (four papers; S2
57; 58; 79; 87), greenhouse gasses (S2 60), and temperature (Fig. 3, S2 86; 110). Twenty-eight
papers considered combined threats of climate change, most commonly the interaction
between SLR and altered rainfall regimes (Fig. 3). Eight papers investigated the effects of
Fig. 1 Map showing study location of reviewed papers. Inset map of eastern USA, no data is lost under inset
map
0
5
10
15
20
1992 1994 1996 1998 2000 2002 2004 2006 2008 2010 2012 2014 2016 2018 2020
Year of Publication
Number of Publications
AlteredRainfall ExtremeEvents GreenhouseGas SLR Temperature
Fig. 2 Number of publications per year of coastal freshwater wetland literature showing the temporal distribution
of research into each aspect of climate change
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SLR and extreme events, while three papers investigated the triple threat of SLR, altered
rainfall, and extreme events (S2 9; 96; 104). Authors rarely investigated the combination of
SLR and changes in temperature (S2 35; 56), greenhouse gas changes and SLR (S2 29; 43;
51), and altered rainfall and extreme events (Fig. 3, S2 47; 69).
3.1.2 Research methodologies
Methods used to investigate climate change impacts on CFWs are commonly (I) observational,
i.e. field surveys, long-term monitoring (43 papers); (II) experimental, i.e. in-situ manipula-
tions, mesocosms, laboratory soil analysis, etc. (58 papers); or (III) modelling, i.e. distribution
change over time, future projections based on current data (30 papers). Twenty-four short-
listed papers used a combination of methods. Observational studies mainly assessed changes in
vegetation distribution, structure, and composition over time in response to SLR (30 papers).
Experimental methods generally assessed responses of vegetation and soil processes to a range
of flooding, salinity, warming, nutrient enrichment, or disturbance treatments (26 papers).
Projections of vegetation distribution and wetland coverage were explored through various
modelling methodologies (18 papers). A list of all review publications grouped by research
methodology is provided in Table 1.
3.2 Observed effects of climate change on CFWs
3.2.1 Sea level rise
Long-term effects of SLR on CFWs are not thoroughly investigated for systems outside of
southeastern USA. Within this area, however, effects of rising sea levels have been observed
for several decades through long-term vegetation monitoring programs and SET monitoring
systems, investigating the influence of SLR on multiple aspects of wetland systems including
Fig. 3 Area proportional Euler diagram showing the number of papers reviewed for each aspect of climate
change with the number of papers investigating multiple aspects shown where the circles overlap
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Table 1 Summary of short-listed publications grouped by broad methodology and its variations
Data collection method (number
of papers)
Study focus Reference (publication number in
supplementary material 2)
Observational (21) Vegetation Anderson and Lockaby (2013; 3), Canepuccia
et al (2009; 18), Cormier et al (2013; 22),
Delgado et al (2018; 26), Ensign et al.
(2014; 31), Hoeppner et al (2008; 47),
Langston et al. (2017; 52), Liu et al (2017;
54), Wang et al (2019; 94), Weston et al
(2014; 97), Williams et al. (1999;103),
Williams et al. (2003;104)
Birds Brittain and Craft (2012; 15), Taillie et al
(2019b; 85)
Fish Hitch et al (2011; 45)
Invertebrates Canepuccia et al (2009; 18)
Surface elevation
monitoring
Cadol et al (2014; 17), Graham and
Mendelssohn (2014; 40), Wang et al. (2016;
93)
Experimental (46) Soil seed bank Baldwin et al (1996; 8), Middleton (2016; 59),
Osland et al (2011; 69), Peterson and
Baldwin (2001; 71)
Vegetated sod Baldwin and Me ndelssohn (1998; 9), Flynn
et al (1995; 33), Gao et al (2018; 34), How-
ard and Mendelssohn (1999; 49), Howard
et al (2016; 48), Li and Pennings (2018; 53),
Markus-Michalczyk et al (2019; 55), Sharpe
and Baldwin (2012; 76), Spalding and
Hester (2007; 78), Watson et al (2015; 96),
Wilson et al (2019; 105)
In situ manipulations Osland et al (2011; 69), Sánchez-Garcia et al
(2017; 73), Sutter et al (2015; 83), Sutter
et al (2014; 82), Tate and Battaglia (2013;
87), Visser et al (2015; 92)
Sediment accretion Beckett et al. (2016; 11), Boyd et al (2016; 14),
Charles et al (2019; 19), Craft (2012; 24),
Graham and Mendelssohn (2014; 40), Hill
et al (2015; 44), Noe et al. (2016; 65),
Nyman et al (1993; 67), Nyman et al (2006;
68), Palinkas and Engelhardt (2016;70)
Soil chemistry Ardón et al. (2017; 5), Choi et al. (2001; 21),
Craft et al. (2009; 23), Gao et al (2014; 35),
Neubauer et al (2018; 64), Noe et al (2013;
66)
Modelling (19) Sea level affects marshes
model
Akumu et al (2011; 1), Geselbracht et al (2011;
36), Geselbracht et al (2015; 37), Glick et al
(2013; 38), Traill et al. (2011;89),Wuetal
(2017; 107)
Digital terrain models and
digital landscape
elevation models
Bayliss et al (2018; 10), Zhong et al (2011;
109)
Bayesian belief networks Field et al (2016; 32)
Sea level over
proportional elevation
model
Doyleetal(2010;30)
Generalised linear models Bowman et al (2010; 13), Smith (2013; 77)
Van Dobben and Slim (2012; 90)
McCarthy et al. (2018; 56), Mo et al (2019; 62)
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vegetation composition, structure and function, sediment accretion, nutrient sequestration, and
microbial function (e.g. S2 52).
Effects of SLR on accretion Accumulation of organic matter in soil and accretion of sediment
is a key factor limiting the capacity of wetlands to maintain position with rising sea levels. For
CFWs in the USA, especially TFFWs, observed accretion rates are not sufficient to keep pace
with current and projected SLR (Fig. 4, S2 24; 31; 65; 81). Accretion rates in TFMs are
slightly higher due to greater belowground biomass of marsh species; however, rates of
accretion are still insufficient in the face of current and projected SLR rates (Fig. 4,S211;
14; 17; 27; 68; 70). Migration of salt marsh into freshwater marsh areas can raise accretion
rates closer to those of SLR; however, this comes at the loss of freshwater marsh vegetation
(S2 24; 68). Reported accretion rates within the USA and globally vary based on local SLR
rates and climate (Fig. 4.). Accretion rates are generally higher in brackish wetlands or salt
marshes compared to TFFWs or TFMs (Fig. 4), and are up to 70% higher in the tidal marshes
of Elbe Estuary, Germany, where tidal marshes, but not freshwater wetlands, appear to be
keeping pace with local SLR (Fig. 4, S2 16). Historic rates of sedimentation in Estonian
coastal wetlands show varied responses to atmospheric pressure and storm surges with
increasing sedimentation rates since the 1960s, most likely in response to SLR, recent climate
change and loss of sea ice but are also considered to be keeping up with local SLR (S2 95).
Freshwater Phragmites and Saueda wetlands in the Liaohe Delta, China, exhibited overall net
increases in accretion rates (0.3 mm year1to 6.9 mm year1, compared to SLR 2.4
5.5 mm year1), suggesting that they could also keep pace with higher sea level conditions,
especially for Phragmites wetlands which might continue to gain elevation at rates higher than
Table 1 (continued)
Data collection method (number
of papers)
Study focus Reference (publication number in
supplementary material 2)
Normalised difference
variation index
Swamp individual-based
tree model
Hoeppner and Rose (2011; 46)
Hydrodynamic marsh
equilibrium model
Alizad et al (2016; 2)
Wetland accretion rate
model of ecosystem
resilience
Throne et al (2018; 88)
Mixedcombination of
observational, experimental,
and modelling methods (24)
Ardon et al (2013; 6), Baldwin et al (2001; 7),
Brittian and Craft (2012; 15), Cadol et al
(2014; 17), Craft et al (2009; 23), Desantis
et al. (2007; 28), Field et al (2016; 32),
Grenfell et al (2016; 41), Grieger et al.
(2019; 42), Kearney et al. (2019; 50), Krauss
et al. (2018; 51), Middleton (2009;58),
Mitsch et al (2013; 60), Neubauer et al
(2018; 64), Rivera-Ocassio et al (2007; 72),
Smith (2013; 77), Stagg et al (2017a; 80),
Stagg et al (2017b; 81), Taillie et al (2019a;
84), Taillie et al (2019b; 85), Van Dobben
et al (2012; 90), Whittle and Gallego-Sala
(2016; 99), Widney et al (2019; 100), Yu
et al (2019; 108)
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SLR (S2 93). Comparing accretion rates to rates of global SLR predicted by the IPCC under
RCP 8.5 scenario we see that only brackish marsh accretion rates are sufficient to keep pace
(IPCC 2013; Fig. 4.).
SLR effects on soil chemical processes and carbon sequestration Most of the papers
investigating the effects of SLR on soil chemical processes and carbon sequestration (11 of
13 papers) come from studies in freshwater marshes in the USA. In both TFMs and TFFWs,
more frequent and longer saline periods have led to increased nitrogen and phosphorous export
(S2 5; 6; 66), stimulated decay of available carbon (i.e. cellulose from litterfall; S2 79), reduced
amounts of available litter (S2 22), and increased sulfidization in high iron wetland soils (S2
74). In coastal Scotland, SLR-induced migration of saline communities into coastal peatland
areas have reduced carbon accumulation and lowered recent carbon sequestration (S2 99).
The carbon sequestration potential of CFWs is a current research priority globally, receiv-
ing significant research attention since 2018. However, the influence of SLR on this capacity is
relatively unknown. Krauss et al. (2018; S2 51) highlighted the sequestration importance of
TFFWs and oligohaline wetlands which can store an average of 721 Mg C ha1. Significantly,
this is higher than the global estimates for seagrasses (140 Mg C ha1; Fourqurean et al. 2012)
and saltmarshes (162 Mg C ha1;Duarteetal.2013) but lower than those for mangroves (856
±32MgCha
1, global estimate; Kauffman et al. 2020). Similarly, Weston et al. (2014; S2 97)
noted that TFMs can be equally or more productive than salt marshes but exhibit large inter-
annual variability in plant productivity. With an expected expansion of freshwater marshes into
inland forested areas in response to climate change, Krauss et al. (2018; S2 51) suggest that
carbon sequestration will increase in coastal landscapes overall due to the greater soil carbon
RCP 2.6
RCP 4.5
RCP 8.5
−5
0
5
10
15
20
0
10
20
(1) (2) (2) (3) (4) (5) (6) (7) (8) (9) (10) (10)
Accretion Rate (mm y1)
SLR (mm y1)
Wetland Type BM TFM TFF
Fig. 4 Reported accretion rates of CFWs and brackish marshes showing mean, min, and max as boxplot. Local
rate of mean SLR shown as red + extended to show min and max local SLR, where mean accretion below mean
local SLR suggests higher likelihood of wetland submergence. Blue horizontal lines show global rates of SLR
under RCP scenarios (IPCC 2013). (1) Palinkas and Engelhardt (2016), (2) Beckett et al. (2016), (3) Craft (2012),
(4) Delgado et al. (2013), (5) Wang et al. (2016), (6) Stahl et al. (2018), (7) Beckett et al. (2016), (8) Noe et al.
(2016), (9) Ward et al. (2014), and (10) Ensign et al. (2014)
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stock observed in marsh sites; however, this prediction does not account for marsh losses from
SLR inundation (S2 21).
Effects of SLR on vegetation of CFWs In general, rising sea levels negatively affect vegeta-
tion cover, structure, composition, and function in CFWs. In the USA, increasingly saline and
more frequently flooded conditions have negatively affected coastal forested wetland areas,
causing declines in forest health and productivity (S2 52; 56), basal area and tree density (S2
22; 54), species diversity (S2 3; 28; 52), and seed germination and regeneration (S2 28; 52;
103; 104), along with increased plant mortality (S2 3; 22; 52). Similarly, altered salinity led to
reduced recruitment and tree growth in Puerto Rico (S2 72). Declines in Melaleuca forest area
of Kakadu National Park, Australia, are associated with SLR impacts, exacerbated by feral
water buffalo (S2 13). Conversely, soil subsidence, not SLR, attributed to changes in species
composition of island marsh wetlands over 15 years of SLR in the Netherlands (S2 90).
Kearney et al. (2019; S2 50) referred to the transitional zone between healthy forest and marsh
as a persistence zonewhere live mature trees persist but regeneration is limited, and
saltmarsh species start to appear. Collapse of these systems, triggered by a large disturbance
such as fire, results in ghost forests(Kirwan and Gedan 2019).
Dieback of freshwater forested wetlands and subsequent migration of salt and brackish
marsh into ghost forestareas is a common response to SLR (see Kirwan and Gedan 2019
and references) and transition can be accelerated under the influence of extreme events such as
fire and hurricanes (S2 84). In other cases, however, dominant wetland species constrain the
migration of saline vegetation into CFWs (S2 77). In the Delaware Estuary (Delaware, USA),
for example, Phragmites australis expanded into areas of SLR-related forest dieback which
restricted transition to saltmarsh (S2 77). Lower canopy cover and greater light availability at
the marsh-to-forest boundary of SLR affected forested wetlands in Connecticut enhanced tree
growth and minimised landward migration of saltmarshes (S2 32). These results highlight the
ecological complexities of landward ecosystem migration which can be influenced by local
geomorphology and hydrology (Brinson et al. 1995; S2 32; 77).
3.2.2 Altered rainfall regimes and extreme events
Only six of 40 observational papers in our short-list investigated responses of CFWs to effects
of climate change other than SLR, specifically altered rainfall regimes and extreme events.
Drought can have significant effects on CFWs by reducing freshwater supply and increasing
saltwater intrusion (S2 47). Often in conjunction with saline intrusion, drought has resulted in
observable negative effects on CFWs, particularly for vegetation structure and productivity (S2
47; 104). Drought and salinity resulted in tree mortality, reduced recruitment, increased leaf
litter biomass, and decreased annual net primary productivity in forested wetlands of Louisiana
and Florida (S2 47; 104). Drought also resulted in delayed and shortened growth periods for
coastal marsh species, suggesting that growth inhibition and dieback could occur over
extended periods (S2 61). For seasonally flooded CFWs in wet/dry tropical regions, an
extended dry season and reduced rainfall at the start of the wet season could reduce vegetation
species richness and facilitate a transition towards drought tolerant species (S2 69).
Extreme events such as hurricanes often exacerbate the effects of drought and salinity in
CFWs. Hurricane impacted wetlands of the Gulf of Mexico exhibited reduced vegetation
regeneration, recruitment, and increased tree dieback in areas of storm surge-induced saline
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intrusion (S2 58; 104). Physical removal of standing trees in these wetlands by hurricanes has
led to a significant change in vegetation structure in a coastal forested wetland of western
Florida (S2 104).
3.2.3 Climate change effects on fauna
Only two papers observed effects of climate change on faunal communities of CFWs. In both
cases, climate change decreased species richness and habitable area. Increased salinity and
relative sea level correlated with increased fragmentation and decreased cover of submerged
aquatic vegetation which resulted in lower fish densities in Louisiana coastal marshes, USA
(S2 45). Increased rainfall and flooding in Argentina, attributed to climate change, reduced
habitat diversity for arthropod assemblages, impacting species with poor dispersal abilities and
specific habitat requirements (S2 18).
3.3 Experimental responses of CFWs to climate change
3.3.1 Sea level rise
SLR and soil processes Effects of SLR on soil processes associated with greenhouse gas
production are explored experimentally by applying saline flooding treatments to soil cores
collected from CFWs. Saline intrusion in CFWs can shift microbial community compositions
increasing CH4and CO2production (S2 25), especially in intermittently flooded soils where
CH4emissions were almost double those of permanently inundated soils (S2 43). However,
net reductions in greenhouse gas emissions occurred in permanently flooded soils (S2 43),
urban pollutant impacted wetlands (S2 29), and in phosphorus enriched wetlands (S2 105).
CFWs experiencing marine incursion may therefore exhibit transitional periods in which their
greenhouse gas emissions exceed those of saltmarsh (S2 25) or act as short- (S2 29) or long-
term carbon sinks (S2 43; 105), depending on their geographical setting.
Rising salinity reduces soil stability in marshes associated with higher decomposition of
soil organic carbon (SOC; S2 99; 102), contributing to elevation decline and further submer-
gence due to reduced plant productivity and root biomass (S2 19; 98; 105). Saline intrusion
also affects soil microbes involved in denitrification, reducing rates by up to 70% (S2 64). In
wetlands experiencing early stages of SLR, dry periods allow for nitrification to occur between
periods of saline inundation, however, under permanent inundation ammonium is released and
macrophyte N storage is reduced (S2 100). Saline conditions in combination with warming,
however, can increase denitrification and ammonium availability (S2 35). Decomposition of
leaf litter increased with initial saline intrusion in Louisiana and South Carolina freshwater
marshes, however, with more permanent saline flooding rates of litter decay could decline,
resulting in greater carbon storage (S2 79; 80). Saltwater intrusion greatly increased the release
of bioavailable phosphorous (PO43
) from salt- and brackish marsh soils in northern Florida
but not in the freshwater marsh where PO43remained available for plant uptake (S2 101).
SLR and vegetation Responses of CFW vegetation to SLR are commonly explored through
experimental applications of salinity and water to TFM mesocosms and soil seed bank
samples. Reduced seed germination and vegetative regeneration typically occur in response
to saline flooding (S2 8; 9; 33; 42; 73; 96), except amongst species commonly found in higher
salinity environments (S2 33; 42; 76; 83). Experimental saline flooding also tends to reduce
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biomass and productivity of emergent seedlings and standing vegetation in mesocosms,
causing mortality in some species (S2 9; 26; 33; 34; 39; 42; 49; 76; 83; 92; 96; 100). Net
ecosystem productivity fell by up to 55% under in situ treatments of elevated salinity, and up
to 75% in flooding treatments, in a TFM in South Carolina (S2 63). Freshwater and brackish
marsh species (e.g. Panicum hemitomon,Sagittaria lancifolia,Distichlis spicata,andSpartina
bakeri) commonly exhibit reduced plant growth and higher mortality in response to SLR. In
contrast, species which are more commonly found in saltmarshes (e.g. Spartina alterniflora,
Juncus roemerianus,andSporobolus virginicus), are not negatively affected (S2 42; 48; 49;
53; 78). Saline conditions increase the dominance and proportion of salt-tolerant species in
mesocosm and soil seed bank germination trials (S2 33; 76; 83), suggesting that these species
will likely become more common in CFWs as saline intrusion occurs. Significantly, Bompy
et al. (2015; France; S2 12) records that seedlings of the common freshwater forested wetland
tree species Pterocarpus officinalis are able to gradually adapt to increasing salinity over a
short period, especially when sporadic flushing with freshwater allowed for desalination of
leaves. Similarly, juvenile individuals of floodplain willows (Salix alba,Salix viminalis)were
resilient to low levels of salinity (up to 2 ppt), however, increased saline flooding negatively
affected morphological traits and reduced breaking resistance (S2 55). Numerous experimental
studies further suggest that the overall negative effects of experimental SLR are likely to be
exacerbated by other biological disturbances such as herbivory (S2 9; 39).
3.3.2 Altered rainfall and extreme events
Experimental treatments of water levels examine the effects of rainfall changes on biotic and
abiotic responses of CFWs. Drought conditions, for instance, increased nitrogen (NH4+)export
from sediments of wetlands undergoing restoration, and to a lesser degree, from forested
wetlands (S2 6). In contrast, experimental flood conditions reduced germination and species
richness of freshwater marshes (S2 7; 42; 71), suggesting likely biodiversity loss and vegeta-
tion change in response to more frequent and severe floods.
Effects of hurricanes on CFWs are experimentally investigated through treatment applica-
tions representing storm surge conditions and debris deposition (wrack). Storm surgeinduced
saline intrusion reduced seed germination in several wetland types (S2 58; 59). Similarly, in
CFWs of eastern USA, storm surge conditions reduced vegetation biomass and cover, and
increased mortality, where wrack application also led to further reductions in cover (S2 59;
87). Salinity from a storm surge event can penetrate wetland soils at a rapid rate and can persist
up to 6 months (S2 57; 87), with potential implications for soil microbial function and
vegetation productivity due to the exposure of the rhizosphere to elevated salinity and
increased availability of soil nitrogen (S2 57; 87).
3.3.3 Temperature
Only two short-listed papers investigate the influence of increased temperatures on CFWs
through soil cores and temperature manipulations. Both studies, conducted in Phragmites reed
dominated wetlands from temperate coastal regions of China, find that increases in tempera-
ture influences wetland function (S2 86; 110). Increases in vegetative litter decomposition,
total organic carbon, soil enzyme activity, microbial nitrogen and phosphorus, and total
nitrogen all occur under warming conditions, suggesting that these wetlands will continue to
172 Climatic Change (2020) 163:161–179
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play important carbon and nitrogen processing and storage roles in a warmer world (S2 86;
110).
3.4 Modelled responses of CFWs to climate change
Eighteen short-listed papers predict the effects of climate change on CFWs using models. Most
of these explore changes in the spatial distribution of coastal wetland types in response to SLR.
Thirteen papers predict declines in the total area of freshwater forest and freshwater marsh (S2
1; 10; 15; 18; 23; 30; 36; 37; 38; 76; 89; 107; 108) while six papers predict an expansion of
saline wetlands into areas of CFW (S2 1; 23; 30; 36; 37; 89). An overall decrease in all wetland
types (freshwater and saltwater) is predicted for western USA coastal estuaries and areas of
coastal Louisiana, due to low local accretion rates and a lack of upland space available for
wetland migration (S2 38; 88). Similarly, Kearney et al. (2019; S2 50) identified that up to
20% of TFFFs on the Delmarva Peninsula lie within a persistence zonewhere trees do not
regenerate and are liable to forest collapse with future SLR and storminess (S2 50). Further-
more, in coastal Louisiana, USA, predicted accretion rates are not sufficient to maintain stable
coastal marshes, leading to submergence of brackish and freshwater wetlands (S2 20; 30; 91).
Predicted effects of SLR on CFWs have focused on overall changes in wetland area with
trajectories of change and degradation of wetlands through time receiving comparatively little
attention. Model simulations of forest decline in response to salinity and flooding over
500 years for Taxodium distichum and Nyssa aquatica trees suggests that even small changes
in elevation and salinity (0.23 m and ~ 16 psu) can increase flood duration, decrease swamp
function, reduce tree basal areas, and induce tree mortality (S2 46).
Changes in CFW habitat for threatened animals in response to SLR can have significant
impacts on their populations. Modelled declines of herbaceous wetland communities, and
expansion of mangrove areas, in south-east Queensland, Australia, suggest greater habitat
availability for an increased population of Xeromys myoides (False Water Rat).However,
model runs revealed that simultaneous expansion of urban areas and feral cat predation are
likely to dramatically reduce, or even eradicate, the population by 2100 (S2 89) Losses,
predicted by Brittian and Craft (2012), of shrub habitat, salt marsh and tidal forest from
SLR, and of oak and pine forests from urban development, highlight the importance of tidal
forests as a refuge for avian forest species (S2; 15). Significantly, the effect of climate-induced
wetland restructuring is dependent on the habitat structural requirements of each bird species,
where the transition to ghost forests benefits species which favour shrubby areas (e.g. common
yellowthroat, northern bobwhite), while canopy-dwelling species are negatively affected (S2
85).
Only four short-listed studies model the influence of climate change on ecological functions
of CFWs. Reductions in ecosystem service delivery, such as productivity and wastewater
treatment, are predicted in response to reduced macrophyte biomass, decreased denitrification,
and nitrogen sequestration under low level SLR predictions (0.520.82 m; S2 23). Soil organic
carbon and organic nitrogen storage potential of Louisiana coastal wetlands will be signifi-
cantly impacted by SLR as projections of up to 6000 km2of CFWs are likely to be inundated
(up to 100 cm elevation) corresponding to the area of greatest soil organic carbon storage (S2
109). Mitsch et al. (2013; S2 60) reported that freshwater wetlands in temperate regions had
the highest carbon sequestration rate (278 g C m2year1; S2 60). Despite this, tropical and
subtropical wetlands have a greater global area and therefore have greater total carbon stock
potential (0.56 × 1015 gCyear
1; S2 60). Louisiana coastal marshes are also predicted to
173Climatic Change (2020) 163:161–179
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increase their CO2uptake and storage with increases in atmospheric CO2due to the increased
length of marsh growing periods (S2 62).
4Discussion
Climate change can affect CFWs via multiple pathways due to their low-lying topography and
position within the coastal plain, but surprisingly little is known of what the effects and
outcomes will be (Conner et al. 2007; Saintilan et al. 2018). Most research to date examines
effects of SLR, with the bulk of the knowledge generated over the last 9 years (Fig. 1)and
mainly in the USA (Fig. 2). More broadly, however, we identified three broad aspects of
CFWs that global research has focused on (I) sediment accretion and changes in elevation; (II)
carbon accumulation, nutrient cycling and, sediment processes; and (III) vegetation structure,
function, and distribution. Despite a diversity of study areas and methodologies, major
learnings have emerged from each of these broad research areas. First, multiple lines of
evidence indicate that current sediment accretion rates are not sufficient to keep pace with
current rates of SLR. As a result, the persistence of CFWs is dependent upon their capacity to
migrate into new available areas, as dramatic changes in ecological function are likely to occur
due to saline inundation and peat collapse. Second, CFWs are increasingly recognised as
important sites for carbon sequestration, sediment accumulation, and nutrient cycling but the
influence of SLR on these processes is relatively unknown. Third, vegetation in CFWs is
negatively impacted by SLR, the effects of which are exacerbated by drought or extreme storm
events. Our review has highlighted the lack of integrated research exploring the effects of
combined effects of multiple climate change drivers. Very few studies explore the impacts of
more than a single axis of climate change; yet, we know that the drivers do not operate in
isolation and the synergistic impacts will likely drive dramatic changes in CFW ecosystems.
More work is also needed to understand the cumulative impacts of climate change drivers on
CFW structure and functioning, including the sequential impacts of extreme events and
changes in temperature.
4.1 Knowledge gaps
In this review, we have synthesised a significant body of research regarding the effects of
climate change on CFWs. However, even within USA, where most of the reviewed research
comes from, there are still gaps in the literature, especially concerning the effects of increased
temperatures on CFWs and how interacting climate drivers will influence CFWs. Globally,
there is a need for research into all aspects of CFWs as this basic information is critical to be
able to make reliable projections regarding their future with climate change. Landward
migration away from the saline influence is a widely recognised mechanism for resilience to
climate change in coastal wetland vegetation literature (Morris et al. 2002). However, this
migration has not been observed in CFWs and there is a lack of knowledge regarding the
mechanisms of potential migration in CFW species, making it difficult to predict the future
distribution of species if they were able to migrate. While the effects of temperature changes
will likely influence the function and productivity of CFWs, research is greatly lacking on this
topic (only two papers in this review). Although CFWs are becoming increasingly recognised
as important sites for carbon sequestration, this research is still emerging and is focused on
CFWs in the USA. Significant further knowledge is required to quantify carbon sequestration
174 Climatic Change (2020) 163:161–179
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in CFWs globally. Also, the impacts of climate change on carbon sequestration and other soil
nutrient processes are relatively unknown, adding to the uncertainty of the future of CFWs.
Impacts of climate change on fauna which inhabit CFWs is also under-represented in this
review suggesting that further research is warranted.
4.2 Assessing impacts of multiple drivers
Compounding effects on CFWs by multiple climate drivers have received minimal research
attention to date. This knowledge is valuable in understanding the processes and responses of
CFWs, enabling the development of more robust predictions of future changes and responses.
Only 20% of papers reviewed here investigate the effects of two or more climate drivers.
Research which only considers one climate driver is valuable in understanding the specific
responses to that driver, but it is important that these responses be considered in the wider
context of climate change given that it impacts many aspects of an ecosystems structure and
function. McCarthy et al. (2018), for example, observed forest decline in Florida, USA over
four decades and attributed gradual decline to increased saline intrusion and flooding as a
result of rising sea levels. However, to understand the recent rapid decline in forest cover, they
learnt that temperature patterns were more informative, as a recent period of extended cold
snaps helped to explain the decline (McCarthy et al. 2018).
Observing the effects of multiple climate drivers is difficult in the field, where changes can
be gradual and take longer to result in a significant quantifiable outcome, and generally longer
than most funding allows. However, for some organisms and ecosystems, the effects of
multiple climate drivers could work synergistically to drive fast and dramatic changes. We
rrecognise the challenges of field-based observations of multiple climate drivers and responses
and highlight the role here of experimental and modelling methods which can have more
intense treatment applications and include predictions over long-time scales. Ideally, research
programs should use local field observations to guide the development of an experiment, with
data from both used to inform the development and validation of a predictive model. This
integrated approach, while resource intensive and requiring a large interdisciplinary team,
would result in a significant increase in knowledge enabling better management of CFWs in
the face of a rapidly changing climate.
Non-climate change threatening factors, like urban expansion, excess nutrients, and feral
animals, were not commonly addressed in conjunction with climate change threats in the
research reviewed (just six studies of the 110 reviewed). Although these non-climatic drivers
of change were not the focus of this review, they can contribute to greater loss and decline of
ecosystems when considered in conjunction with climate change threats. Future work should
couple examination of climatic and non-climatic threats, as the expansion of urban develop-
ments in coastal areas continues to threaten coastal wetlands, and in many locations around the
world there is no space available between them and the built urban environment or
unfavourable topographies, largely reducing the potential migration area (Thorne et al.
2018). Encroachment into CFWs by salt marsh and mangrove systems as they migrate inland
results in coastal squeeze.
4.3 Management considerations
Conservation of CFWs under climate change requires a significant increase in the global
knowledge base for adequate management and policy decisions to be made. This requires a
175Climatic Change (2020) 163:161–179
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far-reaching research initiative to encompass all aspects of CFWs but also site-specific
investigations due to the inherent differences in hydrology, topography, and species compo-
sition between CFWs. Long-term monitoring of CFWs is required to understand the processes
of vegetation migration, impacts of climate change on species productivity and regeneration
and, the anticipated change in functioning of CFWs.
Many CFWs which are currently affected by SLR likely exist in a relicstate, or what
Kearney et al. (2019) have termed a persistent state, not able to reproduce or regenerate, and
liable to collapse if a disturbance event, such as a hurricane or fire, removes the standing
vegetation, resulting in ghost forestswhere trees have been replaced by intertidal vegetation
(Baldwin and Mendelssohn 1998; Kirwan and Gedan 2019; Langston et al. 2017;Williams
et al. 1999;Williamsetal.2003). This collapse has been observed in freshwater forested
swamps of Chesapeake Bay and coastal Florida, USA, where standing trees in SLR affected
areas died and were uprooted after the wind, storm surge, and saline intrusion of recent
hurricanes (Desantis et al. 2007;Langstonetal.2017;Middleton2016a;Williamsetal.
2003). Regeneration post hurricane in these sites has been dominated by salt-tolerant species
(Desantis et al. 2007;Langstonetal.2017; Middleton 2009; Williams et al. 2003). These
studies signal a significant imminent threat to the future of CFWs. Many CFWs may not be
able to exist in their current state for much longer, with a transition to species which are salt-
tolerant already observed and likely to continue. CFWs often lack space for migration and are
directly affected by encroachment of large urban developments in coastal regions and to date
there have been no observations of landward migration, highlighting their vulnerability to
encroachment. Recognising the squeeze on CFWs, we suggest that it is important to leave
space for wetland migration through the expansion of existing coastal protected areas and for
research to be conducted into the migration potential and dispersal mechanisms of CFW
species. The conversion of coastal agricultural lands to CFWs in areas which have reduced
agricultural productivity due to increased inundation and salinity is an emerging opportunity
which could provide space for wetland migration and regeneration, as well as reduce runoff of
agricultural fertilizers and chemicals into coastal and nearshore environments (Waltham et al.
2019).
The other mechanism which could contribute to wetland persistence is sediment accretion.
However, accretion rates in CFWs reviewed here were generally found to be outstripped by
SLR. To aid in sediment accretion processes and increase rates of vertical elevation gain,
sediment remediation could be implemented as sufficient sediment supply was found to be a
contributing factor to elevation gain. The restoration of flow regimes in highly modified rivers
could also help to reduce the impact of saline intrusion facilitated by minimal freshwater flows
and deliver sediment supplies for accretion processes. Similarly, providing periodic freshwater
flushing of salt impacted CFWs could improve the resilience of species which are not salt-
tolerant, providing short periods in which regeneration and migration could occur (Mauchamp
and Mesleard 2001).
Active restoration and mitigation actions can lead to further impacts on the coastal region,
and therefore require careful consideration, planning, and consultation with the relevant
stakeholders to achieve an optimal outcome of enhanced wetland resilience while maintaining
the services and function of CFWs and surrounding lands. The impacts of salinity, tempera-
ture, and water regime on sediment accretion processes all require further investigation to be
able to accurately predict the future of CFWs. Implementing further widespread long-term
monitoring programs which quantify accretion rates during periods of climatic change would
also help untangle this process.
176 Climatic Change (2020) 163:161–179
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5 Conclusion
Coastal freshwater wetlands are key components of the coastal landscape which are signifi-
cantly threatened by global climate change. While there has been strong growth in research
from 2010 to 2020 exploring SLR effects on CFWs, very little attention has been given to
other aspects of climate change. Furthermore, there is a lack of studies which investigate the
impacts of compounding climate threats to CFWs. The key findings of our review are that (I)
current sediment accretion rates are not sufficient to keep pace with current rates of SLR; (II)
the effect of climate change on carbon sequestration, sediment accumulation, and nutrient
cycling processes are unknown; and (III) vegetation is negatively impacted by the effects of
SLR which are further exasperated when combined with drought or extreme storm events. We
suggest that future research attention should be given to CFWs in areas outside of the USA
(but not to their exclusion) and that research should focus on the effects of temperature
changes on CFWs, the carbon sequestration potential of these wetlands, and the impacts of
multiple climate drivers on CFWs.
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... The structure and distribution of coastal wetland vegetation is largely explained by biophysical regimes which include salinity, elevation, inundation and vegetative relationships, and form the environmental gradient structure of coastal wetlands (Rogers et al., 2014;Saintilan and Wilton, 2001;Woodroffe et al., 2016). Sea-level rise (SLR) has been identified as a factor driving forest retreat in some coastal wetland environments (Grieger et al., 2020;Kirwan and Gedan, 2019). With anticipated environmental change (IPCC, 2014;Justic et al., 2016;Wong et al., 2014), particularly in the coastal zone, it is imperative that the dynamics and characteristic structure of coastal wetland vegetation are understood. ...
... Wetlands in the supratidal zone (i.e. above Highest Astronomical Tide) are relatively understudied in Australia (Grieger et al., 2020;Saintilan et al., 2019). Currently, there is a lack in understanding of the structure, function, and distribution of supratidal Coastal Swamp Oak Forest (CSOF) in southeast Australia. ...
... Consequently, this community is vulnerable to changes in wetland hydrology (including SLR) (Saintilan et al., 2019). Historically, large areas of CSOF wetland have been lost in this region due to land-use change in the coastal zone, while observations of localised plant stress and/or dieback in some extant populations is of significant concern (Grieger et al., 2020;Kelleway et al., 2021). Data on the aboveground biomass structure of CSOF ecosystems is limited, though recent assessment shows aboveground biomass densities Fig. 1. A. Minnamurra River estuary and the focal study site. ...
Article
Coastal Swamp Oak Forest (CSOF), a supratidal wetland community dominated by Casuarina glauca, is a widely distributed coastal ecosystem along Australia's east coast. These wetland communities are highly valuable for providing ecosystem services, including carbon sequestration. Positioned within the supratidal zone of estuaries – and often abutting upper intertidal saltmarsh and/or mangrove – CSOF may be vulnerable to salinity intrusion and increased tidal inundation due to sea-level rise. To understand spatial patterns of vegetation composition and structure in CSOF, field-based (in-situ) and remote-sensing approaches were employed on the Minnamurra floodplain, New South Wales, Australia. In-situ vegetation surveys within 23 field plots located along seaward to landward transects revealed large variations in mean tree height (2.5–13.1 m) and tree densities (100–8700 trees/ha). Unmanned Aerial Vehicles with Structure from Motion (UAV-SfM), and airborne Light Detection and Ranging (LiDAR) approaches returned mean plot canopy height estimates ranging between 0.1 and 12.8 m. Comparison of vegetation metrics between remote sensors (UAV-SfM and LiDAR) demonstrated similar capacities (R² values > 0.85) to capture CSOF vegetation height. Comparison of field and spatial metrics elucidated a moderate correlation between the datasets for maximum canopy height (R² > 0.6) which can be partially explained by the different spatial scales of measurement among these approaches. Canopy height, Normalised Difference Vegetation Index (NDVI), and point density (i.e., vegetation density) estimates were each positively correlated with elevation above mean sea-level. This coincides with indications of plant stress and/or mortality at the seaward edge of CSOF, and in topographic depressions. These findings suggest physico-chemical gradients exert a strong control on CSOF vegetation structure and health, with implications for the current acceleration of sea-level rise. When combined, remote sensing and field-based datasets are useful to characterise and quantify CSOF structure and distribution and can therefore be employed in future assessments of this understudied ecosystem.
... Although SLR imposes obvious threats to all coastal wetlands, most previous wetland vulnerability assessments focused on tidal saline wetlands (e.g., salt marshes and mangroves) [11][12][13]. There are significant knowledge gaps on how freshwater and brackish wetlands respond to projected SLR [7,13]. ...
... Although SLR imposes obvious threats to all coastal wetlands, most previous wetland vulnerability assessments focused on tidal saline wetlands (e.g., salt marshes and mangroves) [11][12][13]. There are significant knowledge gaps on how freshwater and brackish wetlands respond to projected SLR [7,13]. Many regional and global monitoring and assessment frameworks, such as the high-precision rod surface-elevation table-marker horizon (RSET-MH) [14][15][16][17], mainly target mangroves and saltmarshes. ...
... Many regional and global monitoring and assessment frameworks, such as the high-precision rod surface-elevation table-marker horizon (RSET-MH) [14][15][16][17], mainly target mangroves and saltmarshes. Other wetlands, such as coastal floodplain forests, swamps, and lagoons, have received much less attention [2,13]. Globally, there is currently a lack of information on the distribution of these wetlands as well as basic information on the physiological ecology of major coastal freshwater wetland species under natural settings; the structure and dynamics of pure and mixed species communities, soil-plant interactions, biogeochemistry, hydrology, soils, wildlife habitat, primary biotic and abiotic functions; and the response of these systems to natural and human-caused disruptions. ...
Article
Full-text available
Many coastal wetlands are under pressure due to climate change and the associated sea level rise (SLR). Many previous studies suggest that upslope lateral migration is the key adaptive mechanism for saline wetlands, such as mangroves and saltmarshes. However, few studies have explored the long-term fate of other wetland types, such as brackish swamps and freshwater forests. Using the current wetland map of a micro-tidal estuary, the Manning River in New South Wales, Australia, this study built a machine learning model based on the hydro-geomorphological settings of four broad wetland types. The model was then used to predict the future wetland distribution under three sea level rise scenarios. The predictions were compared to compute the persistence, net, swap, and total changes in the wetlands to investigate the loss and gain potential of different wetland classes. Our results for the study area show extensive gains by mangroves under low (0.5 m), moderate (1.0 m), and high (1.5 m) sea level rise scenarios, whereas the other wetland classes could suffer substantial losses. Our findings suggest that the accommodation spaces might only be beneficial to mangroves, and their availability to saltmarshes might be limited by coastal squeeze at saline–freshwater ecotones. Furthermore, the accommodation spaces for freshwater wetlands were also restrained by coastal squeeze at the wetland-upland ecotones. As sea level rises, coastal wetlands other than mangroves could be lost due to barriers at the transitional ecotones. In our study, these are largely manifested by slope impacts on hydrology at a higher sea level. Our approach provides a framework to systematically assess the vulnerability of all coastal wetland types.
... It is causing large-scale degradation and loss of wetlands through direct and indirect effects of changes in temperature, precipitation and humidity, and subsequently in patterns of evapotranspiration, alterations in hydrological regimes, and increases in the frequency of extreme climate events such as floods and droughts (Erwin, 2009;Davidson, 2014). Sea level rise and the increasing frequency of tidal surges, with associated salinization of soil and freshwater resources, pose additional, more proximate but less predictable threats to coastal wetlands (Herbert et al., 2015;Grieger et al., 2020). ...
... Nevertheless, there are several impacts of climate change affecting the short-and medium-term suitability of wetland environments that our models are not able to detect. Sea level rise and the altered frequency of tidal surges could increase the risk of permanent saltwater intrusions into coastal freshwater wetlands, shifting the vegetation towards salt-tolerant associations and altering the structure and processes of coastal wetland ecosystems (Barlow and Reichard, 2010;Herbert et al., 2015;Grieger et al., 2020), with significant negative implications for our model species. Beyond climate change, these species could be impacted by the destruction and modification of natural habitats resulting from habitat loss and land-use change, but we have not evaluated these factors. ...
Article
Wetlands, one of the most biodiverse ecosystems in the world, are increasingly subjected to area loss and degradation due to land-use and climate changes. These factors impact their unique biodiversity, including numerous invertebrates that depend on them. Here we investigated the current and future habitat suitability of the aquatic spiders Argyroneta aquatica and Dolomedes plantarius. We evaluated future trends in their geographic range, aiming at assessing their extinction risk according to the International Union for Conservation of Nature (IUCN) Red List criteria, at both global and regional levels. We investigated present and future distribution ranges using species distribution models for two integrated emission scenarios (SSP1-2.6 and SSP5-8.5) and combining three general circulation models. These were combined with knowledge on species' dispersal limitation to account for the possibility that these species will not be able to move beyond the current range in the next decades. We found a significant future northern shift in the geographic range and a global reduction in habitat suitability for both species, corresponding to a loss of 28.9% for A. aquatica and 38.1% for D. plantarius in the next 10 years. The application of the IUCN criteria qualifies A. aquatica as Near Threatened and D. plantarius as Vulnerable. Regional assessments provided similar patterns of range reductions and population vulnerability across all European regions, particularly for Central-Eastern and Western Europe. Conversely, Northern Europe is expected to become a climatic refugium for both species. This work goes beyond the available studies on the conservation of these species by taking account their dispersal abilities in quantifying future trends in their habitat suitability using the most up to date knowledge. Conservation strategies should be directed towards limiting the impact of climatic and non-climatic stressors on wetlands, and towards implementing management plans and restoration programmes to increase habitat suitability and connectivity among wetland patches.
... Because considerable glacier loss is already underway due to historic emissions, its contribution to sea level rise is not likely to differ significantly between 1.5°C and 2°C global warming (Marzeion et al., 2018). However, even modest sea level rise can substantially alter the character of low lying coastal freshwater ecosystems, rapidly shifting them into more brackish or saline systems unable to support their characteristic freshwater biodiversity (Grieger et al., 2020). As Siegert and Pearson (2021) note, under high emissions scenarios, we cannot rule out 2 m of sea level rise by 2100, and up to 5 m by 2150. ...
... In many places, warming and increased CO 2 levels, as well as altered water and disturbance regimes, are resulting in woody encroachment and thickening of non-wooded wetland ecosystems (Saintilan and Rogers 2015). In coastal regions, however, saline intrusion has already resulted in widespread tree mortality in coastal swamps, generating 'ghost forests' (Grieger et al., 2020). ...
Article
Full-text available
Freshwater ecosystems are highly vulnerable to global warming because 1) their chief drivers, water quality and flow regimes, are highly sensitive to atmospheric warming, and 2) they are already extremely threatened by a wide range of interacting anthropogenic pressures. Even relatively modest global warming of 1.5°C poses a considerable threat to freshwater ecosystems and the many critical services these provide to people. Shifts in the composition and function of freshwater ecosystems are widely anticipated with adverse consequences for ecosystem services, including those underpinning water and food security. While the extent and severity of effects is likely to be significantly reduced if global warming is limited to 1.5°C, concerted efforts to implement widely recognised priorities for policy and management are required to mitigate unavoidable impacts and reduce the likelihood of perverse outcomes of climate mitigation and adaptation efforts in other sectors—all of which rely on fresh water supply. Freshwater ecosystems and their services, including provision of fresh water, must therefore be considered first and foremost when developing and implementing any climate action.
... Their ability to remove nitrate (NO 3 -) from surface waters is particularly important as it helps ameliorate coastal eutrophication (Hansen et al., 2018, Jordan et al., 2011. Research considering how coastal wetlands respond to climate change stressors has predominately focused on brackish and marine systems, with less attention focused on the response of coastal freshwater wetlands (Grieger et al., 2020). However, these ecosystems are vulnerable to salinization due to sea-level rise, which can have such a significant impact on soil physicochemistry and wetland biota that ecosystem stability and functioning are threatened (Herbert et al., 2015), and ecosystem services such as NO 3 removal may be impaired (Ardón et al., 2013, Larsen et al., 2010. ...
Article
The salinization of coastal freshwater wetlands may impact the biogeochemical cycling of critical nutrients by altering soil microbial community structure and function. Nitrate (NO3–) reduction pathways appear particularly sensitive, though the interactive effects of salinization intensity and duration of exposure remain unclear. To address this knowledge gap, we performed a transplant experiment that exposed soil from a freshwater (≤0.1 ppt) wetland to oligohaline (1 ppt) and mesohaline (14 ppt) conditions for two years. In transplanted soils, we found physicochemical changes characteristic of salinization events, including elevated porewater concentrations of ammonium and sulfate and decreased soil organic matter and redox potential. Amplicon sequencing (16S rRNA) revealed that mesohaline levels of salinization caused rapid community restructuring; distinct transition communities were evident by the first sampling event (5 months) and persisted for ∼2 years (19–22 months). In contrast, freshwater communities transplanted to oligohaline conditions were highly resistant to restructuring, and it took nearly 2 years of salinization for differences to manifest. For both transplants, community shifts included changes in the distribution and abundance of taxa capable of NO3– reduction, including several groups also known for sulfur redox metabolism. Nitrate (¹⁵NO3⁻) reduction assays were performed to determine how rates were affected. Dissimilatory nitrate reduction to ammonium (DNRA) increased for all sampling events, but only under mesohaline conditions, whereas denitrification responses depended more on the duration of exposure than salinity level. These findings may be useful for determining when and how the ability of wetlands to remove nitrogen will be impacted by sea-level rise. Further, these results suggest that efforts to synthesize and generalize prior research regarding salinization effects on wetland microbial community structure and function must explicitly consider both salinity intensity and exposure length.
... Changes in salinity and hydroperiod in wetlands, in addition to the hydrodynamic effects on channels, result in ecological changes which may have geomorphic impacts. Coastal freshwater wetlands are among the environments most likely to be impacted by climate change, but there has been scant research on those impacts (Grieger et al., 2020). Tidal freshwater forests are 'sentinels for climate change' according to Stahl et al. (2018), with sea-level rise likely to lead to forest death due to saltwater intrusion and submergence. ...
Article
The fluvial‐estuarine transition zone (FETZ) of the Neuse River, North Carolina features a river corridor that conveys flow in a complex of active, backflooded, and high‐flow channels, floodplain depressions, and wetlands. Hydrological connectivity among these occurs at median discharges and stages, with some connectivity at even lower stages. Water exchange can occur in any direction, and at high stages the complex effectively stores water within the valley bottom and eventually conveys it to the estuary along both slow and more rapid paths. The geomorphology of the FETZ is unique compared to the estuary, or to the fluvial reaches upstream. It has been shaped by Holocene and contemporary sea‐level rise, as shown by signatures of the leading edge of encroaching backwater effects. The FETZ can accommodate extreme flows from upstream, and extraordinary storm surges from downstream (as illustrated by Hurricane Florence). In the lower Neuse—and in fluvial‐to‐estuary transitions of other coastal plain rivers—options for geomorphological adaptation are limited. Landscape slopes and relief are low, channels are close to base level, sediment inputs are low, and banks have high resistance relative to hydraulic forces. Limited potential exists for changes in channel depth,width, or lateral migration. Adaptations are dominated by formation of multiple channels, water storage in wetlands and floodplain depressions, increased frequency of overbank flow (compared to upstream), and adjustments of roughness via vegetation, woody debris, multiple channels, and flow through wetlands.
Article
Supratidal wetlands are threatened by agricultural production and are highly vulnerable to climate change, particularly through sea level rise (SLR). While vegetation structure and composition of supratidal wetlands will likely change under projected SLR with run‐on effects for ecosystem service provision, these changes can provide opportunities for restoration of adjacent agricultural land. Here, we investigated the natural regenerative potential of supratidal wetlands on abandoned agricultural land in Southeast Queensland, Australia, specifically, responses of wetland vegetation communities to simulated SLR, through tidal reinstatement. In 15 years since crop abandonment, distinct communities of typical supratidal wetland vegetation have naturally re‐established, in predominately freshwater conditions, with minimal management intervention. Reinstating tidal floodwater increased the flooded extent and permanence of brackish water. Four repeat surveys of vegetation composition, structure, and condition were conducted in permanent plots established in Casuarina swamp, Melaleuca swamp, herbaceous marsh, and riparian zone vegetation communities, to observe change over time. Species richness decreased in all regenerating communities (Herbaceous marsh, Casuarina, and Melaleuca) post flood gate removal. Understorey vegetation cover also decreased in Melaleuca and Casuarina plots, but increased in herbaceous marsh plots, with increased cover of salt tolerant species throughout. Changes in woody vegetation community and structure were not observed during this short study (2.5 years), although the regenerative capacity of woody and herbaceous species was reduced. Supratidal wetland vegetation communities can naturally re‐establish in areas of abandoned agricultural land, however, increased saltwater flooding (likely with SLR) will put these communities at risk of transition to salt‐tolerant vegetation. This article is protected by copyright. All rights reserved.
Article
We argue for improved conservation of freshwater ecosystems at catchment or eco-regional scales by explicit assignment of values to all river sections and wetlands, recognising current disturbance, and aiming for ‘no further harm’ to the commons. The need is indicated by the global deterioration of biodiversity and ecosystem services of rivers and wetlands, increasing demands on water and land resources, and climate change. Regional pressures include multiple jurisdictions, competing demands, piecemeal management, pollution and habitat impacts. Effective resource and conservation management needs to integrate multiple uses via governance of activities of stakeholders, recognising hydrogeomorphic, water quality and ecological properties of ecosystems. Complete ecological protection is impractical amidst water-resource and land-use development, but we suggest that all river reaches and wetlands be given a conservation rating based on habitat, biodiversity and connectivity values. We present a straightforward approach to spatial conservation rating of freshwaters, using hydrogeomorphic typology and assignment of conservation values on the basis of available information and expert elicitation. We illustrate the approach by using the large Burdekin River catchment in north-eastern Australia. This approach is complementary to more spatially focused conservation prioritisation and could greatly improve management for sustainability, reduce further decline in conservation values, and facilitate rehabilitation.
Article
The river's role as the aquatic continuum, transporting and transforming terrestrial material in transit to sea, has long been appreciated. Along this aquatic continuum lies an enigmatic river stretch known as the tidal freshwater zone (TFZ). Because it oscillates along the daily tidal cycle, yet records no salinity, TFZs are often overlooked or sporadically studied. Additionally, research efforts into TFZs have been disproportionately focused on the intertidal zone rather than the subtidal zone. The limited studies to date do, however, highlight the subtidal TFZ's importance in both the removal and transformation of terrestrial material before exchange at sea, and, as a primary production hotspot. The shifts between biogeochemical activity within the TFZ vary in a semi-predictable manner based on hydrologic state. Presented here is a conceptual model, defining the TFZ as inseparable from the traditionally studied estuary by reviewing the relevant literature. The TFZ acts as a “fluidized bed reactor” which depends on marine and aquatic material delivery and tidally prolonged retention times. Subsequently, TFZ biogeochemical byproducts affect the saline estuarine reach's chemistry, water quality, and ecology. Specifically, TFZ biogeochemistry contributes significantly to episodic acidification and deoxygenation in the saline reaches, due to a hydrologic switch. Therefore, conceptualizing the TFZ and estuary as a single entity oscillating jointly between river and ocean dominated forces, within the framework of the pulse-shunt concept, developed in low order stream networks, is most apt. TFZ ecological and hydrological restoration efforts, like those occurring in the traditional estuary, will be needed to mitigate hydrologic switch events in the future.
Article
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Mangroves sequester large quantities of carbon (C) that become significant sources of greenhouse gases when disturbed through land- use change. Thus, they are of great value to incorporate into climate change adaptation and mitigation strategies. In response, a global network of mangrove plots was established to provide policy-relevant ecological data relating to interactions of mangrove stocks with climatic, tidal, plant community, and geomorphic factors. Mangroves from 190 sites were sampled across five continents encompassing large biological, physical and climatic gradients using consistent methodologies for the quantification of total ecosystem C stocks (TECS). Carbon stock data were collected along with vegetation, physical and climatic data to explore potential predictive relationships. There was a 28-fold range in TECS (79 - 2,208 Mg C ha-1) with a mean of 856 ± 32 Mg C ha-1. Belowground C comprised an average 85% of the TECS. Mean soil depth was 216 cm, ranging from 22 to > 300 cm, with 68 sites (35%) exceeding a depth of 300 cm. TECS were weakly correlated with metrics of forest structure, suggesting that aboveground forest structure alone cannot accurately predict TECS. Similarly, precipitation was not a strong predictor of TECS. Reasonable estimates of TECS were derived via multiple regression analysis using precipitation, soil depth, tree mass and latitude (R2 = 0.54) as variables. Soil carbon to a one meter depth averaged 44 % of the TECS. Limiting analyses of soil C stocks to the top 1 m of soils would result in large underestimates of TECS as well as in the greenhouse gas emissions that would arise from their conversion to other land uses. The current IPCC Tier 1 default TECS value for mangroves is 511 Mg C ha-1, which is only 60% of our calculated global mean. This study improves current assessments of mangrove C stocks providing a foundation necessary for C valuation related to climate change mitigation. We estimate mangroves globally store about 11.7 Pg C: an aboveground carbon stock of 1.6 Pg C and a belowground carbon stock of 10.2 Pg C.
Article
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Ghost forests created by the submergence of low-lying land are one of the most striking indicators of climate change along the Atlantic coast of North America. Although dead trees at the margin of estuaries were described as early as 1910, recent research has led to new recognition that the submergence of terrestrial land is geographically widespread, ecologically and economically important, and globally relevant to the survival of coastal wetlands in the face of rapid sea level rise. This emerging understanding has in turn generated widespread interest in the physical and ecological mechanisms influencing the extent and pace of upland to wetland conversion. Choices between defending the coast from sea level rise and facilitating ecosystem transgression will play a fundamental role in determining the fate and function of low-lying coastal land. A review of the phenomenon of low-lying ‘ghost forests’, and the physical and ecological mechanisms that control their occurrence in the context of sea level rise, with a focus on the Atlantic Coast of North America.
Article
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The retreat of coastal forests as sea level rises is well documented; however, the mechanisms which control this retreat vary with the physical and biological setting of the interface between tidal marsh and forest. Tidal flooding and saltwater intrusion as well as flooding and wind associated with storms can kill trees. Even if these processes do not kill stands, they may halt regeneration because seedlings are more sensitive to stress. We present a case study of a coastal pine forest on the Delmarva Peninsula, United States. This forest contains a persistent but nonregenerating zone of mature trees, the size of which is related to the sea level rise experienced since forest establishment. The transgression of coastal forest and shrub or marsh ecosystems is an ecological ratchet: sea-level rise pushes the regeneration boundary further into the forest while extreme events move the persistence boundary up to the regeneration boundary.
Article
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Managers are moving toward implementing large-scale coastal ecosystem restoration projects, however, many fail to achieve desired outcomes. Among the key reasons for this is the lack of integration with a whole-of-catchment approach, the scale of the project (temporal, spatial), the requirement for on-going costs for maintenance, the lack of clear objectives, a focus on threats rather than services/values, funding cycles, engagement or change in stakeholders, and prioritization of project sites. Here we critically assess the outcomes of activities in three coastal wetland complexes positioned along the catchments of the Great Barrier Reef (GBR) lagoon, Australia, that have been subjected to restoration investment over a number of decades. Each floodplain has been modified by intensive agricultural production, heavy industry and mining infrastructure, urban/peri urban expansion, aquaculture development and infrastructure expansion. Most development has occurred in low-lying coastal floodplains, resulting in major hydrological modifications to the landscape. This has left the floodplain wetlands in a degraded and hydrologically modified state, with poor water quality (hypoxic, eutrophication, sedimentation, and persistent turbidity), loss of habitat, and disconnected because of flow hydraulic barriers, excessive aquatic plant growth, or establishment of invasive species. Successful GBR wetland ecosystem restoration and management first requires an understanding of what constitutes “success” and must be underpinned by an understanding of complex cause and effect pathways, with a focus on management of services and values. This approach should recognize that these wetlands are still assets in a modified landscape. Suitable, long term, scientific knowledge is necessary to provide government and private companies with the confidence and comfort that their investment delivers dividend (environmental) returns.
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Coastal ecosystems throughout the world are increasingly vulnerable to degradation as a result of accelerating sea-level rise and saltwater intrusion, more frequent and powerful extreme weather events, and anthropogenic impacts. Hardwood swamp forests in the Big Bend region of Florida’s Gulf of Mexico coast (USA) are largely devoid of the latter, but have degraded rapidly since the turn of the 21st Century. Photographs of the forest, collected on the ground since 2009, were used to guide an analysis of a 60 km2 study area using satellite images. The images confirm that the coastal forest area declined 0.60% from 1982 to 2003, but degraded rapidly, by 7.44%, from 2010 to 2017. The forest declined most rapidly along waterways and at the coastal marsh–forest boundary. Additional time series of aerial-photographs corroborated the onset of degradation in 2010. Degradation continued through 2017 with no apparent recovery. Previous research from the area has concluded that increased tidal flooding and saltwater intrusion, of the coastal plain, represent a chronic stress driving coastal forest decline in this region, but these drivers do not explain the abrupt acceleration in forest die-off. Local tide gage data indicate that sea-level rise is 2 mm yr−1 and accelerating, while meteorological data reveal at least two short-term cold snap events, with extreme temperatures exceeding the reported temperature threshold of local vegetation (−10 °C) between January 2010 and January 2011, followed by more extremes in 2016. The Big Bend hardwood forest experienced acute cold snap stress during the 2010–2017 period, of a magnitude not experienced in the previous 20 years, that compounded the chronic stress associated with sea-level rise and saltwater intrusion. This and other coastal forests can be expected to suffer further widespread and lasting degradation as these stresses are likely to be sustained.
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The response of coastal wetlands to sea-level rise during the twenty-first century remains uncertain. Global-scale projections suggest that between 20 and 90 per cent (for low and high sea-level rise scenarios, respectively) of the present-day coastal wetland area will be lost, which will in turn result in the loss of biodiversity and highly valued ecosystem services1-3. These projections do not necessarily take into account all essential geomorphological4-7 and socio-economic system feedbacks8. Here we present an integrated global modelling approach that considers both the ability of coastal wetlands to build up vertically by sediment accretion, and the accommodation space, namely, the vertical and lateral space available for fine sediments to accumulate and be colonized by wetland vegetation. We use this approach to assess global-scale changes in coastal wetland area in response to global sea-level rise and anthropogenic coastal occupation during the twenty-first century. On the basis of our simulations, we find that, globally, rather than losses, wetland gains of up to 60 per cent of the current area are possible, if more than 37 per cent (our upper estimate for current accommodation space) of coastal wetlands have sufficient accommodation space, and sediment supply remains at present levels. In contrast to previous studies1-3, we project that until 2100, the loss of global coastal wetland area will range between 0 and 30 per cent, assuming no further accommodation space in addition to current levels. Our simulations suggest that the resilience of global wetlands is primarily driven by the availability of accommodation space, which is strongly influenced by the building of anthropogenic infrastructure in the coastal zone and such infrastructure is expected to change over the twenty-first century. Rather than being an inevitable consequence of global sea-level rise, our findings indicate that large-scale loss of coastal wetlands might be avoidable, if sufficient additional accommodation space can be created through careful nature-based adaptation solutions to coastal management.
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