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Root productivity contributes to carbon storage and
surface elevation adjustments in coastal wetlands
Brooke M Conroy
University of Wollongong Faculty of Science: University of Wollongong Faculty of Science Medicine and
Health https://orcid.org/0000-0001-5907-8265
Jeffrey J Kelleway
Kerrylee Rogers
Research Article
Keywords: Coastal wetlands, Root productivity, Surface elevation change, Carbon sequestration
Posted Date: November 4th, 2024
DOI: https://doi.org/10.21203/rs.3.rs-5279548/v1
License: This work is licensed under a Creative Commons Attribution 4.0 International License.
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Abstract
Background and aims: Additions of organic matter in coastal wetlands contributes to blue carbon
sequestration and adjustment to sea-level rise through vertical growth of substrates. To improve models
of carbon sequestration and adaptation to sea-level rise, data of root mass and volume additions across
tidal gradients are required. This study aims to characterise the inuence of vegetation zonation and
tidal position on root mass and volume dynamics within substrates. Methods: The root ingrowth
technique was coupled with sediment cores to quantify below-ground root mass and volume production,
standing stocks and turnover across two years to 90 cm depth at Kooweerup, Victoria, Australia.
Measurements of vertical accretion quantied mineral sediment additions at the surface. Results: The
results indicate a complex non-linear relationship between root production and tidal position, which is
driven by variation in vegetation structure across mangrove (442–3427 g ne root mass m-2 yr-1),
saltmarsh (540–860 g m-2 yr-1) and supratidal forest (599 g m-2 yr-1) zones. Fine root volume additions
ranged from 274 to 4055 cm3 m-2 yr-1 across sampling locations. Root production was greatest for older
mangroves and tidally dened optimal zones of production were evident for mangrove and saltmarsh.
The live rooting zone extended beyond depths typically measured in studies, and for forested sampling
locations, live roots were found as deep as 1.0 m. Conclusion: These data can be used to improve highly
parameterised models accounting for carbon sequestration and substrate vertical adjustment across an
intertidal gradient by quantifying both root mass and volume additions across the live rooting zone.
Introduction
Root production is an important process in coastal wetlands, underpinning multiple ecosystem services
including, binding substrates to minimise erosion, carbon sequestration and adaptation to sea-level rise
through surface elevation adjustments (Krauss et al. 2014; Arnaud et al. 2023). Both above- and below-
ground root structures trap autochthonous and allochthonous sediments, protecting against erosion
while promoting organic matter additions, leading to carbon accumulation and volumetric contributions
to substrates (Allen 2000; Cahoon et al. 2006; Saintilan et al. 2013; Kelleway et al. 2017). The ne
fraction of roots, generally distinguished as < 2 mm diameter, represent a major source of soil carbon
and volume in coastal wetland substrates (Alongi 2014; Lin et al. 2023; Adame et al. 2024a; Sun et al.
2024). Below-ground carbon in coastal wetlands accumulates primarily via carbon transfer facilitated by
organic matter additions to substrates, which is mediated by root growth and decay. Edaphic conditions
in substrates inuence organic carbon accumulation, with anaerobic conditions dampening rates of
aerobic decomposition and saline conditions dampening methanogenic pathways of decomposition
(Bartlett et al. 1985; DeLaune et al. 1990; Adams et al. 1990; Watson et al. 2000; Chmura et al. 2003;
Poffenbarger et al. 2011; McLeod et al. 2011). Organic matter accumulation enhances carbon stocks
and substrate volume which can drive vertical elevation gain where additions exceed losses (Cahoon et
al. 2006). This is particularly evident in settings with low sediment supply, where root additions are the
primary component of substrates, allowing for peat development (Nyman et al. 2006; Mckee et al. 2007).
Root volume can be signicant in coastal wetlands due to the root aerenchyma of mangroves which may
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account for up to 70% of root volume (Hogarth 2015), which is largely pore space (Curran et al. 1996).
Depending on sediment supply, root additions can signicantly inuence the capacity of coastal
wetlands to maintain intertidal positions under sea-level rise (Morris et al. 2002; Krauss et al. 2014;
Woodroffe et al. 2016; Rogers et al. 2019). Adequately accounting for root contributions to these
services in different settings requires improved understanding of root additions and losses and the
processes inuencing these dynamics.
Processes contributing to below-ground root mass accumulation are poorly understood due to
diculties measuring additions through production and losses through decomposition and establishing
relationships with processes. Accordingly, there is an abundant focus on above-ground biomass and
production given the relative ease of these assessments (Alongi 2020), despite the below-ground
component overwhelmingly contributing to long-term carbon sequestration and coastal protection
services (Krauss et al. 2014; Arnaud et al. 2023). Root addition and decay occurs within coastal wetland
substrates in the space that is available for accumulation of sediments: commonly termed
‘accommodation space’ (Jervey 1988; Rogers et al. 2022). Distinct zonation of vegetation communities
and the type and character of root additionality and decomposition can be the lateral expression of
changes in tidal position and is dened by available accommodation space. Tidal position inuences
edaphic conditions that have ecophysiological effects on coastal wetland vegetation (Ball 1988, 1998;
Lovelock et al. 2006, 2016). The distribution and structure of coastal wetlands is related to edaphic
factors at the site-scale governed by inundation including anoxia, salinity and nutrient availability
(Lovelock et al. 2006; Feller et al. 2010), resulting in the zonation of distinct vegetation communities
such as mangrove, saltmarsh and supratidal forests (Semeniuk 1980; Feller et al. 2010; Woodroffe et al.
2014; Conroy et al. 2022). Within these broad vegetation zones, both above- and below-ground
productivity is inuenced by variation in vegetation structure, composition and age, and edaphic factors.
For example, in homogenous tidal marsh settings, organic matter additions peak at optimal tidal
positions, diminishing under less optimal conditions (Morris et al. 2002; Kirwan and Guntenspergen
2012). Studies of mangrove above-ground productivity indicates optimal conditions of inundation and
salinity inuences species richness and growth, with greater height, leaf area and biomass (Ball and
Pidsley 1995; Ball 1998). More recently, edaphic factors such as nutrients and salinity have been shown
to inuence root production and biomass, although these effects are highly variable within and across
sites (Castañeda-Moya et al. 2011; Cormier et al. 2015; Pierfelice et al. 2017; Stagg et al. 2017). Greater
root production and root biomass has been associated with high mangrove forest density (Adame et al.
2014; Robertson and Alongi 2016; Lamont et al. 2020; Arnaud et al. 2021), and/or greater forest biomass
was associated with stand age (Komiyama et al. 1987; Zhang et al. 2021). Evidently, the complex links
between vegetation distribution, structure and production, and edaphic factors inuence root production.
The adaptation and use of root ingrowth techniques in coastal wetlands has enhanced understanding of
below-ground sediment dynamics (Mckee et al. 2007). However, gaps in our understanding of the spatial
and temporal variation in root production and turnover remain. Previous efforts have largely focussed on
quantifying additions within the upper 30 to 50 cm range (Cormier et al. 2015; Lovelock et al. 2015;
Hayes et al. 2017; Santini et al. 2019; From et al. 2021; Lin et al. 2023), a range commonly explored in
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terrestrial settings where rooting depths are typically shallower (Raich et al. 1994; Jackson et al. 1996;
Vogt et al. 1998). As such, there is evidence of active rooting depths in coastal wetlands below 30 cm
(Castañeda-Moya et al. 2011; Muhammad-Nor et al. 2019; Arnaud et al. 2021). However, these
measurement depths may be insucient and potentially introduce signicant uncertainty to current
estimates of carbon storage and additions to substrate volume for maintaining surface elevation.
Emphasis on understanding root additions within ecosystem types, including mangrove forests
(Poungparn et al. 2016; Hsiao et al. 2024) and saltmarshes (Kirwan and Guntenspergen 2015; Stagg et
al. 2017) has resulted in few studies investigating root zone dynamics across complex vegetation zones
or across inundation gradients that establishes the conditions for complex zonation. Further, coastal
wetland root systems exhibit greater volumetric contribution to substrates relative to mass contributions
per root due to pore space within root structures in mangrove aerenchyma (Lovelock et al. 2015) and
saltmarsh (Ampuero Reyes and Chmura 2022). Nevertheless, most studies focus on quantifying root
mass production in the context of carbon storage (Xiong et al. 2017; Santini et al. 2019; Zhang et al.
2021; Arnaud et al. 2023) and/or make indirect links to volumetric contributions (Arnaud et al. 2021; Lin
et al. 2023). While a focus on root mass can be readily translated to information to support
understanding of carbon dynamics, it does not provide crucial information about the contribution of
roots to substrate volumes and adjustment to sea-level rise.
Addressing these gaps is critical for projecting the effects of environmental change on root production
and turnover and estimating the impacts of climate change on root-related ecosystem services. While
mineral sediment addition is typically described using linear relationships with inundation (Rogers,
Wilton and Saintilan, 2006; Kirwan and Megonigal, 2013), the same does not hold true for root production
(Morris et al. 2002; Kirwan and Guntenspergen 2012). Root production appears to be related to
vegetation structure and/or age (Arnaud et al. 2021), while decomposition may be related to
environmental conditions (Ouyang et al. 2017; Adame et al. 2024b), typically established by tidal
inundation (Stagg et al. 2018). Further research describing root production and turnover within the
context of complex settings characterised by multiple vegetation zones and differing root morphologies,
and within the context of processes varying along inundation gradients, may resolve these knowledge
gaps.
This study seeks to address knowledge gaps regarding the inuence of vegetation structure and tidal
position on root production and turnover across different root morphologies. To ensure the different root
morphologies are suciently described, this study focusses on describing root additions to depths
beyond 30 cm. The aim of this study is to characterise the inuence of vegetation zonation and tidal
position on root mass and volume dynamics within substrates to a depth of 0.9 m across a two-year
period using the root ingrowth technique. This study reports both the mass and volume of root
production to greater depths than typically reported and leveraged a tidal gradient with complex
vegetation structure to explore relationships with tidal position and vegetation zonation. It was
hypothesised that vegetation structure and tidal position inuence the relative contribution of mineral
and organic material to substrates, and the contribution of organic material to carbon stocks and
substrate volume. The specic objectives of this study are to:
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1. Quantify the mass and volume of below-ground root standing stock and characterise sediment
composition in the context of tidal position
2. Estimate root production and root turnover across rooting depths
3. Develop relationships between the mass and volume of root standing stock, root production and
root turnover and a) tidal position and b) vegetation zonation
4. Develop relationships between vertical accretion, root production and surface elevation change
This study will provide critical information of root mass and volume contributions to ~ 1 m depth and
across tidal positions, which is needed to inform highly parameterised models of coastal wetland carbon
uxes and surface elevation gain. The information derived from these spatially applied models will assist
decision makers in planning as coastal wetlands adjust to sea-level rise.
Materials and Methods
Site description and sampling regime
Kooweerup lies on the northern shore of Westernport Bay, Victoria, Australia (38°13'35"S, 145°24'31"E)
(Fig.1a). The region has a temperate climate with mean annual rainfall of 800 mm and is typically wetter
in the winter months (Boon et al. 2015). The coastal wetland at Kooweerup is characterised by high
mineral sediment supply and high vertical accretion rates (Rogers and Saintilan 2021), leading to
extensive mudat development that creates suitable conditions for shoreline progradation and seaward
extensive of mangroves which has been observed over recent decades (Rogers et al. 2022). The
semidiurnal tidal regime with a tidal range of up to 3.1 m near the study site (Water Technology 2014)
contributes to the development of several distinct vegetation zones which have a relatively broad width
of distribution. As such, mangroves extend laterally from the shoreline by ~ 100–300 m, saltmarshes
cover ~ 200–600 m of the wetland and supratidal forest extend across ~ 100 m near the terrestrial
boundary of the coastal wetland at Kooweerup (Fig.1a). These conditions, combined with existing data
of sedimentation and surface elevation change at the site (Rogers et al. 2006, 2022; Rogers and Saintilan
2021) make Kooweerup an ideal location for investigating the contribution of root production to below-
ground carbon and substrate volume in distinct zones characterised by relatively high rates of mineral
sediment supply. Tides amplify along the Northern Arm of Westernport Bay, particularly along the
shoreline of Kooweerup, resulting in signicant vertical and lateral accommodation space for tidally
borne mineral sediments to accumulate (Rogers et al. 2022).
Sampling was conducted along a transect that traversed an inundation gradient extending from the
upper half of the intertidal zone into the supratidal zone to capture variation in vegetation structure and
species composition. Sampling locations along the transect included the intertidal seaward mangrove
(shrubby
Avicennia marina, ~
0.3–2.0 m tall) and landward mangrove (taller
Avicennia marina
, ~ 2.0–4.0
m); the supratidal seaward marsh (marsh shrubland dominated by
Tecticornia arbuscula
) and landward
marsh (herbaceous co-dominated by
Salicornia quinqueora
&
Sporobolus virginicus
); and the supratidal
swamp paperbark forest (dominated by
Melaleuca ericifolia
, up to ~ 12.0 m tall) (Fig.1). Seaward marsh
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also comprised
Salicornia quinqueora, Samolus repens, Suaeda australis
and other less-dominant
herbaceous plants. Other herbaceous plants in the landward marsh included
Disphyma crassifolium,
Suaeda australis
and
Samolus repens
, and the supratidal forest ground cover included
Selliera radicans,
Disphyma crassifolium
and other less-dominant herbaceous plants. The sampling locations complement
placement of existing surface elevation tables and marker horizons (SET-MH network) established in
2000 (Rogers and Saintilan 2021). Approximate sampling locations were pre-determined upon the
elevation gradient specied by a digital elevation model (DEM) of the site (Fugro 2009).
To quantify sediment composition and root standing stock, three adjacent sediment cores (~ 1.0 m
apart, ~ 1.0 m length) (18 cores in total) were extracted in March 2022 at each sampling location using
percussive hand coring in aluminium tube (7.2 cm diameter). To quantify root productivity over a two-
year period, root ingrowth cores of ~ 5.5 cm diameter were placed in the evacuated holes remaining
from sediment core extraction (18 ingrowth cores in total) to a depth of ~ 0.9 m. The ingrowth core
length of 0.9 m was governed by the maximum length of the mesh material used for construction. Root
samples were analysed to this depth to correspond to the standard measurement depth for blue carbon
accounting (Howard et al. 2014). Root ingrowth cores were constructed using synthetic plastic mesh ~ 2
mm thick, with ~ 5 mm apertures and were lled with sand. Commercially available, washed quartz-rich
sand was used to ll evacuated holes because it contains negligible organic material and is regarded to
be inert (Götze 2009).
The previously deployed root ingrowth cores were collected at 6- (September 2022), 12- (March 2023)
and 24-month (March 2024) intervals post-deployment to quantify root productivity. The 24-month
ingrowth core in the landward marsh sampling location was damaged throughout the deployment period
and therefore this data point is missing from analyses. Upon extraction of sediment cores and the later
collection of root ingrowth cores, the cores were sealed and transported to the University of Wollongong
where they were placed in cold storage at 4℃ for later processing and analyses. All sample preparation
was undertaken in laboratory conditions. One replicate sediment core for each sampling location was
sub-sampled at 10 cm increments with a portion (25% of total volume) reserved for sediment
composition analyses, a portion reserved for root standing stock analyses (50% of total volume) and the
remaining portion (25% of total volume) archived in cold storage. A compaction correction factor was
applied to sediment core samples after analysis. Root ingrowth cores were also sub-sampled at 10 cm
increments with the entire sample used for analyses. The specic procedures for data processing are
detailed in the following sections.
To establish relationships and test hypotheses aligning with the objectives, statistical analyses were
undertaken in JMP (JMP Pro 17; SAS Institute Inc., Cary, NC, USA) using an alpha level of 0.05. The
condence in relationships was assessment using coecient of determination (r2) and where possible,
root mean square error (RMSE).
Standardising tidal position
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The absolute surface elevation (relative to the Australian height datum: AHD) and coordinates of each
sampling location were determined using a real-time kinematic global positioning system (RTK-GPS)
(Fig.1b). The surface elevation was modied to indicate the tidal position of each sampling location by
calculating a standardised tidal positioning index (STPI) using the equation:
Equation 1
Where: SE is the surface elevation (m AHD), MTL is the mean tide level (m AHD), and HAT is the highest
astronomical tide (m AHD). MTL and HAT were calculated from the available water-level data for Stony
Point tide gauge (Bureau of Meteorology, 2021). HOBO water-level loggers (SOLINST 189 LTC Edge,
HOBO U20-001-04) were placed at each sampling location to characterise inundation patterns and to
infer the effects of inundation on surface building processes of sedimentation. Tidal inundation
frequency was quantied using water-level logger data of pressure measurements at 30-minute intervals
over an 11-month period (13/03/2023–19/02/2024). This period included signicant variation in water
levels encompassing the spring and neap tidal regime, and rainfall events, and is an appropriate
representation of tidal patterns at the site. Inundation frequency (%) was determined as the number of
occurrences (30-minute increments) the logger was submerged relative to the total number of
occurrences. Tidal position was used to develop relationships for objective III and data is provided in
Supplementary Materials, Fig. S1, Table S2.
Below-ground sediment composition
Dry bulk density (g cm− 3), percent organic carbon (%Org C) and grain size distribution were quantied at
10 cm increments along the length of sediment cores. To determine dry bulk density, sub-samples were
oven dried at 60℃ until constant mass. To estimate %Org C using standard loss-on-ignition (LOI)
approaches, a sub-sample (~ 5 g) was taken after determining dry bulk density and dried in a mue
furnace at 550℃ until constant mass. LOI was determined using Eq.2, from Heiri, Lotter and Lemeck,
(2001), which converts LOI values to %Org C. Soil organic carbon density (g C cm− 3) was calculated
using the dry bulk density and %Org C values for each segment.
Equation 2
Where: LOI550 is the LOI (%) at 550℃, DW550 and DW60 is the dry mass (g) after heating at 550℃ and
60℃, respectively (Heiri et al. 2001).
Samples for grain size analyses were selected based on visual changes in sediment character to
describe variability down core. A sub-sample (~ 1 teaspoon) of the remaining dry bulk sediment was
treated with hydrogen peroxide (HO 30%) to digest organics prior to grain size analysis. Samples were
STPI
=
SE
−
MTL
HAT
−
MTL
LOI
550 =
( )
× 100
(
DW
60−
DW
550)
DW
60
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analysed in a laser diffraction particle size analyser with a detection range of 0–2000 µm
(Mastersizer2000; Malvern Panalytical, Worcestershire, UK). To establish links between organic matter
additions and mineral sediment additions, linear regression analyses were undertaken between %Org C,
bulk density and grain size (See Supplementary Materials, Table S3 and Fig. S4).
Root standing stock and root productivity
Both the root standing stock and root ingrowth samples were processed following a standardised
procedure for quantifying root mass and volume. Roots were extracted from samples by rinsing with tap
water and collecting roots using stacked sieves (1 mm and 5 mm). Separation of roots into live and dead
fractions was achieved using the oatation approach, whereby live roots oat. Roots were then
separated into size categories, delineating ne (≤ 2 mm), medium (2–5 mm) and coarse (≥ 5 mm) roots.
Despite successful use of LUDOX solutions to separate live and dead ne roots of mangrove species
(i.e.,
Rhizophora stylosa
and
Ceriops tagal
) in previous studies (Robertson and Dixon 1993; Robertson
and Alongi 2016), the small quantity of roots in some cores (resulting in high variance in separation
eciencies (Robertson and Dixon 1993)) and resource constraints meant that visual separation was the
most feasible approach. After air drying roots for ~ one week, root volume (cm3) was determined by
displacement in a test tube or in a pycnometer (Eq.3). Root mass was then determined by oven drying
roots at 60 ℃ to a constant mass and recording the dry root mass (g).
Where:
Pw
represents the pycnometer mass with water (g),
S
represents the air-dried sample mass (g),
Pws
represents the pycnometer mass with water and the sample (g) and ρ
w
represents the density of
water (1 g per cm3). Measurements were divided by the cross-sectional area of cores to calculate root
mass (g m− 2) and root volume (cm3 m− 2). The root standing stock measurements were corrected for
compaction and were standardised to 10 cm increments.
Values for root mass and volume for root ingrowth core segments were calculated per unit area to a
depth of 10 cm and adjusted to an annual productivity rate (g or cm3 m− 2 yr− 1). Root productivity was
only calculated for roots that penetrated the mesh aperture ( ~ ≤ 5 mm), and therefore does not account
for coarse root productivity. It was expected that coarse roots would not develop in the two-year study
and therefore this approach is appropriately representative of total root growth in two years.
Linear regressions were conducted to determine annual rates of root mass and volume productivity for
both (1) ne, and (2) all size classes (total). The predicted annual root production values for ne roots
were used to calculate ne root turnover, whereas the total dataset was used to report root carbon
accumulation rates and develop relationships between root production and tidal position and vegetation
zonation. A linear regression was considered the best model to indicate annual rates of root productivity
due to the relatively short deployment period (maximum of 24 months). Linear regressions were applied
Root volume
(
cm
3
)
=(
Pw
+
S
)−
Pws
ρ w Equation
3
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to the summed ne and total root mass (g m− 2) and volume (cm3 m− 2) additions to 90 cm depth, with
time (deployment period) for each sampling location. The suitability of linear regressions was assessed
using coecient of determination (Supplementary Materials, Fig. S5). The exception was for root volume
additions in the seaward marsh (Fig. S5h) where the coecient was not signicantly different and
reects the very low rates of annual productivity in this zone. For this reason, the model was deemed
suitable despite the low coecient of determination, for reporting annual rates of productivity in this
zone.
Previous studies have focussed on root additions in the upper 30 cm, while this study seeks to determine
whether this analysis depth suciently describe additions of root mass and volume. Paired t-tests were
performed to determine whether there was a signicant difference between root production in the upper
30 cm compared to total additions to 90 cm. Analyses were conducted for both mass and volume
measurements to assess differences in the total dataset (all sampling locations) (n = 14), across time
(6-, 12-, 24-month deployment periods) (n = 5 for 6- and 12-months and n = 4 for 24-months) and for each
sampling location (n = 3 for all sampling locations except for landward marsh where n = 2).
A linear regression analysis was performed to assess the relationship between root mass and volume
using all measurements taken for both the root standing stock and production (n = 343). Further linear
regression analyses were conducted to assess how the relationship varied across sampling locations
and root size classes.
The total annual root mass production rates (all size classes) for each sampling location derived using
the linear regression were converted to a rate of root carbon addition (Mg C ha− 1 yr− 1) using carbon
conversion factors (Table1).
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Table 1
Carbon conversion factors used for each sampling location sourced from J. Kelleway, unpublished data.
AG refers to above-ground
Sampling
location Dominant
species C conversion
factor Derived from Source
Seaward
mangrovea
A. marina
0.47
A. marina
, ne root Kelleway et al.,
(2018)
Landward
mangrovea
A. marina
0.47
A. marina
, ne root Kelleway et al.,
(2018)
Seaward
marshb
T. arbuscula
0.47
A. marina
, ne root Kelleway et al.,
(2018)
Landward
marshc
S. quinqueora
0.42
S. quinqueora
&
S.
virginicus
bulk AG J. Kelleway,
unpublished data
Supratidal
forestd
M. ericifolia
0.48
M. ericifolia
, woody
AG J. Kelleway,
unpublished data
Footnotes:
a Represents the mean ne root carbon concentration (%) (Kelleway et al. 2018).
b No carbon conversion factors were available for this species so the
A. marina
factor was used here
as woody plant structures generally contain more carbon than herbaceous plants due to higher lignin
which is carbon rich (Ma et al. 2018).
c No root carbon conversion factors were available for the roots of these species so a value was
derived from above-ground %C values for these species in southeast Australia (0.42 ± 0.70, J.
Kelleway, unpublished data). This value is within range of root %C reported for grass-dominant
marshes in North America (43.0–43.5%) (From et al. 2021), and
Salicornia perennis spp alpini
in
Spain (40.3%) (Palomo and Niell 2009).
d No carbon conversion factor for the roots of this species was available so a factor for the above-
ground woody component of
M. ericifolia
from southeast Australia was used (0.48 ± 0.85, J.
Kelleway, unpublished data).
Root turnover
Root turnover for ne root mass and volume for each sampling location was calculated using ne root
stock values and the predicted rate of ne root production from ingrowth cores to depths of 90 cm
(Eq.4). Root turnover time was also calculated for each sampling location and is the inverse of turnover
rate (Eq.5).
Fine root turnover rate
=
fine root production
fine root biomass or volume Equation
4
Fine root turnover time
=1
fine root turnover rate Equation
5
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Relationships between root mass and volume, tidal position
and vegetation zonation
The relationship between the measurements of root mass and volume from root standing stock, root
turnover and root productivity and tidal position was determined using linear regression analysis. The
input values were derived by taking the sum of measurements across the depth intervals to 1.0 and 0.9
m depth for root standing stock and root production, respectively. The input values for root turnover
represent the total turnover to 0.9 m depth. STPI was used as a proxy for tidal position and regressions
were performed using data for all sampling locations with the inclusion of mudat (where root stock and
productivity = 0) (n = 6). Previous studies indicate peak models describe relationships between
productivity and elevation or tidal position in monospecic zones (Morris et al. 2002; Kirwan and
Guntenspergen 2012). However, given the effort to collect suitable data within zones, the capacity to
explore peak models in a statistically rigorous manner was limited. Irrespective, gaussian models were
applied to all data, the mangrove zone (n = 4) and the saltmarsh zone (n = 4). To strengthen this analysis,
root productivity for these broad zones was presumed to be zero at the boundaries. The four data points
used for the mangrove model include the two data points from landward and seaward mangrove, the
seaward marsh data point and a zero value at the mudat where mangrove root production was
presumed to be zero. The four data points for the saltmarsh model include the seaward and landward
marsh, supratidal forest and a zero value at the mangrove tidal position where saltmarsh root production
is presumed to be zero. Given the small number of data points for this model, this analysis was
exploratory only and the statistical suitability of the model could not be determined (coecients
included in Supplementary Materials, Table S7). The mass and volume of root stock and root productivity
was plotted against vegetation zone to assess variation with vegetation zonation.
Relationships between vertical accretion, root production and surface
elevation change
Marker horizons were deployed using the mesh material used for root ingrowth cores. The mesh was cut
into ‘marker horizons’ of 15 cm × 30 cm (three at each sampling location) and pinned onto the wetland
surface using metal pegs. The vertical accretion of sediments above marker horizons was determined by
measuring the depth of sediments above the mesh at different time intervals (6-, 12- and 24-months
after placement). Multiple measurements of marker horizons at each sampling location were taken
(between 8–13 individual measurements) per time interval, with the average used to determine the rate
of vertical accretion across the two-year sampling period. Large twigs and leaf litter were observed on
marker horizons in the supratidal forest and oysters and large twigs were observed on marker horizons
in the mangrove zones. These were not included in the measurements of accretion.
The rate of surface elevation change (mm yr− 1) at each sampling location was determined using the
modelled rates of surface elevation gain per metre of accommodation space, determined from 210Pb
dating of sediments at Westernport Bay (Rogers et al. 2022). This reported rate of surface elevation
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change for Kooweerup was 4.4 mm per metre of accommodation space per year, and values at each
sampling site were determined based on calculations of available accommodation space, in this case
dened as the depth below HAT. Water-level data from the Stony Point tide gauge were adjusted for tidal
amplication at this study site to determine HAT, as per the approach of Rogers and Saintilan (2021).
To assess the role of root production in modifying vertical accretion and surface elevation change, linear
regression analyses were conducted to determine if there was a relationship between the rate of root
volume addition and the measured vertical accretion rate with modelled surface elevation change. The
relationship between total addition rate (organic + mineral) and modelled surface elevation change was
also assessed using a linear regression.
Results
Root mass and volume of below-ground standing stock
Root biomass and volume for saltmarsh and supratidal forest sampling locations were concentrated in
the upper 30 cm of wetland substrates, whereas the mangrove sampling locations had a more uniform
distribution of root standing stock to 1.0 m depth (Fig.2a & b). The landward marsh and landward
mangrove sampling location had the greatest total root biomass stock; however, the landward mangrove
had the greatest total root volume stock, with supratidal forest and landward marsh having comparable
root volume. Both the mangrove and supratidal forest stored proportionally more volume than mass
relative to the saltmarsh zone. The proportion of ne, medium and coarse roots and living and dead
roots varied between zones and with core depth (Fig.2c & d). In particular, ne roots dominated the
vertical distribution of root structure to 1.0 m depth contributing to 74% and 66% of total root biomass
and volume, respectively. Dead roots were the primary constituent of root stock, comprising of > 74% of
root biomass and volume across all sampling locations. Forested zones (i.e., mangrove and supratidal
forest) exhibited diverse root size class distribution compared to saltmarshes, which were almost
entirely ne roots. The proportion of the mass and volume of different size classes and living and dead
roots were broadly similar; however, the live coarse roots in the mangrove zone exhibit proportionally
greater root volume compared to mass. The live rooting zone was variable between sampling locations
and for the well-established forested wetlands (i.e., supratidal forest and landward mangrove), live roots
were recorded in the deepest interval of 0.9–1.0 m.
Root productivity by mass and volume additions
Root productivity was dominated by ne root additions, which comprised 97% and 96% of the total mass
and volume additions, respectively, across all deployment periods (6-, 12- and 24-months) (Fig.3). Dead
roots constituted 73% and 65% of ne root mass and volume production, which is presumed to be
representative of the live to dead root turnover cycle within the study period. Consequently, root
productivity is reported for ne roots and represent both the live and dead portion.
Page 13/40
Fine root production varied with core depth and across the sampling locations after 24-months of
deployment (Fig.3a & b). Root productivity was generally concentrated in the upper 30 cm of wetland
substrates after 24-months, although root additions were recorded to the lower depth limit of ingrowth
cores (80–90 cm) (Fig.3a & b). Fine root mass and volume production was greatest for landward
mangrove followed by landward marsh. Root volume additions were proportionally greater than mass,
particularly for the forested sampling locations with both the mangrove and supratidal forest storing
proportionally more volume than the saltmarshes. The total ne root mass and volume production rates
across the sampling period generally increased across time, particularly for the landward mangrove with
an acceleration in root production between 12- and 24-months (Fig.3d & c) In contrast, seaward marsh
had a reduced rate of root volume between 12- and 24-months (Fig.3d).
While root productivity was generally concentrated in the upper 30 cm of wetland substrates across the
experimental period, root additions were evident across the full range of root ingrowth depths (Fig.3).
Deeper root additions were particularly evident in the seaward marsh (~ 84% of additions below 30 cm),
supratidal forest (~ 55%), and were still detectable in the seaward mangrove (33%), landward marsh
(13%) and landward mangrove (8.6%). The large proportion of roots stored below 30 cm for the seaward
marsh were found almost entirely in the 30–40 cm depth interval (Fig.6a).
Paired t-tests indicated the additions at depths beyond 30 cm were signicant, resulting in detectable
differences in root mass and volume production across all root size classes (|t|<0.0001) (Supplementary
Materials, Table S8). These differences were also signicant across the deployment periods for root
mass and volume, except for the 24-month ingrowth core for root volume (|t|<0.1171). When assessed
for each sampling location, there was a signicant difference (|t|<0.05) between 0–30 cm and 0–90 cm
for root mass for landward mangrove, seaward marsh and landward marsh however, for supratidal forest
there was no signicant difference (|t|<0.0619). Further, there was no signicant difference for root
volume for each sampling location (Table S8).
The linear regression analysis comparing the pooled data from root stocks and productivity indicate
mass and volume are related when using all data, each sampling location and root size class
(Supplementary Materials, Fig. S6a-i). A positive relationship was found between root mass and volume
measurements for all data (r2 = 0.73, Fig. S6a). The relationships were the greatest for the upper tidal
positions of the transect in the landward marsh (r2 = 0.99, Fig. S6e) and supratidal forest (r2 = 0.97, Fig.
S6f) sampling location, whereas a lower coecient was found for landward mangrove (r2 = 0.66, Fig.
S6c). The strength of the linear relationship between root mass and volume also varied with root size
class. The strongest relationship was found for ne roots (r2 = 0.88, Fig. S6g), followed by coarse roots
(r2 = 0.86, Fig. S6h) and medium roots (r2 = 0.67, Fig. S6i).
Root turnover
The rate of ne root turnover varied across sampling locations, and generally decreased with distance
from the shoreline (Fig.4a). In contrast, ne root turnover time increased with distance from the
Page 14/40
shoreline (Fig.4b). Forested zones exhibited greater rates of ne root turnover and were the highest for
mangroves ranging from 0.42 to 1.11 yr− 1 and 0.17 to 0.74 yr− 1 for mass and volume, respectively. The
saltmarsh zone and supratidal forest had the greatest ne root turnover time ranging from 10.6 to 14.6
yrs. and 6.47 to 15.6 yrs., for mass and volume, respectively. In the more seaward sampling locations
(i.e., seaward marsh, landward mangrove, seaward mangrove), rates of ne root turnover were greater
for root mass compared to volume and indicate volume additions take longer to replenish the initial root
standing stock compared to root mass. Turnover rate and turnover time for mass and volume appeared
to be related to tidal position with higher turnover rates and lower turnover time associated with
mangroves at lower tidal positions.
Relationships between root mass and volume, tidal position
and vegetation zonation
The rate of root mass and root volume turnover was positively correlated with tidal position (STPI) (r2 >
0.9, Fig.5a & b). Root mass was negatively correlated with tidal position (r2 = 0.81), whereas the
coecient of determination was lower for the negative correlation between root volume and tidal
position (r2 = 0.37) (Fig.5c & d). Both root mass and volume productivity were not well correlated with
tidal position (Fig.5e & f). The relationship between the mass and volume of root stock and root
production appeared to vary with vegetation zonation (Fig.5g–j), with a general pattern of higher root
stock and production in the landward sampling positions for both mangrove and saltmarsh zones. The
rate of root carbon addition also varied across sampling locations, with the landward mangrove
sampling location having the greatest carbon stock contributions in the two-year period (Fig.5i). Root
mass and volume productivity appear align with a gaussian peak relationship (Fig.6a–d), where the
amplitude of the peak for the mangrove zone was greater than the saltmarsh zone.
Relationships between vertical accretion, root production
and surface elevation change
Vertical accretion rates and the modelled rate of surface elevation change decrease with increasing
landward position of sampling locations, with the greatest accretion and surface elevation change
occurring at the seaward mangrove position (Table2). Vertical accretion was correlated with modelled
surface elevation change indicated by a positive linear t (r2 = 0.96, Fig.7b), whereas root volume
addition was not correlated with modelled surface elevation change (r2 = 0.003, Fig.7a). Combining
vertical accretion from mineral sediments and root volume addition resulted in a similar positive linear
relationship (r2 = 0.97, Fig.7c). The pattern of organic and mineral sediment additions aligns with
modelled surface elevation change (Rogers and Saintilan 2021) (Fig.7d).
Page 15/40
Table 2
Rafe of vertical accretion (± standard error of mean) for each sampling location. The modelled rate of
surface elevation change is based on the two-year observation period
Mean vertical accretion
rate
(mm yr− 1)
Modelled rate of surface elevation change (mm
yr− 1)
Seaward
mangrove 39.00 ± 2.04 8.51 ±
Landward
mangrove 8.50 ± 1.75 2.96 ±
Seaward marsh 0.21 ± 0.15 1.08 ±
Landward marsh 3.05 ± 0.67 0.69 ±
Supratidal forest 1.11 ± 0.82 -0.21 ±
Mean 10.37 ± 7.30 2.61 ± 1.56
Discussion
Inuence of vegetation structure on root growth
Greater root standing stock mass in the landward mangrove, landward marsh and supratidal forest at
Kooweerup suggest vegetation structure, age and preservation inuences root stock. This has been
observed elsewhere in both mangrove (Komiyama et al. 1987; Lang’at et al. 2013; Robertson and Alongi
2016; Adame et al. 2017; Zhang et al. 2021) and saltmarsh settings (Ampuero Reyes and Chmura 2022).
The well-established, tall, forested sampling locations of landward mangrove and supratidal forest
exhibited deep live rooting zones and diverse root size class distributions characteristic of older forests
(Tamooh et al. 2008). Here, the large root standing stock is attributed to the older forest age, taller trees
and higher tree density. Older vegetation stands have had more time to accumulate roots in substrates,
and when combined with conditions favourable for preservation, can lead to well-developed root
standing stock. The large root standing stock in landward mangrove is also inuenced by the high root
production rates, associated with greater stand density as the mangrove forest approaches maturity.
Proportionally greater root volume standing stock in the mangrove and supratidal forest was indicated,
which is inuenced by diverse root size classes and mangrove root aerenchyma structures that are
important for oxygen transport for mangroves occupying anaerobic substrates (Hogarth 2015). This gas
space contributes to the volume of root stocks allowing for the ratio of root volume to mass to be
markedly higher in mangrove roots compared to other zones. The large root stock in the landward marsh
is a consequence of the relatively high root production rates and preservation conditions. Here,
decomposition may be limited due to elevated salinity and reduced tidal inundation (Huxham et al. 2010;
Hayes et al. 2017), which inuences microbial communities responsible for decomposing organic matter
(Trevathan-Tackett et al. 2021). The inverse can occur where increased tidal inundation promotes
Page 16/40
preservation by saturating substrates and lowering redox potentials (Feller et al. 1999; Mckee and
Faulkner 2000; Poret et al. 2007); however, high root turnover in the landward mangrove suggests this is
not occurring at this sampling location. The results of this study suggest decomposition may be limited
across saltmarsh and supratidal elevations, leading to preservation of root stock.
Root production rates for landward mangrove were substantially greater than observed for other
sampling locations. In the landward mangrove, high root production rates were aligned with the greater
tree density and height, consistent with other studies (Adame et al. 2014; Arnaud et al. 2021). The
shorter stature and sparse cover of the prograding seaward mangrove margin was associated with low
root production, predominance of ne root fractions, low soil organic carbon density and high root
turnover, and is likely an outcome of the juvenile nature of mangroves at this location. This is supported
by observations of low root biomass stocks in sparse mangrove stands relative to dense mangroves at
higher tidal positions (Komiyama et al. 1987). Despite the negligible difference in tidal position between
the two saltmarsh sampling locations, the landward position had greater root production, reecting
differences in species composition related to greater distance from the shoreline. The shrub-dominated
Tecticornia
marsh in the seaward position may have lower growth rates even if it’s at an optimal tidal
position. Here, additions likely match losses due to decomposition. However, characteristic growth and
production for
T. arbuscula
could not be determined due to limited data on this species.
Live rooting depths were generally greater in the forested zones, with saltmarsh below-ground
productivity largely concentrated to the upper ~ 40 cm. Despite the low root stock and root production at
the seaward mangrove position, the young trees exhibit the capacity to store roots to depths of up to
0.9–1.0 m depth. This corresponds to studies elsewhere reporting mangrove root growth beyond 30 cm
(Castañeda-Moya et al. 2011; Cormier et al. 2015), up to at least ~ 60 cm in southeast Australia (Lamont
et al. 2020), and live mangrove roots have been reported to the extent of measurements at 100 cm depth
in China (Lin et al. 2023). The live rooting depth of the supratidal forest to 0.9–1.0 m depth in standing
stock cores and to 80–90 cm in ingrowth cores is supported by observed root growth up to 60 cm depth
by the non-tidal wetland species,
Melaleuca halmaturorum
, which can extend roots to source water
deeper in substrates root in response to lower water tables (Mensforth and Walker 1996). The depth of
root production across the tidal gradient at Kooweerup indicated woody vegetation has greater rooting
depths compared to saltmarshes, aligning with the different root morphologies of the herbaceous
saltmarsh versus the wooded tree dominated zones of the mangrove and supratidal forest. Based on
this study, it is recommended that methods to assess root production are conducted to at least 1.0 m
depth for mangroves and forested wetland communities and to 0.5 m depth for saltmarshes to capture
the entire active rooting zone of coastal wetlands.
Inuence of tidal position on root growth
Tidal position was linearly related to standing stock root mass, in accordance with existing studies in
mangroves (Castañeda-Moya et al. 2011; Lamont et al. 2020; Zhang et al. 2021), indicating a link
between root accumulation and edaphic factors. Allocation of biomass and carbon to roots has been
associated with reduced tidal inundation and increased salinity in
A. marina
in southeastern Australia
Page 17/40
(Saintilan 1997), and in this case may inuence the high root stocks found in the landward mangrove
sampling location. In contrast to root mass stocks, tidal position and root volume stocks were not
related, and this may arise due to the mangrove root aerenchyma resulting in proportionally higher
volume stocks compared to mass (Hogarth 2015; Ampuero Reyes and Chmura 2022). While root
turnover was correlated with tidal position, this correlation may be inuenced by the high root turnover
rates at the seaward mangrove where the young age and sparse distribution of trees inuenced the low
root standing stock mass. Although tidal position appears to inuence root standing stock and turnover,
the patterns observed are primarily explained by vegetation structure. Moreover, tidal position appears to
have a secondary inuence on root growth, and largely inuences root stock and turnover by setting up
the conditions for the zonation and associated diversity in vegetation structure which mediates
preservation by inuencing edaphic conditions.
Root production was not related to tidal position in this study, unlike studies undertaken in more
homogenous settings where inundation parameters were more clearly associated with variation in root
production (Morris et al. 2002; Kirwan and Guntenspergen 2012, 2015). However, when accounting for
structural variation, production tended to be higher towards the landward margin of the mangrove and
saltmarsh zones. Higher tidal positions have been associated with greater root productivity. For example,
Mckee et al. (2007) observed higher root productivity in the more elevated mangroves, which was
associated with ooding limitations on root growth (McKee 2001). In prograding settings similar to
Kooweerup, older mangrove forests at high intertidal positions exhibited greater organic matter
additions (Zhang et al. 2021), which is contrary to more stable settings or settings with shoreline erosion
where older forests may be more seaward. However, as tidal position transitions towards less optimal
ranges (Morris et al. 2002; Poungparn et al. 2016) or when younger individuals are expanding landwards
(Kelleway et al. 2016), organic matter additions can decline. Prioritising sampling to capture the
complexity of vegetation zones may have limited the capacity to fully characterise the inuence of tidal
position on root production.
Root production in the saltmarsh and supratidal forest zones were found to be less productive than
mangroves in this study, aligning with greater below-ground carbon values in mangrove relative to
saltmarsh in nearby settings (Kelleway et al. 2016). The landward mangrove (2486 g m− 2 yr− 1) was more
productive than previously reported for
A. marina
in eastern Australia (74–683 g m− 2 yr− 1) (Hayes et al.
2017, 2019) and China (576 g m− 2 yr− 1) (Xiong et al. 2017), although there is relatively limited data on
this species globally and some studies only measured root additions in the upper 30 cm of substrates
(Hayes et al. 2017, 2019). The older forest was targeted in this study and values may be within range of
A. marina
peak production at the site.
A. marina
in China is competing with adjacent mangrove species
and thus may explain why the monospecic mangrove stand at Kooweerup is more productive (Xiong et
al. 2017). Supratidal forest root mass production rates (592 g m− 2 yr− 1) were at the lower end of
reported values for tidal forested wetlands in North America (267–1814 g m− 2 yr− 1) (Pierfelice et al.
2017; Stagg et al. 2017; Li et al. 2020; From et al. 2021), which may be explained by differences in
vegetation composition, structure and abiotic conditions (e.g. freshwater settings). Further, saltmarsh
Page 18/40
root mass production (503–806 g m− 2 yr− 1) was generally lower than observed outside of Australia,
particularly in comparison to North American marshes (907–5828 g m− 2 yr− 1) (Kirwan and
Guntenspergen 2012; Pierfelice et al. 2017; Stagg et al. 2017; From et al. 2021), which may reect the
high tidal position and different species composition of marshes in the present study.
Conceptualising the interacting effects of tidal position and
vegetation structure
The demonstrated inuence of forest age and density on root production in the mangrove zone is linked
dynamically to tidal position, accommodation space and sediment accumulation. Further, the structure
and position of saltmarsh and supratidal forest in the tidal frame was related to accommodation space
and sediment accumulation, such that losses in root stock volume were replaced by root production.
Since vegetation structure has a profound inuence on root production, the inuence of tidal position on
root production only becomes evident within a zone, and not across zones. At Kooweerup, where mineral
sediment supply is abundant, high vertical accretion rates and mangrove progradation at the shoreline
were observed and root production was lower amongst the juvenile mangroves at the seaward margin
(Rogers and Saintilan 2021; Rogers et al. 2022) (tidal position 1, Fig.8). Continued accretion has
supported maturing of juvenile mangroves at optimal intertidal positions for root production, storing
considerable live and dead root mass (tidal position 2, Fig.8.). This study shows that mature, landward
mangroves have higher carbon storage and volumetric additionality. The geomorphological history of
relatively high sediment supply and shoreline progradation at Kooweerup has a profound inuence on
root additions in the mangrove zone by controlling age-vegetation structure dynamics. It is expected that
given the high root production rates of the landward mangrove, this sampling location has not yet
reached maturity, where tree density generally declines until senescence at ~ 70 years (Jimenez et al.
1985; Walcker et al. 2018) and would result in reduced root production (Arnaud et al. 2021).
In contrast, root mass is predominantly concentrated in the upper 30 cm of saltmarshes, representing
the depth of active root production and turnover to replace losses (tidal position 3 & 4, Fig.8). In this
zone there is relatively little available accommodation space for further vertical additionality of root
material, and production rates are likely to reect both losses and differences in vegetation composition.
For example, the herbaceous marsh had higher root production rates which may indicate these plants
are more productive than woody
Tecticornia-
dominated marshes. Further, salinity variation in the
saltmarsh zone may result in variable decomposition rates that inuence production of new root
material. In the supratidal forest, where tidally borne sediment supply is negligible, organic addition to
substrates is critical to limit tidal inundation and ensure maintenance of conditions suitable for
supratidal forests. This is exhibited by root additions with proportionally higher volume contributions
(tidal position 5, Fig.8). The limited available accommodation space for additions within the saltmarsh
and supratidal zone, where inundation frequency is now low to negligible, restricted additions and
inuenced root turnover. In these zones where tidal inundation is negligible, substrate additions are
dominated by root additions to remain beyond the available accommodation space. Available
accommodation space has been linked to carbon storage (Rogers et al. 2019) and here saltmarsh root
Page 19/40
production has been linked to tidal position. The higher tidal positions of coastal wetland substrates at
this study site, particularly compared to locations that have been adapting to sea-level rise for millennia
(e.g., tidal marshes in North America) (Morris et al. 2002; Kirwan and Guntenspergen 2012; Pierfelice et
al. 2017), acts to limit space for processes of root mass addition to occur. This results root production in
saltmarshes being largely constrained to the upper 40 cm of substrates where the bulk of root stock lies
and additions are replacing losses. As such, feedback between sediment accumulation and tidal position
have resulted in distinct zones of varying tidal position, vegetation age, composition, and structures,
which effectively inuences the complex patterns of root growth and accumulation.
Modelling root production and contribution of material to substrates requires quantication of complex
patterns across tidal positions and/or across vegetation structural gradients. The strong inuence of
vegetation structure and composition is demonstrated by the dual peak of root production across the
tidal gradient in this study (Fig.8a). In contrast, tidal position is a primary control on root production in
homogeneous settings, such as the grass-dominant marshes (e.g.,
Spartina alterniora
,
Schoenoplectus
americanus
,
Spartina patens
) in North America where root growth has been modelled using an inverted
parabolic relationship with tidal position (Morris et al. 2002; Kirwan and Guntenspergen 2012, 2015).
While parabolic models can optimise model performance to observations, they do not suciently
describe root production beyond observations, and gaussian-style relationships, as applied in this study,
are more appropriate.
Implications for adjustment to sea-level rise
Coastal wetland surface elevation gain is inuenced by mineral and organic matter additions to
substrates that increase soil volume, and elevations are modied by processes of autocompaction and
deep subsidence (Krauss et al. 2017; Cahoon et al. 2019; Rogers and Saintilan 2021; Sun et al. 2024).
Quantifying the contributions of organic matter to soil volume requires information about root volume,
rather than mass. Many studies have focused on root mass and ignored volumetric relationships; here,
the dual quantication of root mass and volume allowed the contributions to surface elevation change to
be explicitly considered. This study supports that root volume additions are proportionally greater than
mass additions in coastal wetlands (Lovelock et al. 2015; Ampuero Reyes and Chmura 2022), with the
relationship between mass and volume varying across sampling locations and root size classes
(Supplementary Materials, Fig. S6). This underpins the importance of developing species-specic
relationships between mass and volume. Surface elevation gain has exceeded the contribution of
mineral sediment additions in southeast Australia, which is attributed to rapid addition of root volume in
substrates (Rogers et al. 2005). This emphasises the importance of considering root volume when
accounting for surface elevation adjustment. Previous work at Kooweerup has demonstrated the
overwhelming inuence of mineral sediment additions to substrates, particularly at lower tidal positions
(Rogers et al. 2006, 2022). Here, vertical accretion was strongly correlated with modelled surface
elevation change, but the relationship failed when considering root volume additions. Incorporating both
mineral and organic matter volume additions to substrates slightly improved the relationship, but this
relationship was only based on root additions over a two-year study period, which were predominantly
Page 20/40
ne roots. A longer period of analysis may have allowed for the contribution of medium and coarse root
material, particularly in the mangrove and supratidal forest zones, to be incorporated in the relationship,
and it is feasible that medium and coarse root contributions to soil volume may be more signicant.
In the mangrove zone, surface elevation change is highly dominated by tidally borne sediment additions,
particularly in the seaward position where greater inundation frequency provides more opportunities for
sediment delivery and deposition. In the mangrove zone, sediment additions prior to autocompaction
were in the order of 39 mm yr− 1, corresponding to previous estimates at Kooweerup of ~ 40 mm yr− 1
derived from radiometric dating techniques (Rogers et al. 2022). In the seaward position, root volume
additions and standing stock were relatively low and inuenced by the young age and sparse cover of
mangroves prograding across the mudat, the predominance of ne roots and the high rate of sediment
supply. This contrasts the landward position in the mangrove zone where mature trees produced more
root volume. Here, the root volume was proportionally greater than the mass, reecting the improved
contribution of mangrove root aerenchyma to the root volume. Considering the variation in mangrove
structure with age between the landward and seaward positions, it is evident that where sediment supply
is high at the prograding front, the contribution of roots to soil volume is low. Further, where sediment
supply diminishes, root volume, particularly mediated by root aerenchyma, contributes more to the soil
volume. In the context of rising sea levels, providing mineral sediment supply is sucient to maintain
tidal position, the lower root volume additions at the seaward margin should be sustainable, providing
sediment supply remains stable and balances rates of sea-level rise. Should sediment supply diminish,
or rates of sea-level rise exceed increases in soil volume, then root volume additions will become
increasingly important. The addition of greater root volume within the landward mangrove indicates
there is some capacity to add more organic material to mangrove substrates as sea-level rise
accelerates.
In the seaward marsh and supratidal forest, organic matter addition is the larger component of surface
building processes. Root production contributed to small, modelled increases in surface elevation at
these positions, where the low rates of mineral sediment addition, in the order of 0.21–1.11 mm yr− 1,
provide limited material for building substrate volume. Across the saltmarsh and supratidal zones, low
supply of tidally-borne sediments makes root volume additions increasingly important for maintaining
substrate positions with sea-level rise. The relatively high root standing stock and root volume additions
in the landward marsh and supratidal forest indicate a history of accumulation of substrate volume
primarily via root additions, thereby maintaining their substrate elevation over time. Despite lower
production rates and negligible mineral sediment supply in the seaward marsh, the relatively high tidal
position of the saltmarsh zone (inundated < 3% of the time), suggest organic matter is sucient for
maintaining substrate elevations. Supratidal forests exhibit an enhanced capacity for maintaining
substrate elevations due to their tendency to store proportionally more root volume than mass, with the
complex size distribution of their root architecture providing a basis for volume additions as the forest
matures. Litterfall, which can be particularly high in
Melaleuca
forests (Congdon 1979; Finlayson et al.
1993) and tidal freshwater forests (Cormier et al. 2012), may also contribute to substrate volume
Page 21/40
(McKee 2011), although was not directly assessed in this study. As sea levels rise, it will become
increasingly important that saltmarshes and supratidal forests continue to produce root volume material
to maintain substrate elevations given negligible mineral sediment supply.
Implications for carbon storage
Fine root production and turnover are the most dynamic component in root systems (He et al. 2021),
representing a major portion of carbon sequestration and ux in coastal wetlands. The preservation of
roots and/or storage of carbon from root decomposition will inuence the fate of carbon produced by
coastal wetland plants. In this study, the contribution of ne roots to total below-ground carbon stocks
was considerable, contributing > 70% to root mass stocks and comprising > 95% of root mass production
compared to medium and coarse roots to the base of cores. Root carbon addition was the greatest for
landward mangrove (11.8 Mg C ha− 1 yr− 1) compared to the other sampling locations (2.0–3.4 Mg C ha− 1
yr− 1). Faster root turnover rates in the mangrove zone indicate ne roots grow rapidly and decompose,
transferring carbon to soil deeper in the soil prole. The dominance of dead root mass (> 70%) found in
root stocks and in the 6- 12- and 24-month productivity deployments across sampling locations,
conrms ne roots grow and perish quickly, or may preserve well, as supported by slower root turnover
rates and high root stocks in the saltmarsh and supratidal zones. This is consistent with other studies
that show ne roots dominate root production (Kihara et al. 2022; Lin et al. 2023), and dead root biomass
is a considerable portion of coastal wetland substrates (Cormier et al. 2015; Robertson and Alongi 2016;
Lin et al. 2023). Both the production and preservation of roots are important factors for long-term below-
ground carbon accumulation, however, vary based on vegetation structure and edaphic conditions.
Fine root mass stocks are refractory with turnover rates exceeding a decade in this study and other
studies indicating a high refractory component accumulation (Middleton and Mckee 2001; Tamooh et al.
2008), which may suggest long-term carbon accumulation. Only short-term (~ two years) root carbon
additions were estimated using ingrowth cores, and losses were not directly quantied, which limits the
ability to predict the fate of below-ground carbon at Kooweerup. However, the results imply
decomposition may be greater in the mangrove zone given the considerable root production rates here
despite relatively similar root stock and comparatively low soil organic carbon density compared to the
saltmarsh and supratidal zone. Tea bag decomposition experiments in Australian coastal wetlands
conrm up to two-fold greater decomposition in mangroves compared to saltmarshes and supratidal
forests, perhaps due to increased leaching-related mass loss mediated by porewater ushing from tidal
inundation (Trevathan-Tackett et al. 2021). Further, the characteristic root structure of some mangroves
enhances root decomposition, such as the presence of permeable roots and the pneumatophores of
A.
marina
which promote oxygenation of substrates (Ouyang et al. 2017; He et al. 2021). Increased nutrient
supply and freshening of soil salinity, which may be occurring at lower tidal positions at Kooweerup
(Ouyang et al. 2017), could account for the well-preserved root stock and higher soil organic carbon
density in the saltmarsh and supratidal forest zone. The pattern of progradation in the mangrove zone
driven by high sediment supply supports a vegetation sequencing relative to age, which in turn inuences
carbon stocks. The higher soil organic carbon density in the saltmarsh and supratidal forest zones may
Page 22/40
reect the long occupancy of vegetation at these positions, allowing for carbon to accumulate in
substrates. As mangroves increase vertical position in the tidal frame, below-ground carbon will continue
to accumulate but this will likely also inuence decomposition rates, creating uncertainty in projecting
the carbon stock dynamics. Further, increases in both above- and below-ground carbon storage are
expected at this site with sea-level rise as mangroves continue to encroach into saltmarsh (Saintilan and
Williams 1999, 2000; Rogers et al. 2006; Saintilan and Rogers 2013; Saintilan et al. 2014; Whitt et al.
2020), similar to patterns elsewhere in Australia (Kelleway et al. 2016). Future studies at the site should
aim to quantify loss dynamics as to improve forecasting of longer-term changes to carbon stocks and
substrate volume as organic matter decomposes.
Conclusion
This study addressed key knowledge gaps in the contribution of root mass and volume across a tidal
gradient at a rapidly prograding coastal wetland. Variation in vegetation structure and composition along
diverse coastal wetland zones imposed a primary inuence on root production, root standing stock and
root turnover. Tidal position exhibited a secondary inuence on these root growth dynamics, such that
variation within vegetation zones was linked to tidal position. The interaction between vegetation
structure and tidal dynamics resulted in a dual peak in productivity, with optimal zones inuencing
spatial patterns in mangrove and saltmarsh production. Shoreline progradation facilitated by high
mineral sediment supply has encouraged a sequencing of mangroves relative to age whereby the
mature, taller and more dense mangroves occupy the upper intertidal and contribute the greatest
amount of root mass and volume across the tidal frame. The mangrove and supratidal forest exhibit
capacity to store proportionally greater root volume than mass due to their unique root morphologies
and greater rooting depths compared to saltmarshes. Rooting depths extended the boundary of typical
measurement depths, emphasising the importance of characterising the live rooting depth in each
setting prior to sampling for root production. The gradient of edaphic conditions along the tidal gradient
appeared to impose an inuence on root standing stock and turnover and enhanced decomposition may
encourage root production.
By linking root mass and volume additions to both patterns of inundation and rates of surface elevation
change, the key relationships have been established for projecting organic matter contributions to
substrates. Specically, understanding the contribution of root volume to substrates to ~ 1 m depth
across a tidal gradient provides crucial information for highly parameterised models of wetland vertical
adjustment. Further, root mass contributions to ~ 1 m depth aligns with the measurement depths for
blue carbon accounting. This information is critical for decision makers interested in the
geomorphological evolution of substrates and increasing carbon sequestration that is anticipated as
wetlands adjust to relative sea-level rise.
Declarations
Page 23/40
Competing interests
The authors have no relevant nancial or non-nancial interests to disclose.
Funding
The authors acknowledge the nancial support provided by the Victorian Government and the Australian
Research Council through a Discovery Project (210100739).
Author Contributions
All authors contributed to the study conception and design. Material preparation and data collection
were performed by Brooke Conroy, Jeffrey Kelleway and Kerrylee Rogers. Data analysis was performed
by Brooke Conroy. The rst draft of the manuscript was written by Brooke Conroy and all authors
commented on previous versions of the manuscript. All authors read and approved the nal manuscript.
Acknowledgments
The authors respectfully acknowledge the Bunurong (Boonerwrung) people of the Kulin nation where this
work was conducted, and recognise their enduring relationship with the sacred land, and waterways,
including the wetlands within Westernport Bay. The authors thank the numerous volunteers in the
laboratory and eld including Grace Bendall, Ryan North, Alysha Johnson, Stephen Rigney, Laura
Mogensen, Ella Magnussen, Jesse Paton and others. The authors acknowledge the generous funding
support from the Victorian Government and Australian Research Council.
Data Availability
The datasets generated during and/or analysed during the current study are available from the
corresponding author on reasonable request.
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Sea-Level Rise. Ann Rev Mar Sci 8:243–266. https://doi.org/10.1146/annurev-marine-122414-
034025
10. Xiong Y, Liu X, Guan W et al (2017) Fine root functional group based estimates of ne root
production and turnover rate in natural mangrove forests. Plant Soil 413:83–95.
https://doi.org/10.1007/S11104-016-3082-Z/FIGURES/7
107. Zhang Y, Xiao L, Guan D et al (2021) The role of mangrove ne root production and decomposition
on soil organic carbon component ratios. Ecol Indic 125:107525.
https://doi.org/10.1016/j.ecolind.2021.107525
Figures
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Figure 1
a) Location map of study site, transect and core locations, b) LiDAR* point data for ground and non-
ground along the transect and c) photos of sampling locations at Kooweerup. *LiDAR sourced from
Fugro (2009)
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Figure 2
Root biomass (a) and root volume (b) for all root size classes separated by live and dead roots to 1.0 m
depth. Sum of root biomass and volume to 1.0 m depth reported at the bottom of each sampling
location data frame. The proportion (%) of root category of total root biomass (c) and volume (d) to 1.0
m depth for each sampling location
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Figure 3
Fine root mass (a) and volume (b) productivity with depth from root ingrowth cores collected after 24-
months. Sum of root mass and volume production to 0.9 m depth reported at the bottom of each
sampling location data frame. Total (summed to 90 cm) ne root mass (c) and volume (d) productivity
from root ingrowth cores at 6-, 12- and 24-months post-deployment. Note that for landward marsh the
values are from the 12-month deployment period due to damage of the 24-month core
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Figure 4
Fine root turnover rate (a) and ne root turnover time (b) for mass and volume for each sampling
location. Turnover was calculated using ne root biomass and volume and the predicted root
productivity values for one year derived from the linear regression across deployment periods. Live and
dead roots are included
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Figure 5
Linear regressions of root mass and volume with respect to STPI for turnover rate (a & b), standing stock
(c & d) and productivity (e & f). Regression data points include all sampling locations with the addition of
a point for mudat where root mass and volume = 0. Total root mass and volume stock and productivity
across sampling locations (g–j). Labels on bars for plot i. represent the root carbon addition rate (Mg C
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ha-1 yr-1). Root productivity data are the predicted values of the linear regressions of root production
across time (intercept at one-year)
Figure 6
Gaussian peak model predicted curves using input values for root mass and volume productivity using
all data (n = 6) (a & b), and data for both mangrove (n = 4) and saltmarsh (n = 4) (c & d). Root mass and
volume productivity values are for all size classes and were derived from linear regression analyses and
represent the intercept at one year. The r2 of each model is provided in Supplementary Materials Table
S7
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Figure 7
Relationships between rate of vertical gain and modelled surface elevation change (SEC) assessed by
root volume addition rate (a), mineral sediment vertical accretion rate (b) and the combined addition rate
from organic (root volume) and mineral sediment (c). Measured rate of root volume and mineral
sediment addition with STPI adjusted for autocompaction at the site (Rogers and Saintilan 2021)(d), with
labelled numbers for each sampling location (1. Seaward mangrove, 2. Landward mangrove, 3. Seaward
marsh, 4. Landward marsh and 5. Supratidal forest). Root volume additions represent the predicted total
of all root size classes for each sampling location and the vertical accretion rate is the mean rate from
marker horizons. Modelled SEC is for the two-year experiment period based on the Stony Point tide
gauge
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Figure 8
Conceptual diagram explaining the relationships between mineral and organic additions and vegetation
distribution in a prograding setting with high allochthonous sediment supply, at Kooweerup, Westernport
Bay. The diagram includes the relationship between tidal position and root production (a), 2-D prole of
vegetation zones and processes (b) and the resulting vegetation response and evidence in this study (c).
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Root standing stock and carbon density is dependent on vegetation age and rates of decomposition
such that old vegetation and slow decomposition rates would result in high carbon stock in substrates
Supplementary Files
This is a list of supplementary les associated with this preprint. Click to download.
KWRmanuscriptPlantSoilsuppinfo.docx