Primary succession in an Atlantic salt marsh: From intertidal flats to mid-marsh platform in 35 years

Journal of Ecology. 2021;109:2909–2921. | 2909wileyonlinelibrary.com/journal/jec Received: 18 February 2021 | Accepted: 5 May 2021 DOI: 10.1111/1365-2745.13692 RESEARCH ARTICLE Primary succession in an Atlantic salt marsh: From intertidal flats to midmarsh platform in 35 years Jesús M. Castillo1 | Blanca Gallego- Tévar1 | Eloy M. Castellanos2 | Manuel Enrique Figueroa1 | Anthony J. Davy3 This is an open access article under the terms of the Creative Commons Attribution- NonCommercial License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited and is not used for commercial purposes. © 2021 The Authors. Journal of Ecology published by John Wiley & Sons Ltd on behalf of British Ecological Society 1Departamento de Biología Vegetal y Ecología, Facultad de Biología, Universidad de Sevilla, Sevilla, Spain 2Departamento de Ciencias Integradas, Facultad de Ciencias Experimentales, Universidad de Huelva, Huelva, Spain 3Centre for Ecology, Evolution and Conservation, School of Biological Sciences, University of East Anglia, Norwich, UK Correspondence Jesús M. Castillo Email: [email protected] Funding information Universidad de Sevilla Handling Editor: Christine Angelini Abstract 1. Although salt marsh is a classic example of primary succession, the underlying mechanisms and their timescales are poorly understood. As salt marsh succession depends on sediment accretion, the amelioration of abiotic conditions associated with increasing elevation suggests potential explanatory roles for facilitation, competition and the stressgradient hypothesis. 2. We present a 35- year longitudinal study of salt marsh development from intertidal flat to a midmarsh platform at Odiel Marshes in southwestern Iberia. Using permanent plots, this work chronicles changes in elevation and marsh morphology, their evolving effects on sediment redox potential and salinity and the colonisation and changing patterns of dominance of halophytic species. 3. Sporadically colonising clumps of the lowmarsh species Spartina maritima trapped sediment to form raised tussocks, which increased in elevation and area. Reduced tidal inundation and locally improved drainage promoted higher redox potentials and allowed colonisation by a sequence of species less tolerant of reducing conditions: Sarcocornia perennis, its hybrid with highmarsh S. fruticosa, and Atriplex portulacoides. Unlike its centrifugally colonising predecessors, A. portulacoides invaded from the tussock edges. Transplant experiments designed to investigate its late establishment on tussocks showed that seedling survival depended on elevational differences as small as 4 cm. 4. After increasing in elevation by c. 1 m (c. 29 mm/year), coalescence of the tussocks formed a marsh platform at a level corresponding to mean high tides. This supports a theoretical punctuated transition from ‘submergence marsh’ to ‘emergence marsh’, previously postulated for this tidal elevation. 5. Synthesis. The unexpected rapidity of this primary succession highlights the central role of facilitation. Vertical sediment accretion, locally engineered by colonising species, progressively alleviates abiotic stress and allows colonisation by species that are less tolerant of chemically reducing conditions but are ultimately better competitors. 2910 | Journal of Ecology CASTILLO eT AL. 1 | INTRODUCTION Succession has been of enduring interest since the inception of ecological theory and remains central to contemporary ecological questions (Chang & Turner, 2019; Meiners et al., 2015). Primary succession involves changes in the abiotic environment that modulate the outcome of biotic interactions but the mechanisms, notably facilitation, inhibition and tolerance, have been contentious (Connell & Slatyer, 1977; Farrell, 1991; Pickett et al., 1987a, 1987b; Pulsford et al., 2016). Nevertheless, the fundamental role of facilitation in the structure and ontogeny of natural ecosystems is now recognised (Brooker et al., 2008); for primary succession, it is widely accepted that tolerance of abiotic stress and facilitation dominate in the early stages, whereas interspecific competition becomes more important as abiotic conditions ameliorate, consistent with the stress gradient hypothesis (He & Bertness, 2014). Because of the timescales involved, much of the evidence has come from chronosequence studies and thus from spacefor- time substitutions. Although valuable, they have limitations and inappropriate inferences have been drawn from them (Walker et al., 2010). Longitudinal studies, coupled with experimentation, are likely to provide more reliable insights into mechanisms. In particular, they may clarify the extent to which successional trajectories are gradual or are determined by thresholds (e.g. Corenblit et al., 2010); nonlinear dynamics have previously been associated mainly with the disturbances initiating secondary successions (e.g. Jager et al., 2018). Coastal salt marshes represent the earliest recorded examples of successional development (Davy, 2000) and succession is integral to our understanding of salt marsh geomorphology (Allen, 2000). However, spacefor- time substitutions have not satisfactorily distinguished it from static vegetational zonation associated with the elevational gradient from sea to land, particularly in the light of changing relative sea levels, and associated sediment dynamics. Because the relationship between tidal submergence and elevation is not linear, a distinction has often been made between lower marshes (dominated by factors associated with submergence) and upper marshes (dominated by those associated with emergence); the threshold between submergence and emergence marsh is widely accepted as around mean high water (MHW) tidal level (Chapman, 1960; Long & Mason, 1983; Ranwell, 1972). Succession implies a transition between these zones, but such punctuated changes and their dynamics have not been observed in longitudinal studies. Longterm studies based on permanent quadrats (Bos et al., 2002; Leendertse et al., 1997; Roozen & Westhoff, 1985) or historical maps and aerial photographs (Gray, 1972; Willis, 2000) have focused on the changing extent and composition of plant communities, offering circumstantial insight into succession. Primary salt marsh succession was initiated at Odiel Marshes (southwestern Iberian Peninsula) in 1977, when the construction of a dyke to protect an estuarine shipping channel allowed sedimentation on sand flats in intertidal lagoons (Castellanos et al., 1994). In a welldrained lagoon, the primary colonist, Spartina maritima (Curtis) Fernald, trapped sediment to form raised tussocks that increased in diameter and elevation; this allowed invasion of the centres of the tussocks by Sarcocornia perennis (Mill.) A.J. Scott, which spread centrifugally, displacing Spartina to the periphery of the expanding tussocks (Castellanos et al., 1994). Continued vertical accretion facilitated similar establishment and centrifugal spread of a hybrid Sarcocornia resulting from in situ hybridisation of S. perennis with the highmarsh species S. fruticosa (L.) A.J. Scott, whose pollen had been dispersed from nearby (Figueroa et al., 2003). Subsequently, this marsh and others in similar lagoons at Odiel Marshes have continued to develop, providing a model system for investigating primary succession in mineralogenic salt marshes over 35 years. The purpose of this paper is to present the changes that have occurred in 18 years since we reported on the establishment of the submergence marsh, to set these in the context of the overall succession, as currently understood, and to explain the processes driving change. We have investigated temporal and spatial patterns of salt marsh structure, manifested as changing species abundance, sediment accretion and the resulting physicochemical environment. We augment longitudinal observation with transplant experiments using Atriplex portulacoides L., as the emerging dominant, in examining the importance of changing position in the tidal frame, and the extent to which this influences abiotic and biotic constraints on the colonisation of plant species. We hypothesised that continued accretion would lead to: (a) a general increase in elevation in the tidal frame; (b) further amelioration of the physiochemical environment (evident as increasing sediment redox potential); (c) colonisation and subsequent domination by potentially more competitive species that are less tolerant of the consequences of submergence; (d) evolution from tussock structure to a marsh platform with dissecting creeks; and (e) a transition from submergence to emergence marsh at an elevation level similar to tidal mean high water. 2 | MATERIALS AND METHODS 2.1 | Field sites The study was carried out at Odiel Marshes (Figure S1), in the joint Estuary of the Odiel and Tinto rivers on the Atlantic Coast of the southwestern Iberian Peninsula (37°08′– 37°20′N, 6°45′– 7°02′W). Odiel Marshes have been protected as a ‘Natural Reserve of the Biosphere’ and represent one of the largest salt marsh complexes in the Iberian KEYWORDS Atriplex portulacoides, elevation, emergence marsh, facilitation, invasion, redox potential, sediment accretion, stressgradient hypothesis | 2911 Journal of Ecolog y CASTILLO eT AL. Peninsula. The tidal regime is semidiurnal, with a mean range of 2.10 m and a mean spring tidal range of 2.97 m. Mean tidal levels relative to Spanish hydrographic zero (SHZ) are mean high water neap (MHWN), +2.44 m; mean high water (MHW), +2.91 m; mean high water spring (MHWS), +3.37 m; and highest astronomical tides (HAT), +3.71 m. Work was located in three accreting intertidal lagoons, two with free tidal drainage (Laguna de Don Claudio and Laguna de El Manto) and another with impeded drainage (Laguna de Ludovico)— henceforth Don Claudio, El Manto and Ludovico respectively. More details of the physiography, climate and vegetation of the sites are provided by Castellanos et al. (1994) and Figueroa et al. (2003). 2.2 | Analysis of vegetation The composition and structure of the vegetation in Don Claudio were investigated over 35 years. Changing conditions necessitated certain modifications to the methodology over this period. Initially, eight raised tussocks formed by colonising clones of S. maritima were identified as permanent plots and the areas occupied by all species present on them were recorded in October– November 1984, 1990, 1997 and 2001. Subsequently, an additional eight tussocks were included as permanent plots in the sampling programme in October– November 2004, 2007, March 2011 and November 2013, 2015 and 2018 (giving a total of 16 tussocks). From 1984 to 2007, the area of each tussock and that of its colonising species was recorded by measuring the minimum and maximum diameter at the base of the canopy and applying the formula of an ellipse. In 2007, as the vegetation became more complex in structure, the area occupied by each species was also recorded continuously along four radial, linear transects up to 5 m long; these originated at the centre of each tussock and were at 90° to one another. Estimates by the two methods corresponded closely: A. portulacoides (52.0 ± 6.2% vs. 67.9 ± 5.8%), Sarcocornia hybrid (18.7 ± 5.1 vs. 19.9 ± 5.6%), S. perennis (26.3 ± 5.6% vs. 25.0 ± 5.9%) and S. maritima (3.8 ± 3.4% vs. 2.8 ± 1.2%). Thereafter, only the latter method was used, as it allowed us to measure the increasing superimposition of species’ canopies and was more appropriate as tussocks became more irregular and started to coalesce. For every year, the cover of each species was calculated as a percentage of the total area of the raised tussock. Previously, aboveground plant biomass (AGB) had been harvested from transects across representative permanently marked colonising tussocks (Castellanos et al., 1994; Figueroa et al., 2003). In January 2005, we resampled one of these tussocks to display the radial distribution of newly invading A. portulacoides. Each transect was a belt of contiguous quadrats (15 cm radially × 25 cm wide) across the radius of the tussock. Dry mass was determined after drying at 80°C for 48 hr. Similarly, measurements of the vertical distribution of belowground biomass (BGB) made up to 2001 were augmented by new samples in 2005. Sediment cores (10.5 cm diameter × 50 cm deep) were extracted from under the largest A. portulacoides patch in five replicate tussocks in April 2005. Cores were divided into 5 cm layers, and roots and rhizomes of all species present were washed from them, before being dried and weighed. Roots of A. portulacoides and S. perennis were distinguished by their colour, size and shape. Spartina maritima roots were distinguished by their white colour and the presence of welldeveloped aerenchyma. Care was taken to maintain the attachment of small roots to the more recognisable larger ones. To examine larger scale plant distribution patterns in relation to the developing creeks, the continuous presence of each species, bare sediment and tidal creeks was recorded along two linear transects (298.5 and 292.5 m long) following the main axis of the lagoon in November 2001, and along a further five transects (2 × 30 m and 3 × 100 m) in June– August 2005. 2.3 | Sedimentary environment Elevations relative to SHZ were surveyed to a resolution of 2 cm using a Leica NA 820 theodolite at the centre and the edge of the 16 permanent tussocks and under the canopy of each plant species (n = 10– 20) each time that plant cover was recorded. Elevations of extreme tidal levels were also determined as reference points. Sediment redox potential (Eh), pH and electrical conductivity (EC) were recorded in parallel with elevation under the canopy area of each plant species (n = 5). Eh was measured in the field using a Crison pH/mV meter and redox electrode system at two sediment depths in the rooting zone (0– 10 and 11– 20 cm). pH and EC (as a measure of salinity) for sediment samples collected in the field between 0– 10 and 11– 20 cm deep were recorded in the laboratory using a pH meter (Crison GLP21, Barcelona; in 1:1 mix of sediment and distilled water) and a conductivity meter (Crison- 522; in 1:2 mix of sediment and distilled water), respectively, and their electrode systems. 2.4 | Transplant experiments with Atriplex portulacoides As the most recent coloniser to attain dominance, A. portulacoides was transplanted into three intertidal lagoons to investigate its survival in different successional settings. Transplants were seedlings chosen for similar size (3– 5 branches, c. 15 cm height). No attempt was made to remove sediment adhering to the roots of transplants. Survival was determined at 2- monthly intervals for 6 months and a transplant was considered dead when no green leaves or shoots remained. Sediment Eh and conductivity were measured, as described previously, at five locations from each transplant/replant site; as EC measurements at the two depths were not significantly different, data were combined for analysis (n = 10). 2.4.1 | Transplants to poorly and welldrained lagoons This experiment examined whether A. portulacoides was able to survive on S. maritima tussocks recently colonised by S. perennis at their centres, in poorly and welldrained settings. Seedlings were 2912 | Journal of Ecology CASTILLO eT AL. obtained from a natural population at the periphery of Ludovico (elevation +2.9 m SHZ), where seven seedlings were also replanted as a control treatment. Seedlings were transplanted to the central areas of tussocks, colonised by S. perennis (patches smaller than 30 cm diameter), in the previous 2 years at Ludovico (poorly drained) and El Manto (welldrained) lagoons. Single seedlings were planted at the centre of each of seven replicate tussocks at each lagoon in April 2005. 2.4.2 | Transplants to different tussock zones The second transplant experiment investigated the survival of A. portulacoides in three different radial zones of tussocks that had been colonised centrifugally by S. perennis and which had begun to develop raised levees at their edges, as they met or formed creek banks. A. portulacoides seedlings were sourced from material previously collected from Odiel Marshes and cultivated for 5 months in a glasshouse in commercial potting compost, to obtain a total of 60 of consistent size. Then, seedlings were transplanted to Don Claudio at the edge of tussocks (levee or creekbank), their inner areas colonised by S. perennis and inner areas still occupied only by S. maritima. Five seedlings were planted in each of the three zones of four different tussocks in May 2007. Surface elevation and sediment water content were determined, as described previously, at five locations for each transplant site. 2.5 | Statistical analysis Analyses were carried out using SPSS v.15.0 (SPSS, Inc.). Data were tested for normality using the Kolmogorov– Smirnov test and for homogeneity of variance using the Levene test. Sediment variables for the transplant experiment to poorly and welldrained lagoons were compared between lagoons using ttests for independent samples. Sediment variables for the transplant experiment to different tussock zones were compared among zones using oneway analysis of variance (ANOVA) and Tukey's honestly significant difference post hoc test. Where normality or homogeneity of variance was not achieved by transformation, means were compared using nonparametric Kruskal– Wallis ANOVA with a post hoc Mann– Whitney U- test. Sigmoidal and linear regressions were fitted using Sigma Plot v11.0 (Systat Software, Inc.) to examine the relationships of Eh and EC, respectively, with surface elevation of the tussocks. Differences in marsh elevation among taxa, years and their interaction were analysed using the Gamma generalised linear model (GLZ) with Chisquare (χ2) of Wald, and Bonferroni post hoc test, since transformations of the data did not achieve homogeneity of variance (Ng & Cribbie, 2017). Plant cover data for largescale spatial distribution analysis obtained from linear transects following the main axis of Don Claudio were grouped into contiguous 50- cm sections. Data were checked for conformation to a Poisson distribution and dispersion coefficients were calculated as the ratio of variance and arithmetic mean (σ2/x); random distribution: σ2/x = 1; regular: σ2/x < 1; contagious: σ2/x > 1. Where contagion was detected, the size of clumps was estimated by the method of block variance (Greig- Smith, 1964). Association between species and tidal channels was tested by a χ2 test for observed and expected frequencies of association, with a null hypothesis that plant species and channels were distributed randomly. 3 | RESULTS 3.1 | Changes in intertidal environment Sedimentation in Don Claudio resulted in generally increasing elevation of its surface over the whole 35- year period of the study. Sporadic initial colonisation by clones of S. maritima (see Section 3.2), when the elevation was little above the mean tidal water level (MW), was associated with increased sedimentation and the formation of raised tussocks. These tussocks increased in area and elevation with time (Figure 1; Figure S1). Their area increased 118 m2 from FIGURE 1 Changes in (A) surface elevation at the centre (●) and the periphery (○) and (B) corresponding tussock area of tussocks in a welldrained intertidal lagoon (Don Claudio) at Odiel Marshes (southwestern Iberian Peninsula) between 1984 and 2018. Values are means ± SE (n = 8– 16). Tidal levels: MW, Mean Water; MHWN, Mean High Water Neap; MHW, Mean High Water; MHWS, Mean High Water Spring; HAT, Highest Astronomical Tide 1980 1985 1990 1995 2000 2005 2010 2015 2020 Tussock area (m2) 20 40 60 80 100 120 140 160 180 1980 1985 1990 1995 2000 2005 2010 2015 2020 Elevation (m over SHZ) 1,8 2,0 2,2 2,4 2,6 2,8 3,0 3,2 3,4 3,6 (A) (B) MHW MHWS MHWN MW Coalesced tussocks 1.8 2.0 2.2 2.4 2.6 2.8 3.0 3.2 3.4 3.6 Year | 2913 Journal of Ecolog y CASTILLO eT AL. 1984 to 2007. The centres of the tussocks were consistently higher than their peripheries, increasing by c. 3 cm/year from 1984 until 2004, amounting to a total accumulation of 72 cm in 20 years. They surpassed the level of mean high water neap tides (MHWN; +2.44 m SHZ) in 1991 and approached the mean high water tidal level (MHW; +2.91 m SHZ) in 2004. After this, elevation change was slower and more erratic, with 16 cm of sedimentation from 2004 to 2018. Elevation of the tussock peripheries reached MHWN much later and oscillated around it from 2000 to 2011, before continuing to increase until 2018, at which time it was still below MHW. Elevation of the tussock peripheries decreased 27 ± 7 cm from 2000 to 2011, while they were moving rapidly in a centrifugal manner to expand tussock area. After 2011, with coalescence of the tussocks, the now stationary edges resumed their increase in elevation (Figure 1A,B). The area of tussocks grew as they increased in mean diameter until 2007, eventually driving the coalescence of neighbouring tussocks. By this time, incipient drainage channels between the tussocks had either been filled with sediment or consolidated into the new marsh drainage system (Figure 1B). Increasing elevation was associated with higher sediment Eh, changing in a sigmoid manner from values c. −300 mV to c. +200 mV, both near the surface (0– 10 cm) and in deeper (10– 20 cm) sediments (Figure 2). Eh values above zero were almost entirely restricted to elevations above MHWN in the superficial sediments, although the deeper ones were generally rather more reducing. Higher Eh (> +200 mV) also tended to be associated with the later stages of the succession, irrespective of elevation. The EC of sediment extracts was significantly, if rather weakly, dependent on elevation as the marsh developed from 2001 to 2018 (Figure 3). Thus, EC from 2001 to 2013 varied between 6.0 ± 0.0 and 17.8 ± 0.7 mS/cm, increasing to values c. 30 mS/cm under the canopy of the hybrid of Sarcocornia and A. portulacoides in 2015. Sediment pH showed no discernible trend over the same period, with little deviation from neutrality, varying between 6.7 and 7.6. 3.2 | Vegetation change The initial colonisation of the Don Claudio salt marsh was carried out by S. maritima; clonal clumps of tillers formed isolated, expanding tussocks. Changes in plant species cover recorded between 1984 and 2018 showed a clear sequence of species invasion and dominance (Figures 4 and 5). By 1984, although the vegetation cover was still dominated by S. maritima (with a mean cover of 88 ± 6%), the centres of the tussocks had been invaded by S. perennis (with a mean cover of 12 ± 4%). Subsequently, the cover of S. perennis increased, at the expense of that of S. maritima until S. perennis was clearly dominant by 2001, when it achieved its maximum mean cover of 82 ± 4%. However, by 1997, other species had begun to invade the tussocks, albeit with low cover. The hybrid S. perennis x fruticosa replaced S. perennis at the centres of many of the tussocks through in situ hybridisation and expanded its cover, again centrifugally, steadily to reach mean cover of 66 ± 5% by 2018 (Figure 4). Also in 1997, the first individuals of the shrub A. portulacoides were recorded, FIGURE 2 Relationship between sediment redox potential (Eh) and surface elevation relative to Spanish Hydrographic Zero (SHZ) on tussocks in a welldrained intertidal lagoon (Don Claudio) at Odiel Marshes (southwestern Iberian Peninsula) between 1990 and 2018 at two depths: (A) 0– 10 cm; (B) 11– 20 cm. 1990– 2001 (●), 2004– 2018 (○). Tidal levels: MW, Mean Water; MHWN, Mean High Water Neap; MHW, Mean High Water; MHWS, Mean High Water Spring; HAT, Highest Astronomical Tide. Regression equations: (A) y = −407.106 + 580.128/ (exp(−(x − 2.127)/0.165)) (R = 0.738, p < 0.0001); (B) y = −343.924 + 496.906/ (exp(−(x − 2.235)/0.139)) (R = 0.526, p < 0.0001) Marsh elevation (m over SHZ) 1.82.0 2.22.4 2.62.8 3.03.2 3.43.6 3.8 –400 –300 –200 –100 0 100 200 300 1.82.0 2.22.4 2.62.8 3.03.2 3.43.6 3.8 Sediment Eh (mV) –400 –300 –200 –100 0 100 200 300 1990–2001 2004–2018 MHWMHWS MHWN MW HAT (A) (B) 2914 | Journal of Ecology CASTILLO eT AL. initially representing a mean cover of only 2 ± 0%. In contrast to other species, A. portulacoides colonised the peripheries of the tussocks and then invaded them centripetally. This process was slow at first, with cover only 4 ± 1% in 2002, but it then rapidly achieved dominance of the tussocks by 2004 and has maintained the highest cover of c. 80 ± 5% from 2011 to 2018. Most of the expansion of A. portulacoides since 2001 has been at the expense of S. perennis. A very low cover of the annual Suaeda albescens Lázaro Ibiza (Suaeda maritima (L.) Dumort) was able to persist throughout the sequence, but it did not show any successional change (Figure 4). The elevational ranges of the four perennial species increased over the last 28 years (Figure 6). Although S. maritima colonised at elevations little above mean sea level (MW), its upper limit was nearer to mean high water (MHW) level, as the marsh accreted. Sarcocornia perennis consistently occupied a zone between MHWN and MHW level. Hybrid Sarcocornia and A. portulacoides both appeared above MHWN level but were predominantly distributed around or above MHW. GLZ showed that elevation was significantly different among taxa (χ2 = 135.216, df = 3, p < 0.0001), among years (χ2 = 337.732, df = 8, p < 0.0001) and their interaction (χ2 = 169.169, df = 20, p < 0.0001). This confirms that S. maritima and S. perennis colonised lower elevations (+2.66 ± 0.02 m SHZ and 2.69 ± 0.01 m SHZ, respectively) than the Sarcocornia hybrid and A. portulacoides (+2.88 ± 0.02 m SHZ and 2.84 ± 0.01 m SHZ, respectively), and all taxa except S. perennis increased their colonised marsh elevations over time (Figure 6). Changes in radial distribution of aboveground biomass on a representative tussock are shown in Figure 7. Four years after its first appearance, A. portulacoides was distributed contagiously (σ2/x = 1.39) forming patches c. 2 m in diameter that were positively FIGURE 3 Relationship between electrical conductivity (EC) of sediment extracts and surface elevation relative to Spanish Hydrographic Zero (SHZ), on tussocks in a welldrained intertidal lagoon (Don Claudio) at Odiel Marshes (southwestern Iberian Peninsula), between 1990 and 2018 at two depths: (A) 0– 10 cm, y = −3.947 + 7.093x, r = 0.222, p < 0.01, n = 155; (B) 11– 20 cm, y = −25.068 + 14.729x, r = 0.339, p < 0.0001, n = 104. Tidal levels: MW, Mean Water; MHWN, Mean High Water Neap; MHW, Mean High Water; MHWS, Mean High Water Spring; HAT, Highest Astronomical Tide 1.82.0 2.22.4 2.62.8 3.03.2 3.43.6 3.8 0 10 20 30 40 MHWMHWS MHWN MW HAT Marsh elevation (m over SHZ) 1.82.0 2.22.4 2.62.8 3.03.2 3.43.6 3.8 Sediment conductivity (mS/cm) 0 10 20 30 40 (A) (B) FIGURE 4 The sequence of change in the fractions of the tussock area occupied by five plant taxa colonising a welldrained intertidal lagoon (Don Claudio) at Odiel Marshes (southwestern Iberian Peninsula) between 1984 and 2018. Values are means ± SE (n = 8– 16) Year 1980 1985 19901995 2000 2005 2010 2015 2020 Plant cover (%) 0 20 40 60 80 100 Spartina maritima Sarcocornia perennis Sarcocornia perennis × fruticosa Atriplex portulacoides Suaeda albescens | 2915 Journal of Ecolog y CASTILLO eT AL. associated with developing tidal channels (χ2 = 24.56, p < 0.0001). By 2005, it was regular or random in distribution (σ2/x = 0.89– 0.98), although still associated with tidal channels (χ2 = 6.10, p < 0.01), if less strongly. This trend was also reflected in the fraction of A. portulacoides plants appearing near the edge of developing tidal channels, which fell from 49 ± 18% in 2001 to only 20 ± 7% in 2005. The dominance of A. portulacoides in the canopy at this stage was reflected in its belowground biomass (Figure 8), where it monopolised the upper 25 cm of the sediment profile, consigning the residual biomasses of earlier colonisers to greater depths. 3.3 | Transplant experiments with Atriplex portulacoides No seedling of A. portulacoides that had been transplanted to the centre of tussocks recently colonised by S. perennis in either poorly drained (Ludovico; Figure 9B) or welldrained (El Manto; Figure 9C) lagoons survived for as long as 6 months. In contrast, survival of the replanted controls at the source locality was 100% (Figure 9A). Sediment Eh at the transplant sites in both Ludovico and El Manto was strongly reducing in comparison with Don Claudio, although even more so in the poorly drained lagoon. EC values were 30% higher at El Manto than at Ludovico (Table 1). Survival was no better in seedlings transplanted to the inner areas of tussocks colonised by S. perennis at Don Claudio, whether into inner areas already occupied by S. perennis (Figure 9E) or S. maritima (Figure 9F). In contrast, no seedlings died after transplantation to levees at the edge of the tussocks within 6 months (Figure 9D). Environmental characteristics at these locations are also shown in Table 1. The favourable locations at the edges of the tussocks were on average only 4 cm higher than the unsurvivable locations but, nevertheless, had significantly lower water contents, higher superficial Eh and lower EC values. FIGURE 5 Photographs illustrating the development of the salt marsh at Laguna de Don Claudio, Odiel Marshes, taken from various aspects between 1986 and 2020 1986 1989 1992 2006 2003 2013 2015 2020 2916 | Journal of Ecology CASTILLO eT AL. Year 1990 1997 2001 2004 2007 2010 2013 2015 2018 Marsh elevation (m over SHZ) 1.8 2.0 2.2 2.4 2.6 2.8 3.0 3.2 3.4 MHW MHWS MHWN MW 1.8 2.0 2.2 2.4 2.6 2.8 3.0 3.2 3.4 1.8 2.0 2.2 2.4 2.6 2.8 3.0 3.2 3.4 1.8 2.0 2.2 2.4 2.6 2.8 3.0 3.2 3.4 MHW MHWS MHWN MW MHW MHWS MHWN MW MHW MHWS MHWN MW (D) Spartina maritima (C) Sarcocornia perennis (B) S. perennis x fruticosa (A) Atriplex portulacoides a b c cc d ddd abc a bc ab c ab abc abc abc aa b bb bb a ab bc bc c bc bc FIGURE 6 Box plots of changes in elevational range (relative to Spanish Hydrographic Zero, SHZ) of four perennial plant taxa (A) Atriplex portulacoides, (B) Sarcocornia perennis x fruticosa, (C) Sarcocornia perennis and (D) Spartina maritima on tussocks in a welldrained intertidal lagoon (Don Claudio) at Odiel Marshes (southwestern Iberian Peninsula), between 1990 and 2018. n = 10– 20. Tidal levels: MW, Mean Water; MHWN, Mean High Water Neap; MHW, Mean High Water; MHWS, Mean High Water Spring; HAT, Highest Astronomical Tide. Different letters indicate significant differences between elevations for the same taxa (Bonferroni test, p < 0.05) FIGURE 7 Stacked bar histogram showing the distribution of aboveground dry mass in a radial transect from one representative, permanently marked tussock in a welldrained intertidal lagoon (Don Claudio) at Odiel Marshes (southwest Iberian Peninsula) that was colonised by Atriplex portulacoides. (A) 1990, (B) 1997, (C) 2001 and (D) 2005. Spartina maritima (green); Sarcocornia perennis (blue); A. portulacoides (red) (A) 1990 Above-ground dry mass (g/m2) 0 500 1,000 1,500 2,000 (B) 1997 0 500 1,000 1,500 2,000 (D) 2005 Distance from the tussock centre (m) 6543210 0 500 1,000 1,500 2,000 (C) 2001 0 500 1,000 1,500 2,000 | 2917 Journal of Ecolog y CASTILLO eT AL. 4 | DISCUSSION Atlantic salt marshes were mostly initiated when coastlines stabilised in their current positions some 6000– 7000 years ago, after the rapid sealevel rise associated with the end of the last glaciation; subsequently, vertical sediment accretion has been accommodated by generally rising relative sea levels and autocompaction of the sediments (Allen, 2000). This long history has obscured our view FIGURE 8 Depth profile of the belowground dry mass of three perennial plant taxa under the canopy of Atriplex portulacoides on tussocks in a welldrained intertidal lagoon at Odiel Marshes (Southwest Iberian Peninsula) in April 2005. Values are means ± SE of five tussocks Below-ground biomass (g/m 2 ) 0200 400600 8001,000 1,2001,400 1,6001,800 Sediment depth (cm) 0–5 6–10 11–15 16–20 21–25 26–30 31–35 36–40 41–45 46–50 Atriplex portulacoides Sarcocornia Spartina maritima FIGURE 9 Survival of seedlings of Atriplex portulacoides after transplantation to different drainage regimes. Transplants to tussocks in poorly and welldrained lagoons: (A) control, (B) poorly drained (Ludovico), (C) welldrained (El Manto; n = 7 tussocks, with single transplants per tussock). Transplants to different zones of tussocks in a welldrained lagoon (Don Claudio): (D) tussock edges dominated by S. perennis (E), interior areas dominated by S. perennis and (F) interior areas dominated by Spartina maritima. Values are means ± SE (n = 4 tussocks, with 5 transplants per tussock) 0123456 0 20 40 60 80 100 0123456 Transplant survivorship (%) 0 20 40 60 80 100 (A) Control Time after transplanting (months) 0123456 0123456 0123456 0123456 (B) Poorly drained lagoon (C) Well-drained lagoon (D) Edges with S. perennis (E) Interior with S. perennis (F) Interior with S. maritima Atriplex portulacoides on the interior of tussocks recently colonised by Sarcocornia perennis Atriplex portulacoides on tussocks totally colonised by Sarcocornia perennis