Long-term change in habitat and vegetation in an ungrazed, estuarine salt marsh: Man-made foreland compared to young marsh development

Estuarine, Coastal and Shelf Science 227 (2019) 106348

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Long-term change in habitat and vegetation in an ungrazed, estuarine salt marsh: Man-made foreland compared to young marsh development

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Karina Hartmannb, , Martin Stockb b

Schleswig-Holstein Agency for Coastal Defence, National Park and Marine Conservation - National Park Authority, Schlossgarten 1, 25832, Tönning, Germany

A R T I C LE I N FO

A B S T R A C T

Keywords: Salt marsh Natural development Geomorphology Vegetation succession Drainage system Wadden sea

Clay-rich salt marshes of mesotidal Wadden Sea coasts and of estuaries have been established mainly within artificial sedimentation fields in front of embankments. Natural salt marsh formation and natural range expansion outside artificial structures were rare. In the last three decades of this century natural marshes along the southern Wadden Sea coast of Schleswig-Holstein, Germany, started to grow outside groyne fields and extended on tidal mudflats. This growth happened without direct human influence and naturally structured marshes of considerable spatial dimension evolved. Due to a spread in recent decades, natural grown marshes in our study area – southern Schleswig-Holstein Wadden Sea coast - are younger than man-made marshes. Vegetation developed rapidly in response to finescaled geomorphological conditions. Meandering creeks and different surface elevation ranges of the developing natural salt marsh are special features. The naturally grown marshes show a high proportion of pioneer vegetation with Spartina anglica and Salicornia europaea. Succession proceeds fast and elevated parts of the marsh were rapidly colonised with marsh vegetation of Puccinellia maritima and Aster tripolium in the lower marsh to late successional stages, like Halimione portulacoides and Elymus athericus, on the higher elevated parts. Strikingly, median elevations of the vegetation zones in the natural marsh were several centimetres lower than those of the man-made marsh. The largest difference between both marsh types was the characteristic and the extent of drainage systems. Naturally grown marshes have a natural developed, fine-branched and four times shorter drainage system than man-made marshes with a dense drainage structure.

1. Introduction Salt marshes of the Wadden Sea have fringed the coastline much more extensively in the past (Wiersma et al., 2009; Vos and Knol, 2015). The major cause for their decline has been land-reclamation (Dijkema, 1987b). Today, total salt marsh area is 39,530 ha. Foreland salt marshes, according to the definition by Dijkema (1987a), make up approximately 50% of the entire area (Esselink et al., 2017). The largest proportion of foreland salt marshes can be found along the mainland coast of the Wadden Sea National Park of Schleswig-Holstein (Northern Germany). Compared to the past, the extent of salt marshes in the Wadden Sea is low today. According to the inventory of Dijkema (1983), most marshes had a clear seaward boarder in the 1970ies and ended either in the outer sedimentation fields or were limited in seaward extension by stone groynes. Natural foreland salt marshes were thus relatively rare and no detailed description exists besides the work by Reents (1995) and Reents et al. (1999). She suggests that a natural creek system of

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foreland salt marshes along the Wadden Sea coast is formed mainly in early marsh development. The first active formation takes place in the new established pioneer zone and is accompanied by shaping the relief of the mudflats. Creeks start meandering and branching. During marsh development creek profiles change from wide and shallow to narrow and deep, at least in smaller creeks. Salt pans are formed over time. The general patterns evolving are mainly depending on hydrodynamic conditions, exposition and sediment supply (Reents, 1995). During the last quarter of the past century many marshes along the west coast of Schleswig-Holstein showed a remarkable spread. The increase in area totalled 2.075 ha salt marshes over the last 28 years with an actual extent of 9.350 ha. About 1.600 ha of this increase happened in a natural way and more than half of the increase occurred outside sedimentation fields and thus without any direct human involvement (Stock, 2018). Coastal salt marshes of Europe have been classified by Dijkema (1987a) according to the origin of the substrate, the main geological forces and the geomorphology in relation to the ecological conditions.

Corresponding author. E-mail addresses: [email protected] (K. Hartmann), [email protected] (M. Stock).

https://doi.org/10.1016/j.ecss.2019.106348 Received 18 March 2019; Received in revised form 20 July 2019; Accepted 14 August 2019 Available online 15 August 2019 0272-7714/ © 2019 Elsevier Ltd. All rights reserved.

Estuarine, Coastal and Shelf Science 227 (2019) 106348

K. Hartmann and M. Stock

upper 60 cm of soil, 40% of the grain size is smaller than 20 μm and 75% is smaller than 63 μm (Müller, 2013). Salinity of the inundating water varies between 14 psμ and 22 psμ with lower values in winter (Becker, 1998). Within the man-made salt marsh regular drainage ditches run through the marsh in a parallel arrangement with a distance of approximately 10 m to each other. They run into collector drains, which are mainly orthogonal to the field drains. From the collector drains the water runs into the main channels, which are up to 30 m wide. Some collector drains and some main channels are naturally developed watercourses, but they were changed by man through straightening and digging. The tidal creek system in a naturally growing marsh is treeshaped. With further distance to the mudflats the structures branch out ever finer. These creeks are developing while the marsh is growing. Following the establishment of the ‘Wadden Sea national park of Schleswig-Holstein’ the traditional coastal protection management and its associated grazing was abandoned on large parts of the mainland coast (Hofstede and Schirmacher, 1996; Stock et al., 1997; Stock and Maier, 2016). With the abandonment of grazing in the beginning of the 1990′s, the artificial drainage systems were set aside, too.

Fig. 1. Overview of the study area. The study area reaches from the former harbour entrance (H, blue line) of Friedrichskoog (F) in the North to the southern end of Dieksanderkoog. The study area is separated into a northern and a southern part by a grazed marsh in between. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article).

2. Methods Vegetation data as well as analogues and digital vertical aerial photos (year of recording: 1986, 1988, 1992, 1994, 2008, 2014) were provided by the Landesbetrieb für Küstenschutz, Nationalpark und Meeresschutz Schleswig-Holstein (LKN.SH) or were taken from the WMS server of the Landesamt für Vermessung und Geoinformation Schleswig-Holstein (LVermGeo SH; year of recording: 2013). The DEM (year of recording: 2016) was provided by the Wasser- und Schifffahrtsamt Hamburg. Within the defined study area a differentiation between the artificial and the naturally grown marsh was determined by the typical structure of man-made ditches and drainage systems on the basis of a vertical aerial photo from 1988, the start of the natural growth of the marsh. Data processing was carried out in ESRI ArcGIS 10.2. Grazed areas and a former clay mining area in the northern part have been excluded from the study area. The horizontal salt marsh growth was calculated by measuring the shift of the outer vegetation line through the years by means of 20 parallel lines in a distance of 250 m to each other. The lines reached from the border between naturally grown and man-made salt marshes to the seaward vegetation border of 2016. Several shorter lines were drawn on these 20 lines to measure the increase and the decrease respectively between the years. Differences in marsh growth within sections and years were tested with a Wilcoxon rank sum test (RStudio Desktop 1.1.383). All tidal creeks and drainage structures, which were perceptible in CIR vertical aerial photos from 2014 or in the DEM from 2016, were manually digitalized into a shapefile. Later on the study area was split into grid cells of 1 ha size to calculate drainage length per area. Cells which could not be attributed to either salt mash type were removed. Differences in creek or drainage system length between marsh types were tested with a Wilcoxon rank sum test. Vegetation data from six datasets (1988, 1996, 2001, 2006, 2011, 2016) were available from the Trilateral Monitoring and Assessment Program (TMAP) (Kellermann, 1995). According to Petersen et al. (2014) 29 different salt marsh vegetation types and 3 vegetation zones were distinguished. On the basis of this typology we described the surface area changes as well as the succession of zones and vegetation types over time. To analyse vegetation development in relation to elevation we used data of an airborne laser scan survey of the area from March 2016 (Arbeitsgemeinschaft smile consult GmbH und Inphoris GmbH ARGE, 2016). The ASCII-data were converted into a point shapefile in ArcGIS and corresponding vegetation zone and vegetation typology data of 2016 were added to the attribute table. Tidal creeks, collector drains

The ‘foreland salt marsh type’ as well as the ‘estuarine salt marsh type’ develops in front of alluvial coastal plains and is divided only by geographic location. The sediment is often clay-rich and of considerable thickness. We consider our study area as a ‘foreland salt marsh type’ within the outer Elbe estuary. In this paper we report results from a local field study on a natural grown salt marsh in comparison to a man-made salt marsh in the outer Elbe estuary. We asked, how do elevation range, drainage pattern and vegetation succession differ between the two marshes in the long-term? We hypothesize that the natural grown marsh can function as a reference situation for mesotidal ‘foreland and estuarine salt marshes' in the assessment of the EU habitats directive. 1.1. Study area The studied foreland salt marsh is located in the northern Elbe estuary and part of the Wadden Sea National Park of Schleswig-Holstein (Fig. 1). We split the salt marsh complex in front of Dieksanderkoog into a man-made and a naturally grown salt marsh. The study area consists of a northern and a southern part because it was intersected by a grazed central part. The man-made salt marsh was built up with brushwood-groyne fields following the establishment of the dyke in 1935 (Trende, 2011). Maintenance was continued following its reinforcement in 1992. The established marsh was artificially shaped, continuously drained and was used for sheep grazing. Most of the marsh area in front of the dyke is thus largely of anthropogenic origin. High sedimentation rates led to a quick expansion. Even when the maintenance of sedimentation fields and ditches stopped in the beginning of the 1990ies, the marsh continued to grow and finally reached its current extent with a width of 2500 m from the dyke to the mudflats. This natural foreland salt marsh in front of the artificial marsh covers an area of approximately 350 ha within our study area. Marsh elevation of the study site ranges from approximately +1.2 m to +2.8 m normal height null (NHN) with a mean elevation of +2.11 m NHN (Müller et al., 2013). The tidal range is 3 m with a mean high tide (MHT) at +1.62 m NHN (BSH, 2014). Based on local tidal data over the last 10 years the upper part of the marsh platform is inundated less than once a year. An area-wide inundation is associated only with extreme tides (> 2,8 m NHN) and during storm surges. In the 2

Estuarine, Coastal and Shelf Science 227 (2019) 106348

K. Hartmann and M. Stock

900

400 Salt marsh area (in ha)

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Upper salt marsh Lower salt marsh Pioneer zone

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Fig. 4. Zonation of naturally grown salt marsh. Vegetation zonation within the naturally grown salt marsh from 1988 to 2016 in ha.

200 100

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0 1996

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2016 Salt marsh area (in ha)

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Fig. 2. Areal development. Spatial extension of the area covered with vegetation in the naturally grown salt marsh from 1988 to 2016.

and field drains were removed from the dataset by overlaying buffers along the lines of the tidal creeks and drainage systems shapefile (30 cm around tidal creeks, 50 cm around field drains, 1.5 m around collector drains). Differences in the elevation of vegetation zones or types were analysed with a Kruskal-Wallis test.

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Upper salt marsh Lower salt marsh Pioneer zone

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Fig. 5. Zonation of man-made salt marsh. Vegetation zonation within the manmade salt marsh from 1988 to 2016 in ha.

3. Results zone. The area of the upper salt marsh has increased from 1 ha to 92 ha over time. The man-made salt marsh was stable in size. The coverage with pioneer vegetation was very low and variable. Striking was the decline of the lower salt marsh zone from 178 ha in 1988 with a small increase in 2006 to the extent of 27 ha in 2016 (Fig. 5). The upper salt marsh zone showed a doubling in size from 161 ha in 1988 to 323 ha in 2016 and has replaced the lower marsh zone.

3.1. Areal development The area of the naturally grown salt marsh in the study area has increased from 71 ha in 1988 to 358 ha in 2016. During the last two surveys the increase was levelling off (Fig. 2). Average width of the naturally grown marsh has increased over time from 187 m in 1988 to 792 m in 2016. Today the seaward vegetation border is up to 990 m away from the man-made salt marsh. Differences in horizontal growth varied between years (Fig. 3) but not between the two sections. Neither the differences between years (Wilcoxon rank sum test, p = 0,026; post-hoc, p > 0,05) nor between sections were significant (Wilcoxon rank sum test, p = 0,9017; post-hoc, p > 0,05).

3.3. Vegetation types Vegetation development according to TMAP-typology in the study site is shown in GIS-maps in Fig. 6, showing changes within the manmade marsh over time and the increase of the naturally grown marsh in front of the established marsh. In both salt marsh types the number of vegetation types has increased over time from 3 in 1988 to 13 within the naturally grown and to 19 within the man-made salt marsh in 2016. In the naturally grown salt marsh the ‘Spartina anglica type’ was the marsh creating vegetation and dominated the growing marsh during the first four years, whereas the ‘Salicornia-type’ played a minor role (Fig. 7). Later successional stages of marsh vegetation started to grow from 1996 onwards. ‘Puccinellia-type’ vegetation occurred from the beginning of the marsh growth and showed its largest extent in 2011. ‘Atriplex portulacoides type’ vegetation established in 2001 and has increased during the last two surveys. High marsh ‘Elymus athericus type’ vegetation appeared first in 1996 and showed a steady increase up to 91 ha in 2016. Beside Spartina and Salicornia-dominated vegetation, Elymus was the third dominant vegetation type today. The man-made marsh was dominated by only three vegetation types in 1988 (Fig. 8). After abandonment of grazing and draining in the area, the vegetation has changed accordingly. The ‘Festuca rubra type’ has increased until 2001 at the expanse of the ‘Puccinellia maritima type’. In the following years ‘Elymus athericus type’ overgrew ‘Festuca rubra’ vegetation and other types of the low marsh. In 2016 more than 317 ha ‘Elymus athericus type’ covered the man-made salt marsh and dominated the vegetation (Fig. 8). In 2011 freshwater vegetation (‘Ruderal salt marsh areas') was mapped the first time, but incidence was less than 1 ha. This vegetation type as well as brackish marsh types and ‘Fresh grasslands' types did not appear within the naturally grown salt marsh.

3.2. Vegetation zonation Spatial growth of the natural marsh was mainly due to an increase of pioneer vegetation from 50 ha in 1988 to 232 ha in 2016 (Fig. 4). The area of lower salt marsh vegetation differed greatly and showed an increase from 9 ha to a maximum of 101 ha in 2011. Afterwards this vegetation type was replaced by vegetation of the upper salt marsh

Fig. 3. Horizontal growth 1988–2016. Horizontal growth of the naturally grown salt marsh from 1988 to 2016. Boxplots illustrating the median growth from one survey to the next with median, 25% and 75% percentiles, minimum and maximum. N gives the number of transects measured per section. The small letter a indicates non-significant difference between years. 3

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Fig. 6. Vegetation development 1988–2016. Development of the vegetation from 1988 to 2016 by vegetation types in the study area. The border between man-made and naturally grown salt marsh is marked with the black line.

within the naturally grown salt marsh than within the man-made salt marsh. These differences were significant, as were the differences within the vegetation zones (Kruskal-Wallis test, p < 2e−16; post-hoc, each p < 2e−16).

3.4. Elevation Salt marsh elevation has increased from pioneer zone over lower salt marsh to upper salt marsh in both salt marsh types (Fig. 9). Median elevations of the three vegetation zones were about 10–14 cm lower 400

Salt marsh area (in ha)

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S.1.0 - Pioneer zone (unspecific) S.1.2 - Salicornia type S.2.0 - Lower salt marsh (unspecific) S.2.3 - Aster tripolium/ Puccinellia maritima type S.3.0 - Upper salt marsh (unspecific) S.3.5 - Artemisia maritima/ Festuca rubra type S.3.9 - Atriplex prostata/ Atriplex littoralis type

2011

2016

S.1.1 - Spartina anglica type S.1/S.2 - Complex pioneer zone/ lower salt marsh S.2.1 -Puccinellia maritima type S.2.4 - Atriplex portulacoides type S.3.3 - Festuca rubra type S.3.7 - Elymus athericus type

Fig. 7. Vegetation types in naturally grown salt marsh. Vegetation types within the naturally grown salt marsh from 1988 to 2016 in ha. 4

Estuarine, Coastal and Shelf Science 227 (2019) 106348

K. Hartmann and M. Stock

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S.6.1 -Lolium perenne, Cynosurus cristatus and other fresh species type

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S.3.14 - Ruderal salt marsh

S.3.10 - Agrostis stolonifera/ Trifolium fragiferum type

S.3.9 - Atriplex prostata/ Atriplex littoralis type

S.3.7 - Elymus athericus type

S.3.5 - Artemisia maritima/ Festuca rubra type

S.3.3 - Festuca rubra type

S.3.0 - Upper salt marsh (unspecific)

S.2/S.3 - Complex lower salt marsh/ upper salt marsh

S.2.4 - Atriplex portulacoides type

S.2.3 - Aster tripolium/ Puccinellia maritima type

S.2.1 -Puccinellia maritima type

S.2.0 - Lower salt marsh (unspecific)

S.1/S.2 - Complex pioneer zone/ lower salt marsh

S.1.2 - Salicornia type

S.1.1 - Spartina anglica type

S.1.0 - Pioneer zone (unspecific)

S.0.1 - Open water in the salt marsh

S.0.0 - Disturbed or no vegetation

Fig. 8. Vegetation types in man-made salt marsh. Vegetation types within the man-made salt marsh from 1988 to 2016 in ha.

formed in the natural marsh with a size of 0.07 ha in 2016. Due to rewetting processes water-logged areas appeared within the man-made marsh first in 2001 (0.55 ha) and doubled in size in 2016 (0.99 ha). 4. Discussion 4.1. Areal Development Natural growth of clay-rich salt marshes has been starting spontaneously on tidal mudflats mainly in the southern part of the National Park of Schleswig-Holstein at the beginning of the late 1980′s. The marsh area has steadily increased over the last 28 years (Stock, 2018). The marsh growth in the studied Dieksanderkoog area was already described as very pronounced in comparison to other regions by Stock et al. (2001) and Wanner et al. (2014). According to Benninghoff and Winter (2019) the tidal basins in the Elbe estuary and the adjoining Meldorf bight region are the only in the German Bight that show both accretion in the subtidal and intertidal areas. Whether this is due to ongoing dredging of the shipping lane in the Elbe or natural sediment transport by the river itself stays unanswered. Most of the increase of salt marshes in southern Schleswig-Holstein took place outside of groyne fields and the development was not influenced by coastal protection management measures. The four-fold increase in our study area is thus caused by natural dynamics and the marsh surface showed natural geomorphological patterns and a natural drainage system. As the spatial development is described by a spread of vegetation, it has to be mentioned that during the first survey in 1988 the Salicornia population was not covered due to insufficient aerial images in that specific year (Stock et al., 2005). Part of the marsh increase between the first two surveys can thus be explained by missing Salicornia mapping in 1988. A subsequent increase can be explained primarily by a progressing seaward expansion of Spartina anglica vegetation. Salicornia vegetation is always growing in front of the Spartina marsh and both vegetation types prepare for further progressive succession. The man-made salt marsh is spatially constrained between the

Fig. 9. Elevation of the salt marsh in 2016. Height above sea level (NHN, left axis) and above mean high water (right axis) within the naturally grown (N, white) and the man-made salt marsh (M, grey) depending on the vegetation zonation. The blue dashed line marks the height of mean high water. Boxplots illustrating the median elevation with 25% and 75% percentiles, minimum and maximum. N (above) gives the number of points with known elevation per vegetation zone and marsh type. Small letters indicate significant difference between marsh types. N = Naturally grown marsh, M = Man-made marsh. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article).

3.5. Tidal creeks, drainage system and salt pans The drainage system in both marsh types was distinctively different (Fig. 10). Naturally grown salt marshes showed a natural developed drainage system of 205 m per hectare on average, whereas man-made salt marshes were traversed by a drainage system with an average length of 747 m per hectare (Fig. 11). The difference between drainage length in both salt marsh types was significant (Wilcoxon rank sum test, p < 2, 2e−16). Draining structures in the investigated man-made salt marsh were approximately four-fold higher than the total length of all tidal creeks in the naturally grown salt marsh. First salt pans have

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Fig. 10. Drainage systems. Comparison of waterrunning systems in a natural grown (A, left) and a man-made (B, right) salt marsh. All tidal creeks and drainage structures were manually digitalized as perceptible in CIR vertical aerial photos from 2014 or in the DEM from 2016. Only perceptible connections between collector drains and field drains were drawn. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article).

(Dijkema, 1984; Esselink et al., 2002). Until today the pioneer zone is still dominating. Within the man-made salt marsh the area of lower salt marsh decreases, while the upper salt marsh area increases. Due to stabilisation of the marsh through management techniques, natural dynamics are more or less absent (Esselink et al., 2009). This leads to a rapid succession towards dominating Elymus vegetation, as shown by Rupprecht et al. (2015). Increase of the pioneer zone from 1988 to 2001 can be explained by two reasons. Close to the dyke clay mining was carried out in 1992 for dyke reinforcement. These lowered areas were later colonised by pioneer vegetation. Secondly, there has been an increase of pioneer vegetation alongside drains and next to the border of the naturally grown salt marsh. Due to succession processes the pioneer vegetation changes to lower marsh vegetation. The slight increase of pioneer zone in the last survey is caused by rewetting processes, causing backward succession with an increase of Spartina anglica.

Fig. 11. Length of drainage systems. Average length of drainage systems per hectare in the naturally grown (right) and the man-made (left) salt marsh. N gives the number of 1 ha plots measured per marsh section.

4.3. Vegetation types

mainland dyke and the naturally grown salt marsh seawards. The latter can protect the man-made salt marsh during storm events (Pethick, 1992). Thus the area of the man-made salt marsh should stay relatively stable. Differences in its extent can be explained by accretion of the drainage systems. As Reents (1995) described, the drainage systems get smaller during development of the marsh and cause an increase in marsh surface. The comparison of the widths of main channels on aerial photos confirmed this. The main channels were up to 25 m wide in 1992, whereas they were only up to 8 m wide in 2014.

Beside missing Salicornia vegetation in the first mapping survey, the number of vegetation types in that year was extremely low and is a direct effect of intensive grazing on 90% of foreland salt marshes in Schleswig-Holstein (Stock et al., 1997). Intensive grazing leads to homogeneous and grass dominated salt marsh vegetation (Stock et al., 1997). Perennial herbs, like Halimione portulacoides in the lower salt marsh or Artemisia maritima in the upper salt marsh, are strongly negatively affected by grazing and therefore do not occur on grazed salt marshes or only in low densities (Leendertse et al., 1997; Schröder et al., 2002; Kiehl et al., 2003). After abandonment of grazing in 1991 the number of mapped vegetation types has increased through spreading populations of perennial herbs (Stock et al., 2005; Wanner et al., 2014b). In the beginning the study area was covered to a large proportion by Festuca rubra vegetation. This species is facilitated by grazing and low salinity in the Elbe estuary (Stock et al., 2005; Arbeitsgemeinschaft smile consult GmbH und Inphoris GmbH ARGE, 2016; Armstrong et al., 1985; Bakker, 2014; Balke et al., 2016; Becker, 1998; Benninghoff and Winter, 2019, BSH, 2014, Dijkema, 1983, 1984, 1987a, 1987b, Esselink et al., 2002, 2009, 2017; Hofstede and Schirmacher, 1996; Jensen and Suchrow, 2005; Kamps, 1962; Kellermann, 1995; Kiehl et al., 2003;

4.2. Vegetation zonation The areal increase of the naturally grown marsh is correlated to the seaward extension of pioneer vegetation. Only plants of the pioneer zone are able to settle on waterlogged mudflats and can deal with periodical flooding (Armstrong et al., 1985). Beside horizontal growth of the marsh, there is also vertical increase in the surface due to sedimentation. In consequence higher elevated parts are less often flooded and the salinity decreases (Bakker, 2014). Over time plants of the pioneer zone are replaced by more competitive plants of the lower marsh. With a further increase in elevation salinity decreases and soil aeration increases. Plants of the upper salt marsh settle and spread 6

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The marshes are artificially elevated and until today in parts heavily drained through a still functioning drainage system. Parts of the drains are silted up and have reinforced homogeneity of the site. Due to artificial elevation the soil is well aerated. Natural watercourses have only developed in former clay mining areas alongside the dyke, where the structures of the field drains have been removed during mining. The establishment of a natural, meandering tidal creek system on the high marsh is very unlikely since the development of those systems is only possible during marsh genesis (Reents, 1995; Esselink et al., 2009). At that time a small scaled relief with bank formation and depressions is formed. Subsequent colonization with other plant species contributes to further surface elevation increase. On better drained and more elevated parts of the marsh settle plants of later successional stages. The differences in length and density of the drainage systems between naturally grown and man-made salt marshes in our area accord to the results of Reents (1995). They compared two man-made marshes in the Netherlands with two naturally grown marshes of the clay-rich mainland-type and two of the beach-barrier type. They found that drainage systems in the man-made marsh type cover about 20% of the entire marsh area, whereas the proportion in the natural marsh is 10%. While both natural reference marshes in Reents et al. study have developed within large sedimentation fields, our naturally grown marsh has established on unbounded tidal mudflats with different hydrodynamic conditions. This might be the reason why we found a much shorter drainage system within our naturally grown marsh. As this marsh still is of low age it has a less developed creek system than a natural marsh of higher age (Kamps, 1962). Due to the applied digital procedures with aerial photos and elevation data in our study it also might be possible that we have underestimated small and shallow tidal creeks as well as artificial drains because of their poor visibility in higher vegetation. According to Reents et al. (1999) natural marshes are furthermore characterised by a proportion of salt pans and waterfilled areas of up to 6% of the marsh surface. First salt pans have formed in the naturally grown marsh in 2011 as constriction of creeks. They establish to a size of 0.07 ha und are thus spatially still underrepresented in the young natural marsh.

Leendertse et al., 1997; Müller, 2013; Müller et al., 2013; Nolte et al., 2019; Petersen et al., 2014; Pethick, 1992; Reents, 1995; Reents et al., 1999; Rupprecht et al., 2015; Schröder et al., 2002; Stock, 2018; Stock et al., 1997, 2001, 2005; Stock and Maier, 2016; Trende, 2011; Veeneklaas et al., 2013; Vos and Knol, 2015; Wanner et al., 2014a, 2014b; Wiersma et al., 2009; Wanner et al., 2014). In the following years Festuca has been replaced by Elymus athericus dominated vegetation. The ‘Elymus athericus type’ spreads out on more than 86% of the area of the man-made salt marsh until 2016. Spreading of Elymus athericus is favoured by the artificial drainage system and thus soil aeration as well as by comparatively high elevation (Armstrong et al., 1985; Veeneklaas et al., 2013; Bakker, 2014; Nolte et al., 2019). The decline of Salicornia vegetation in the beginning of the mapping period (Stock et al., 2005) did not continue over time. Within the natural grown salt marsh the area of ‘Salicornia type’ vegetation doubles from 2001 to 2016. The variation in occurrence during the surveys might have been caused by weather variations (Stock et al., 2005), weather induced sea level variability or other tidal constituents (Balke et al., 2016). The higher total number of vegetation types within the man-made salt marsh can be explained by marsh age and by succession to brackish and freshwater vegetation. This marsh is more than 50 years old and thus further developed than the natural grown salt marshes in front of it. The man-made marsh escapes tidal influence more and more due to elevation. 4.4. Elevation and plant colonization We found different elevation ranges of pioneer vegetation in our study area. The 50% percentiles of the pioneer zone in the natural grown marsh range from 0.17 to 0.29 m above MHT. Thus, the pioneer zone in the natural marsh is not flooded daily. Jensen and Suchrow (2005) already showed that the elevation range of Spartina anglica and Salicornia europaea at mainland coasts of Schleswig-Holstein are above the ranges named in literature. They found ranges for Spartina between −0.23 m and +0.09 above MHT and for Salicornia between +0.05 m and +0.43 m above MHT (Jensen and Suchrow, 2005). Their data fits our findings. The smaller distributional range for Salicornia in our study site can presumably be attributed to a higher tidal range in the Elbe estuary. Balke et al. (2016) generalised the lowest elevation at which vascular salt marsh plants can colonize tidal flats within different tidal ranges worldwide. This critical elevation roughly corresponds to mean high water of neap tides and is more depending on inundation frequency rather than duration. The pioneer zone of the man-made salt marsh is even higher in surface elevation and both pioneer vegetation types are on the same elevation. Their occurrence is mainly caused by secondary succession on water-logged areas. Conspicuously, all vegetation zones are growing 10–15 cm higher in elevation on man-made marshes compared to natural marshes. These elevation differences are caused mainly by two reasons. The man-made marsh is older and sedimentation has been facilitated within sedimentation fields and by regularly excavated ditches until abandonment of the drainage system. (Hofstede and Schirmacher, 1996).

5. Conclusion The studied natural grown salt marsh has developed spontaneously by dynamic processes without human interference. A naturally branched tidal creek system with meandering water courses has evolved. A beginning salt pan formation is recognizable. Vegetation pattern and succession follow natural geomorphological elevation within the marsh. Thus, clear differences in the parameter elevation range, drainage patterns and vegetation succession appeared in contrast to the man-made marsh. The differences in recent vegetation distribution of both salt marsh types are related to their genesis and age. The natural grown marsh has a higher proportion of early succession stages because of its low age and the progressing seaward increase. Furthermore the same TMAP types have been detected in both salt marsh types, besides of freshwater indicators which can only be found within the higher elevated and older man-made salt marsh. Due to the close vicinity of late successional stages of the man-made marsh, succession runs fast on elevated parts of the naturally grown salt marsh and follows the predictions of Rupprecht et al. (2015), leading to a dense cover with Elymus athericus on higher elevated locations. After abandonment of grazing and set-aside of drainage systems, the higher elevated man-made salt marshes show little signs of rewetting. Due to a still existing and functioning drainage system, the artificially elevated marsh surface and low hydrodynamics on those super-elevated locations progress towards more natural circumstances will last centuries. Secondary, succession takes place only on higher elevated marshes to a small extent in wet depressions. The expected rewetting on those locations is thus insufficient to stop dominance of Elymus vegetation (e.g. Kiehl et al., 2003) since the rapid

4.5. Tidal creeks, drainage system and salt pans The drainage system between both salt marsh types is strikingly different. The man-made marsh shows systematic drainage patterns of excessive extension, a homogenous and artificially enlarged surface elevation, while the naturally grown marsh is characterised by a four times shorter natural drainage, a natural geomorphological structure, natural elevation patterns with first salt pans, and a corresponding distribution of plants. Artificial structural components of the man-made marsh in our study area are clearly detectable even 25 years following the set-aside. 7

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spread of Elymus athericus is favoured by accretion and by high elevation in comparison to the mean high water (Veeneklaas et al., 2013; Nolte et al., 2019). Whether the pioneer zone of the naturally grown salt marsh will increase in future as strong as in the past, and whether the freshwater indicators will spread further within the man-made salt marsh, as described by Esselink et al. (2009) for the Ems estuary, remains to be seen. Due to the habitat and vegetation characteristics of the naturally grown marsh we argue that the studied marsh can function as a natural reference for the ‘foreland and estuarine salt marsh type’ in the Wadden Sea and thus can be used as a baseline for the assessment in the framework of the EU Habitats Directive.

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