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17Accretion rates, defined as the vertical growth of salt marshes measured in mm per 18 year, may be influenced by grazing livestock in two ways: directly, by increasing soil 19 compaction through trampling, and indirectly, by reducing aboveground biomass and thus 20 decreasing sediment deposition rates measured in g/m² per year . Although accretion rates 21 and the resulting surface elevation change largely determine the resilience of salt marshes to 22 sea-level rise (SLR), the effect of livestock grazing on accretion rates has been little studied. 23Therefore, this study aimed to investigate the effect of livestock grazing on salt-marsh 24 accretion rates. We hypothesise that accretion will be lower in grazed compared to ungrazed 25 M A N U S C R I P T A C C E P T E D ACCEPTED MANUSCRIPT2 salt marshes. In four study sites along the mainland coast of the Wadden Sea (in the south-26 eastern North Sea), accretion rates, sediment deposition rates, and soil compaction of grazed 27 and ungrazed marshes were analysed using the 137 Cs radionuclide dating method. Accretion 28 rates were on average 11.6 mm yr -1 during recent decades and thus higher than current and 29 projected rates of SLR. Neither accretion nor sediment deposition rates were significantly 30 different between grazing treatments. Meanwhile, soil compaction was clearly affected by 31 grazing with significantly higher dry bulk density on grazed compared to ungrazed parts. 32Based on these results, we conclude that other factors influence whether grazing has an effect 33 on accretion and sediment deposition rates and that the effect of grazing on marsh growth Introduction 38Many coasts of the world show an enhanced rate of sea-level rise (SLR) over the past 39 century, and studies predict it to accelerate in the future (IPCC, 2007; Vermeer and 40 Rahmstorf, 2009). Global SLR was 3.1 mm yr -1 between 1993and 2003 (IPCC, 2007 unique flora and fauna (Schmidt et al., 2012). 54Given that lateral erosion is not occurring, the resilience of salt marshes to SLR is 55 largely determined by their ability to compensate higher water levels by increased vertical 56 accretion and/or reduced soil subsidence rates leading to increased surface elevation. Only if 57 accretion rates and the resulting increase in surface elevation are higher than rates of SLR, a 58 salt marsh will be able to keep pace with relative SLR. The surface elevation change in salt 59 marshes is the sum of sediment accretion, erosion, compaction processes, and possibleregional crustal movements (French, 1993 French et al., 2003). Many studies have investigated accretion rates in salt marshes (e. g. 74Cahoon and Turner, 1989; Dijkema, et al. 1990; Dijkema, 1997; Bellucci et al., 2007; 75 Baustian et al., 2012), and several models exists to predict the future development of salt 76 marshes (e.g. Allen, 1990;Temmerman et al., 2003; Bartholdy et al., 2004; French, 2006, 77 Schuerch et al., 2013). Yet, the question of whether accretion rates and the resulting surface 78 elevat...

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Questions In the early 1990s, grazing and drainage have been abandoned in many artificially created mainland salt marshes of the Wadden Sea to increase their natural character and heterogeneity of vegetation composition. We asked: how did vegetation succession in the low and high marsh respond to this change in management from a large‐scale and long‐term perspective, and what were the major drivers of successional change? Location Wadden Sea coast, Schleswig‐Holstein, Germany. Methods We used vegetation data from the Trilateral Monitoring and Assessment Program (TMAP) for around half of the Wadden Sea mainland salt marshes to identify main successional pathways of the TMAP vegetation types Puccinellia maritima and Festuca rubra, dominating low and high marsh areas, respectively, prior to the change in management. Drivers of succession were analysed using classification and regression tree models, a non‐parametric vegetation modelling technique. Results Since the abandonment of grazing and drainage, progressive succession towards the TMAP vegetation types Atriplex portulacoides and Elymus athericus strongly increased and mostly occurred in abandoned areas at medium to high elevation. Retarded and retrogressive succession decreased and mostly occurred in grazed and low‐lying areas. Persistance of the early successional vegetation type Puccinellia maritima in the salt marshes of the northern Wadden Sea suggests that large‐scale gradients of salinity, inundation frequency and sedimentation lead to geographical variation in the pace of succession. Conlusions Considered from a large‐scale and long‐term perspective, abandonmnent of grazing and drainage caused spread of grazing‐sensitive plant species from mid‐ and late‐successional stages. This change in vegetation composition affects species diversity and ecosystem functions of the Wadden Sea mainland salt marshes. Grazing regime and elevation were found to be the most important drivers of succession. To achieve and maintain large‐scale heterogeneity in vegetation composition, we recommend management to maintain a mosaic of grazed and ungrazed areas, and to consider the role of large‐scale gradients of environmental factors driving succession.