About 150 years of dune stabilizations have degenerated a sand conveyor belt of a transgressive dune spit and its potential for adjusting barrier island geomorphology to an accelerating sea level rise. We first discuss the processes of dune aging and its implications, and then add suggestions for the rejuvenation back to a dynamic dune landscape.
As reviewed by Provoost et al. (2011) for coastal dunes in NW-Europe, a phase of increased dune mobility by overgrazing with livestock and introduced rabbits, wood clearance, harvesting marram grass and cutting sods preceded a reverse phase of increasing dune stability. At many sites, the ‛Little Ice Age‛ supported the trend of general aeolian activity. Since then, aeolian sediment dynamics at many sites have decreased, because of a phasing out of grazing and other uses of dune vegetation, epidemic diseases in rabbits, introducing exotic plants, atmospheric nitrogen deposition, and measures of coastal defence like planting marram grass and pine forests.
A similar environmental history applies to the dune spits of the island of Sylt. A royal decree for dune protection issued already in 1539, repeat lawsuits over grazing rights, introduction of rabbits in 1732, harvesting of marram grass for fodder and thatching (Fischer 1938; Jessen 1914; Panten 2003) together indicate reduction of dune vegetation until the nineteenth century. Concomitant dune mobility prevailed. Due to nutrient poor acidic soil and dune migrations, no woodland could develop. After the war of conquest by German armies against Denmark in 1864, a war against dune dynamics commenced. Almost all grazing and marram grass harvesting stopped, and washovers and blowouts became precluded by organized plantings of marram grass. Sprawl of military and touristic infrastructures in the dunes were another incentive to stop dune dynamics after 1914.
Our analysis of dune migration and vegetation cover is concerned with the latter phase. Bare dunes near the exposed western shore became stable by building brushwood fences and planting marram grass since the 1860s as a measure of coastal defence. Where military infrastructures and residential areas were built near mobile dunes, these have been similarly stabilized in the 1930s and 1940s, and occasionally thereafter. These stabilizations account for most of the decrease of bare sand between 1936 and 1958. The subsequent loss of bare sand areas occurred until the 2000s with less direct human interference, like the succession of dune vegetation, its increase in terms of area and, especially, introduced plants.
Large migrant dunes not subjected to planting campaigns continued to move at variable speed. Previous estimates on shorter intervals (Priesmeier 1970; Straka and Straka 1984) are somewhat higher than our averages of 2.2 to 2.9 m a−1, with or without including old maps, respectively. Comparing the migration rates with other mobile dunes around the North Sea and the Baltic Sea unravels a rather irregular pattern. For the large Danish mobile dune ‘Råbjerg Mile’ Anthonsen et al. (1996) have calculated higher annual migration rates of 11 to 13 m. On the other hand, Labuz et al. (2018) mention that the Polish mobile dune ‘Lacka’ close to Leba merely proceeds with 1 to 2 m annually. Besides methodological differences, arguably, each transgressive dune field has its own distinct characteristics. Hesp (2013) considers a complex interplay of multiple factors, which in turn determine the migration speed of single dunes. Furthermore, individual dune migration may vary in multi-annual cycles: Years of slow migration occur when westerly winds primarily increase dune height. This gradually creates steep slip faces, which advance fast by sand avalanches (Priesmeier 1970; Jakob 2014). These avalanches eventually decrease dune height again and dune migration slows down. Length of phases may depend on storms and precipitation (own observations).
Washovers and blowouts in the vicinity to the exposed beach gradually disappeared. We attribute this to stabilization efforts of coastal engineering. In addition, the building of access paths to the beach, fences and information signs helped preventing people from trampling on dune vegetation from the 1970s onward (Klug and Klug 1998; own observations).
Later, repeated sand nourishments (4 times between 1992 and 2016; www.schleswig-holstein.de) at this beach generated a new foredune ridge in front of the cliff, bolstering it and turning it passive. Sand nourishments reversed former beach and dune erosion into dune aggradation, as they substantially increased sediment availability. However, a higher sediment flux from the beach to the hinterland did not occur. Instead, the added sand formed ephemeral embryonic dunes and brushwood trapped sand to form a new foredune ridge, further stabilized by planted marram grass. Therefore, the artificial sand supply does not reach the transgressive dune field in the hinterland. Similar observations have been made at the so-called sand motor at the Dutch coast, where a new foredune acted as a sediment trap (Arens 2015).
Natural succession versus anthropogenic decline in dune dynamics
Our result, that bare sand and marram grassland areas declined while heath became dominant in the dunes, may be the outcome of natural succession in nutrient poor and acidic dune sand (de Groot et al. 2017a). Also Hesp (2013) describes a large-scale overall trend of transgressive dune fields to stabilize. However, in transgressive dune fields, frequent natural disturbances by waves (washover) and wind (blowouts, migrating dunes) tend to set back plant succession from heath to bare sand. Dunes overgrown with marram grass hold an intermediate position between bare sand and heath. They are still mobile to some extent, driven by wind, and their resilience to wind disturbances is high. Dunes overgrown by heath have become immobile. However, there resilience is low if disturbance initiates a blowout. The deflation widens and on sand deposits at the leeside marram grass takes over (see Hesp et al. 2017). Especially beach erosion and foredune retreat initiate aeolian sediment redistribution (Psuty 2004). Early maps of Sylt support this tendency. Thus, without human interference, dynamic processes with washovers, blowouts and parabolic dune formation near the beach would have prevailed.
We have found, though, that this was not the case (Fig. 9). For dunes on Sylt, there is ample evidence of direct human interference by stabilizing dunes for coastal and infrastructural protection, culminating in the 1940s and still lingering on (see above). This may explain most of the loss of bare sand area and the eventual dominance of heath. However, this does not exclude the role of other facilitating or confounding factors. Grazing by domestic animals stopped around 1970 in our study area but continued in the adjacent dune area to the north. Surprisingly, no difference in vegetation on either side of a fence could be detected (Beinker 1996) and is still not apparent after 40 years (own observation). An explanation may be that sheep in this area can choose between grazing on dune and salt marsh vegetation, and clearly prefer the latter. Variable grazing by rabbits could have had an effect too. Since 1927, a causeway connects the island with the mainland, and a fox population established subsequently. We assume, this excludes rabbits from most dunes and relegates them to residential areas. Due to this timing, vegetation changes that occurred after 1958 (as shown in Fig. 6) cannot be attributed to fading effects of rabbits.
Also there has been no remarkable shift within the regional water regime. The consequences of ongoing climate change such as slightly increasing winter precipitation (DWD 2016) may interact with eutrophication and then facilitate scrub proliferation and grasses (Bakker et al. 2016). This may drive future change in dune vegetation but may not account for changes in the past. Groundwater extraction by wells in the dune field adjacent to our study area occurred from 1936 until 2006 (Petersen 1982; B. König, pers. communication). This might have led to lower groundwater level, while sea level rise would cause it to rise again (Lindhorst et al. 2008). It is unlikely that this affected the vegetation categories in our study.
Instead, increased eutrophication (N-deposition in Fig. 9) accelerates plant succession in nutrient poor and acidic coarse dune sand, where it often results in grass encroachment (Provoost et al. 2011; Veer and Kooijman 1997). In the Wadden Sea region, atmospheric nitrogen deposition increased in the twentieth century until it reached a peak of up to 40 kg N ha−1 y−1 in the 1980s and then decreased again. Current values for atmospheric nitrogen deposition in Germany are within a range of 9–12 kg N ha−1 y−1 (de Groot et al. 2017a; Lammerts et al. 2009). Nevertheless, these values still have an impact on dune vegetation. Based on studies from dunes at the Baltic coast, Remke et al. (2009) conclude, that even nutrient inputs as low as 5–8 kg N ha−1 y−1 result in grass encroachment. However, we observed no encroachment with Carex arenaria in our study area (Leguan 2013, own observation).
Arguably, introduced plants constitute a severe indirect human effect on dune succession on Sylt, too (Beinker 1996; Leguan 2013). Particularly from residential areas there is a sprawl of high shrub and trees into the adjacent dunes which is detectable on aerial images. However, most trees (mainly species of Pinus, Picea, Populus, Betula and Sorbus) remain small and scattered in the dunes up to now. Most prominent are shrubs of Rosa rugosa, occurring at roads and residential areas, at former military infrastructures and on the leeside of foredunes. The latter may be due to beach visitors relieving themselves and thus increasing the nitrogen supply for this exotic plant. R. rugosa is generally invading dune islands in the Wadden Sea region (Isermann 2008), and on Sylt may cover at least 3% of total dune area (H. Hoffmann, pers. communication). Another important invader is the heath star moss Campylopus introflexus. In our study area it spread since in the late 1980s and became particularly abundant within grey dune vegetation (Skowronek et al. 2017) and on bare sand areas at the transition between marram grass and heather vegetation (own observation). It contributes to the decline of bare sand, and may facilitate dune stabilization (Essl et al. 2014; Skowronek et al. 2017). We conclude that next to the direct human efforts of dune stabilization, the spread of introduced plants has contributed most to the decline of bare sand and marram grass areas. The overall effect is diminishing habitat diversity and increasing dominance of a few plants at the expense of pioneer species (see also de Groot et al. 2017a, b). The encroachment by high shrubs and trees from residential areas is still at an early stage but is expected to displace characteristic dune vegetation in the long run. Reviving dune dynamics would roll back these long-term trends.
Reviving dune dynamics
Priesmeier (1970) has documented the wane of the aeolian sediment conveyor belt. Our findings demonstrate that this process has exacerbated since the 1960s. For reasons of nature conservation (de Groot et al. 2017a, b) and long term coastal safety (Oost et al. 2012) this development is detrimental. In the face of accelerating sea level rise in the wake of global warming (Clark et al. 2016; Nerem et al. 2018), traditional strategies for coastal protection require revision. Hofstede and Stock (2018) have outlined a support strategy for tidal flats and salt marshes in the northern Wadden Sea growing with the sea. Similar to coastal wetlands, dune islands should be able again to grow as sea level rises. Pioneering experiments for dune revitalization have already been conducted in The Netherlands (Arens et al. 2013) and in Wales (Pye et al. 2014).
Currently, however, vertical accretion of dune areas is largely impossible due to all the former coastal protection measures still in place (Oost et al. 2012). Gateways for aeolian sand transport across barrier islands will be necessary for stimulating aggradation and resilience in response to an accelerating sea level rise. On crowded recreational islands such as Sylt, dune dynamics adjusting deflation plains to a rising sea and the build-up of new massive dunes surely will cause conflicts with existing infrastructures including residential areas and roads. Furthermore, debates will emerge that may resemble those on managed coastal realignment (i.e., Ahlhorn 2018; Schernewski et al. 2018). New strategies in dune management would constitute a substantial policy change. To gain experience and knowledge, Huitema et al. (2011) stress the importance of pilot experiments. Furthermore, the success should become visible locally before a general paradigm shift is ripe for acceptance.
Suggestion for two pilot experiments
As a pilot site, we suggest the investigated transgressive dune field which is merely dissected by two roads (Figs. 2 and 3). The residential areas in the vicinity would remain unaffected by reviving dune dynamics. Successful aeolian sand transfer from the exposed beach to the backshore dunes will depend (1) on the amount and frequency of nourished sand to the beach, (2) on local dune topography, and (3) the vagaries of wind, rain and other weather conditions, storm surges in particular.
The aim of the experiment is to initiate parabolic dunes moving inland from the beach with the prevailing wind. Formation and landward migration of parabolics will depend on perpetual supply of sand from the shoreface. This also requires repeated sand nourishments to the beach. In the selected corridor, these parabolics could migrate for two to three centuries before approaching the first road. They would probably fuse with formerly stabilized dunes on their route (see Fig. 3), and then could form a new migrant dune in the far future, progressing with higher speed across the island. The deflation plain left behind it would increase in elevation as groundwater level rises in concert with sea level rise (Lindhorst et al. 2013). Besides preventing nourished sand from being washed back to the sea again, mobile dunes thus raise the elevation of barrier islands, adjusting them to the level of the sea.
At the selected corridor, safety is not at risk and stabilizations of dunes are possible at any time if that would deem necessary. Nourishments have already occurred since the 1990s at this site. However, we endorse shortening intervals and increasing the supply of sand in an effort to initiate substantial aeolian sand movement in backshore direction. Therefore, existing brushwood fencing and planting of marram grass in the foredunes should stop. If storm surges cause washovers, this would facilitate landward sand transport and thus be appreciated. Nevertheless, an immediate success is not certain, and the experimentally increased sand nourishments should continue over a decade or more.
Secondly, we suggest examining to what extent migrating dunes can be reconciled with existing infrastructure. Our study site offers a suitable location for that purpose, too. Since 1965, a main road runs alongside one of the last migrating dunes (Fig. 8). Next to that road the dune (No. 4 in Fig. 2) was stabilized for protecting the nearby road. In the meantime, the still mobile part has been overtaking the formerly stabilized part. Thereby the dune gained in height and now is turning towards the road. Instead of (partially) stabilizing this dune once more, we now suggest to examine another solution in detail: A tunnel for the traffic could allow the dune to pass over the road and proceed to the sheltered beach. Maybe that would be the first time of building a tunnel before a mountain appears.
For a long-term adjustment of the barrier spit to the expected sea level rise, more than a narrow corridor would be necessary. In the meantime, this pilot project could provide experience with proper management, and then could serve as a vivid example needed for societal choices. This will not only concern financial implications when dunes approach roads or even residential areas but also trade-offs between e.g. short-term benefits and long-term debts.