Erosion Control Ecosystem Service Provided by Salix Acutifolia Willd. Neophyte on the South Baltic Coast: Insights from Wolin Island, Poland

Global warming is accelerating coastal erosion (Mentaschi et al. 2018). The rate of erosion depends on sea abrasion, mass movements, and water and aeolian erosion. The dynamics of these processes are influenced by hydrometeorological and morpholithological conditions as well as vegetation. With above-average values of hydrometeorological parameters, such as heavy rainfall, heat waves, droughts, strong winds, and storm surges, significant transformations of the coastal topography and vegetation take place (Žilinskas 2008, Tylkowski et al. 2021, 2023a). Continued global warming is projected to exacerbate the frequency and intensity of severe events (Arias et al. 2021). Coastal erosion will intensify along most coasts, especially on sandy shores (IPCC 2021).

Maritime administration intensifies coastal erosion-mitigation measures. Hard defences, artificial sand nourishments (Hanley et al. 2014), and soft strategies based on managing natural coastal ecosystems (Feagin et al. 2015, Gracia et al. 2018, Schernewski et al. 2022) are used to achieve this goal. However, these techniques are not fully effective. Sometimes, they are counterproductive (Nordstrom 2000, Pruszak, Zawadzka 2008). Biotechnical coastal protection measures used to control erosion cannot be equated with coastal nature-conservation tasks aimed at protecting biodiversity. However, the two types of measures can be skilfully combined (Feagin et al. 2015). Non-native plant species used for shoreline stabilisation may be invasive in coastal habitats and may threaten biodiversity (Weeda 2010, Hacker et al. 2012). However, they can benefit coastal protection (Charbonneau et al. 2017). Olšauskas (2009) listed publications on coastal erosion-control plantings that used geographically alien species such as Salix daphnoides Vill. and Pinus mugo Turra and Rosa rugosa Thunb., in the Curonian Spit, already in the years 1807–1938. The 14 villages were buried under the shifting spit sand at the end of the 18th and the beginning of the 19th centuries. For the same reason, these plant species (also S. acutifolia Willd.; syn. S. daphnoides Vill. var. acutifolia (Willd.) Döll.) were used on the Polish Baltic coast (Herbich, Warzocha 1998).

In the CICES V5.1 classification (Haines-Young, Potschin 2018), in the section Regulation & Maintenance (Biotic), the service 2.2.1.1 Control of erosion rates is indicated. It belongs to the group: Regulation of baseline flows and extreme events. It was highlighted that controlling erosion occurs owing to the stabilising effects of plant species and vegetation diversity (Science 2015). Therefore, protecting and restoring biodiversity and well-functioning ecosystems are targets in The European Union’s biodiversity strategy for 2030 (European Commission, 2021). Mapping ecosystems and their ecosystem services are essential for measuring progress towards these targets.

In the European Union, to ensure the longterm sustainability of biodiversity and reverse the degradation of ecosystems, the Natura 2000 network has been established. The tool to achieve this goal is the management plan drafted for the N2000 site (Article 6 in Council Directive 92/43/EEC). Such a document is to ensure the favourable conservation status of the Natura 2000 species and habitats while considering human well-being. It outlines the actions needed to be taken to eliminate threats or mitigate negative influences that are associated with an unfavourable-bad status. Threats are identified according to the list of the European Environment Agency (European Environment Agency, 2019). One of the threats (code I01) is the biological invasion by an alien species. The management plan may mandate the elimination of invasive alien species (IAS). Thus, the way to restore ecosystems and enhance their services is to combat IAS. Combating IAS has started as the implementation of target 5 of the EU Biodiversity Strategy to 2020 (European Commission, 2011). It is continued with an emphasis on applying the new EU biodiversity management framework (European Commission, 2021).

In both the EU Biodiversity Strategies (2020 and 2030), the term biodiversity (biological diversity) is consistent with the definition given in the Convention on Biological Diversity. In the specific region, they are indigenous species (naturally occurring, natives). Non-indigenous plants (introduced accidentally or deliberately into natural or semi-natural ecosystems, aliens) can become IAS and develop their secondary range and cause severe damage to natives and their ecosystems, and therefore losses to ecosystem services. The Strategy to 2020 (European Commission, 2011) indicated the need to create a legal basis for combating IAS. Guidelines for adopting such formal acts have been published in the regulation on the prevention and management of the introduction and spread of IAS (Regulation 2014). It provides directions to the identifying of IAS, to controlling pathways of their unintentional introduction and to taking actions aimed at their early detection and rapid eradication. In 2021, the act on alien species was passed in Poland (Act 2021), which implements the EU Regulation guidelines.

There are 14 Natura 2000 sites on the south Baltic coastline in Poland. Among other things, they protect 1628 ha of Natura 2000 habitat 2120 Shifting dunes along the shoreline with Ammophila arenaria (white dunes) and 2144 ha of priority habitat 2130* Fixed coastal dunes with herbaceous vegetation (grey dunes). Both habitats occur in the coastal technical belt and constitute the area of direct interaction between the sea and the land. Its border is determined by the maritime office, considering the average sea level for the last 10 years. The task of the belt is to protect the coast against erosion while taking into account the conservation measures provided in the management plans for nature-conservation areas (Act 1991). Maritime offices implement the program for coastal protection 2003–2023, aiming to protect the coastline against shore erosion. They put into use hard and soft engineering techniques. From the first half of the 20th century, they performed windbreak planting. S. acutifolia and S. daphnoides were planted on the younger dunes, while also P. mugo and R. rugosa on the older ones. Both hard and soft techniques have a negative impact on N2000 habitats, directly or indirectly. Pruszak and Zawadzka (2008) found the intensification of erosive phenomena within all geomorphological landforms of the coastal system in Poland. They cite the accelerated sea-level rise and extraordinary processes (including storms more often, stronger, and longer lasting) under the influence of climate change as the reasons. Based on data from 1883 to 2000, they showed an increasing value of the average rate of coastal retreat (at an average rate of 10 cm · 100 a⁻¹) and a decrease in the accretion processes (sediment starvation). Their optimistic scenario assumes a sea-level rise of 30 cm, while the pessimistic one is by 100 cm. Łabuz et al. (2018) reported that only 15% of the foredune coast in Poland is in the accumulative phase. Protective systems have already been developed for 26% of the Polish coastline (Pruszak, Zawadzka 2008). The dynamics of open coast shoreline changes clearly indicate the need to intensify protective measures. Establishing the N2000 network almost along the entire length of the Polish coast prompts us to look for coastal hazard-protection measures with the least negative impact on biodiversity. Regional average relative sea-level rise will likely continue throughout the 21st century. Approximately two-third of the global coastline has a projected regional relative sea-level rise within ± 20% of the global mean increase. Coastal erosion will intensify along most coasts, especially on sandy shores (IPCC 2021). Forecasts indicate the necessity of developing adaptation strategies for coastal areas under erosion.

On the Polish Baltic coast, four out of fourteen N2000 sites have a management plan, and several drafts of the management plans are pending approval. In these documents, the occurrence of S. acutifolia in dune habitats has been classified as a threat of biological invasion. The willows were ordered to be gradually cut or uprooted manually, with the removal of biomass. It has been forbidden to use fences made of live willow shoots. A. arenaria and Leymus arenarius (native grasses) were ordered to be planted in the white dunes. In the draft of the management plan for Wolin National Park (N2000 site PLH320019 Wolin and Uznam), Ziarnek and Ziarnek (2013) classified this willow as alien species, particularly invasive in the habitats of white (Natura 2000 habitat type 2120) and grey dunes (2130). They ordered it to be ‘cut regularly and dig up if possible’. The expected result of the willow removal is to be favourable conservation status of habitats 2120 and 2130. Popiela et al. (2015) recognised S. acutifolia as highly invasive on the whole coast of Wolin Island. The assessment of invasiveness was made only based on the number of records (natural and artificial sites) and the phytosociological diagnosis of vegetation at the sites. They have, however, not conducted detailed scientific studies authorising the inclusion of S. acutifolia in the list of IAS, i.e. IAS taxa ‘whose introduction or spread threatens biodiversity and related ecosystem services with a significant contribution to human well-being’ (Regulation 2014). Mandatory studies include the analyses of those features of the structure and dynamics of the IAS population that determine the degree of invasiveness of the species, especially to the extent necessary to enable the regression of these features to a non-invasive state. Only the results of such studies oblige to determine the ways of restoring the natural state of ecosystems degraded by IAS. All dune habitats lie within the technical belt of coastal protection (Regulation 2016). In the management plans, the institutions designated to carry out active conservation measures in the technical belts are the maritime offices that once introduced Salix sp. plantings as biotechnical anti-erosion protection and still use it as essential protection. Biotechnical coastal protection efforts are, therefore, contrary to coastal nature conservation. There is an urgent need to reach a consensus between coastal protection and nature conservation.

We adopted the thesis that removing S. acutifolia to achieve favourable conservation status can radically weaken the erosion-control service. There is also an apparent conflict between coast protection and nature conservation. To provide premises for management decisions, we undertook research on the spontaneous occurrence of the S. acutifolia neophyte on the south Baltic coastline in Poland, on Wolin Island. The research aimed to analyse: (1). the distribution pattern of S. acutifolia and their plant communities; (2). the role of S. acutifolia in 2.2.1.1 Control of erosion rates ecosystem service; (3). plant biodiversity of embryonic dunes with S. acutifolia; and (4). the biological invasion forecast for S. acutifolia.

The study area was the coastline of Pomeranian Bay on Wolin Island. It was the entire strip (meter by meter) of 22 km between the seashore and the top edges of foredunes or cliffs, from 53°54′47.7″N, 14°18′56.3″E to 54°00′22.04″N, 14°40′50.13″E excluding Międzyzdroje resort where only random observations were made (Fig. 1). Most of the area was located in the Wolin National Park (N2000 site PLH320019 Wolin and Uznam). In the physics-geographical regionalisation of Europe (Borzyszkowski et al. 2021), it is the Uznam and Wolin Islands mesoregion (313.21) in the province of Central European Lowland (31).

Fig. 1.

On the southern coast of the Baltic Sea of Wolin Island, in the years 1960–2019, the mean annual air temperature was 8.7°C, with a statistically significant upward trend of 0.3°C every 10 years. Significant warming has been observed since the 1990s. In the indicated 60 years, the mean annual precipitation reached 546.7 mm. The annual sum of precipitation did not show a statistically significant long-term trend. From 2010 to 2020, a statistically significant trend of extending the vegetative season by 3 days every 10 years has been stated. The mean sea-level rise of ~2 mm · a⁻¹ was found. The dynamics in the maximum level rise was twice as high and amounted to 4 cm every 10 years. Environmental changes lead to the high frequency of severe winter storms causing the increased intensity of sea erosion in the coastal zone (Winowski et al. 2022, Tylkowski et al. 2023a; Fig. 2A).

Fig. 2.

In 2017, when sites of S. acutifolia and their ecological conditions were studied, the mean annual air temperature was 9.6°C. From June to August, the average monthly air temperature exceeded 15°C, and the maximum in August was 18.7°C. In these months, 20 hot days were recorded (Tmax >25°C). The hottest day temperatures (Tmax >30°C) did not occur. There were 55 days with ground frost. The average annual relative humidity was 79.6% and 669.3 mm of the annual total precipitation. There were 182 days with precipitation, mainly with low rainfall <10 mm. The lowest value was recorded in February (21.6 mm) and the highest in July (147.2 mm). The snow cover remained for 43 days in 2 months and had an average thickness of 4 cm, with a maximum of 14 cm. The growing season lasted 235 days, from 11 March to 31 October. The prevailing winds were from the NW and SE–SSE directions. The winds with the highest average speeds (~4 m · s⁻¹)blew from WNW and NW (October–December, February), and the lowest winds (~1.5 m · s⁻¹) came from the ENE–E sector. The average speed was 1.5 m · s⁻¹ (Tylkowski et al. 2018). From 2009 to 2022 (Tylkowski et al. 2023b), winds from the NW and SSE directions were dominant based on the annual frequency of wind directions. However, the wind distribution differed if we consider the average wind speed from a particular direction. Notably, there was an increase in the percentage of stronger winds from the SSW–NW sector. The average annual wind speed in 2009–2022 was 2.8 m · s⁻¹. The maximum average value of 3.3 m · s⁻¹ was recorded in 2009, while the minimum average speed of 2.4 m · s⁻¹ was noticed in 2012 and 2016. The highest average wind speeds (≥3 m · s⁻¹) were recorded in the cold half-year months (November–February) and in July and October.

About 15 km of the examined coastline was a flat coast covered by dunes in different development stages (Fig. 2B). The sediment sources for dune development included fluvioglacial sands from eroded shores, river-discharged sand and older eroded dunes. Only a few kilometres of coastline (Świna Gate Sandbar) were in the accumulative phase. The chain of embryonic dunes and foredunes together was up to 60 m wide. In Poland, it was one of the largest coastal aeolian areas, nourished by the Świna river mouth. A new, 5–7 m high foredune was developed every 10–15 years. The diameter of sand found in the foredunes of the Polish sea coast ranged from 0.16 mm to 0.25 mm. About 90% of its composition was light quartz grain. Aeolian processes leading to the development of dunes occurred mainly in spring and autumn. The springtime allows for dune development due to the increase in wind speed from a direction oblique to the coast. During autumn, stronger onshore winds reappeared with offshore directions (Hojan 2009, Łabuz 2013). In late autumn and winter, we observed the degradation of the landforms (Fig. 2C).

The next analysed coastal segment (7 km) was the sandy clay moraine cliff coast (Fig. 2D), up to 93 m asl, sloping 30–700 towards the northwest. The cliffs were areas of active litho- and morphological processes. Great dynamics of mass slope movements (Fig. 2E, F), water (Fig. 2G, H) and aeolian (Fig. 2I) erosions characterised them. The greatest morphodynamics was observed in the section Biała Góra – Świdna Kępa. Significant transformations occurred, especially during storm surges, efficient precipitation, and strong winds. The feet of the cliffs were abraded when the maximum water level was at least 90 cm higher than the average. The average abrasion rate of cliffs was 7.5 m in the last 35 years. The greatest annual dynamic was recorded in typically sandy sections, 0.32 m · a⁻¹. The lowest was in the sandy-clay parts, at 0.12 m · a⁻¹ (Kostrzewski et al. 2015). Extreme, short-term storm surges in 1995, 2004 and 2017 retracted the cliff by an average of 2.74 m, accounting for 35% of the total coastal land loss in 1984–2020 (Tylkowski et al. 2021, Winowski et al. 2022, Tylkowski et al. 2023a).

In 2017, we examined the locations of spontaneous S. acutifolia Willd. (syn. S. daphnoides Vill. var. acutifolia (Willd.) Döll.) and their ecological conditions during the growing season. We also conducted occasional field observations between 2018 and 2023. Willow was used there for anti-erosion plantings in the technical-protective coastal strips. It escaped from these strips and became a neophyte in the spontaneous flora of dune vegetation. All spontaneous locations of S. acutifolia in the study area have been documented, and the data have been archived. In natural plant communities with the share of spontaneous locations of S. acutifolia, 30 phytosociological relevés were taken using the Braun–Blanquet method, as well as 10 such relevés in Salix plantings located in technical coastal belts in the national park and the Międzywodzie resort. The relevé area depended on the patch’s homogeneity and ranged from 30 m² to 120 m². The syntaxonomic rank of developed vegetation among both spontaneous and Salix plantings was analysed using Matuszkiewicz’s (2022) classification. The results were used to identify the N2000 habitats with the Herbich (2004) manual. Names of plant species were given after Mirek et al. (2020). The length of the parent (main) stem of each S. acutifolia individual was measured. Germination strength of willow was tested in laboratory conditions. Ripe seeds collected from 23 individuals were placed immediately after being sown on wet sand collected from the beach where these individuals were found. The results of 7-year observations of vegetation dynamics against the background of morphodynamic processes were used. Data from environmental monitoring were taken into account in the interpretation of the results.

Our multi-aspects research has shown that S. acutifolia is characterised by a low degree of invasiveness, soft intervention on biodiversity and a high level of the provision – the ecosystem service 2.2.1.1 Control of erosion rates. Considering several environmental reasons, there are no grounds for performing such radical active protective treatments such as cutting or removing the root system.

We found that the primary source of propagules forming spontaneous locations of S. acutifolia were biotechnical plantings of this shrub in the coastal protection zone. During maintenance pruning, the wind-dispersed cuttings of shoots remained within the plantings and took root in the cliff side habitats. In this way, pioneer sites of the secondary geographical range of S. acutifolia have been developed. The aeolian process in such locations drove the willow adaptive strategy, simultaneously activating several ecological functions (morphological and developmental features of the willow) constituting the ecosystem service 2.2.1.1 Control of erosion rates. Hein et al. (2006) pointed out that natural processes at a sandy shore are the mainspring for creating ecosystem services. Adapting S. acutifolia to aeolian processes resulted in phytogenic aeolian forms in the shape of shadow dunes or small hillocks. The latter can be considered as aeolian landforms analogous to phytogenic forms created by vascular plants, typically by shrubs and small trees, in desert areas (Wang et al. 2006, Li et al. 2007, Zhang et al. 2011, Pool et al. 2013, Quets et al. 2013, Yang et al. 2014, Zhou et al. 2015). El-Bana et al. (2002) named such sites ‘vegetation islands’ and those landforms of aeolian accumulation he recognised as good indicators of aeolian erosion. Rahmanov et al. (2011) discussed the formation of fertility islands under canopies of S. acutifolia in plantations controlling aeolian erosion. Knowledge of biogeomorphological mechanisms in the evolution of phytogenic aeolian forms was used in environmental bioengineering to control desertification (El-Bana et al. 2003, Zhao et al. 2011, Lavaine et al. 2015). On Wolin Island, S. acutifolia, at its spontaneous locations, created habitats with ecological niches favourable for numerous native psammophytes from Elymo-Ammophiletea and Koelerio-Corynephoretea. These habitats were also favourable for the development of many plants’ characteristic of these phytosociological classes (including Artemisietea, Molinio-Arrhenatheretea, Querco-Fagetea), which, according to Matuszkiewicz (2022), are bioindicators of plant communities with higher trophic requirements. The source of soil fertility is probably the annual in situ decomposition of litter and also matter brought by the wind. By developing aeolian phytogenic landforms of the ‘fertile islands’ character, S. acutifolia contributes to the increase of biodiversity, not the loss of biodiversity. Within the forms, plant species absent from neighbouring habitats occurred relatively often. Fałtynowicz (2018) reported 24 species of lichens on the bark of Salix individuals occurring on the shore of the Baltic Sea in Poland. Six are on the red list of threatened species (four endangered [EN], two vulnerable [VU]), and two are protected species in Poland. The cutting of willows required by the management plans for N2000 sites on the Polish coast is therefore tantamount to destroying a microhabitat important for biodiversity.

The level of ecosystem service 2.2.1.1 provided by vegetation-forming phytogenic aeolian forms with spontaneous locations of S. acutifolia was affected by a high share (72% in the total list of flora) of species sporadically co-occurring with willow. Among them, there were numerous perennials that represented rhizome caulophytes (e.g. Epipactis atrorubens) or rhizocaulophytes (as Calamagrostis epigeios, Tussilago farfara), according to the classification developed by Łukasiewicz (1962) for the dune species of the Łebska Spit. Their common features comprising the biology of development, important for the adaptation to the conditions of aeolian erosion, were the ability to develop a dense network of rhizomes, i.e. underground creeping shoots, as well as a highly developed system of adventitious roots growing from shoots covered by windblown deposits (Borysiak 2022). Due to the latter feature, willow was used to control aeolian erosion in the degraded areas of the Błędów Desert (Rahmanov et al. 2009, 2021).

Alien plants can get out of their intentional plantings, stabilising coastlines and reaching the stage of biological invasion that reduces biodiversity (Weeda 2010, Hacker et al. 2012). In the study area, the degree of invasiveness of S. acutifolia can be considered very low. Among other things, no seedlings were found for 7 years of observation (2017–2023). The reason for this could be the very low germination capacity of the willow seeds, confirmed in the laboratory. The willow was planted far from its natural range, which includes areas in Eastern Europe and Central Asia with a different climate than on Wolin Island, which may be the reason for the very poor generative reproduction, and therefore invasiveness.

It was found that only remnants of anti-erosion plantings with S. acutifolia remained within the boundaries of the national park, and the erosion of sea waters was the destructive factor. It cannot be ruled out that they could also be (and will still be) the source of pioneering individuals developing spontaneous sites originating from the rooting of shoots that were mechanically torn off by the wind or sea waves. Together with the remnants of plantings destroyed during dune scarping generally in periods of high-water level, the potential source of vegetative and generative propagules will undoubtedly disappear. As previously mentioned, the main source of willows growing in spontaneous sites were cuttings from S. acutifolia plantings in the technical belt protecting the coastline. In turn, this potential source of spreading geographically and ecologically alien species can be easily eliminated. Cuttings must be removed immediately after maintenance cuts have been made to landfills far enough under the coastal line.

Significant losses were observed in the remnants of not only the oldest plantings of S. acutifolia located within the national park’s boundaries but also in spontaneous sites of the willow’s secondary range. In both cases, they were the result of coastal erosion. Pruszak and Zawadzka (2008) found that the western regions of the southern coastline of the Baltic Sea in Poland (including Wolin Island) in 1883–2000 were characterised by a gradually increasing trend of average rates of the recession of the cliff and dune foot and general shoreline, and also a decrease in the accretion processes. They considered that the increased rate of coastal change mainly reflected the increasing number of storms, which was one of the main factors for sea coast erosion. Winowski et al. (2022) found statistically significant upward trends for the mean and maximum levels of the Baltic Sea. This is related to the rising threat of cliff coast abrasion in the future. Such a forecast is sufficient reason to believe that, in the long-term perspective, the willow will not maintain its spontaneous positions along the cliffs. Therefore, it should not be treated as a potential invasive species.

Winowski et al. (2022) found that during weak and moderate sea dynamics on Wolin Island, sediments eroded in the upper parts of the sandy clay moraine cliff coast systematically pass into the lower parts. At the foot of the cliff, sediments develop alluvial fans and talus cones. Then, windblown deposits cover these newly formed landforms. During such periods of geodynamics, on these colluvial-aeolian units, we observed rooted vegetative propagules of S. acutifolia brought by the wind from anti-erosion plantings, where maintenance treatments were performed. It is possible that willow seeds would be able to germinate there if they were present there. During storm episodes following a period of weak and moderate sea dynamics, the colluvial sediments are rapidly accumulated at the foot of the cliff as a result of mass movements. They partially or completely cover the habitats created and occupied by the willow in the previous period of geodynamics. At the same time, there are episodes of dune scarping. The periodically alternating morphodynamics in the sea–land interaction zone undoubtedly is the reason for the small size of S. acutifolia we found (26% of individuals up to 1 m of the length of the parent branch) and their less frequent occurrence than on the sandy flat coast sections. Due to the low stability of habitats, the service 2.2.1.1 Control of erosion rates is provided by S. acutifolia only periodically.

Phytogenic aeolian hillocks with S. acutifolia along the flat coast provided the service 2.2.1.1 Control of erosion rate at a much higher level. Sand accumulation around the willows was on a similar basis as in Dryas aeolian landforms in the Arctic deflationary tundra described with quasi-continental polar climate conditions (Borysiak et al. 2020) or phytogenic aeolian forms in desert regions (Goudie 2022). Plant species’ role in forming phytogenic aeolian hillocks varies depending on the life form (Gilbert, Ripley 2010). For life forms most effectively trapping sand, Mycielska-Dowgiałło et al. (2008) recognised small- and medium-sized phanerophytes. Therefore, the willow which can reach up to 5 m in height (Seneta et al. 2021), during maintenance in the technical belt of coastal protection is cut to small sizes. This treatment stimulates the development of many shoots, which raises the level of service provision. The adaptability of S. acutifolia in the areas under aeolian erosion effects was used to stabilise the dunes of the Baltic coast (Herbich, Warzocha 1998, Olšauskas 2009). The use of S. acutifolia to reduce deflation in the anthropogenic Błędów Desert is another example (Rahmanov et al. 2021). More pronounced phytogenic aeolian hillocks in this desert were formed by S. repens L. subsp. repens var. arenaria introduced for the same purpose as S. acutifolia. It effectively controlled erosion with extensive horizontal physiognomy (Rahmanov et al. 2009). In Poland, on the Baltic coast, S. repens subsp. repens var. arenaria is a native species. It forms natural thickets low to the ground. They are the stage of succession following the Helichryso-Jasionetum grey dune swards. Forest succession is limited by very strong winds (Matuszkiewicz 2022). S. repens subsp. repens var. arenaria occurs on the Wolin Island (e.g. in location 53°59′03.45″N, 14°34′57.46″E in 2023, our observation). Due to the relatively high level of the ecosystem service 2.2.1.1 provided, it could be used in N2000 sites instead of the alien S. acutifolia. It would be an example of effectively using natural shoreline defence (Hanley et al. 2014). Feagin et al. (2015) provided management considerations for using plant species to protect high-energy shorelines. Rahmanov et al. (2021) proved that in anti-erosion plantings, S. repens subsp. repens var. arenaria meets many of these considerations, and natural processes can continue to evolve sustainably, despite a soft intervention.

We observed the extraordinary vitality of old willows on the Wolin Island in Międzyzdroje, on the front dune degraded by storms (see chapter 2). Shrubs were planted there by testing the horizontal laying of cuttings of S. acutifolia, replacing vertical planting (Błaszczak 1979). In the first year, the cuttings rooted strongly, developing adventitious roots and sprouting shoots of about 1 m in good vitality. In order to maintain this vitality, 3-year protection with brushwood fences against backfilling and erosion was necessary. The test results confirm the possibility of rapid rooting of vegetative propagules of S. acutifolia blown from the places where anti-erosion plantings are cared for. At the same time, the lack of technical protection would probably cause the newly rooted willows to retreat from their pioneering positions quickly. This proves low species’ invasiveness. Olšauskas (2009) reported that during Hurricane Anatoly, on the Curonian spit seashore in 1999, anti-erosion willow plantings had suffered severely. In the spring of 2000, part of the willow was reborn from its living roots. Salix regenerated there much better than other phanerophytes used for erosion control. Generally, however, the willow is gradually disappearing under the influence of, among others, the activity of seawater, similar to our study area.

An alternative to anti-erosion S. acutifolia plantings along the coastal line is constructing sand-trapping fences with Ammophila species. We recorded a constant share of A. arenaria (with a cover of 25–75%) in natural phytocenoses with spontaneous S. acutifolia sites. Keijsers et al. (2015) found that the level of aeolian control in frontal dunes covered by A. arenaria increases with higher vegetation cover. To remain vigorous in this psammophilous grass, however, it is necessary to bury its individuals in a certain amount of sand trapping on a year-to-year basis. This biological feature is responsible for the dune system’s ability to keep up with rising sea levels. Van der Biest et al. (2017) estimated the volume of sand accumulated per year by Ammophila vegetation using an indicator of the sedimentation rate. Multiplying it with the area of dunes covered by Ammophila grasses, they estimated the value of the natural supply of sand. They used the cost to replace the natural supply of sand with artificial nourishments of dune feet as an indicator of the economic value of sand accumulation. They also calculated that dunes dominated by young vegetation successional stages might create up to ~50% higher economic benefits than complexes dominated by the vegetation of late stages of succession (shrub formations), developing in response to the interruption of sand transport between the sea, the beach and the dunes. Thus, they showed that as the early, dynamic stages of plant succession subside, there is a change in the provision of ecosystem services. Earlier, Everard et al. (2010) noted the potential public value of coastal dune systems.

Based on the indicator for the maintenance of coastal safety provided by Van der Biest et al. (2017), it can be concluded that in the case of the coexistence of A. arenaria with S. acutifolia in the phytogenic aeolian hillocks in Wolin National Park, there is an enhancement of the ecosystem service 2.2.1.1. Control of erosion rate. A. arenaria, F. villosa and L. arenarius, which co-built the Salix hillocks-controlled erosion independently of the service provided by the willow. All three species of grass are rhizomatous caulophytes forming one complex adaptative system of tangled roots under the conditions of aeolian processes (Łukasiewicz 1962, Borysiak 2022), overgrowing the freshly trapped sand. The coexistence of willow and grasses (also other accompanying plants) gives a higher level of vertical accretion. It contributes to the faster development of dunes less susceptible to degradation in response to severe storms under the continued global warming. In the case of habitats 2120 (white dunes) and 2130 (grey dunes) of Natura 2000 sites, ordering the removal of neophyte S. acutifolia in management plans to ensure their favourable conservation status would reduce the level of erosion control service 2.2.1.1 and its role for nature conservation.

The study has shown that the neophyte S. acutifolia in its spontaneous locations in Wolin National Park is not an invasive species and does not threaten biodiversity. On the contrary, it contributes to developing biodiversity patterns and provides erosion-control ecosystem services. Therefore, the current conservation practice of removing the willow from coastal habitats is not justified. We have formulated the scope of scientific research that should be conducted before making any management decisions regarding the elimination of neophyte in order to achieve a favourable conservation status of dune habitats.