Effects of Geomorphological Processes and Phytoclimate Conditions Change on Forest Vegetation in the Pomeranian Bay Coastal Zone (Wolin National Park, West Pomerania)

The current functioning of plant associations is determined both by global factors through climate change and local factors related to, inter alia, the dynamics of erosion-denudation processes. The stability of the substrate, resulting from the interaction of mainly abiotic properties of habitats, is an important factor responsible for the succession and harmonious development of plant associations, including forest associations. Therefore, it is of great importance to indicate the relationships between morpholithological and hydrometeorological conditions and the intensity of the erosion-denudation processes. The above relationships determine the spatial and temporal transformations of habitats and the dynamics of vegetation development. Determining the impact of abiotic components of the natural environment on the development of plant associations is particularly important in edge zones that are characterised by the relief energy, in the form of e.g. significant height differences and slopes as well as the presence of active mass movements.

An example of an edge zone in which there is a significant influence of hydrometeorological conditions and the dynamics of geomorphological processes on the changes and development of plant associations is the part of terminal moraine occurring in the Wolin National Park (WNP) undercut by the cliff coast of the Wolin Island. Therefore, it is important to recognise the relationship between the features of glacial, fluvioglacial and aeolian topography and the activity of geomorphological processes and soil features in zones of high relief energy. In these zones, morphogenetic activity is the most intense, and sedimentary covers are the least stable, which has an impact on the dynamics and structure of the plant cover. Such a situation of increased erosion of quaternary sediments in the WNP occurs mainly in the zone of geomorphologically active cliffs of the Pomeranian Bay.

The individuality of the cliff topography, among other active elements, consists of landslides and rockfalls, and erosion-denudation valleys cutting through the cliff's top (Kostrzewski et al. 2015). An intensification of mass movements, water and aeolian erosion occurs mainly on the slope and at the foot of the cliff. Significant transformations of the topography occur especially during storm surges, efficient precipitation and strong winds (Tylkowski 2018).

Climate and sea level changes, also visible in the Southern Baltic coastal zone, affect the occurrence of geohazards (Paprotny, Terefenko 2017, Hojan et al. 2018, Tylkowski 2018, Winowski et al. 2019, Rutgersson et al. 2022, Meier et al. 2022a), such as intense shore erosion or rapid mass movements, resulting in e.g. degradation of coastal dunes associations and the development of specialised plant associations on the cliff top, e.g. thermophilic orchid beech (Tylkowski et al. 2021).

The morphodynamics of the top cliff edge does not currently show a clear increasing linear trend of its erosion. A general analysis of the linear trend in cliff top recession on a total coast length of 1.36 km tested area, registered since 1984, revealed no statistical dependence. Despite the increased frequency in recent years of extraordinary hydrometeorological events, no intensive cliff top abrasion has been observed. A distinctive feature of the coastal cliff is the cyclical activity of cliff top recession connected with occasionally occurring extreme phenomena. Twenty-year cycles of cliff recession activity were observed with 10-year intervals ±2–3 years between the maximum and minimum of the cycle (Kostrzewski et al. 2015). The greatest erosion of the cliff occurs during episodic, extreme storm surges, e.g. 3 and 4 November 1995 (161 cm above average sea level with maximum cliff top recession of 8.35 m). These extreme morphodynamic events are random, with no clear relationship to climate change. In the effect of predicted global sea level rise in 2100 in the range between 69 cm and 111 cm (Bamber et al. 2019), the frequency of extreme Baltic Sea levels will rise (e.g. Hieronymus, Kalén 2020). However, extreme changes of sea level in relation to average sea level will not be statistically important, because it is predicted that wind velocities will stay unchanged (Christensen et al. 2022). In the Polish coastal zone of the Baltic Sea, the mean annual air temperature may rise in the 21st century by 2–3°C, with a concurrent rise in total precipitation of 0–10% during summer and 10–20% during winter (Collins et al. 2013). Therefore, in the longer term, climate changes will cause changes in the species composition of forests (Tylkowski 2015a).

In the coastal cliff area, individual components of the natural environment have been well recognised, including: geological structure, lithology of surface sediments, and topography (Borówka et al. 1982, 1999, Kostrzewski, Zwoliński 1987a, b, Winowski 2015), morphodynamics of the cliff edge (e.g. Kostrzewski 1987, Kostrzewski, Zwoliński 1987c, 1995, Kostrzewski et al. 2015, 2017, Winowski et al. 2019, 2022), climatic conditions and the water cycle (e.g. Tylkowski 2013, 2018, Tylkowski, Hojan 2018). The type and occurrence of plant associations were also determined (e.g. Piotrowska 1955, 1979, 1993, 2003). There are no studies showing the relationships and cause–effect relationships between abiotic conditions and the occurrence and functioning of plant associations. The existence and development of some forest habitats (e.g. the Baltic orchid beech Cephalanthero rubrae-Fagetum between Biała Góra and Grodno) is conditioned by the occurrence of secular erosive processes (Tylkowski et al. 2021), and, in turn, other plant associations (e.g. the Elymo-Ammophiletum between Grodno and Wisełka) are disappearing due to extreme erosion of the sea coast.

The dynamics of erosion and denudation processes and the development of plant associations in the area of the cliff coast are determined by hydrometeorological conditions. Occasionally, during the occurrence of extreme hydrometeorological events, rapid but spatially limited morphological changes of the cliff edge and degradation of phytocoenoses occur. The temporal distribution of hydrometeorological threshold values initiating mass movements as well as water erosion (Winowski 2015, Tylkowski 2018) and aeolian erosion (Hojan et al. 2018) was analysed.

The present study aims to determine the influence of cliffed coast morphodynamics and phytoclimatic conditions' changes onto the development of plant associations in the WNP coastal cliff zone in the second decade of the 21st century. The succeeding parts of the paper offer an elaboration concerning associations inferred between the floral diversity in the Wolin National Park and parts of a coastal cliff that are subject to coastal erosive activity and therefore undergoing rapid degradation.

The research area is located on the South Baltic Coast (313), in the Szczecin Coast macroregion (313.2–3), and in the Uznam and Wolin mesoregion (313.21) (Kondracki 1998). Morphogenetically active Pomerania Bay cliffed coast on Wolin Island is located on the territory of WNP. No activities of coast erosion protection are conducted within the Park area. Cliffed coast erosion is the effect of natural erosion-denudation processes' occurrence, induced mainly by storm surges. This is why, in this region, the morphodynamics of the Baltic Sea coastal cliff and changes in the type and extent of vegetation associations are especially important.

The studied cliff edge zone of the Pomeranian Bay, approx. 14.5 km long, extends in the section of the sea coast, between Międzyzdroje and Międzywodzie (Fig. 1), and constitutes a strip of the moraine plateau of the Wolin Range (Kostrzewski, Zwoliński 1994). The complex of terrain forms in the research area is a glacitectonically dammed moraine, built of heavily disturbed glacial and fluvioglacial sediments, cut by steep cliffs from the sea coast (Borówka, Tomaszewski 1978). The Wolin Range consists of extensive moraine surfaces, often as isolated hills, sometimes reaching heights of over 100 m a.s.l. and numerous depressions in the form of undrained hollows, deeply erosive valleys, and contemporary erosional cuts.

Fig. 1

The cliff edge zone of Pomeranian Bay (Figs 2 and 3) is located within the northern part of the Wolin frontal moraine. It breaks off in the north with steep cliffs reaching 93 m a.s.l. (Gosań Hill), and its slope ranges from 30º to over 70º. The altitude of the terrain varies from 0 m to 108 m a.s.l. The lowest part of the area is the beach at the foot of the cliffs. The greatest relief diversity can be observed in the section Biała Góra – Świdna Kępa. The landscape features numerous hills separated by erosion-denudation valleys. Valleys have slopes with a gradient of up to 30–35°. The coastal zone is dominated by active cliffs of varied geological structures (sandy, loamy, sandy-loam). Most of the cliff coast slopes have a northwest aspect. To the east of Świdna Kępa, the altitude of the terrain drops quite clearly to about 20 m a.s.l., while the cliff slopes assume the northern exposition. In the section between Grodno and Wisełka, the cliff coast is clearly stabilised by pine forests, and in its foreground there is a well-formed front dune. The part of the coast stretching to the east of Wisełka is characterised by an altitude of about 5–8 m a.s.l. This lowering of the terrain is crowned with a front-moraine hill with a level of up to 75 m a.s.l. For the next 4 km, the topography is characterised by a significant rhythm, with numerous elevations and depressions. The coastal zone is dominated by active, clay and sandy cliffs. The eastern part of the analysed area is a vast depression located near the base of the Dziwnów Peninsula. In this place, the cliff edges give way to dune coasts, and the altitude of the terrain is approx. 10 m a.s.l. in the crest of dunes and 2–5 m a.s.l. in their back.

Fig. 2

Fig. 3

Coastal cliff morphodynamics, being the effect of erosional-denudational processes' occurrence, is mostly dependent on short-term hydrometeorological conditions' changes. Spatial differentiation of mass movements and coastal erosion geohazards has been described. Spatial valorisation of risk to forest stands of the cliffed coast of WNP has been performed, taking into account the type of plant associations and type and morphodynamics of the sea coast.

In arriving at a decision concerning the extent up to which there has been development of the dominating Fagus sylvatica forest stands in the coastal zone of WNP, it has been necessary to consider the threshold values corresponding to the changing phytoclimatic conditions characterising the cliffed coast zone. It also pointed to the climate aridity index (AI) and the temporal changeability and forest productivity index.

In this elaboration, hydrometeorological and phytoclimatological analyses based on raw data were used; additionally, results of earlier studies in the literature pertaining to coastal cliffs morphodynamics were used (Kostrzewski et al. 2015, Winowski et al. 2019, 2022). Cliff edge retreat rates data from the years 1984–2020 and results of morphometric measurements of the terrain surface from airborne scanning from the years 2012–2020 were also included. The analysis of the erosive events occurrence determined by exceeding the hydrometeorological threshold values and the phytoclimatic assessment for the period 2010–2020 based on the daily sea level data from Świnoujście (data from the Institute of Meteorology and Water Management) and own meteorological data from the Environmental Monitoring Station of the Adam Mickiewicz University in Biała Góra, which is located directly in the cliff edge zone of the Pomeranian Bay of the WNP. Hydrometeorological conditions for coastal cliff erosion analysis were assessed according to methods adapted from Winowski (2015), Hojan et al. (2018), Tylkowski (2018) and Tylkowski and Hojan (2018).

Analyses of plant associations' occurrence in the cliffed coast zone based on the real vegetation map from 2014 (WNP data) and filed mapping from 2021 were also performed. Plant associations were determined based on Matuszkiewicz's (2014) classification.

The analysis of coastal, fluvial and aeolian erosion events' frequency was performed for the cliffed coast based on the occurrence of hydrometeorological threshold values in the second half of the 21st century. Many years of research on the cliff coast have made it possible to distinguish the threshold value of sea level for cliff abrasion. When the maximum water level is at least 90 cm higher than the average, the foot of the cliff is abraded (Winowski 2015, Tylkowski 2018). Research conducted on the coast of Pomeranian Bay (Tylkowski 2015b) showed that on the dune coast, sea abrasion occurs most often when the water level exceeds the mean sea level by 94 cm. Then, the frontal dune is abraded, e.g. in the edge zone near Grodno. In the case of atmospheric precipitation, the criterion for the occurrence of mass movements (e.g. landslides) was the amount of precipitation within 15 days ≥90 mm and the simultaneous precipitation within 2 days ≥40 mm (Winowski 2015). The study included the threshold values of weather elements that trigger deflation on the cliff slope as well as the transport of mineral matter and its accumulation at the top of the cliff, such as: average daily wind speed ≥4.4 m · s⁻¹, average daily air temperature >0°C and no precipitation atmospheric conditions for 2 days. It was assumed that aeolian processes occurred while meeting all the above threshold values (Hojan et al. 2018). For this purpose, the daily data of sea level, air temperature, precipitation and wind speed were considered. The time analysis of exceeding the hydrometeorological threshold values for the research period 2010–2020 allowed to indicate the long-term and seasonal distribution of abrasion events, mass movements and deflation, which consequently cause floristic changes. Direct studies of mass movements on the cliff coast of Wolin Island allowed for the determination of threshold values initiating geomorphological processes such as landslides, sediment fall, and slumping (Kostrzewski et al. 2015, Winowski 2015).

The trend of changes in phytoclimatic conditions was determined based on the following indicators (Mayr 1909, De Martonne 1926, Ellenberg 1988, Tylkowski et al. 2021): De Martonne AI, Ellenberg quotient (EQ), Forestry Aridity Index (FAI), and Mayr tetratherm (MT) index. The application of the above phytoclimatic indexes will allow the presentation of annual thermal-precipitation changeability in the context of forest stands' development. Multi-annual (2010–2020) tendency and the classification of following years in the scope of optimum or restrictive thermal-precipitation conditions for the development of the dominating tree species, that is Fagus sylvatica, were determined. Phytoclimatic indices were compared to the threshold values of the species Fagus sylvatica (Budeanu et al. 2016): De Martonne AI (De Martonne 1926) with optimal thresholds for the beech forest in the range of 35–40 (Satmari n.d.), EQ (Ellenberg 1988) with an optimal threshold favourable for beech development <30 and its disappearance threshold >40 (Stojanovic et al. 2013), FAI with climatic conditions favourable for beech trees <4.75 (Führer et al. 2011) and MT (Mayr 1909) with optimal thermal conditions for a beech forest of 13–18°C (Satmari n.d.).

In addition, to show the relationship between weather conditions and biomass productivity, the radiant index of climate aridity (RIA) (Budyko 1975) was analysed and the forest productivity of the WNP cliff edge zone was assessed based on the climate vegetation productivity (CVP) index (Paterson 1956).

Research of cliffed coast morphometrics and morphodynamics influencing the functioning of forest stands was performed in the 500-m land stripe extending inland from the shoreline, based on terrain altitude data from airborne laser scanning (ALS, ISOKproject, 2011) and ALS data from 2012–2020, gained from the Marine Office in Szczecin. The point cloud density ranged from 3 pts · m⁻² to 8 pts · m⁻². Owing to these data it was possible to delimit cliff sections of different erosion dynamics (m³ · m⁻¹ · a⁻¹): low ≤5.25, moderate 5.26–9.50, high 9.51–13.75 and very high >13.75 (Winowski et al. 2022).

Based on geoinformation analyses and direct field studies, the areas particularly vulnerable to the intensification of erosion and denudation processes, which may lead to the degradation of plant associations, especially forest associations, have been identified. The valorisation was based on the coast type and its morphodynamics and occurrence of forest associations. To determine the degree of erosion risk of forest associations in the cliff edge zones of the WNP, a three-level scale was created: class 0 – no risk, class I – moderate risk and class II – high risk. Sections showing no risk (class 0) were separated, corresponding to the fragments that are usually protected by the dune belt. These are primarily stabilised cliffs and inactive cliffs, where mass movements occur in points or are not currently found, and thus they do not affect the neighbouring forest habitats. Sections of the active cliff that are subject to the influence of sea abrasion, resulting in intensification of the processes of loose sediment sliding (for which phenomenon rinsing processes also serve as a secondary causative factor), were assigned a moderate level of risk (class I). In the event of high risk (class II), on very active cliff sections, the risk of degradation of forest associations due to the increased susceptibility of erosive processes, the formation of landslides and increased intensity of aeolian processes, also extends inland, beyond the area of the cliff top itself, resulting in, on the one hand, its accelerated withdrawal, and on the other, the instability of the subsoil, leading to significant surface changes in the morphometric and soil structure of the forest.

Erosion of the cliff edge zones of the Pomeranian Bay of the WNP is mainly caused by sea abrasion, initiating mass movements. The measurements of the rate of the cliff top recession carried out annually since 1984 on five test sections showed that the average annual rate of cliff recession was 0.22 m (Winowski et al. 2019). Geomorphological changes in the cliff edge zone of the WNP, especially the slopes and the top of the cliffs, depend mainly on the dynamics of sea abrasion and slope erosion (Tylkowski 2018). The moraine coast of the southern Baltic Sea area is under the most threat of retreat, particularly as the projected sea-level rise accentuates its vulnerability to storm surges and wave erosion (Räisänen 2017). The high sea level during storm surges, intense rainfall and strong winds lead to the transformation of the cliff coast, which is manifested, among others, by the retreat of the cliff top and disappearance of forest associations (Czyryca et al. 2021, Tylkowski et al. 2021). The largest changes were observed in the upper parts of the active cliff as a result of mass wasting triggered by the loss of the slope stability due to erosion of the lower part of the slope, e.g. in the time period 2008–2012, on the 2 km of the Wolin cliff coast, a negative sediment balance of −33,000 m³ was found (Dudzińska-Nowak, Wężyk 2014). As a result of morphodynamic processes, the vegetation in the dune edge zone is also subject to changes (Łabuz 2004, 2015). Also, the foredunes of the Pomeranian Bay are objects to significant morphodynamic changes (Tylkowski 2014). Coastal dunes are eroded especially during storm surges with sea level >1 m above the average level (Łabuz 2013). The highest storm surges in the time period 2002–2019, with the highest storm sea level >1.3 m (the Amsterdam mean sea level), triggered a heavy erosion of the dune coast at the Świna Gate Sandbar (Łabuz 2022). Such erosive storm surges occurred almost every 2 years. The heaviest surges resulted in an average 5.2 m and 7.0 m dune retreat on the high-beach-accumulative coast and on the low-beach-erosive coast, respectively (Łabuz et al. 2018). Therefore, for the Pomerania Bay cliffed coast, the threshold value of erosive sea level is 0.9 m above the mean sea level (Winowski 2015, Tylkowski 2018), which is about 0.1 m lower than that for the dune coast (Łabuz 2013).

In the cliff edge zone of the WNP, exceeding the threshold values of hydrometeorological conditions (including maximum sea level, total precipitation and wind speed) (Winowski 2015, Tylkowski 2015b, 2018, Hojan et al. 2018) generates intensive erosion and denudation processes causing coast erosion and stimulates the evolution of landforms and phytocoenotic changes (the disappearance of the stand on the slope and top of the cliff). In the long term, due to bank erosion, the extent of the soil cover, e.g. of the cliff naspas, changes. Therefore, the evolution of the habitat conditions in the cliff edge zone causes changes in the range and species composition of plant associations. Climate change will affect the distributions of native species too. An increase in the proportion of deciduous tree species (except Alnus incana) and some reduction in the proportion of conifers (Picea abies and Pinus sylvestris) are expected in the Baltic region (Ozolinčius et al. 2014). The dominant conifers of the mixed conifer/northern hardwoods zone, Picea abies and Pinus sylvestris, retreat from the south and west while Fagus sylvatica and other temperate hardwoods spread to the north (Sykes and Prentice 1996). Generally, climate changes are projected to lengthen the growing season for terrestrial plants, and to promote northward shifts in the boundaries of species occurrence (Smith et al. 2008). Increased temperatures in the Southern Baltic coast may reduce the growth of coniferous trees, where the growth tends to be limited by water availability (Lindner et al. 2010).

Changing climatic conditions affect the limitation of water resources in the cliff edge zone of Pomeranian Bay, which is clearly illustrated by the clustering (2013–2016 and 2018–2020) of high values (>1) of the RIA index. If the RIA >1, then in a given year there are conditions typical for drought, because most of the precipitation evaporates, and thus, there is no possibility of increasing water resources and there is limited availability of water for the development of vegetation. On the other hand, if RIA <1, this can be taken as an indication of the prevalence of humid conditions, owing to rainfall remaining as the principal mode of liquid water supply and water retention being possible. The Earth's forest ecosystems occur in climates with a dryness index of 0.33 < RIA < 10. The most favourable ecological conditions for vegetation are provided by climates with values of 0.8 < RIA < 1.0 (Kożuchowski 2013). The analysis of the relationship between the annual value of net primary production (PPN) and the climate also allows the assumption to be made that the maximum PPN corresponds to the value of RIA = 0.8. The value of the radiation dryness index RIA = 0.8 is therefore the optimal relation for forests between the radiation balance and precipitation (Budyko 1975).

Among the many methods of assessing forest productivity, Paterson's (1956) CVP index is suitable for determining the productive potential of forests in the cliff edge zones of the WNP. The CVP index does not apply to stands or species; hence, it can be used for the general assessment of forest and vegetation productivity (Siedlecki 2018). Unfavourable phytoclimatic conditions, and frequent and long-lasting droughts as reflected by RIA >1, result in a decrease in CVP and a reduction in the productivity of coastal forest ecosystems.

The geomorphological development of the cliff edge zone and the dynamics of changes in forest associations largely depend on meteorological and hydrological conditions. In the Polish coastal zone of the Baltic Sea in the 21st century, the average annual air temperature may increase by 2–3°C, with an uncertain increase in total precipitation by 0–10% in summer and a more possible increase by 10–20% in the winter season (Collins et al. 2013). The frequency and duration of droughts (Orlowsky, Seneviratne 2012) and heat waves (Nikulin et al. 2011) are also expected to increase. The increase in air temperature is likely to be accompanied by higher precipitation, especially in the winter season. However, in the growing season, the sum of precipitation is unlikely to increase, which, with increased thermal conditions and high evapotranspiration, will reduce the resources of surface and ground waters. Plant associations, including forest ecosystems, will therefore have limited access to water.

For the Baltic Sea, the current rate of sea level changes is approx. 2–3 mm · a⁻¹ (Stramska, Chudziak 2013, Tylkowski et al. 2021, Weisse et al. 2021). According to various scenarios, based on the forecasts of emissions to the atmosphere of compounds resulting from the combustion of fossil fuels (mainly CO₂ and CO, and CH₄), it is expected that by 2100 there will be a further global sea level rise from 0.82 cm (RCP 8.5) to 26 cm (RCP 2.6) (Nauels et al. 2017). Thus, the forecast of the average global sea level increase (including the Baltic Sea), also taking into account the instability of the Antarctic and Greenland ice sheets in the RCP 8.5 scenario, is about 1 m for 2100 (Grinsted 2015), over 7 m for 2200 and almost 15 m for 2500 (Popkiewicz et al. 2018). In the RCP 8.5 scenario (IPCC 2022) for the Baltic Sea, the occurrence of sea level Historical Centennial Events (IPCC 2022) is predicted to take place in about 50 years, for a study area amounting to approx. 1.7 m of the Normal-Null reference level (Wolski et al. 2014, Tylkowski, Hojan 2018). Thus, the long-term functioning of the edge zones of the WNP and their forest associations will be exposed to degradation caused by a high frequency of extreme sea levels, increased erosion and probably a rapid and significant retreat of the cliff edge. Mean annual Baltic Sea surface water temperature may rise until the end of the 21st century by about 2–4°C, with a frequency and periods of marine heat waves surface water temperature ≥20°C. These changes will be especially visible south from 60° N and in the near coast area (Meier 2022b).

However, it should be emphasised that within the cliff coast of Wolin Island there are also unique plant associations, the functioning of which is conditioned by the geomorphological activity of the cliffs' slope. Such a community is the thermophilic orchid beech (Cephalanthero rubrae-Fagetum). In the case of orchid beech trees, the appropriate degree of morphogenetic activity of the cliff is the basic factor determining the good condition of the community (Tylkowski et al. 2021). This is related to the formation of a specific type of soil – the cliff naspas, the genesis of which is closely related to the aeolian deposition of material originating from the cliff face slope (Prusinkiewicz 1971, Hojan 2009). Along with the change of morphogenetic activity to the stabilisation of the cliff, the gradual disappearance of this valuable plant community is noticeable.

The morphodynamics of the edge zone of the Pomeranian Bay within the WNP is clearly characterised by the stages of development during which the cliff sections change their morphodynamic functions, in the form of extreme episodes of intense erosion separated by periods of coast stabilisation. Thus, the dominance of the sea factor was found, through abrasive storm surges with a sea level 90 cm above mean sea level, which generates the greatest geomorphological transformations of the cliff edge and coastal plant associations. The processes of water and aeolian erosion are relatively less important in the dynamics of erosion-denudation processes and in habitat changes. In the edge zone in the period 2010–2020, for only almost a month, there were favourable hydrometeorological conditions for the initiation of mass movements (landslides, falls). Then, for 3 weeks, there were favourable conditions for cliff abrasion during storm surges, and only 1 week for slope erosion as a result of efficient precipitation. In the analysed 11-year period, favourable meteorological conditions for the occurrence of aeolian processes in the edge zone occurred for almost 7 months.

Cyclical dynamics of coast erosion as a result of storm surges were observed, which is manifested in relatively short periods of increased degradation with a relatively higher frequency of events with exceeding abrasive sea level (2011–2012 and 2016–2019), separated by a period of relative stabilisation with sporadic abrasive floods storms (2013–2015). Mass movements in the coastal area of the cliffs show a clear seasonal disconnect. They are most often initiated during storm surges, especially from October to March. On the other hand, water erosion and mass movements caused by precipitation occur rarely and mainly in the summer season. For mass movements, no significant statistical tendency was found to change their annual turnout. On the other hand, aeolian processes do not show such a clear seasonal regularity, they appear throughout the year and there is a statistically significant increase in the annual frequency of their occurrence.

Phytoclimatic indicators AI, EQ, FAI and MT show a statistically significant trend towards climatic conditions beyond the climatic optimum for Fagus sylvatica. The indicators confirm the climatic stress, especially in the last 3 years, for the dominant tree species, which is the common beech. The consequence of climate change is a high RIA index, the values of which reflect unfavourable conditions for the development of forests and a decrease in their productivity expressed by the CVP index. In the forests of the cliff edge zone of the Pomeranian Bay of the WNP, there is a statistically significant trend of decline in their productivity, at a rate of −0.1 m³ · ha⁻¹ · a⁻¹. In the longer term, it is also possible to reconstruct the species composition of forests in Wolin, as the phytoclimatic conditions for boreal species will be unfavourable, and thermophilic trees will enter.

Ongoing climate changes, for a longer time, will cause the transformation of forest associations. We anticipate that the effects of phytoclimatic condition changes that play a significant role in determining the pace of forests' development will be altogether perceptible, especially during the second half of the 21st century. The transformation of forest associations will cover the whole WNP area. However, currently observed plant associations changes are of restricted spatial range and limited mostly to the cliffed coastal zone of Pomeranian Bay, owing to plant associations growing on the edge zone of the Pomeranian Bay being mainly related to mass movements regulating the state of the denudation system of the cliffs in Wolin Island. The highest rate of top retreation occurs primarily on sandy cliffs, and it is in their case that the greatest threat to forest associations is observed. This risk varies across time and space as the morphodynamic functions of the cliff change. The spatial analysis of the threat to plant associations for the more-than-14-km section of the Pomeranian Bay coast showed that 45% are classified as areas with no significant threat to forest associations, whereas for 36% of the coastal zone length these threats are moderate, and for 19% of the length of the studied coastal section, there is exposure to big threats. The most vulnerable to the impact of geomorphological processes are Luzulo pilosae-Fagetum along the 5.1 km section, Betulo-Quercetum robris loniceretosum xylostei along the 1.53 km section and Cephalanthero rubrae-Fagetum along the 0.85 km section.

It is worth emphasising that, while cliffed coast morphodynamics and phytoclimatic changes are both causing modifications in the type and extent of plant associations, these two factors are not correlative, because their various influences on plant associations pertain to different time scales. Short-term (seasonal and few-years') changes of cliffed coast morphodynamics are mostly influencing the limitation of plant association occurrence, e.g. the vanishing of Cephalanthero rubrae-Fagetum association. On the other hand, long-term (multi-decadal) changes of phytoclimatic conditions are transforming forest associations with the domination of boreal species, i.a. Empetro nigri-Pinetum.

It should be emphasised, however, that the cause–effect relationships between hydrometeorological conditions and the dynamics of erosion-denudation processes and the transformation of forest associations are complicated and heterogeneous for individual sections of the coastal zone. The dynamics of erosion of the Pomeranian Bay cliff edge zone are determined by many other conditions that disturb the simple relationship between hydrometeorological conditions and the intensity of erosion. The most important factors influencing the dynamics of the erosion of the sea coast include: morpholithological conditions of the cliff, slope exposure towards the sea wave approach, morphology of the underwater slope, qualitative and quantitative conditions of sediments on the beach and shallow coastal zones, dynamics and frequency of extreme hydrometeorological events in the preceding period, as well as anthropogenic factors.

Due to phytoclimatic conditions' changes and sea level rise, as well as the expected intensification of erosion and denudation processes in edge zones in the 21st century, effective forms of protection of coastal forest associations are difficult to define and there is no such need in the area of WNP. The inferences gleaned from the present study indicate that possible future changes of forest associations in the edge zones of the Pomeranian Bay can be anticipated to arise as a consequence of global and regional conditions, and that the application of local safeguards would most likely prove ineffective in the long term.