Conclusions and summary
The topic of this work is the coastal dunes and their vegetation in Finland, their development and the factors affecting this during the last 100 years. These sites and biotic communities are especially interesting because of their continuously diminishing area, and are rendered unique by land uplift and the brackish water of the Baltic. Only a small number of species can survive the poverty of the soil and the severity of the microclimate. The sites were studied by transect analysis, and their current state was compared with that at the beginning of this century. During this time the dunes have stabilized, as grazing on the dune meadows has ended and the climate has been slightly damper than earlier. Strong winds have been frequent in the 1990's, and these have moulded the beaches. Because of the rise in sea level and the more frequent strong SW winds, shoreline displacement has already reverted to a transgressive régime on the southern coasts of Finland. In view of the simultaneous increase in sea erosion and deflation on all the dune coasts, the reason for this must be climatic.
Aeolian transport of sand is most pronounced during autumn storms, although it can also be detected at other seasons. Sublimation in late winter releases sand grains on the dune crests for wind transport, and the water flowing down the frozen distal slopes forms sand tongues on the snow.
The coastal dunes of Finland have been formed on the slopes of glaciofluvial formations as the land has risen above the sea. The characteristics of the source material have a clear effect on the aeolian material, the median grain-size of which varies between 0.15 and 0.56 mm and it is well or very well sorted. The coarsest and most poorly sorted aeolian material was found in the hummock dunes of the deflation surfaces and the transgressive dunes on the distal sides of these surfaces. The finest-grained material was found in the parallel dune ridges of the very gently sloping beaches. The skewness of the grain-size distributions of the aeolian material is often near normal or slightly skewed in the direction of the coarser material.
The foredunes forming nowadays on the coasts of Finland are typical transverse coastal dunes, varying in height between 0.5 and 8 m, but usually about 2 m high, and occurring either alone or in swarms, in parallel or partly overlapping. The windward slopes of the foredunes (5o - 15o) are usually more gentle than the leeward slopes (15o - 35o), but may also be steepened into bluffs by wave or wind erosion. Small embryo or lee dunes may occur on the beach. Active slipfaces have been formed leeward of the most trampled blowouts. Low sand tongues occur leeward of the smaller wind-scoured gaps. Broad deflation flats are dotted with separate hummock dunes.
The most important factor affecting beach development regionally is the grain-size of the source material, which controls the steepness of the beach. The beach profile in turn affects the characteristics of the approaching waves, which may be either eroding or accumulating. The beach slope affects the breadth of the sandy flats that are left above the water level at times of low water, and thereby the supply of sand for aeolian transport. The beach slope also affects the moisture content of the soil, which in turn regulates the development of the vegetation.
The broad coastal sandy flats are characterized by longitudinal bars and swash bars running parallel to the shore, and material moves from these formations to the beach. Prograding beaches are only gently sloping and consist of fine material, providing suitable conditions for the formation of parallel dune ridges. The material of these ridges may be almost entirely aeolian, or else aeolian material may alternate with washover beds. These ridges may be formed quickly during one storm, apparently when it is calming down, or gradually over a longer period of time. Ridges over one metre high that consist of fine material are mainly built by the wind.
Small cusps characterize beaches on which the sediment budget is either negative or in balance. The form of the beach affects the local sediment budget, in that arcuate embayments effectively trap material, while material can be transported away from straighter streches. The highest foredunes are formed on beaches where the sediment budget is in balance and the shoreline remains at the same place for a long time. This suggests that the classification of the Irish coastal dune topography presented by Carter (1990: 34, fig. 3), depending on the local sediment supply, is also valid for the coasts of the Baltic in spite of the yearly ice cover and the insignificant tides.
When the material of a beach is so coarse that there is only a small supply of grains suitable for aeolian transport, the beach slopes more steeply and the foredune is replaced by a low beach face berm deposited by waves and winds together. Landward of this ridge there are shield like hummock dunes bound by dwarf shrubs. The manner and speed of growth of the dune forming plants affects the size and the topography of the dunes.
In the following scheme the coastal morphodynamics are added to the topographic/morphological sediment budget matrix proposed by Psuty (1992: 4-6). On the coasts studied by him the sediment supply depended on the beach drift. In the coastal dune fields of Finland grain-size, beach slope and the rate of land uplift are the most important factors affecting the sediment supply. The significance of beach drifts is very local, but it can be seen clearly at Yyteri, for example.
When the beach budget is highly positive, the beach progrades rapidly and only beach ridges or low, parallel foredunes are formed. Such beaches belong to the dissipative domain, with very gentle slopes and fine grain-sizes. The high erodibility of silt and fine sand leads to smoothing out of the beach and the dunes. When the shoreline is displaced more slowly there is more time for sediment accretion, and higher, parallel foredunes are formed.
As shoreline displacement slows down further, as in the middle part of Yyterinsannat, only one high foredune ridge is formed. These beaches belong to the intermediate domain, with a slightly coarser material and steeper beach profile than in the dissipative domain. As a beach is eroded, the foredunes grow higher and blowover beds are formed. The dunes of the northern part of Yyterinsannat and of Tulliniemi in Hanko have been formed in this way. A foredune may grow higher as a result of marine erosion, but protracted erosion will lower it. A foredune may totally vanish or be replaced by separate hummock dunes, as has happened on the Hietasärkät dune field at Kalajoki. Powerful waves deposit coarser material onto the beach, and the beach profile also becomes steeper. On the other hand, as the foredunes are eroded, the material is moved to the bars and the beach becomes gentler and shallower. Of the sites studied here, only the Tulliniemi beach in Hanko belongs to the reflective domain most of the time.
The formation of spits, tombolos and lagoons is an essential part of the development of sandy beaches. The depressions between dunes are sometimes lagoons invaded by peat bogs, sometimes dune slacks formed by marine erosion followed by aeolian accumulation, and sometimes areas moulded by deflation. Heavily trampled beaches feature vast deflation flats and bowls with flat topped erosion remnants. The broadest deflation flats are formed as a result of grazing by sheep.
Based on the data collected here, the vegetation cover can be added (Fig. 58) to the model of sediment supply and dune topography presented by Carter (Fig. 1). This matrix of dune geomorphology as a product of sediment supply and vegetation cover is suitable for all the coastal dune fields in Finland. The grain-size of the source material and the gradient of the shore will have affected the sand supply when the coastal dunes that are now lying inland, far from the modern beaches, were formed. Shoreline displacement will have been rapid on the gentler, lower slopes of the glaciofluvial formations, leading to the formation of swarms of parallel dune ridges or lower beach ridges.
Fig. 58. The current Finnish coastal dune topography as a function of beach sand supply and vegetation cover.
Dune ridges can be broken down not only by marine erosion and deflation, but also by trampling, which destroys the vegetation, and they can be smoothed out as a consequence of the easily erodible grain-size of their material, storms filling the dune slacks, water erosion, deflation and trampling, which intensifies the latter two factors. Lowering down of the intermediate dunes behind the incipient foredune can be brought about by changes in the soil and vegetation accelerating water erosion.
Some plant species that are classified as threatened in Finland belong to the wrack flora, so that even small sandy beaches are important for the survival. Honkenya peploides, Agrostis stolonifera, Cakile maritima and Lathyrus japonicus often grow on the windward slopes of foredunes. The pH of the sand near the shoreline is always over 6, but a higher calcium content was found only at Padva, and even there the soil is rapidly leached further away from the shoreline.
The most important factors affecting the time-succession of the geomorphology and plant communities behind the beach zone are pedological processes. Behind the foredunes Leymus arenarius becomes sterile, and the amounts of pathogenic and hydrophobic micro-organisms, especially fungal hyphae, increase in the soil. This and the discontinuity of vegetation cover cause an acceleration of water erosion and intensified deflation. The pH of the soil in the zone of intermediate, lowering dunes is below 5.9, and the dunes are covered by mosses (especially Ceratodon purpureus and Polytrichum piliferum), Cladonia lichens, Festuca species, Hieracium umbellatum and Rumex acetosella. The grain-size of the material also affects the ecological succession, with Agrostis stolonifera and Festuca rubra frequent on fine material, usually with a higher moisture content, and Festuca ovina most frequent on coarser, drier material and in grazed areas. The Festuca species are replaced by Deschampsia flexuosa on stabilizing sand surfaces.
As leaching continues the amount of soluble phosphorus diminishes and the soil becomes more acid, so that the dry dune meadows (pH 5.5 - 5.7) are replaced by Empetrum hermaphroditum heaths (pH 5.2 - 5.4). Racomitrium canescens often grows over the pioneer mosses. The damp heaths are characterized by Salix repens, Juncus balticus and a continuous moss cover. As the pine saplings grow, the dune heaths are invaded by forest, so that the deflation flats which were open grazing land at the beginning of this century are rapidly becoming pine forest. With the increase in organic matter and humic acids in the soil, weathering of the sand grains releases plant nutrients, and as the soil becomes more acid the solubility of plant nutrients, especially phosphorus, increases.
Sand movement prevents the ecological succession and the accumulation of organic matter in the soil, although the sand may become more acid because of leaching. The effect of exposure on the distribution of the vegetation is most obvious on the older dunes that are partly covered by forest, and grain-size also has its effect on the older dunes. Finer sand can absorb moisture and nutrients and can maintain a herb-rich forest on the leeward dune slopes.
One part of the flora of the coastal dune fields in Finland has a clear southerly distribution and another part a northerly distribution. The flora of the southern Hanko Peninsula differs most obviously from that of the other coasts, especially in the replacement of Festuca ovina by F. polesica. The nutrient-poor, vulnerable nature of the habitat has an effect on the flora of the coastal dune fields which possess many northerly characteristics, and some species characteristic of peat bogs can be found on these. The most important factors affecting the ecological succession on the Finnish coastal dune fields are geographical location, soil moisture, sand movement, time, pedological processes and distance from water line.
Almost completely untouched coastal dunes can be found in Finland only in the protected areas of the Tauvo Peninsula and in the archipelago, although patches where human influence has been insignificant can also be seen here and there in private areas around summer cottages and in military areas. Around tourist attractions, on the other hand, intensive trampling is destroying the vegetation cover and causing erosion and impoverishment of the flora. If the trampling is not intensive or continuous, however, it may create new habitats for the endangered pioneer species. Old dunes should in any case be protected against destruction of their vegetative cover, as bare sand surfaces on these are especially vulnerable to water erosion.
Besides construction, trampling and forest invasion, the coasts of Finland are threatened by eutrophication caused by water and air pollution. Many sandy shores on the Gulf of Bothnia have acquired a continuous vegetative cover that has brought aeolian activity to an end, but elsewhere, e.g. at Vedagrundet near the town of Hanko and at Vihaspauha in the Kalajoki district, sand is accumulating to form new coastal dunes.