Factors affecting the development of coastal dune fields

The primary factors affecting the development of coastal dunes are time and temporal changes in climate, the sea, the sand supply and variations caused by man and animals. Other factors, such as groundwater level, are dependent on these primary ones. The ecological and geomorphological succession depends on all these factors.

Climate

The sea makes the coastal climate milder than further inland, the spring colder and the autumn warmer. The growing season becomes shorter northward (Atlas of Finland 1987: folio 131: 9), and precipitation and cloudiness are lower on the coasts of Finland than inland. Precipitation is also lower in the southern Bothnian Bay than in the Bothnian Sea or the Gulf of Finland (Atlas of Finland 1987: folio 131: 19; Table 1). The dryness of the climate around the Bothnian Bay assists aeolian processes. Solar radiation can be very powerful on open shores, and the temperature of the sand surface can rise up to 60-70^oC (Jämbäck 1995: 21). At the more northerly sites studied here, near Oulu, the ground is covered by snow from the end of October to the beginning of May, while in the south, near Hanko, the snow cover lasts from the end of December to the middle of April (Atlas of Finland 1987: folio 131: 20). Meltwater from the snow increases the dampness of the terrain in spring, especially in deflation bowls, which are covered with ice. Both very severe and very mild winters have been experienced during the last fifteen years (Merentutkimuslaitos 1992: 14), but the annual mean temperature in the study region has slightly risen in the past few years (Heino 1994). The microclimate of dune fields can vary greatly according to the local topography, exposure, vegetation cover and soil water content (Wartena et al. 1991: 24-25).

The climate affects the vegetative cover that binds the sand surface. Moisture alone will bind the sand grains together (e.g. Belly 1964) as well as nourishing the vegetation. Koutaniemi (1990) claims that precipitation has diminished in southern Finland over the past few years, based on the records for Stockholm and Helsinki, whereas Heino (1994), examining all the climatical records in Finland, points to a slight increase in precipitation over the last decade. Average records for consecutive normal periods at the coastal weather stations, as presented in Table 1, show an increase in precipitation with the exception of the Åland Islands, Vaasa, Helsinki and Oulu, but the variation from year to year is great. Deflation surfaces are covered with green mosses during the rainy summers, but the dying mosses are covered with sand or blown away in dry summers.

Table 1. Annual total precipitation at coastal weather stations in Finland (see Appendix 1) during the standard periods 1931-60 and 1961-90. (Helimäki 1967; Climatological Statistics in Finland 1961-90).

Rain can cause considerable erosion on open sandy dune slopes. During one extreme storm about 2.5-3 kg of waterlogged sand was eroded per a square metre of dune slope in the Netherlands to form mudflow tongues (Jungerius & Dekker 1990: 191). The distribution of rain on dune slopes depends on the strength and direction of the wind, and wind speeds will be highest on the upper parts of windward slopes and lowest on the bottom parts of leeward slopes (Frank & Kocurek 1996; Wiggs et al. 1996). Thus strong winds will blow the rain away from windward slopes, while leeward slopes will receive ample amounts of rain and snow, drifting with the wind (Franssila 1949: 149; Geiger 1966: 419-420).

The occurrence of dune coasts necessitates at least occasional winds that are strong enough to transport sand. The prevailing winds on the Finnish coasts are from the south and southwest, but high winds blowing from the north and west are also quite common. Average wind speeds at coastal weather stations tend to be 4-7 m/s, measured at 10 m above the ground surface (Heino 1976), while aeolian transport begins when the wind speed is over 4 m/s, measured at 2 m above the ground surface (Bagnold 1941: 101). Since most of the sand is transported by saltation (Bagnold 1941: 37), aeolian transport increases expotentially with wind speed (Kuhlman 1958: 67-68; Borówka 1980). The power of winds and waves depends on the fetch (Schou 1945: 213). The most effective dune building winds blow onshore and transport material from the shore to the dunes. This is shown by the orientation of dunes, following the shoreline. Only further away from the shore are the dunes oriented according to the resultant wind vectors (Robertson-Rintoul & Ritchie 1990).

Winds can blow onto open shores with their full strength. The most effective wind speeds for dune building seem to vary between 10 and 13 m/s (10 min averages) measured 5 m above the ground surface, often during short but sometimes heavy showers (Arens 1996: 16). Rain may slow down aeolian transport, but does not stop it (Rutin 1983: 132; Wicherek 1989). The splash erosion caused by the rain drops may even increase the transport of larger grains during short showers (De Ploey 1980; Arens 1996: 15). Prolonged rainfall makes the sand surface hard, however, and wind transport declines to zero regardless of the wind speed (Arens 1996). The turbulence of the air flow affects the amount of sand transported (Warren 1979).

Winds are most powerful over the open sea and weaken landward because of friction. This can be seen in Table 2, which gives records of average wind speed, the occurrence of strong winds and gales or storms and the annual distribution of wind directions for three coastal weather stations. Kuuskajaskari is more sheltered by islands than Russarö, and the weather station of Oulunsalo is located on the mainland. A weakening of wind power can also be detected at weather stations located nearer to each other. Thus the average wind speed in the period 1961-90 was 5.3 m/s at Kuuskajaskari and 4.3 m/s at Pori, and correspondingly, 4.3 on Hailuoto and 4.0 m/s at Oulunsalo.

Table 2. Average wind speed, annual wind days and wind distribution among eight principal directions at three coastal weather stations. Measurements are 10 min averages recorded four times daily at a height of 10 m. (Meteorological Year Books of Finland 1981 - 1991; Climatological Statistics in Finland 1961-90).

According to data collected by Fairbridge (1992), the extra-warm climatic cycles during the Holosene have been characterized by higher storm frequencies, and warming has similarly been shown to be accompanied by stormy winds during this century (cf. Heino 1994: 93). Especially since 1994, strong winds and autumn storms seem to have transported sand and moulded beaches and dunes. According to Kuusisto (1993), the changing wind climate can already be seen in earlier records. Four coastal weather stations (Utö, Nyhamn, Rankki and Valassaaret) recorded an average of 111 days with winds >10 m/s per year in the 1980's but only 99 such days per year in the 1970's. At the stations of Russarö and Rankki, where the measuring equipment has remained at the same height for decades, 83 and 49 days of this kind were recorded per year on average during the period 1961-90 and 102 and 72 days per year in 1991-95, respectively.

The most obvious change has taken place since autumn 1994. Before that the ecological succession continued on the Finnish coasts for decades without interruptions, but now a notable change can be observed in the southern part of Yyterinsannat, for example, in that the dunes have grown higher and have been levelled out in places where the slacks have been filled with sand (Fig. 11). In the northern part of Yyterinsannat marine erosion has steepened the windward slope of the foredune and thick blowover layers have accumulated on the leeward slopes. Winds have eroded the windward slopes, and the foredune has begun to move at the most severely trampled places, so that advancing slipfaces have been formed (Fig. 10). At Tulliniemi in Hanko the steep windward slope of the foredune has retreated landward and the dune has grown higher. In places, waves have breached the dune ridge and thrown sand, pebbles and algae behind the foredune. Sand has covered the dune meadow and smoothed the surface, and has also suffocated the lichens (Coelocaulon, Stereocaulon). On the other hand, the real dune plants (Leymus arenarius, Festuca polesica, Carex arenaria) have flourished. Landward of the foredune new stony deflation surfaces have appeared and the pines have become buried more deeply in the sand. Marine erosion has also levelled the beach ridges of Lappohja and eroded a cliff into the slope of the First Salpausselkä end moraine, which is partly covered by dunes.

Borówka (1980), having studied the amount of aeolian transport on the Polish coast, notes that stormy winds seem to be the most important factor, even though they last for only short periods of time. Aeolian transport on the Polish coast is most effective in autumn and winter. The humidity of the ground does not affect the rate of transport to any notable extent, especially with high winds, although Pluis (1992) reports that in the Netherlands aeolian transport is least effective in winter in spite of the fact that the winds are most powerful at that time. The winds do transport sand in the winter in Finland, too (Laulajainen 1914: 210; Mattila 1938: 5), and pile it on top of the snow on the leeward slopes of dunes to form tongue-like niveo-aeolian accumulations (cf. Koster & Dijkmans 1988; Dijkmans 1990; Pye & Tsoar 1990: 251-253). Strong winds are most common here in January - March and in October - November (Keränen & Korhonen 1952: 114; see also Table 2). The soil is frozen in winter, of course, which diminishes aeolian transport, but when the temperature is close to zero, the sublimation of the ice in the pores loosens the sand and yields large quantities of sand grains for transport (McKenna-Neuman 1990a, 1990b; Van Dijk & Law 1995). The wind power is concentrated on the wind-scoured gaps breaching the dune ridges, and even clods of wet sand can be transported up the slopes of these gaps when assisted by a hard, ice-covered surface.

Snow can also drift with the wind, even at wind speeds of 5-7 m/s (Rudberg 1968: 183). The dune crests remain snowless for a long time in the autumn, and the thin snow cover melts early at the same points in spring. Without the sheltering snow cover the plants would be exposed to freezing (Seppälä 1971: 28). Thus the snow cover in winter is an important factor regulating the distribution of the plant communities on coastal dune communities, as elsewhere (Hiltunen 1980; Clark et al. 1985; Haapasaari 1988). According to the findings of Sarre (1989: 25), the accumulation of sand is regulated by the seasonal variation in wind and by the vegetation cover. Most of the accumulation takes place on windward slopes covered by vegetation.

At Yyteri on 13. March 1994 the air temperature was above zero and the sun was shining and melting the snow. The preceding week had been rainy, and in spite of the strong wind and the bare state of the dune crests, aeolian transport was very slight, as the ground was mostly frozen or the sand was saturated by water. Some sand grains were transported as the surface was dried by the sun and wind, and these were accumulating on the leeward slopes to form thin, tongue-like covers. The sand layers between the snow layers were thin, the thickest of them being only about 5 mm. At least some of them had been formed after rains or melting, when waterlogged sand had flowed down onto the snow. Mudflows of this kind are common on the leeward slopes of foredunes. Rainwater also accumulates on leeward slopes at times of strong winds. Niveo-aeolian layers are characteristically laid down grain by grain, but the material of the mudflow tongues consists of clods. It is obvious that aeolian transport is insignificant in Finland during the coldest winter months, but it can begin early in the spring, when sublimation makes the sand surface rough. Especially significant sublimation features are hollowed sand crusts resting on dead tufts of Leymus arenarius on dune crests. These sand crusts 1-2 cm thick will have been formed on snow or ice layers, and 5-10 cm hollows will have been left when the latter sublimated. The sand crust remains, bound by cohesion between the moist sand grains, and the sand is easily transported away by the wind when the crust dries out. Koster and Dijkmans (1988) describe similar formations on the Kobuk dunes in Alaska.

The sea

The sea in front of dune coasts is shallow. The greatest fluctuations in sea level in the Baltic are caused by the weather, and reach their maximum at the ends of the gulfs (Fig. 41). The highest sea level recorded at Kemi, at the northern end of the Bothnian Bay, is +201 cm and the lowest -134 cm. The tide is only 10 cm at its maximum (Atlas of Finland 1986: folio 132: 3). At times of stormy westerly or southwesterly winds the sea level can rise at a rate of 1 m/h. These winds are often accompanied by rain. A low water level can be caused by northerly or easterly winds blowing at a time of high pressure, whereupon the sea level can fall 1.0-1.5 m and expose the sand bars (Lisitzin 1958, 1964; Vartiainen 1980: 13). Examples of the seasonal variation is shown in Figure 42. The winds are powerful in the autumn and the early winter and the sea level is high. The maximum values are usually measured during severe storms. There are exceptions, however, e.g. in November 1993, when winds lowered the level of the northern Baltic 30 cm below the theoretical mean sea level (Kahma 1994: 10). The winds are usually weak in spring and early summer, so that the sea level is below the mean value and variations are at their minimum (Atlas of Finland 1986: folio 132: 6). Thus the effect of northerly winds as dune builders is often assisted by a low sea level (cf. Davidson-Arnott & Law 1990: 199). The sea level has a direct effect on the groundwater table in the coastal dune field, and the latter regulates the end level of deflation and the spread of vegetation on the deflation surfaces (cf. Seppälä 1984: 48, 1995: 801). In the case of the Gulf of Bothnia, land uplift is always a significant factor affecting the shaping of the coastals (Fig. 43). According to Granö and Roto (1989: 52), shoreline displacement at Vaasa, where land uplift is rapid and sea level fluctuations are small, takes about 260 years to proceed from the lower to the upper limit of the littoral zone, whereas at Hamina, where land uplift has slowed down and the sea level fluctuates considerably, displacement is much slower, about 1300 years.

Fig. 41. Distribution of sea level fluctuations in the northern part of the Baltic Sea (cm) at the 99% probability level in 1926-55, after Atlas of Finland (1986, folio 132: fig 5a).

Fig. 42. The annual course of sea level heights at Hanko and Oulu, as recorded by tide gauges (monthly averages by Merentutkimuslaitos 1995).

Fig. 43. Distribution of land uplift in the northern pars of the Baltic Sea area in 1989, after Kakkuri (Atlas of Finland 1990, folio 125: fig. 35b). Isobases are drawn relative to mean sea level (mm/year). For further details, see text.

The level of the oceans has been rising, and this, too, can be detected on the coasts of Finland. According to Kahma (1993: 14-15), the longest series of records shows that the mean sea level of the Baltic has been higher since 1975 than could be expected on the grounds of the earlier even trend, the statistical significance being 90 %. Similarly, the tide gauge records for Hanko and Oulu (Fig. 44) show how the sea level has settled at a higher level than before since the 1970's. The same tendency can be seen in the mean values for yearly records, i.e. the floods have been higher and perhaps also more frequent. The records of the tide gauges are compared in Figure 44 with the theoretical mean sea level, a long-term average calculated separately for each year and each tide gauge. This value has been corrected since 1975 for the influence of land uplift and eustatic sea level rise, a correction factor which at Oulu has been +7.1 mm/year for the period 1975-1989, +6.9 mm/year in 1990-1992 and +3.9 mm/year 1993-1994, and at Hanko +3.1 mm/year, +2.73 mm/year and -0.27 mm/year, respectively. Thus the eustatic sea level rise is already more than the effect of isostatic land uplift on the southern coast of Finland, implying a marine transgression and consequently a smaller supply of sand for aeolian transport. According to measurements performed by the Finnish Institute of Marine Research, the sea level has been rising by 3.8 mm/year over the past few years (Kimmo Kahma, oral presentation 1995). The change in winds in the basin of the Baltic could also have this kind of effect, in which case the rise would be only temporary. Eustatic sea level rise is a global phenomenon, but changes in winds and weather have also played a part in the unexpectedly rapid rise in the Baltic sea level during the last ten years. When the sea level is rising the bars move towards the shoreline (Keränen 1985: 46-47), and if the sand supply is limited, aeolian sand may accumulate only as a thin veneer on beach ridges moulded by waves (Carter 1985: 29-30). On the other hand, on the Landes coast of France, a littoral transgression has led to the formation of the highest coastal dune in Europe (Bressolier et al. 1990).

Fig. 44. Annual averages of the tide gauge records (Merentutkimuslaitos 1995) for Hanko and Oulu (upper diagrams; the values for Hanko are missing in 1940-42 and 1949, and those for Oulu in 1935-36 and 1938), and the ten-year running means (lower diagrams; means of less than ten years are marked with broken lines) as deviations from the theoretical mean sea level.

Beach drift is driven by winds, and in the Baltic it usually moves anticlockwise, westwards on the Finnish coast of the Gulf of Finland and northwards on that of the Gulf of Bothnia. Admittedly their speed is low (only 2 cm/s on average) and they exist for only about 16 % of the time (Atlas of Finland 1986: folio 132: 3-4). In addition, the shape of the bottom causes obvious vortices, often as wide as the embayments. The beaches sheltered by headlands are reached by waves from roughly the opposite direction from that prevailing on the windward side of a headland (Yasso 1965; Jacobsen & Schwartz 1981: log-spiral beaches). This is caused by wave refraction, and to lesser extent diffraction and reflection, and leads to a movement of sediment towards the headland on its lee side. With time this will give the beach an arcuate shape, concave with respect to the sea. Sand is deposited in this way at Yyteri and Kalajoki, accumulating on the southern parts of these beaches. The shallows and islands refract the waves and have an effect on the beach drifts.

Waves can mould loose sand very effectively, and the amount of sediment in motion varies a lot according to the strength of the wind and waves. Constructive spilling breakers predominate on gently sloping beaches, but the waves grow higher during storms and may steepen to form destructive plunging breakers (Chorley et al. 1984: 379; Uusinoka 1986: 56-57). If, in addition, the wind is piling water against the beach, this may lead to considerable marine erosion and shifting of foredune material to submerged bars and swash ridges (Keränen 1985: 45). The formation of successive parallel dune ridges may be caused by an alternation between destructive winter storms and constructive waves during calm summer periods, or by oscillations in sea level (Bird 1990: 21).

Wave energy correlates with wind force and duration, fetch and depth of water (Klijn 1990b: 5). The waves regulate the sediment budget, the balance that depends on the beach profile (Uusinoka 1986: 60-61) and material transport along a beach. The movement of sand, gravel and pebbles on the coasts of SW Finland has been studied by Pyökäri (1979; 1984; 1986), who notes that most of this transport takes place during severe storms (Pyökäri 1984). The wider the fetch, the more material is moving with beach drift or longshore drift (Pyökäri 1979: 114).

Continuous moulding by waves keeps the material loose and prevents the formation of a binding vegetation cover. In addition, the plants suffer from the salt of sea water and the severe microclimate of the shoreline. In the swash and spray zone, however, the moisture and salts bind the sand grains together and reduce aeolian transport (Knottnerus 1980; Nickling & Ecclestone 1981; Pye 1983: 532, 537-538). According to Sarre (1987: 163-164) the presence of organic material increases the significance of moisture content as a sand binding factor. Since the salinity of the surface water on the coasts of Hailuoto is less than 3 per mil and that at Hanko about 6 per mil (Atlas of Finland 1986: folio 132: 7), salinity is not a significant factor on the coasts of Finland, but other impurities in sea water can also bind the sand grains together.

There is loose sand on the shores of the Baltic in spring, pushed up by the sea ice. Ruz and Allard (1994) conclude that in the subarctic climate on Hudson Bay the beach ridges formed by the autumn storms and the ice-foot that carries the sand to the shore are the most important sources of sediment for niveo-aeolian accumulations. The sea ice also carries sand onto the shores in Finland.

All the major dune coasts in Finland are associated with sandy shallows, and the waves transport this material towards the shoreline in the form of submerged bars. All the major dune fields are also connected to the open sea, providing a long fetch, so that winds can blow freely onto these beaches. This is assisted by the fact that most of the beaches open in the direction of the prevailing westerly winds, which are sometimes also the most powerful. The exceptions are the northern shores of the peninsulas of Tauvo and Letto. The shallows off the Tulliniemi beach, which opens to the southwest, are narrow and the waves erode the shore. In spite of the sea level rise, winds and waves can also deposit sand onto the beaches in the Hanko Peninsula and the shoreline can prograde, e.g. at the tombolos of Vedagrundet and Henriksberg, which border onto broad shallows. Almost all the dune coasts studied here are arcuate embayments that effectively trap the sediments. The only clear exception is the fairly straight northern coast of the Tauvo Peninsula at the mouth of the River Siikajoki.

Sand

Availability of sand

One premise for the formation of coastal dunes is that there should be enough sand suitable for aeolian transport. According to Aartolahti (1976: 88) the grain-size of dune sand in Finland usually varies in the range 0.06-0.6 mm. Material that is finer than this, silt or clay, does not move easily with the wind because of the strong cohesive forces between its small particles (Warren 1979: 328), or else it is washed out to sea (Pyökäri 1979: 115; Uusinoka 1984: 103, 198) or transported further as a suspension in the wind. Material that is coarser than this will move only in extremely high winds.

The amount of material suitable for aeolian transport, varies from beach to beach. All the coastal dune fields of Finland are located in connection with glaciofluvial formations or sandy till areas. Most suitable esker material for dune formation is found on the sandstone deposits connected with the Satakunta and Muhos formations (Atlas of Finland 1990: folio 123-124: 3, 24), and around the Bothnian Bay. Thus dune fields are most frequent on these coasts. The material is transported and sorted by rivers, beach drifts and waves. Aeolian sand accumulates particularly on the slopes of eskers that are emerging from the sea (Okko 1949: 65; Aartolahti 1976: 84), in shallow accumulation bays, at river mouths, on tombolos, on spits and in the shelter of headlands (cf. Carter 1991: 30).

The small amount of sand suitable for aeolian transport limits the formation of dunes on the coasts of Virolahti and the Hanko Peninsula (Lappohja, Syndalen, Kolaviken, Tulliniemi) and at Padva and Koppana. All these shores are bordered by a berm built by combined wave and wind action instead of a foredune. Landward from this berm, a dry deflation surface is dotted by separate, low hummock dunes. Higher dunes than those found on the present coast exist further away from the shore in all these areas. On the southern coast of Finland at least a part of the reason for the poor aeolian sand supply is the rate of shoreline displacement, which has slowed down and reverted to a marine transgression. A steepened coastal slope can have a similar effect. The foredune of Tulliniemi consists of aeolian material, but nowadays the high waves are eroding this fine sand. On the Koppana beach the coarse material of the esker core limits the growth of dunes and results in the formation of shield-like hummock dunes bound by dwarf shrubs. Coarse material is also restricting aeolian activity in some of the other areas, e.g. Kolaviken. On the rocky shores of Pietarsaari the grain-size of the sand is fine, but the small amount of sand and the extent of trampling have prevented the formation of a continuous foredune ridge.

After emerging from the sea, the porous sand dries out quickly and moves with the wind. When the transporting wind encounters an obstacle, its transport capacity weakens and the sand accumulates behind and around the obstacle. Lee dunes and embryo dunes are formed on a shore in this way, often around boulders, pieces of drift wood and plants. When the accumulation is caused by an abiotic obstacle, the size of the dune is restricted by the size of the obstacle. A barren, loose sand surface alone can also reduce the wind speed and cause more sand to accumulate. This requires a sand surface 4-6 m wide in the direction of the wind (Bagnold 1941: 183, psammogenic genesis of dunes; Kuhlman 1958: 70, 1960: 72). Stengel (1992: 219) has described a transition from sand veils to sand patches, barchan patches, and laterally coalescent transverse sand shields. When the sand has started to accumulate, it affects the air flow and traps sand more and more effectively (King 1972: 424-426, 438), creating new secondary eddies (Warren 1979). In this way an interrelation is formed between the wind and the soil surface, and the forms may remain unchanged even under conditions of an irregular flow, creating their own regular flow patterns (Embleton et al. 1979: 65). The height of the dune affects the amount of accumulation (Sarre 1989), as the higher it is, the more effectively it can trap sand.  

Grain-size parameters and topography

The slope of a shore depends on the grain-size, usually being steeper, the coarser the material is (Pyökäri 1979: 113). The slope at the base of a foredune is compared with the foredune morphology and the median grain-size of material in Figure 45. In the upper picture the grain-size is the median value for the foredune material, as the source material has an obvious effect on the characteristics of the dune material. In the lower picture the grain-size is the median for the shore material under the dunes. The grain-size of the swash-built berm material was not been used in these diagrams, as it varies considerably with the power of the waves (e.g. the waves at Tulliniemi have sometimes deposited fine sand and sometimes gravel). The aeolian morphology of a coast varies along with the characteristics of the waves. Carter (1988: table 6) defined limiting mean grain-sizes of > +2, +2-0 and < 0 in phi-values, or < 0.25 mm, 0.25-1 mm and > 1 mm, for dissipative, intermediate and reflective coasts, respectively, with corresponding limiting values for the shore slope of < 0.017, 0.017-0.05 and > 0.05. At the same time the surf zone becomes narrower and the waves change from constructive to destructive (spilling -> plunging breakers). The wave characteristics are also affected by wind strength, sea level and marine transgression or regression. The shore of Lappohja is not included in the diagrams, as it is very steep (0.125) and eroded by waves. Earlier it also had dune ridges (Md = 0.29 mm) on the upper part of the cut and built terrace.

Fig. 45. Relationships of the three types of foredune topography on the Finnish coasts to grain-size (median values of the foredune and shore material) and slope tangent under the foredunes. For further details, see text.

The most important characteristics of a sand surface as far as aeolian transport is concerned are its coarseness, cohesion and the grain-size of the material (Warren 1979: 325; Chapman 1990; on grain-size, see Bagnold 1941: 88; Sarre 1987: 173). Though the median grain-size of dune material is usually between 0.15-0.35 mm (Uusinoka 1984: 19), winds can also transport much coarser sand than this (Fig. 46). Mattila (1938: 15) found at Vattaja sand grains 2 mm in diameter attached to the bark of trees at a height of 2 m above the soil surface. The amount of the aeolian transport is also clearly affected by the sorting of the material (Bagnold 1941: 105-106). Dune sand is usually well or very well sorted.

Grain-size parameters are regulated by sedimentation processes, grainfall, avalanching and ripple migration. Grains that are 0.1-0.3 mm in diameter are most easily transported by saltation in stormy winds, and these tend to form dunes. Smaller grains than this are easily transported further in suspension, and coarser grains usually move by rolling along the soil surface (Pye & Tsoar 1990: 101-102). The wind does not sort the grain-sizes very effectively, because of turbulent flow and the great variation in the characteristics of sand grains. Grains that are transported by saltation are deposited when the wind calms down or when they reach the lee side of a stone or plant. When such grains bombard coarser ones and cause reptation, impact ripple marks are formed, the wave length of which depends on the wind speed (Seppälä & Lindé 1978).

The maximum angle of repose for dry dune sand is usually 34^o (Uusinoka 1984: 195). If a dune slope is over-steepened because of accumulation, sand avalanches are formed (Fig. 46). These usually occur on the leeward slopes of unvegetated dunes. Coastal dunes are often dominated by low-angle crossbeds, and consist of the steep foreset beds of the slipfaces (30-35^o) and more tightly packed, more gently sloping (5-20^o) beds of the windward slopes that are sometimes concave with a general upward trend. In cold, humid climates, when snow accumulates between the sand layers from time to time, the leeward slopes usually have a gradient of only 14-18^o (Fig. 47) or considerably less than the angle of repose for dry sand (Steidtmann 1982). When the ground is frozen, sand grains can be loosened only by sublimation or abrasion. Sand transport by saltation is very effective on a frozen surface, however (McKenna-Neuman 1993: 148). In shadow dunes the two groups of foreset laminae dip in opposite directions away from a median ridge (Pye & Tsoar 1990: 237). Thus the internal structure of a dune depends greatly on its form, which in turn reflects the influence of winds and the vegetation cover (Pye 1983: 545).

Fig. 46. A blowout wall near the headland of Herrainpäivät (Pori). A sand avalanche has formed after over-steepening of the wall through sand deposition by a strong wind. The ripple-marks consist of darker, coarser grains moving upwards. Wind direction is from right to left (photographed on 26 July 1990).

Fig. 47. The leeward strata of this foredune in the middle part of Yyterinsannat (Pori), with a dip of 20-25°, are formed by blowover and niveo-aeolian accumulation (photographed after prolonged rains on 7 July 1996).

The sand samples sieved for the present purposes are listed in Appendix 2, and a summary of the grain-size parameters is given in Table 3. The grain-size parameters are somewhat interdependent. As the grain-size becomes smaller the material becomes better sorted (correlation coefficient r = -0.61***), more leptokurtic (r = 0.39***) and more negatively skewed (r = -0.52***). According to Borówka (1990b: 297), an increase in wind velocity causes the standard deviation of wind velocity to increase and leads to poorer sorting. As the sorting improves, the material becomes more leptokurtic (r = -0.322**) and more negatively skewed (r = -0.4***). There is no significant correlation between sorting and kurtosis (r = 0.038). (*** is a statistically significant difference at the probability level of 99.9 %, ** at the 99 % level and * at the 95 % level.)

Table 3. Summary of grain-size parameters for the sand samples.

*The dune ridge sample from the west coast of Virpiniemi is excluded because of its exceptionally coarse material (Md = 0.52 mm).

The coarsest material was found on the deflation surfaces, which are enriched in coarse lag deposits. The median grain-size of the samples from the deflation surfaces of Hietasärkät in Kalajoki is 0.8 mm, and the bases of the dunes and the lower shore consist of quite coarse material in some areas, particularly at Koppana, Hietasärkät and Padva. The coarsest aeolian material was found in the transgressive dunes and separate hummock dunes located distally to the deflation surfaces. Some coarse material from the deflation surfaces has been able to climb onto these dunes by reptation, and to some extent by saltation during severe storms. Coarse grains are also enriched by deflation on the crests and windward slopes of these dunes. The most poorly sorted samples originate from the deflation surfaces, transgressive dunes and hummock dunes of Vattaja and Hietasärkät, and the most positively skewed grain-size distributions were found in the last-mentioned deflation surfaces. The glaciofluvial material of Padva is highly platykurtic.

The smallest median grain-sizes in the samples originated from the low, swash-built berms on the western coast of Hailuoto (Md = 0.14-0.16 mm), the 20 cm high embryo dune in the Karhuluoto area at Yyteri and the parallel dune ridges in the southern part of Yyterinsannat, where the low berms and the embryo dune have been formed by weak summer winds. In the southern part of Yyterinsannat and in the Karhuluoto area there are broad shallows in front of the shore that weaken the power of the waves. Both of these shores have ample supplies of sand, which accumulates to form successive parallel ridges. The material of Ulkonokanhietikko, at the mouth of the River Siikajoki at Tauvo, has a very small grain-size, and that of Letto, which belongs to the delta of the River Kalajoki, is much finer than that of the nearby Hietasärkät area. The material of the swash-built ridges is fine and very well sorted, as is that of the foredunes of Ulkonokanhietikko, Letto and the Karhuluoto area, the blowover strata of the northern part of Yyterinsannat and the hummock dunes of the rocky shores at Vexala. The most negatively skewed grain-size distributions were found in the material deposited by swash action (particularly on the coastal plain of Marjaniemi) and in a dune ridge of the southern part of Yyterinsannat. A highly leptokurtic grain-size distribution was found in beach material from Marjaniemi, Padva and Pietarsaari and in a low hummock dune at Tulliniemi.

The grain-size of the material becomes smaller from the core parts of glaciofluvial formations towards the borders, and this can also be seen in aeolian material at Vattaja and Hietasärkät. According to Alestalo (1971: 116), the Karhi dunes of Vattaja consist of finer material than the Laakainperä dune, because the dune sand in the Karhi area has apparently mixed with finer fractions of till worn loose by the surf nearby. This is possible, but the finer material in eskers is normally transported to the lower slopes and bordering parts at the time when the eskers are formed, while later, as the esker emerges from the sea, the waves level out the esker and sort the material further. Thus differences in the source material could explain the observed differences in this case. The fact that the large, old transgressive dunes of Vattaja are located on the core part of the esker does not exclude the possibility that the coarser material in these dunes may be due to stronger winds at their time of formation, as proposed by Aartolahti (1976: 91). The descriptions by Mattila (1938) of stony pavements and aeolian transport support this idea. Anyway, the source material does have an effect on the grain-size distribution of dune sand. Heikkinen and Tikkanen (1987: 257) describe the material of the transgressive dune of Tahkokorvanpakka as being finer in the southern part of the dune field because the primary esker material is finer there, further from the esker core. This can also be seen in the samples collected for the present purpose from the windward slope and crest of the transgressive dune.

Hailuoto is exceptional in that the material is finer to the south of Marjaniemi than on the Pajuperä dune field even though the Marjaniemi dune field is located nearer to the core of the glaciofluvial formation. The primary material on both beaches is easily erodible sand (Md = 0.25 mm), which may have contributed to the formation of the broad, flat shore. Similar broad, sandy and at times subaerial flats border the shores of Letto (Kalajoki), the peninsula of Tauvo and the Karhuluoto area, the material of the latter being finer than elsewhere in Yyteri. This finer material is obviously mixed with till from the surrounding headlands, as shown by the abundant mica scales in these samples. The dune sample from Ulkonokanhietikko (Tauvo) has 95 % of its sand grains smaller than 0.25 mm but larger than 0.125 mm (Md = 0.17 mm). This kind of material is highly erodible, which explains the alternating growth and disappearance of dunes in this area and the formation of a broad coastal plain. The situation is about the same in the Letto area at Kalajoki.

The grain-size of the sand strata on the beach of the Karhuluoto area varies greatly, reflecting oscillations in wave power, and similar variation was clearly to be seen in the berm of the Koppana beach. The material grades towards finer fractions in the direction of net shore drift due to the decreasing flow energy, and at the same time the beach face becomes more gently sloping (Jacobsen & Schwartz 1981: 41). The beach sand at Yyterinsannat is coarsest in the middle part of the arcuate beach (Fig. 13) and becomes finer to the north and south. It has become coarser near the tombolo of Munakari as a result of storms, and the beach face has become steeper. At Hietasärkät the material is again coarsest in the middle part of the beach. At times of prevailing SW winds the waves refract around Keskuskari and transport large amounts of material onto the southern part of the beach.

The material that has accumulated on the swash-built berms (distal slopes usually only 10 cm in height) is finer and better sorted and more leptokurtic and negatively skewed than the other beach material. The finest-grained sample originates from the sand flat of Marjaniemi and the coarsest from the finest sand layer of the berm on the Koppana beach.

The coarsest sample from embryo dunes (Md = 0.31mm) originates from the top of a cliff worn by waves at Hietasärkät, Kalajoki, and the finest (Md = 0.15) from the Karhuluoto area at Yyteri. The grain-size distribution of this sample, taken from a 20 cm high embryo dune ridge, was also the best sorted, the most leptokurtic and highly negatively skewed. The whole of this sample is aeolian. The poorest sorted sample representing an embryo dune originates from the upper limit of the broad, fine-grained coastal flat at Marjaniemi. This material, found in a chain of dune mounds, is coarser than that of the emerged sand flat and may include layers deposited by swash.

When the material is coarse and the beach profile is quite steep, wave action can build a berm veneered with aeolian sand instead of a proper foredune. This has happened at the upper limit of the beach face at Koppana, where the berm is 1.5 m high and consists of quite coarse material (Md = 0.5 mm). The distal slope of this ridge is only 20 cm high. Yrjänä Bay at Tauvo has a series of the 20 m wide ridges separated by wet slacks and consisting of fine material (Md = 0.2 mm) deposited by combined wave and wind action, whereas the parallel dune ridges in the southern part of Yyterinsannat (Md = 0.18 mm) are largely formed of aeolian sand, but the youngest ridge is washed by waves during severe storms, as shown by the driftwood that has collected behind it. These dune ridges have grown higher as a result of storms, but they may initially have been formed as settling lag ridges, as they consist mainly of concentric internal bedding.

The dune ridges that consist of fine material are higher than those of coarse material (1-2 m vs. 20-50 cm). A sample from the southern part of Yyterinsannat is the best sorted, but also the most platykurtic. This shows that the energy of the deposition agent has varied a lot and there have been storm waves at times. The grain-size distribution of this sample is also highly negatively skewed, i.e. there are more coarse grains than in a normal distribution. The poorest sorted sample representing the dune ridges originates from Kolaviken, the most leptokurtic from Padva and the most symmetrical from Yrjänä. A sample from the youngest dune ridge of Yrjänä at Tauvo (Md = 0.20 mm, Sd = 0.46, Sk = -0.08, KG = 0.91) does not differ much from the foredune material of Haikaranhietikko (Md = 0.21 mm, Sd = 0.46, Sk = -0.17, KG = 0.91). At the time of sampling, the crest of the Yrjänä dune ridge rose more than a metre above sea level and was bound by Leymus arenarius and Phragmites australis. These dune ridges are asymmetrical, their leeward slopes being considerably steeper and shorter than the windward ones.

The grain-size of the foredune material seems to depend somewhat on that of the original beach material (r = 0.48), but no significant correlation was found between the sorting, skewness and kurtosis values of adjacent beach and dune samples. The foredune samples could be classified into three groups on the grounds of median grain-size. One foredune of Vattajanhietikko consists of coarser material than the others (Md = 0.41 mm), while an older parallel dune ridge is composed of finer material (Md = 0.22 mm). This indicates that the amount of energy on the open seashore has fluctuated. The ridge consisting of coarser material may also include washover layers in which the material is only moderately sorted (the poorest sorted of the foredune samples). The finest-grained foredunes (Md = 0.15-0.19 mm) are the parallel dune ridges of the Karhuluoto area and the southern part of Yyterinsannat, Ulkonokanhietikko (Tauvo), Marjaniemi (Hailuoto) and Letto (Kalajoki), the samples from all of which are negatively skewed. The median grain-size of the foredunes is usually between 0.21-0.3 mm and the sand is very well sorted, as there were only five samples in which it was moderately well sorted.

The sorting of the material usually improves from the beach to the foredunes, but there are exceptions. The best sorted foredune sample was from Ulkonokanhietikko. The beach sand samples are usually more negatively skewed than the dune sand, with the exception of parallel dune ridges, which also include material laid down by waves and again have negatively skewed grain-size distributions. There is no statistically significant difference in skewness between the dune and beach material, but the foredune material is finer (significant at the 99 % level) and better sorted (at the 95 % level) than the beach sand. Mitra and Ahmed (1990) found on the coasts of Bangladesh that the dune sands are finer, better sorted and more negatively skewed than the beach material. Individual samples cannot be classified as dune or beach material only on the grounds of grain-size parameters, however, for as already mentioned, finer and better sorted material may accumulate on the beach than that on the adjacent dunes through wave action at times of calm weather. It has been concluded earlier (Shepard & Young 1961; Moiola & Weiser 1968; Tikkanen 1976; Simola 1979; Hellemaa 1980) that aeolian and beach material cannot be identified only by grain-size parameters, nor was it possible with the combinations of parameters used here, although the grain-size and sorting values of dune material are clearly less variable than those of beach sand.

The grain-size distribution of the foredune sand is more mesokurtic than that of the beach sand (Table 3), which varies from highly platykurtic (Kalajoki) to highly leptokurtic (Marjaniemi), but there is no statistically significant difference between these groups. The most leptokurtic foredune sample was from the Karhuluoto area and the most platykurtic ones from Marjaniemi and the southern part of the Hietasärkät dune field. The most positively skewed were the samples from the middle and northern part of Yyterinsannat.

All the highest incipient foredunes (> 3 m) are located at sites where the shoreline has remained unchanged for a long time or prograded seawards very slowly. The median grain-size of all these dunes is 0.2-0.3 mm. The low parallel dune ridges, consisting of finer material than this, are formed on quickly prograding beaches.

The grain-size of the material in the separate hummock dunes varies greatly. The coarsest material was found in the low hummocks (< 0.5 m) on the deflation surface of Hietasärkät and the higher hummock dunes of Koppana, while the finest material was from the dune hummocks of the Karhuluoto area at Yyteri, the dune fields of the Tauvo Peninsula and Marjaniemi. The hummock dunes usually consist of moderately well sorted material, the grain-size distribution of which is symmetrical and mesokurtic. Sorting in the hummock dunes varies between moderately sorted (in the northern part of Hietasärkät and at Kolaviken in Hanko) and very well sorted (the Karhuluoto area, the rocky shore of Vexala, Pietarsaari, and the Tauvo Peninsula). A highly negatively skewed grain-size distribution was found in the Marjaniemi dune field, which consists of fine material, and the most positively skewed sample originated from a hummock on the distal slope of the First Salpausselkä formation at Lappohja, consisting of quite coarse material. A highly leptokurtic grain-size distribution was found in a hummock dune on the deflation surface of Tulliniemi and a highly platykurtic one at Koppana. The sand in the hummock dunes is significantly coarser (at the 99.9 % probability level) and less well sorted (at the 99 % level) than that in the foredunes.

The coarseness of material at Koppana is due to the proximity of the esker core. Though the swash-built ridges usually consist of quite fine material, a ridge of this kind at Koppana included coarse layers and contained many more coarse grains (5.2 % by weight of diameter over 1 mm) than the dune ridge on the beach face (2.6 % > 1 mm; Md = 0.5 mm), the hummock dunes on the deflation surface (1.3 % > 1 mm; the deflation surface itself contains 22 % > 1 mm grains), or the older hummock dunes covered by forest (1.4 % > 1 mm; Md = 0.56 mm). On the other hand, the berm contains less fine grains than the hummock dunes (Fig. 48), from which it can be concluded that the hummocks consist of aeolian material in spite of its coarseness. Separate hummock dunes that consist of the same kind of material (Md = 0.57 mm) are found further inland, at least at Pohjankangas, where they are again located on the core of a glaciofluvial esker (Hellemaa 1980: 65-73). All the above-mentioned samples from Koppana were collected by puncturing different parts of the formations in question and were analysed with a wider-spaced sieve mesh interval than the other samples. For this reason they are not included in Appendix 2.

Fig. 48. Grain-size distributions of samples from the Tauvo Peninsula (Siikajoki) and Koppana (Oulunsalo). The samples, from the fines (Md) to the coarsest, originate from: (a) leeward slope of a dune at Ulkonokanhietikko, (b) an Empetrum hummock dune, (c) upper beach and (d) a berm at Haikaranhietikko, (e) a beach ridge at Yrjänä and (f) a foredune at Haikaranhietikko. The material of Ulkonokanhietikko is the finest-grained and best sorted. The grain-size distributions of the hummock dune and the berm are negatively skewed. The Koppana samples consist of coarser grains: (1) a stratum of fine sand in a berm, (2) a younger hummock dune, and (3) an older hummock dune.

The coarsest sample from the transgressive dunes (Md = 0.48 mm; Table 3) originates from the windward slope of Tahkokorvanpakka at Kalajoki and the finest-grained one (Md = 0.31 mm) from the leeward slope of Tarkastajanpakka at Vattaja, although the material of the parabolic dune at Virpiniemi in Haukipudas is even finer (Md = 0.2 mm) and well sorted. The material of transgressive dunes is usually only moderately well sorted and the grain-size distributions are symmetrical and mesokurtic. The present transgressive dunes contain a very small amount of fine grains, the proportion under 0.125 mm in diameter being 1.6 % at most (a sample from the deeper layers of the Tahkokorvanpakka dune), while that of coarse grains, over 2 mm in diameter, is also less than 1 %, although they can reach 8 % by weight on the deflation surface of the windward slope. The material obtained here from the windward slope and crest of Tahkokorvanpakka is slightly coarser than that described by Heikkinen and Tikkanen (1987: 254), possibly because the present samples were taken from higher up the windward slope, where the wind is stronger and the influence of deflation is more obvious. Deflation and accretion alternate on the windward slopes of dunes, which causes crossbedding. The material in the transgressive dunes of Vattaja is almost as coarse as that in Tahkokorvanpakka, the coarse material of the deflation surfaces having been mixed with the dune sand as the dune has moved. Heikkinen and Tikkanen (1987: 252) found a bed of very coarse material (Mz = 0.55 mm) on the leeward slope of Tahkokorvanpakka and concluded that it was derived from the deflation surfaces and had been deposited on the distal slope of the dune at times of gale-force winds. Aartolahti (oral communication) has found the same kind of coarse bed in the stabilized slipface of the Viitapakka dune at Kalajoki. No beds of this degree of coarseness were found in the slipfaces at the present sites. In fact, comparison of the samples from the windward and leeward slopes of the transgressive dunes suggests that the median grain-sizes of the slipfaces are even a little finer and more leptokurtic than those of the windward slopes.

The material of the transgressive dunes is coarser and less well sorted than that of the foredunes (differences significant at the 99.9 % level), the hummock dunes (M_Z at the 99 % level and Sd at the 95 % level) or the beach sand (at the 99.9 % and 99 % levels respectively), although this latter difference is simply due to the fact that the coarse beach beds were not sampled. The timing of sampling obviously has a similar effect, as the winds are weak in summer. The fact that the transgressive dunes are no longer moving denotes that they may have been formed at a time when the winds were more powerful than those prevailing nowadays.

The material in old stable or stabilizing dunes that have stayed in the place of their formation is finer than that in transgressive dunes. The coarsest samples of this type originate from the west coast of Virpiniemi and the 3 m high dune at Lappohja and the finest samples are from the southern part of Yyterinsannat. The median grain-size of all the preserved old dunes is 0.2 mm. These samples are well sorted, and only those from Monäs and the southern part of Yyterinsannat are moderately well sorted. The grain-size distributions of the older dunes are negatively skewed or symmetrical and mesokurtic. Only one sample from Lappohja is more leptokurtic. One lichen-covered dune ridge in the southern part of Yyterinsannat that has a highly negatively skewed grain-size distribution belongs to the succession of parallel ridges mentioned earlier and obviously consists at least in part of wave-accreted material.

The aeolian material at Yyteri does not include a very high proportion of grains over 1 mm in diameter, and in some dune samples the proportion of grains over 0.5 mm is still under 0.5 %. On the other hand, the proportion of grains finer than 0.125 mm in diameter is usually less than 2 %. The proportion of fine material of this kind in cover sand is 3 % and that in the parallel dune ridges in the southern part of Yyterinsannat 4-5 %. The cover sand sample is well sorted, mesokurtic and negatively skewed. One sample taken from the bottom of a deflation bowl proximal to the highest dune of Yyteri, the 20 m high Keisarinpankki, gave a median grain size of 0.29 mm. This sample obviously consists of the old beach deposits at the base of the dunes. It is moderately sorted, highly negatively skewed and mesokurtic. The material on the crest of Keisarinpankki is finer-grained (Md = 0.26 mm) and well sorted (Sd = 0.50), symmetrical and mesokurtic, while the sand of the incipient foredune on the same transect is slightly coarser (Md = 0.27 mm), well sorted (Sd = 0.39), positively skewed and mesokurtic.

A sand sample taken from the upper leeward slope of the higher arm of the parabolic dune at Virpiniemi is slightly better sorted and less negatively skewed (Md = 0.20 mm, Sd = 0.38, Sk = -0.18) than one taken from a regular beach ridge located seaward of it (Md = 0.20 mm, Sd = 0.40, Sk = -0.26). The parabolic dune and beach ridges were formed before the Little Ice Age, during which period a dune ridge about 2 m high accumulated on the west coast of Virpiniemi. This ridge contains coarser, less well sorted, symmetrically distributed material (Md = 0.52 mm, Sd = 0.53, Sk = 0.089), which supports the idea that the prevailing winds during the Little Ice Age were stronger than nowadays. The negative skewness of the grain-size distributions in the beach ridges that consist of fine material suggests that they were deposited by wave action. All these aeolian formations have accumulated on the Runtelinharju end moraine (Koutaniemi 1986: 155).

At Vattajanhietikko and at Kalajoki the bases of the transgressive dunes lie 5-8 m above sea level, and in both areas the transgressive dunes have moved concurrently and originate from the Little Ice Age (Heikkinen & Tikkanen 1987: 263-265). Many dunes lying further inland were activated at the same time, and many other dunes began to accumulate on the coasts (Aartolahti 1976: 91). At Hailuoto, Siikajoki, Monäs and Tulliniemi, for example, the bases of these old parabolic dunes are about 5 m above sea level. Younger dunes than this have moved only a little. The highest dune of Yyterinsannat also began to form during the Little Ice Age, and its material is as fine as that of the incipient foredune in the same area, on account of the fineness of the source material. The Keisarinpankki dune has not notably moved from its original place, as shown by the fact that there is no unbroken deflation surface on its proximal side but only some separate deflation bowls, nor have any soil layers been found under the dune. The dune has simply grown higher as the forest behind it has reduced the power of the winds. The climatic conditions of the Little Ice Age obviously favoured the formation of dunes. Cold climatic periods often have less rain. The subsequent warming, however, was evidently accompanied by strong winds and more frequent storms (cf. Fairbridge 1992), and it was at this period that the dunes moved and broad deflation surfaces were formed.

Marrs and Gaylord (1982) suggest that wind velocity can be deduced from the grain-size of dune sand, for which purpose one needs to know the minimum and maximum prevalent grain-sizes, obtainable from the positions of natural breaks in the size distribution curves. In addition, the modal grain-size is used. These values are more or less constant for coastal dune fields where both younger and older dunes occur and have remained immobile (the minimum grain-size is everywhere 0.125 mm, the maximum 0.56 mm and the modal grain-size for incipient foredunes between Hanko and Kalajoki is 0.25 mm and that for stabilized dunes 0.23 mm). The grain-size values for the transgressive dunes of Vattaja and Kalajoki are coarser, however (minimum 0.129 mm, maximum 0.96 mm, modal 0.297 mm at Vattaja and 0.42 mm at Kalajoki). Accordingly the winds seem to have been about the same at the time of older dune formation as they are nowadays, except at Vattaja and Kalajoki. The modal grain-size of the foredunes on the bathing beach of Vattaja and the northern part of Hietasärkät at Kalajoki is 0.297 mm, which can be attributed to the coarseness of the source material and the location of these sites on open headlands. The correlation coefficient between adjacent samples taken from aeolian material and source material on all these beaches is 0.90**. The crest of the transgressive Tahkokorvanpakka dune at Kalajoki reaches almost 20 m above the open beach in places, which may partly explain the higher wind velocity and coarser grain-size. Judging from this, the winds at the time when the transgressive dunes were formed were not notably stronger than nowadays, in spite of the coarseness of the material. 

Chemical characteristics of the material

The bedrock of Finland consists mainly of granite, gneiss and other acidic rocks, and the major part of the sand in the coastal dunes is composed of resistant quartz and feldspar. As a consequence the dunes are barren, acid habitats. Sand containing more nutrients was found only at Padva (Table 4), and even there the calcium is quickly leached away from the upper horizons. All the samples of material from Padva were taken less than 30 m away from the shoreline. The highest pH value measured here, 6.7, was for the beach sand of Padva, and the calcium content of the beach face, covered by Honkenya peploides, was as high as 500 mg/l. This sample also included abundant remnants of algae. The calcium content of the veneered beach ridge, covered by Elytrigia repens, Galium verum, Artemisia vulgaris and Tortula ruralis, on the distal slope is lower, 400 mg/l (pH 6.4), and again this sand contains a lot of organic debris, but that of the deflation surface, characterized by Festuca ovina and Arctostaphylos uva-ursi, is only 200 mg/l (pH 5.8). The amounts of potassium and manganese are also unusually high in the sand of Padva, and correspondingly decrease within a distance of less than 20 m (K: 90 -> 30 mg/l; Mg: 50 -> 15 mg/l), along with the phosphorous content.

Table 4. Summary of the soil samples. The lower figures (in parentheses) are averages.

*¹ Amount of organic material: 0 = none, 1 = a little, 2 = some, 3 = abundant.
*² The Padva samples are not included in the other values for this Table.

The soil becomes poorer and more acid as the sand is leached by rains and nourishment with new material decreases further from the beach (Lundberg 1987). Nutrients are poorly absorbed by sand and silt. In the zone where the lyme-grass of the foredunes is sterile the pH of the soil is below 5.9. Fertile lyme-grass is found growing on the hummock dunes (pH 5.8) and the transgressive dune ridge (pH 5.4) at Kalajoki, although admittedly it is shorter and sparser than that on the foredunes. The mobility of the sand also affects the ecological succession, the hummock dunes, which trap sand, have a higher pH than the surrounding deflation surfaces, which are partly covered by mosses.

As the acidity of the soil increases, phosphorous dissolves from the mineral material contained in it, although the amount of phosphorous is still quite small in the hummock and transgressive dunes located further from the beach. These formations are uncovered and fairly open sites, and effectively leached. At Monäs the amount of phosphorous is higher in the moss-covered forested deflation surface than in the bare sand surface of an older dune. The phosphorous content is usually quite low under a continuous moss carpet, and it is possible that the mosses absorb phosphorous, although another explanation could be that the moss carpet does not prevent leaching of the underlying sand at all. These areas do not receive any new, nutrient-rich sand. The grasses in the herb layer are replaced by dwarf shrubs as the amount of phosphorous present decreases.

The other plant nutrients analyzed, potassium (K) and manganese (Mg), also decrease in concentration with time as nourishment with new sand diminishes, usually accompanying an increase in the distance from the shoreline. When the shoreline remains in place, the influence of leaching can already be clearly seen on the leeward slope of the foredune. It seems that potassium and manganese are released into the soil as organic material and humic acids increase and the sand grains become weathered. 

Water repellency of the sand surface

The coarsest aeolian material is found in connection with the coarsest glaciofluvial source material on the Hanko Peninsula and at Vattaja Cape, Kalajoki and Koppana. All these shores rise steeply from the sea, and as a result the deflation surfaces are dry. Water percolates easily through the coarse sand and the capillar rise from the groundwater table is not as pronounced as it is in silt.

The dampness of a sand surface is affected by its height above sea level, grain-size, vegetation cover, amount of organic material and exposure. Six sand samples collected from Yyteri less than an hour after a light shower of rain (1 mm) gave an average water content for the surface layer in the foredunes of 0.1 %. The intermediate dunes with sterile lyme-grass, which are eroding and partly covered by mosses, had a water content of 2.5 %. The average proportions of organic material were 0.2 % in the foredunes and 0.5 % in the intermediate dunes. The situation may be opposite during a shower of rain, of course, so that a foredune sample taken in the rain had a water content of 5.2 % (organic material 0.3 %) and one from an intermediate dune 4.9 % (organic material 0.4 %).

Water repellency increases with the amount of organic material (Table 5). A soil is considered water repellent if the water drop penetration time exceeds 5 seconds. Dry and acid soils are water repellent, and some humic acids obviously reduce water penetration (Dekker & Jungerius 1990: 174).

Table 5. Water repellency, pH and organic content of 19 sand samples collected from the dune fields of Yyteri. The following classes were distinguished for the water drop penetration time (WDPT) after Dekker & Jungerius (1990: 176): wettable, non-water repellent (<5 s), slightly (5-60 s), markedly (60-600 s), severely (600-3600 s) and extremely water repellent (>3600 s).

In the case of the soil samples from the foredunes of Yyteri the water drops always penetrated the smoothed surfaces of air-dry sand immediately, but the samples taken from the zone of sterile lyme-grass showed great variation according to the presence of clods bound by fungal hyphae in the samples. Fungal proliferation, which increases leeward from the incipient foredunes, seems to promote water erosion and accelerate the smoothing and lowering of dunes. The water drops persisted for some time on the top of all the samples taken from the bare sand surface of the windward slope of Keisarinpankki, and if the sample was tilted the drops would roll along its surface. It can be concluded from this that water erosion is extremely significant on the bare sand surfaces of old dunes, as the water does not penetrate into the dry sand. On slopes this leads to the formation of rills and sandflow tongues. These are not found on the slopes of mobile dunes, however, as the mobility of sand prevents the accumulation of hydrophobic organic materials. In spite of the small number of samples, there is an obvious difference between those taken from the windward slopes of Keisarinpankki and those from the slopes of foredunes. This experiment was repeated, with same results. One sample taken from the deeper layers of an intermediate dune contained less organic material and was immediately penetrated by the water drops. The uppermost soil layer is the most water repellent, while the deeper layers of sand are more wettable as they contain a smaller amount of organic material (Dekker & Jungerius 1990).

Not all differences in water repellency between the samples can be explained by differences in the amounts of organic material, however. One sample from an intermediate dune contained more plant debris and fungal hyphae than one from Keisarinpankki and yet it was penetrated more quickly by the water. This was not due to the grain-size, either, as the material of the intermediate dune is slightly finer (Md = 0.20 mm) than that of Keisarinpankki (Md = 0.26 mm). Water repellency is affected also by the acidity and weathering of the sand. The sample from Keisarinpankki is more acid, and the sand of this older dune seems when viewed under a microscope to contain more very small particles, both organic and mineral material. Rhizomes and fungal hyphae, without cell walls, are also to be seen. These fine particles, which may clog the pores between the sand grains, may have been transported onto the old dune in suspension, to form allochthonous cover sand. On the other hand, weathering caused by frost and humic acids yields fine material and can even break up quartz grains (Pye & Tsoar 1990: 259-260). Water repellency may likewise be affected by brown iron crust, which adheres to the sand grains, but there was no significant difference in this respect between the samples from the intermediate dunes and those from Keisarinpankki.

Soil formation seems to affect water repellency, and also the amount of overland flow (cf. Brooks & Richards 1993). The influence of water erosion can be seen clearly in the field, in the form of sand terraces of varying sizes that have accumulated on the upper sides of the plant tufts. Fine material has also accumulated on the surfaces of the dune slacks.

Vegetation cover

Dune plants

The formation of coastal dunes necessitates that the vegetation cover should be sparse or absent for at least a part of the year, as the plants bind the soil surface and the stems reduce the power of the winds (Chepil & Woodruff 1963). In Finland the shortness of the growing season limits plant growth and their capacity for binding the sand surface. It is hard for plants to spread over loose, dry, barren sand surfaces, which may be lethally hot in the sunshine. On dune coasts the plants also have to cope with the drying and tearing action of the wind, abrasion and burial in sand. Seedlings of most species could not survive complete burial, while erosion causes most plants to die on account of dessication of their roots (Maun 1994). In addition, the wind blows the seeds away from the open sand surfaces and thus hinders the spread of a vegetation cover. The mobility of the sand clearly regulates the range of plant species (Moreno-Casasola 1986).

Even a sparse vegetation cover will weaken the power of wind, shelter the ground surface and trap sand grains (Wolfe & Nickling 1993). Dune plants are xeromorphic, i.e. they have adapted to tolerating dryness. They have long roots that reach down to the lower, humid sand layers, and their stems can survive bending by the wind. The leaves of many dune plants, such as Ammophila arenaria, are in different positions during dry and humid periods, while the leaves and stems of Leymus arenarius, the main dune grass in Finland, have a wax cover that reduces transpiration and gives the plants their characteristic bluish green colour. Vegetative reproduction through roots and rhizomes is very effective in dune plants, and after burial in sand they can form adventitious roots and negative geotropic roots extending into the new sand layers (Lemberg 1933: 116-117; Alestalo 1971: 44). Mycorrhizal fungi play a key role in the nutrition of coastal dune species (Rozema et al. 1986), and the root systems act together with fungal hyphae to bind the sand grains together.

Real dune plants not only tolerate burial in sand, but demand it in order to retain their vitality (Warren 1979: 348), so that their growth follows the annual accumulation of sand. Pioneer species such as Elymus farctus, Leymus arenarius and Ammophila arenaria can withstand a yearly accumulation of 0.6 m of sand at the most (Ranwell 1972), but begin to degenerate if the accumulation of sand comes to an end. If the dunes are not rapidly invaded by ecologically different plants, an attack by wind and water erosion may destroy the dunes at this stage (Daubenmire 1968: 123).

Research on the European coasts has usually distinguished incipient (primary or embryo) foredunes, bound by plants such as lyme-grass (Leymus arenarius), which tolerate salt spray and soil salinity, from white, or established dunes bound by less tolerant species such as marram-grass (Ammophila arenaria). Lyme-grass is a nitrophilous and halophilous species that tolerates occasional floods of salt water (Lemberg 1933: 45; Vartiainen 1980: 42). For this reason it grows nearer the shoreline than marram-grass, which is at the extreme northern border of its distribution in the south of Finland. Marram-grass does not promote the proper dune formation in Finland, whereas lyme-grass binds all the foredunes bordering the coasts. The stems and leaves of these species form a semipermeable obstruction which reduces wind velocity and causes sand accretion. Leymus arenarius is a tall species, capable of growing 1.5 m high in places, and it readily forms tufts and long outward-extending rhizomes. As a result its tufts are more widely spaced than those of Ammophila and for this reason it loses out in competition between the two (Lemberg 1933: 49). They seem to be of the same value as sand binders, however (Lundberg 1987: 473).

The species that occur on all or almost all coastal dune fields in Finland are Leymus arenarius, Honkenya peploides, Hieracium umbellatum, Rumex acetosella, Deschampsia flexuosa, Empetrum hermaphroditum, Ceratodon purpureus, Polytrichum piliferum, Cladina rangiferina and C. arbuscula. Juniperus communis, Salix phylicifolia, S. repens, Arctostaphylos uva-ursi, Festuca ovina, F. rubra, Agrostis stolonifera, Calamagrostis stricta, C. epigejos, Juncus balticus, Lathyrus japonicus, Polytrichum juniperinum, Racomitrium canescens, with the Cetraria, Stereocaulon (S. paschale and S. tomentosum) and Cladonia lichens are very common, too. The most common trees on the coastal dune fields are Pinus sylvestris, Alnus incana, Betula pubescens and Sorbus aucuparia, in this order. Plants frequently to be found at the forest edge, often on the leeward slopes of dunes, include Vaccinium vitis-idaea, Maianthemum bifolium, Trientalis europaea, Dicranum polysetum, D. scoparium and Pleurozium schreberi.

The flora of the coastal dune fields varies in accordance with the vegetational zones of Finland (Ahti et al. 1968; Kalliola 1973: 180-188). Cakile maritima seems to be absent from the northernmost coasts, and Thymus serpyllum, Galium verum and Rumex crispus have a clearly southern distribution pattern in that they occur as far north as Yyteri. Elymus repens and Taraxacum spp. are found from Padva southwards. Ammophila arenaria and Viola canina occur only on the steep sandy slope of Lappohja, and some species such as Festuca polesica, Carex arenaria, Coelocaulon aculeatum and Cetraria islandica ssp. crispiformis are found only on the Hanko Peninsula. Thus the flora of the south coast of Finland differs significantly from that of the other coasts, the most divergent being the flora of the Hanko Peninsula. The soil at Padva contains more nutrients than the other shores studied, and some calcareous species are found there. Festuca polesica replaces the other Festuca species on the Hanko Peninsula, while elsewhere F. rubra seems to thrive better than F. ovina in soils of finer grain-sizes which have a better water absorbing capacity.

Juncus balticus is more common from Pietarsaari northwards than in the south, and Salix repens is also more common in the north, as there are more suitable moist habitats for it there. Vaccinium uliginosum grows on dry sandy soils north of Vattaja, and so does Ledum palustre, on Hailuoto and at Virpiniemi. The climate is more humid in the north because of the lower evaporation rate. Drosera rotundifolia and Gymnocolea inflata are mire species that also occur in damp depressions on the Yyteri dune field, where the acid sandy soil seems to provide a suitably poor habitat for them.

Cetraria nivalis also occurs in Yyteri. It is a characteristic species of northern oligotrophic heaths and is found infrequently in the south of Finland in dry lichen-rich forests and in the central parts of raised bogs (Ahti 1981: 21). Empetrum also occurs on both fells and peat bogs. The coastal subspecies, Empetrum hermaphroditum, similarly has a northern distribution pattern. In addition, the birches of the coastal dune fields have multiple, contorted stems and in this way resemble the mountain birch. The drying effect of the sun and winds is obvious both on the fells of Lapland and on the coastal dunes. In both kinds of habitat the wind blows the sheltering snow cover away in the winter. Thus the open, unprotected nature of the habitat itself affects the flora and the appearance of the plants.

The list of the 42 main dune plants in Finland published by Lemberg (1933: 133) does not include Deschampsia flexuosa or the Cladonia subgenus Cladonia lichens, but there are species such as Lecidea uliginosa, Cladina stellaris (which is more common in the north) and many southern species such as Salsola kali, Ammophila arenaria and Festuca polesica. There are also some species characteristic of damp seashore meadows (Carex nigra and Puccinellia distans ssp. borealis). The coasts studied here are located in a more limited area than those studied by Lemberg, as Karelia and the islands of the Gulf of Finland are excluded and the material is restricted to shores on which aeolian processes are active.

As a result of the ecological succession, vegetation zones are formed which run parallel to the shore, the succession on the dunes differing from that on the slacks between them or the deflation flats. The succession also depends on the exposure and differs between the windward and leeward slopes of the dunes (Daubenmire 1968: 125). Wind and water deposit organic debris and seeds on the lower parts of the leeward slopes, where the microclimate and soil humidity also favour plant growth. Salix bushes occur often in the lee of the foredune, which increases the accumulation of organic material on the leeward slopes.

The swash continuously brings new sand to a beach, containing nutrients such as nitrogen, calcium and salts. The substrate becomes poorer and more acid as the mobility of the sand is reduced and the material is leached by rain water (Daubenmire 1968: 197; Lundberg 1987). At this phase the dune grasses are still present and the ground is covered by mosses and lichens. Lichens do not grow on moving sand. The dunes become greyish in colour and the first saplings of trees appear in the lee of the dune ridges. There is often a deflation surface, a windy heath with small, separate hummock dunes, between the foredunes and the forest edge. The old, large transgressive dunes which were still moving actively at the beginning of this century are located behind deflation surfaces, and even in the middle of the forest there are often open, sandy surfaces on the windward dune slopes, showing how difficult it is for vegetation to spread over such surfaces and how vulnerable the vegetative cover on dunes is.  

Effect of plant species on the dune topography

Phytogenic dunes grow together with the plants, and often move slowly with the wind. The wind tears and dries the plants on windward slopes, and at the same time growth on the leeward slopes is stimulated by sand accretion. An embryo dune that is bound by a single plant grows until the plant dies and the sand is transported away by the wind. The manner of growth also affects the dune topography. If the plant grows rapidly upwards, the dune will also grow higher, e.g. dune ridges bound by lyme-grass, while if the plant grows out laterally, the dune will be low and broad in shape (Daubenmire 1968: 123). In this way the flora and its vitality can affect the morphology of dunes.

Honkenya peploides is a low herb, but is relatively tolerant of burial in sand, spreads laterally and binds sand effectively. It binds shield-like embryo dunes on the beach and small terrace-like dunes on the windward slopes of foredunes. Small shadow or lee dunes can be formed, which become elongated in the direction of the wind in the lee of individual seedlings of Honkenya. Similar forms are also caused by Agrostis stolonifera, Calamagrostis stricta, Festuca rubra and stones on the beaches. Densely growing stems of Juncus balticus form higher, elongated mounds (Fig. 49).

Fig. 49. Low, elongated Juncus balticus dunes at Letto in Kalajoki in July 1987. Besides accumulations and windward sand ramps, semipermeable obstacles have caused the formation of pits behind the plants by wind vortexes. Wind direction is from right to left.

Seedlings of lyme-grass (Leymus arenarius) are often to be seen growing in a row on a berm parallel to the shoreline, the swash having accumulated the seeds on the beach together with other material. The seedlings form a semipermeable obstruction which causes precipitation of sand grains. The base of a foredune accumulates in this way (Fig. 18). If the sand supply is abundant, both the lyme-grass and the dune will grow rapidly in height, preventing the sand from being transported further inland. The lyme-grass community becomes continuous and denser as new shoots grow up from the sand-binding rhizomes. The individual lyme-grass tufts may bind the sand into hummock dunes, which cause turbulence in the air flow (Fig. 50).

Fig. 50. A horseshoe-like vortex has formed a wind channel around this lyme-grass tuft in the Herrainpäivät area (Pori), photographed in July 1990. Wind direction is from left to right.

Festuca ovina grows in dense tufts and binds low, only slowly growing shield-like hummock dunes. The largest Festuca hummock dune found here is situated on the gentle windward slope of the Vonganpakka dune in Vattaja. This round hummock is about 4 m in diameter and 0.5 m high. Even Polytrichum mosses can bind sand to form low, very gently rising mounds. The largest shield-like moss mound found on the present shores is 2 m in diameter but only 10 cm high. The most conspicuous and most frequent forms bound by mosses are table-like erosion remnants that have been left when deflation has lowered the level of the surrounding ground (Fig. 51). The plant cover of these remnants binds sand transported by the wind and in this way they can grow higher (cf. Thorarinsson et al. 1959: 168-169; Seppälä 1974: 218).

Fig. 51. The flat top of this table-like erosion remnant is bound mainly by mosses, Honkenya is growing in the loose sand of the slopes in July 1987. The windward slope of the Keisarinpankki dune (Yyterinsannat, Pori) is in the background.

Empetrum hermaphroditum is a dwarf shrub which grows slowly in small patches. It does not have underground rhizomes, but spreads laterally as shoots buried by the sand take root and break away. Empetrum binds shield-like hummock dunes (Fig. 38). The profiles of the larger hummocks are symmetrical, or else the windward slopes are longer and more gentle than the leeward slopes. Empetrum becomes more frequent on old dune ridges as the lyme-grass dies away. Erosion forms bound by Empetrum often rise quite steeply from the surrounding wind-eroded sand surface (in Fig. 36 the windward slopes have been steepened by deflation).

Hummock dunes bound by Salix repens are always low (10-30 cm) and shield-like. They usually rise up as spots of more bare sand from damp, moss-covered surfaces. The higher Salix bushes bind higher, roundish hummocks. Sand also accumulates around Juniperus communis bushes, and tree trunks on dune fields are often encircled by a low sandy mound. The forested dune hills of Vattaja and Kalajoki can be regarded as dunes bound by trees. Bird-cherry trees (Prunus padus) and lingering or dead pines are often buried in deep sand.  

Statistical analysis of the vegetation cover

Principal Components Analysis was used to find indicator species that account for most of the variance in the vegetation data (186 sample plots). Meanwhile all the species which were significantly intercorrelated were dropped out. In this way the indicator species selected for damp surfaces is Salix repens and not Juncus balticus or Cephaloziella divaricata, for example. Thirteen indicator species were left without any significant intercorrelation, after which those species were excluded which occurred only in a few plots. In this way Festuca polesica, the indicator species for a southern plant community, was left out of the analysis at first. After this the whole vegetation data were processed by Cluster Analysis, using the indicator species as a basis for the clustering. A hierarchical dendrogram was used to determine the number of clusters. With five clusters only the dampest and the most forested plots were clearly distinguishable, but the use of more clusters enabled 21 plant communities to be defined, the distributions of which are compared below with the a priori classification based on the geomorphology and vegetation cover.

The Leymus arenarius communities can be divided into the relatively pure Leymus stands of the foredunes and the more widely spaced Ceratodon - Leymus cover of the intermediate dunes. A purer, thicker Honkenya carpet can be separated from the Honkenya - Leymus communities of beach ridges and foredunes. Deschampsia flexuosa meadows occur on dry deflation surfaces and both the more widely spaced association of Deschampsia - Honkenya - Ceratodon and the denser Deschampsia - Ceratodon association occur on the windward slopes of older dunes. The Festuca ovina - Leymus community can be divided into a sparser type on dry deflation surfaces and intermediate dunes and a denser type that binds hummock dunes. A Deschampsia - Empetrum - Festuca ovina community is also found on dry deflation surfaces. The intermediate 'grey' dunes are characterized by a Ceratodon - Cladina community. Empetrum - Ceratodon heaths can be divided into an Empetrum association of dry, sandy deflation surfaces with hummock dunes and an Empetrum - Salix repens association of damper surfaces. Really damp surfaces are characterized by a Salix repens - Ceratodon community. A more lichen-rich Cladina - Empetrum community can be distinguished from the Empetrum - Cladina - Pleurozium community of the forest edges. An Empetrum - Pleurozium community is also found at the forest edges and on the leeward slopes of old dunes, and a Pleurozium - Deschampsia community is frequent there, too. When Festuca polesica is added to the clustering, the southern dry dune meadows, of which it is characteristic, emerge as a separate cluster. The Lathyrus japonicus and Agrostis stolonifera communities of beaches and dune slacks can also be recognized as distinct clusters, as can the thick Calamagrostis epigejos communities that occur on some windward slopes of old dunes.

The whole set of vegetation data was compared with the a priori classification using Discriminant Analysis. The plots representing sand flats, the upper beach, beach ridges with a thin aeolian veneer and foredunes were clearly distinguishable as groups of their own (Fig. 52 and Table 6: sample set A). The vegetation cover of the foredunes can also be divided from that of the intermediate dunes and dune slacks, but the latter two cannot be separated from each other. The abundance of Leymus arenarius is a sufficient distinguishing factor (Table 6: sample set B). In the same way the intermediate dunes cannot be clearly separated from the hummock dunes or dry deflation surfaces. The plots representing dry and damp deflation surfaces partly overlap, although the extreme cases can be clearly separated (Table 6: sample set C). The thicker vegetation cover on the hummock dunes differs significantly from that of the surrounding deflation surfaces. The vegetation on the foredunes, the intermediate dunes and the windward slopes of stabilizing old dunes forms a continuum in which the plant communities grade from one to another with a decreasing abundance of lyme-grass (Table 6: sample set D). The windward slopes of some old dunes where aeolian processes have remained active for a long time form the only exceptions in this continuum (Fig. 53). These plant communities are clearly distinguishable from the flora of the forest edge and the leeward slopes of old dunes, which in turn do not differ significantly from each other.

Fig. 52. A plot of discriminant functions. The floras of sample plots (n=50) can be separated in accordance with an a priori classification into 1) sand flats of the lower beaches, 2) upper beaches, 3) beach ridges with aeolian veneer and 4) foredunes. The discriminant functions are formed by linear combinations of independent variables (cover % by plant species), multiplying each independent variable by its corresponding weight and adding the products together (STATGRAPHICS 1995: 5). The independent variables that account for most of the differences are Agrostis, Leymus, Hieracium, Galium and Lathyrus.

Table 6. Discriminant Analysis summary comparing the vegetation sample sets (A - D) with an a priori classification. The sample sets are: A) sand flats of the lower beach, upper beaches, beach ridges with aeolian veneer and foredunes (n = 50); B) beach ridges with aeolian veneer, foredunes, intermediate dunes and dune slacks (n = 62); C) intermediate dunes, hummock dunes and dry and damp deflation surfaces (n = 92); D) foredunes, intermediate dunes, forest edges and windward and leeward slopes of old dunes (n = 78). Variables for the functions are selected by stepwise regression from the whole set of vegetation data.

Fig. 53. A plot of discriminant functions (n=78). The floras of 4) foredunes, 5) intermediate dunes, 10) forest edge, and 11) windward and 12) leeward slopes of old, partly forested dunes form a continuum in which the exceptions are a few old dunes which have remained active for long time. The most useful independent variables for this classification are Leymus, Pleurozium and Pinus.

When comparing the whole set of vegetation data with the Kalliola's (1973: 186) vegetational zones for the Finnish coastal flora (including all coastal plant communities) using Discriminant Analysis, the only clear groups formed concern the southernmost plots, those on the Hanko Peninsula, and some of the most northerly plots in the Bothnian Bay region (Fig. 54).

Fig. 54. A plot of discriminant functions (n=118). Ordination of the coastal floras of sample plots in accordance with the vegetational zones of Kalliola (1973: 186): 1) Gulf of Finland, 2) SW archipelago (including the Hanko Peninsula), 3) Bothnian Sea, 4) the Quark, and 5) southern and 6) northern Bothnian Bay. The most useful independent variables for this classification are Festuca polesica, F. ovina and Elymus. For the first function: eigenvalue = 0.762, relative % = 80.90, chi-square = 82.39, D.F. = 15, P-value = 0.0000).

The significance of environmental factors was examined more closely only for a smaller set of data collected alongside the soil samples. Many environmental factors are significantly intercorrelated (Table 7), so that as the distance from the shoreline increases, for example, the height above sea level also rises, the tree cover becomes denser, the acidity of the soil surface increases and the amount of nutrients decreases. On the other hand, the amount of phosphorus in the surface soil decreases at first, but increases again further from the shoreline with increasing acidity and better solubility of phosphorus, and the same trend can also be seen in potassium and manganese (Fig. 55). The amount of organic material in the soil decreases as the vegetation cover becomes more sparse and trampling increases. As almost all the plots are situated on the Gulf of Bothnia, land uplift and fluctuations in sea level both increase northwards. There are more coastal sandy plains consisting of fine grained material in the north.

Table 7. Correlation matrix (Pearson product-moment correlation coefficient) for environmental variables. Abbreviations for the variables: DIST = distance from water line (m), HEIG = height above sea level (m), pH = soil pH, Md and Sd = mean diameter and sorting of sand, NOR = northernness (km) in the Finnish coordinate system, UPLIF = rate of land uplift (mm/year), FLUC = sea level fluctuation (cm), MOIS = soil moisture content (%), SLO = slope tangent, BARE = proportion of bare sand surface (%), TREE = tree cover (%).

Fig. 55. Manganese and potassium content of the upper soil in relation to distance from the shoreline (38 samples).

Canonical Correlations Analysis and Discriminant Analysis were used to determine the main environmental factors affecting the ecological succession in the herb layer (Honkenya -> Leymus -> Deschampsia -> Empetrum). These factors are changes in the surface layer of the soil, i.e. pedogenic processes, especially increasing acidity (Table 8). The mobility of the sand also affects the ecological succession, as can be seen in the Tahkokorvanpakka dune at Kalajoki, where in spite of the acid soil of this old transgressive dune, the vegetation cover of the windward slope and the dune crest resembles that of incipient foredunes. The biotic soil factors that are harmful to dune plants are apparently eliminated during aeolian transport (Woldendorp 1996: 10). Thus, the organic content of the soil and the intensity of trampling have an effect on the ecological succession.

Table 8. Canonical Correlation Analysis ordination of environmental factors affecting the ecological succession (number of sample plots = 38).

The factors affecting the succession in the moss layer (Ceratodon -> Cladonia -> Polytrichum -> Racomitrium -> Dicranum -> Pleurozium) were analyzed separately. The main indicator species, Cladonia subgenus Cladonia and Cladina arbuscula lichens, Dicranum scoparium and Pleurozium schreberi, were selected by Principal Components Analysis. The environmental factor that correlates most significantly with these indicator species is the tree cover. Trees are frequent only on stabilized surfaces, where litter fall increases the amount of organic material in the soil and the shade lessens the extremity of the microclimate (cf. Heikkinen 1991: 216). Other important factors are the distance from the shoreline and the height above sea level, which affects soil humidity (Table 8). The amount of open sand surface is the next factor in importance, as it is indicative of the mobility of the sand.

When the species of both the herb and moss layers were analyzed together, the principal components describing the ecological succession were Honkenya, Leymus, Ceratodon, Empetrum and Pleurozium. The environmental factor that correlated most significantly with these was the cover of the tree canopy, which is itself part of the ecological succession. Other highly significant factors are the pH of the surface soil, the distance from the shoreline and the height above sea level, being indicative of the leaching of the sand as time passes and land uplift continues. Of the nutrients examined, manganese correlates best with the change in vegetation cover. The organic content of the soil and the intensity of trampling form another important group of factors, indicating the stability or mobility of the sand surface. The eigenvalue and chi-square value are naturally larger for all the significant factors together than for the factors separately, denoting more significant correlations. According to Kuusipalo (1985: 75), the most important factors regulating the distribution of plant communities in the upland forests in southern Finland are the cover of the tree canopy, the fertility and acidity of the humus layer and soil moisture content. All these factors also have a clear effect on coastal dune fields, alongside sand mobility.

As can be concluded from the above Cluster Analysis results, the main factors affecting the distribution of plant communities on the Finnish coastal dune fields are: 1) soil-forming processes, which cause an ecological succession from the nutrient-rich beach to the continuous forest cover, 2) soil water content, which depends on grain size and the steepness of the slope, and 3) geographical location, as the flora becomes poorer northwards and species favouring more humid conditions become more frequent. When the indicator species for the southern plant community is included in the analysis, geographical location becomes a significant factor (eigenvalue = 0.468, chi-square = 20.484, D.F. = 7, p = 0.0046), whereas if the indicator species for damp surfaces is included, the importance of height above sea level increases. Calcium content is naturally a significant environmental factor for the calcareous plants of Padva. Thus, the significance of the above environmental factors depends on the area selected for analysis.

The exposure, direction and inclination of the slopes does not seem to have a significant effect on the flora of low and open foredunes. It is true that the bushes and mosses are regularly represented first on the leeward slopes of foredunes, but a more significant difference can be seen between the sunny and partly bare windward slopes and the shady, densely vegetated leeward slopes of the old, usually higher, stabilizing dunes. In earlier studies of stabilized dunes and esker slopes (Jalas 1950; Aartolahti 1973; Påhlsson 1974; Oksanen 1983; Rajakorpi 1987; Heikkinen 1991) the aspect of the slope is repeatedly reported as an important factor regulating the distribution of plant communities. The amount of solar radiation in particular varies on slopes of different aspects and causes differences in the thickness of the humus layer, soil acidity and relative humidity which are mentioned as significant factors (Aartolahti 1973: 43; Oksanen 1983; Rajakorpi 1987; Heikkinen 1991). Differences between north and south-facing slopes are also reported with respect to the thickness of the snow cover, length of the growing season and frost frequency (Rajakorpi 1984). Some plant and animal species do occur on the steep south-facing slope of Lappohja that are not found on other shores in Finland. The impact of inclination may be greater on sunny slopes, as it strengthens the effect of aspect (Rajakorpi 1984; Heikkinen 1991).

Among the other environmental factors, trampling favours the occurrence of Festuca ovina and clearly reduces the possibilities for mosses and lichens to flourish. Cladina and Stereocaulon lichens and the moss Racomitrium in the present data are particularly found to be more abundant on only lightly trampled areas. The grain-size affects the moisture content of the sand, and therefore seems to explain the occurrence of Festuca ovina, which thrives in drier areas, and of F. rubra, which favours more humid surfaces.

Changes caused by man and animals

Human activity has been delaying the natural invasion of sandy shores by forest at least since the fifteenth century. At the beginning of this present century forest fires, the felling of trees and the grazing of animals on seashore meadows caused sand drifting and the formation of broad deflation surfaces and transgressive dunes (see Alestalo 1971: 126-127, 1979; Heikkinen & Tikkanen 1987). The formation of these large transgressive dunes coincided with the common practice of grazing sheep on seashores and islands (Skult 1955), so that many districts around the Gulf of Bothnia had as many sheep as they did inhabitants (Gebhard 1901). The sheep evidently destroyed the sand-binding lyme-grass.

Signs of human activities can be seen on almost all coasts, those that have been preserved untouched being located in the nature conservation areas on the Tauvo Peninsula and in military areas and private areas around summer cottages. A considerable number of summer cottages have been built on the coasts of Finland since the 1970's, and at the same time paths have been converted into roads. At least so far the cottagers have not greatly harmed the vegetative cover of the dune coasts. Achillea millefolium and Poa pratensis are more frequent around cottages, apparently being able to benefit from the proximity of man, as also does Rosa rugosa, the agressive spread of which has destroyed the original dune plants in places on the coasts of Europe (Hellemaa & Doody 1991: 16). This NE-Asiatic invader is already found on the sandy coasts from Hanko to Oulunsalo. It can become dominant in the absence of grazing and should be considered a weed (Hämet-Ahti, oral communication 1997).

Building by man may have totally destroyed a dune coast, as in the harbour of Ykspihlaja, Kokkola, and in some other places man has considerably changed the character of a coast, e.g. by turning it into a golf-course, as in the Karhuluoto area of Yyteri. Thus destruction of dune coasts can occur in a quantitative as well as a qualitative sense (Udo de Haes & Salman 1990: 11). Besides harbours and golf-courses, camping sites, hotels, roads and parking lots have also been built on the coastal dune fields of Finland. Dune coasts make excellent bathing beaches and recreation areas, but intensive trampling impoverishes the vegetation and causes deflation.

The impoverishment of flora can clearly be seen by comparing sites trampled to differing extents, e.g. at Kalajoki and Yyteri. Only Leymus arenarius, Festuca ovina, Hieracium umbellatum and Rumex acetosella occur very sparsely on the most heavily trampled areas of the Hietasärkät dune field in Kalajoki. The vulnerability of the forest vegetation to tourism on this dune field has been studied by Jämbäck (1995). Numerous tourist attractions and cottages have been built on this coast over the past few years, as it is one of the most popular recreation areas in Finland. As a consequence trampling has increased and destroyed in particular the vegetative cover of the pine forest behind the dunes. At the most trampled sites the paths form unified bare sand areas, but where trampling is not too heavy and continuous the destruction of the vegetation may offer new habitats for pioneer species.

At Padva and Pietarsaari the beaches are kept vegetationless by harrowing and the aeolian sand is burying the bases of the tree trunks behind these beaches. In August 1987 the plants had somewhat recovered on the harrowed beach of Padva in spite of trampling, so that 6 plant species were found in a plot one metre wide and 20 m long, and as many as 15 on the better preserved cottage beach nearby. The former flowering dune meadow and the rare calcareous species had vanished as a consequence of the harrowing.

Aeolian sand does not tolerate intensive trampling or heavy loads (Aartolahti 1976: 93), and trampling in coastal recreational areas, and also motor vehicles, especially in military areas (Hellemaa 1980: 56), will break up the vegetation cover. The building of roads and hotels on the dunes increases trampling. Raabe (1973 cit. Skytén 1978: 44) has described how the dune crests are broken down by trampling. At the most trampled sites, deep, wind-scoured gaps break up the foredunes. In Halland in the south of Sweden, the holes dug by sunbathers caused so much erosion that the dunes began to move (Norrman et al. 1974), and strong winds have activated the dunes at the most severly trampled sites in Finland during the past few years.

Coastal ecosystems are also affected by human-induced pollution. In the summer the plants bind and use nutrients originating from sewage and fertilizers washed into the waterways from arable land and forests, from coastal fish farms and in Ostrobothnia especially from fur farms. Fur farms have been estimated to produce 1000 tn of phosphorus and 5000 tn of nitrogen a year during their best years, which is equal to the additional load caused by one million inhabitants (Ulfvens 1992: 44). Nowadays the loading from fur farms is only a fourth of this (Tikkanen 1994: 7). By far the largest proportion of the nutrients originates from agriculture.

The pH of rain water is 4.6 or lower on the Finnish coasts (Atlas of Finland 1987: folio 131: 27). Strong acids spread from the Central Europe with air flows, especially to SW Finland. The amount of nitrogen in rainwater decreases from Virolahti (about 80 mg/m²/month) towards the top of the Gulf of Bothnia (40 mg/m²/month; Atlas of Finland 1987: folio 131: 27). The permeable, barren aeolian sand is very sensitive to the fertilizing effect of pollutants, and as a consequence of eutrophication Phragmites australis is spreading vigorously to the coasts and islets and impoverishing their flora and fauna (Ulfvens 1992). The thick vegetative cover effectively binds sand and stops aeolian processes. The shallow beaches on the Gulf of Bothnia have been invaded particularly vigorously by reedbeds.

Grazing, burrowing and trampling by animals is insignificant on the coastal dune fields of Finland nowadays, and as a result forest is spreading over dune fields that were largely open at the beginning of this century. The same kind of development is also common elsewhere on the European coasts (e.g. Van Dijk 1992) now that grazing by farm animals has ended and the release of fertilizing pollutants has increased.

Ants are also worth mentioning when we are speaking of the natural fauna of the dune coasts, as they bring loose sand to the soil surface (Wallin 1980: 94). Formica cinerea, which lives in the coastal dune fields from the Hanko Peninsula to the island of Hailuoto, particularly favours open sand fields and brings new sand up from the deeper soil horizons for wind transport, thereby reducing the formation of a continuous vegetation cover (Kilpiäinen et al. 1977; Jämbäck 1995: 28). On the Koppana beach at Oulunsalo the hummock dunes have grown higher as a result of the work of ants. Elsewhere in the world rabbits have locally caused sand to move by burrowing and grazing on dunes (Klijn 1990a: 93; Wilson 1990: 77; Rutin 1992). Rabbits do not occur in Finland, but hares have been known to gnaw the bark of pines and accelerate the death of trees, at least on the leeward slope of the Tahkokorvanpakka dune at Kalajoki (Heikkinen & Tikkanen 1987: 259). Many birds nest on the uninhabited dune coasts of Finland, most notably shore birds such as Larus, Sterna, Calidris and Charadius.