The geomorphological and ecological succession
The beach zone means here the area located seaward of the foredune, where wave processes are most powerful. On dune coasts a broad cut-and-built terrace is formed, reaching from the sublittoral zone to the geolittoral and consisting of wave-moulded material with numerous bars. Longshore bars are found at almost all the beaches examined. Longshore bars and swash bars have clearly made progress in terms of beach progradation and provide a supply of sand for wind transport. On the cape of Vattaja the beach is prograding most rapidly in the area of an arcuate accumulation bay bordered by longshore bars (Fig. 56). Longshore bars also line the tombolos that are typical features of sandy beaches, e.g. at Yyteri, Monäs and Vattaja. At Hietasärkät in Kalajoki the shallows have caused refraction of the waves, which have eroded the southern part of the beach and moved some of its material to the bars.
To the north of the sand spit of Lahdenkrooppi at Vattajanhietikko the shoreline is quite straight and has remained in the same place for a long time. Even on this shore, further from the beach, there are large longshore bars, but nearer the shore they are oriented perpendicularly to the shoreline. These bars are quite stable, and the largest of them have maintained the same location for decades (Fig. 56). According to Keränen (1985: 38), large, stable transverse bars of this kind (surf cusps, Carter 1988: 119) indicate a limited supply of sand. On the shores of Vattaja they have been formed as the genly sloping esker has emerged above sea level. Erosion and deposition seem to be in balance on this stretch of coast. Beach drifts can carry material away from a straight shoreline.
Fig. 56. Aerial photographs of the Vattaja dune field (Lohtaja) in June 1971 (left; no. 71154:64) and in May 1992 (right; 92144:55). Forest invasion has been slow in the military area. The tombolo of Kalsonnokka has grown broader and the shoreline has shifted seawards in the arcuate accumulation bays with numerous longshore bars. Large transverse bars join the beach in the straight stretch of shoreline, the bars and the shoreline having remained stationary for decades. There are large longshore bars seaward of these. By permission of © Maanmittauslaitos, permit no. 51/MML/08.
At Tulliniemi on the Hanko Peninsula there are swash cusps consisting of coarse material and the beach is bordered by arcuate submerged bars, which are in accordance with the subaerial cusps. According to Carter (1988: 120, 124), cusps of this kind are often accumulation forms. Small, rhythmic swash cusps are characteristic of shores which belong to the reflective domain, with prevailing destructive waves.
In the southern part of the Hietasärkät dune field the upper limit of the geolittoral zone is marked by a steep cliff 2 m high eroded into the esker slope (Heikkinen & Tikkanen 1987: 246). In the northern part of Yyterinsannat marine erosion has steepened the windward slope of the foredune, and at Tulliniemi, Monäs and the bathing beach of Vattaja the foredunes have been eroded by waves, too. At Lappohja waves have eroded both old aeolian and glaciofluvial accumulations. On all these beaches the sediment budget is either in balance or negative.
The perennials Agrostis stolonifera, Festuca rubra, Calamagrostis stricta and the annual Juncus bufonius thrive on moist coastal sandy plains. Agrostis in particular may form very broad but widely spaced populations. In drier places, e.g. on the Tauvo Peninsula, the rhizomes of grasses bind the aeolian sand to form miniature dunes. The highest lee dunes of coastal sand flats to be found here were about 20 cm high and bound by Festuca rubra. Lemberg (1933: 38, 57) found 0.5 m high Agrostis dunes at Yyteri and 1.5 m high Festuca rubra dunes in Karelia. The horizontally growing, thin rhizomes of F. rubra start to grow upward if sand accretion is more abundant (Lemberg 1933: 57). On the shores where Juncus bufonius and Agrostis stolonifera are most abundant some Phragmites australis can also be found. The abundance of these species may predict the invasion by a reedbed.
The swash zone of a drift sand beach, where the habitat is affected by moisture from the sea water and nutrients from wave-cast organic debris, is usually unvegetated. This is due to the instability of the sand surface. A wrack flora may be found to some extent, for example Galeopsis bifida, Rumex, Polygonum and Atriplex species. This nitrophilous and halophilous flora changes from year to year and the plants often seem to be stunted. Agrostis stolonifera, Cakile maritima, Lathyrus japonicus and the tall Phalaris arundinacea are quite regularly found on the upper beaches. The fruits of the annual Cakile maritima consist of two segments, each with one seed. The distal fruit segments are dispersed with the wind, but the proximal ones remain attached to the parent plant (Watkinson & Davy 1985: 491; Maun 1994: 60). Consequently the plant is found in the same proven habitats year after year. The seeds, roots and nutrients that have accumulated on berms offer favourable initial material for a new embryo dune ridge (Gerlach 1992).
The sand on a smooth, unvegetated surface easily drifts with the wind. Saltation forms transverse ripple marks, but parallel aerodynamic sand flows can form low longitudinal ridges or sand veils at times of strong winds (Warren 1979: 337; Bourman 1986). These formations are incidental.
A Honkenya - Leymus community often dominates the upper part of the geolittoral zone. At Virolahti, Padva and Koppana Honkenya peploides forms thick carpets on the beach faces. Its long-branched rhizomes bind sand very effectively (Vartiainen 1980: 29), making the sand surface more suitable for other plants. Honkenya tolerates burial in the sand, and the growth of its stems follows the annual accumulation of sand. On the beaches it binds low embryonal dune mounds which grow until the next storm washes them away. Leymus arenarius also forms low embryo dune ridges and mounds in the geolittoral zone, which may grow to form foredunes. Developments of this kind have been monitored on the coasts of Yyteri and Vattaja, for example.
Berms veneered with aeolian sand
The shores of Virolahti, Kolaviken, Lappohja, Padva and Koppana possess beach ridges veneered by aeolian material instead of proper foredunes in the upper part of the geolittoral zone. At times of high water levels washover layers are formed, which makes the soil more fertile. The crests of these ridges are bound by a flourishing vegetation cover in which Honkenya and Leymus are frequent, as are Hieracium umbellatum, Tanacetum vulgare, Epilobium angustifolium, Rumex crispus, Cakile maritima, and Lathyrus japonicus, and also Festuca ovina (F. polesica in the Hanko Peninsula) and Deschampsia flexuosa, which grow mostly on the lee side. At Padva and Kolaviken (in Hanko) Leymus is partly replaced by Elymus repens, which has long, branching rhizomes and is nitrophilous, so that it benefits the organic debris. At Lappohja Leymus is mostly replaced by Ammophila arenaria. Rosa rugosa bushes are frequent on the beaches of the town of Hanko. On the southern coasts the plant communities of these beach ridges include Galium verum, Artemisia vulgaris and Taraxacum species (section Palustria). Mosses (Tortula ruralis, Campylium polyganum, Brachythecium salebrosum and Ceratodon purpureus) cover the beach ridge crest on the calcium-rich sand of Padva, where both the moss and the herb layer are fairly continuous.
The foredunes on the Finnish coasts vary between 0.5 and 8 m in height, but are usually only about 2 m high. Dune ridges may occur alone or in swarms, or partly overlapping, but always parallel to the shoreline. New foredunes are usually formed when the waves deposit sand on the prograding beach.
Harsh conditions prevail on the windward slopes of the foredunes, and there lyme-grass often occurs alone. Even this cannot survive in the presence of intensive deflation, however, and in places windy slopes are covered only with sand drifts or deflation surfaces. Other species that occur on the windward slopes of the foredunes are Honkenya peploides, Lathyrus japonicus, Agrostis stolonifera and Festuca rubra.
The windward slope usually has a more gentle gradient than the leeward slope, but windward slopes that have been eroded by wave or wind action are very steep and are usually bound by the root systems of plants, which prevent them from collapsing. On eroding shores the windward slopes are characterized by avalanche and slump structures and the leeward slopes by blowover beds. The vegetation cover on these dunes indicates that they have been in the same position for a longer time. The highest foredunes, in the middle part of Yyterinsannat, to the north of Lahdenkrooppi and on the bathing beach at Vattaja, have their windward slopes steepened by marine erosion and deflation. The total sediment budget of all these areas has been slightly positive and the dunes have had time to grow in height, as the shoreline has remained stationary. Trampling has had the effect of loosening the sand for aeolian transport on the bathing beaches. In the Karhuluoto area at Yyteri the foredune has grown in height rapidly because of the ample supply of sand from the broad, flat coastal plain.
The present foredunes have only exceptionally moved from their place of formation, although there are minor bends landwards at blowouts and wind-scoured gaps. The foredune at Yyteri has clearly become lower at the blowouts on the bathing beach and the leeward slopes have proved to be active slipfaces. The power of the wind is concentrated in these gaps, and the wind can also transport coarser material along them to the dune crests and beyond (Fig. 46). Some smaller wind-scoured gaps have lower blowover 'deltas' and various forms of lee dunes bound by plants. The gaps are often formed around pathways. Intensive trampling may break the foredune into separate mounds bound by lyme-grass. The material of the foredunes is usually so permeable that there is no overland flow, although rain and the melting of the snow can cause water erosion and form sand tongues on the frozen leeward slopes.
The parallel dune ridges in the southern part of Yyterinsannat have formed gradually on the beach ridges. Over the past few years powerful storms have deposited higher ridges within a short space of time. These largely wave-built ridges at the end of a shallow accumulation bay consist of very fine-grained material.
The pH of the sand in young foredunes is always over 6, but it is lower in older dunes, even if they are still growing. Hieracium umbellatum and Rumex acetosella are frequent on the dune crests and the leeward slopes. Hieracium tolerates burial in sand (Lemberg 1933: 124), but the flowers of R. acetosella are often a very dark red on aeolian sand and its numerous long stems begin just at the sand surface. When only a small amount of sand is accumulating, R. acetosella forms numerous rhizome-like roots with new shoots (Lemberg 1933: 66).
Festuca ovina is frequent on dry, trampled dune fields. It is most frequent when the sand surface begins to stabilize, but cannot grow through a very thick sand layer. In loose sand the tufts are widely spaced. In the middle part of the Kalajoki dune field, where marine erosion has destroyed the former foredune, F. ovina binds low dune mounds (15-30 cm high) on the top of a coastal cliff. These mounds are elongated in the direction of the shore and they have gentle windward slopes and steeper, shorter leeward slopes. On the Hanko Peninsula F. polesica and Coelocaulon aculeatum had spread to the crest of a foredune during a long period of stable conditions, but new, thick blowover sand beds have now almost wiped out this rare lichen.
Besides the above species, Festuca rubra, Equisetum arvense, Achillea millefolium, Calamagrostis epigejos, C. stricta, Poa pratensis and Tanacetum vulgare are quite often found on the leeward slopes of foredunes. According to Vartiainen (1980), the last of these is very frequent on the sandy shores of islands in the northernmost part of the Bothnian Bay. Ceratodon purpureus has often spread to the leeward slope of the youngest foredune, as have Salix bushes, and sometimes there are also saplings of pines and alders. A ribbon of high alders and birches may be seen growing in the shelter of the foredune at Monäs, while in the northern part of Yyterinsannat pines and junipers are growing on the steep dune cliff.
In addition to the species that occur on the foredune ridges, Rumex crispus, Vicia cracca, Valeriana sambucifolia, Epilobium angustifolium, Artemisia vulgaris, Deschampsia flexuosa, Carex nigra, Polytrichum piliferum, P. juniperinum and Cladonia lichens are found growing in depressions between the ridges. Sometimes the bottom of a dune slack is covered by a fairly continuous Deschampsia - Ceratodon community. Very thick Lathyrus japonicus communities are found in the dune field of Vattajanhietikko, and Galium verum and G. palustre as well as Erigeron acer occur in the dune slacks of Yyteri.
The dune slacks on prograding beaches are usually formed as a result of wave erosion followed by aeolian accumulation (Gares & Nordstrom 1988; Jämbäck 1995: 24), so that they should not be regarded as blowouts formed by deflation.
Foredunes trap aeolian sand so effectively that the older dunes do not receive any new material from the beach. The soil becomes poorer and more acidic, but at the same time the amount of organic material increases. The lyme-grass becomes lower, more widely spaced and sterile. The pH of the sand in this zone is below 5.9. The lyme-grass suffers from a lack of nutrients on the one hand, and from increasing amounts of pathogenic micro-organisms on the other (cf. Woldendorp 1996). These harmful factors are eliminated at times of aeolian transport. Plant diseases are more common under deteriorating conditions. Black spots caused by infection with Ustillago smut fungi can be seen in old lyme-grass communities (cf. Lemberg 1933: 52). Mosses, lichens and widely spaced herbs spread to bind the sand surfaces as the lyme-grass dies off, and these plants together with the decomposing stems of the lyme-grass and the dark clods bound by fungal hyphae give a grey colour to the dunes. At this stage erosion by wind and water has often lowered the dune ridges to form mounds or a gently undulating sand field.
The sand surfaces of older dunes are usually more water repellent, which increases erosion by overland flow. This could at least partly be due to hydrophobic fungal hyphae, as the dune plants usually have mycorrhiza (Rozema et al. 1986). The water repellency of a sand surface increases markedly with the organic material content (Dekker & Jungerius 1990), and at the same time the structure of soil becomes more tolerant of deflation. Wallén (1980) has demonstrated on the dunes of Southern Sweden that the production of organic material in a one-year-old dune is much greater above the soil surface than beneath it. This situation changes after a few years. In a ten-year-old dune the biomass production is already six times greater beneath the soil surface than above it. The water drops flow away from the surface, which is bound by fungal hyphae, towards the depressions, in which the hydrostatic force increases infiltration. Hydrophilous algae thrive in such places and may also increase infiltration, so that the water repellency of the soil surface varies from place to place, which in turn affects the topography. Especially after long droughts in summer, water erosion can be very powerful.
Loose sand grains are always exposed to splash erosion by rain drops, and sheet wash can lead to levelling of the topography on older dunes. The power of erosion is considerably smaller on moss-covered surfaces. The erodiblity of slopes depends on the consistency of the plant cover. The depressions are filled and the inclination of alluvial lower slopes is typically less than 6o (Jungerius & Dekker 1990: 193). Sand tongues of varying sizes are already common on the leeward slopes of the youngest foredunes (Fig. 40).
The smoothing down of parallel dune ridges has been most obvious in the areas of Karhuluoto (Yyteri), Letto (Kalajoki), Tauvo and Marjaniemi (Hailuoto), where the fine grain-size of the loose dune material has increased the effectiveness of erosion. At Marjaniemi grazing has totally prevented foredune formation at times (Alestalo 1986: 153). In the southern part of Yyterinsannat the dune ridges have been smoothed rapidly when powerful autumn storms have filled the slacks with sand. Thus the factors which cause the smoothing of dunes are water erosion, trampling, grazing, storms and material of an easily erodible grain-size.
Ceratodon - Leymus communities and further from the beach both Deschampsia - Honkenya - Ceratodon and grey Ceratodon - Cladina communities are frequent on the intermediate dunes. Mosses usually spread to the dune slopes prior to lichens, the pioneer species being Ceratodon purpureus, Polytrichum piliferum and P. juniperinum, which tolerate partial burial in sand. Pohlia nutans, Bryum spp. and the liverwort Cephaloziella often appear on sheltered, stabilized surfaces.
The first lichens to appear usually belong to the genus Cladonia, the most frequent of them being C. cornuta, C. gracilis ssp. turbinata, C. deformis and C. botrytes. The two last-mentioned species occur on humus and rotting wood (Ahti 1981: 33), so that their occurrence on dunes can be explained by their growing on dying mosses and remnants of lyme-grass. Other lichens that frequently occur on intermediate dunes are Cladonia uncialis, C. fimbriata, C. cervicornis and C. pyxidata. Sometimes Placynthiella uliginosa, a dark crustose lichen growing abundantly on mosses is the first to appear, or else the tin-like Streocaulon, that thrives on bare sand. Coelocaulon, which occurs on the Hanko Peninsula, has been mentioned above. The lichens of the subgenus Cladina appear slightly later than those of the subgenus Cladonia. C. rangiferina and C. arbuscula often occur with species of the subgenus Cladonia. C. stellaris grows slowly and is frequent only on some intermediate dunes at Vattaja. Cetraria species, especially C. ericetorum, often grow along with Cladina lichens. The northern species Cetraria nivalis is found at the edge of a lichen-rich forest in the southern part of Yyterinsannat. Grey dunes with a lichen cover can be formed only in places where no trampling occurs. The thin crust formed by lichens and mosses on a sand surface is very easily broken during dry seasons.
The amount of Honkenya and Leymus decreases and they linger and often become sterile on intermediate dunes, while Hieracium umbellatum and Festuca species become more frequent. Rumex acetosella and Salix bushes often occur, too. Deschampsia flexuosa becomes more abundant as the dunes become older, and eventually it replaces the Festuca species. Empetrum dwarf shrubs appear later.
The fungal hyphae that bind sand grains together, Ceratodon, Polytrichum and Cephaloziella mosses and Placynthiella and Cladonia lichens play an important role in the stabilization of open sand surfaces caused by wind and water erosion. An algal crust (especially of Cyanobacteria; cf. Pluis & De Winder 1990) may be formed in damp depressions.
Deflation surfaces and hummock dunes
Deflation surfaces vary in size from broad flats to minor blowouts, and tend to be covered with coarse grains as the wind transports the finer material away. These coarse grains protect the surface against further erosion. Deflation may lower the sand surface to the level reached by capillary rise from the groundwater table, whereupon the soil moisture binds the sand surface and it begins to be covered by plants (cf. Seppälä 1984: 47-48). Thus deflation surfaces can be divided into dry and moist ones, since the soil moisture clearly affects the plant cover. Broad, flat deflation surfaces are common behind the foredunes, and these surfaces are dotted with small hummock dunes and having their landward edges marked by old, higher dunes, usually covered by forest.
Many wide deflation bowls elongated in the direction of the shoreline are found at Yyteri, some of which have a dry bottom and are still active, like the one on the proximal side of the Keisarinpankki dune. A still broarder, continuously growing deflation bowl lies seaward of the holiday village, and landward of this surface the cover sand layers have turned the forest floor white. Lyme-grass, growing on the cover sand, has assumed a lingering aspect in the past few years, showing that the rate of deflation has decreased in this area, probably because of the foredunes, which have grown higher and provide more shelter. The deflation surfaces which have reached the groundwater table can be detected from a distance by their dark moss cover. Trampling increases deflation considerably (cf. Skytén 1978: 44) and reduces the number of plant species present. As the deflation bowls have grown larger, table-like erosion remnants have been left between them (Fig. 51).
The wind has eroded a flat deflation surface in the esker slope of the Hietasärkät dune field at Kalajoki, but this surface is shielded against continued deflation by the coarse material. As evidence of gradually progressing deflation there are minor bluffs (5-10 cm) and table-like remnants bound by mosses and Festuca ovina. Deflation has made the dune hummocks higher at the upper edge of this flat surface, and a buried soil layer under the hummocks has become visible (Heikkinen & Tikkanen 1987). These Leymus-hummocks are separated by wind-scoured gaps.
At Lappohja a deflation surface has been formed on the distal slope of the First Salpausselkä end moraine, and small hummock dunes have accumulated on this slope. At Syndalen trampling has caused the formation of active blowouts on the dune slopes, the bare sand supporting rows of Carex arenaria, one of the plants that benefit from trampling and tend to die out in a thickening vegetation.
Leymus, Honkenya, Rumex acetosella and Hieracium umbellatum spread one by one to the loose sand of the most severely trampled surfaces. In any case widely spaced Leymus and Honkenya occur as relics further from the beach. Festuca ovina is also very tolerant, and the grasses that dominate on dry sandy surfaces take the form of Festuca ovina - Leymus and Deschampsia communities. F. ovina is abundant at Koppana, at Hietasärkät in Kalajoki and on the gravelly fields on the proximal side of the Kommelipakka dune at Vattaja. Elsewhere in Vattajanhietikko, Tauvo, Vexala and Yyteri the dominant grass is Deschampsia flexuosa. The best developed Deschampsia meadow is to be found in the Herrainpäivät area of Yyteri, where there are no vast deflation surfaces landward of the foredune but gently undulating aeolian sand beds instead, the grain-size parameters of which are similar to those of the foredune.
Further from the beach the soil becomes more acid, and the phosphorus content in particular decreases. At the same time Empetrum becomes more frequent. In this way a dune meadow (Deschampsia - Empetrum - F. ovina community; pH 5.5-5.7) changes into a heath dominated by dwarf shrubs (pH 5.2-5.4). Scattered junipers and saplings (Pinus sylvestris, Betula pubescens, Alnus incana) are found growing on these meadows and heaths.
In Yyteri Deschampsia has gradually replaced Festuca rubra. It is common for this to happen in the course of time and further away from the shoreline. Deschampsia does not tolerate trampling as well as does F. ovina, which frequently occurs in grazed areas, even on moist surfaces and on very fine-grained material, e.g. the Marjaniemi dune field. It apparently profits from grazing.
A characteristic species of the dry dune meadows of the Hanko Peninsula is Festuca polesica. These meadows are dotted with the pink flowers of Thymus serpyllum, the dark-coloured lichen Coelocaulon aculeatum and the yellow Galium verum nearer the shore. Cetraria, Stereocaulon and Cladina lichens are also found in other dune fields. Dwarf shrubs form green patches and are frequent further from the beach, first Arctostaphylos uva-ursi and then Empetrum. Both of these species have secretions in their leaves which prevent the germination and growth of other plants (see Vanha-Majamaa 1993), a means of competition that is evidently useful in an infertile habitat.
Mosses grow between patches of dwarf shrubs, and Empetrum - Ceratodon communities are frequent. The pioneer species Ceratodon purpureus, Polytrichum piliferum and P. juniperinum cover the dry deflation surfaces and also tolerate partial burial in sand. Racomitrium canescens then grows over these pioneer species (Lemberg 1933: 78) and may gradually form a more continuous moss carpet. Vaccinium uliginosum and Leontodon autumnalis are found only on the northern dune fields.
On damp sand surfaces such as those of Letto in Kalajoki, Festuca rubra is more frequent than the other Festuca species, mosses and Juncus species, especially Juncus balticus with horizontal rhizomes, are usually found on the damp heaths in addition to Empetrum. The moist surfaces are almost always covered consistently by mosses. Alongside the species found on dry heaths (especially Ceratodon and Polytrichum juniperinum), the sand between the Empetrum patches is covered by the dark Cephaloziella divaricata, and later often by the crustose lichen Placynthiella uliginosa. Salix bushes are also frequent on damp surfaces (Empetrum - Salix repens community).
Salix repens replaces Empetrum in places which are regularly flooded at times of high water level (Salix repens - Ceratodon community), while Carex nigra and Drosera rotundifolia often grow in the damp parts of dune fields and Myrica gale occurs on some shores. The damp depressions of the dune fields are quickly invaded by forest, usually birch thickets. The wet coastal flat of Ulkonokka, on the Tauvo Peninsula is partly covered by a Carex meadow. Real oasis vegetation types are excluded from this work.
The flora of the damp sand surfaces depends on the topography, so that Salix repens, for example, occurs only in the areas where the groundwater table is near the surface. Damp coastal plains emerging through land uplift are more like salt marshes than deflation surfaces, and their moist soil surface, cohesion between small sand grains, impurities in the sand and rapidly spreading vegetation reduce the effect of deflation.
Leymus, Empetrum, Salix phylicifolia and S. repens in particular bind hummock dunes on flat deflation surfaces. Leymus hummocks occur on the driest surfaces, on the seaward parts of surfaces and further from the beach, in connection with old dune ridges. These hummocks usually vary in height between 0.5 and 1.5 m and in diameter between 1.5 and 4 m. In a later phase of development, these hummocks bound by lyme-grass may be invaded by Empetrum, which can also bind hummock dunes by itself, although such hummocks are usually lower, only 10-50 cm high and 0.5-4 m in diameter. There are higher Empetrum hummocks on the Koppana beach at Oulunsalo. Hummock dunes can grow together and form complexes, and mosses and lichens will grow on older hummocks, too.
Salix hummocks are formed by a single plant, e.g. Salix repens, which binds small sand mounds, about 20 cm high and 2.5 m in diameter. Higher Salix bushes bind higher dunes, but usually not more than 1.5 m high and about 4 m in diameter. Juniperus communis hummock dunes are frequent at Yyteri, where sand accumulates around the bushes and the lower branches are buried in sand. As a result these junipers are often partly dead.
The sand of the hummock dunes is less acid and contains more nutrients than that of the surrounding deflation surface. The microenvironment of the loose sand of the hummocks differs from that of the surrounding surfaces, which are often stabilized, damp and covered by mosses. In addition to the plant that binds the hummock, the species that usually occur on the hummock dunes are Leymus, Empetrum, Festuca spp. (especially F. ovina), Rumex acetosella, Hieracium umbellatum, Deschampsia flexuosa, Polytrichum mosses (which thrive on loose sand), Ceratodon purpureus and Cladoniaceae lichens. Viola canina is still flowering on the hummock dunes of Lappohja. Pine saplings grow on many hummocks, having apparently germinated in the shelter of a hummock and later caused it to grow further. The shadow provided by a sapling and the needle litter dropped by it serve to increase the vegetation cover. The situation on forested dunes is the opposite, however, so that the bases of the trees are often surrounded by bare sand.
A dry forest with a thick moss and lichen cover usually begins in a dead deflation surface. At the forest edge the vegetation changes rapidly and the total number of species is greater than in an open dune meadow or in a forest. Cladina - Empetrum and Empetrum - Cladina - Pleurozium communities are common at forest edge sites, and broad Arctostaphylos and Racomitrium carpets and thick Deschampsia flexuosa communities are also found. Lichens of the subgenus Cladina, Polytrichum piliferum and Ptilidium ciliare are frequent on the bare sand patches, and there are often pure communities of reindeer mosses landward of these. Cetraria and Cladonia subgenus Cladina lichens occur together quite regularly (cf. Oksanen & Ahti 1982: 286-287). Dicranum scoparium, D. polysetum and Pleurozium schreberi gradually invade the moss layer, and the herb layer is dominated by Vaccinium vitis-idaea and Calluna vulgaris in addition to Empetrum. Further away from the beach the forest becomes drier and the proportion of reindeer mosses increases again.
Old dunes are found in the distal parts of almost all recent dune fields. They are absent only in the youngest areas that have only recently emerged from the sea, e.g. the Karhuluoto area in Yyteri. These rolling, winding ridges are usually parallel to the shoreline, just like the present foredunes. The highest old dunes on the Finnish coast are precipitation ridges which have grown higher as the forests on their distal sides have slowed down the wind speed and prevented the advance of dunes (Mattila 1938: 15; Pye 1983: 541). Obvious parabolic dunes have been formed after the Little Ice Age only at Hailuoto, Siikajoki, Vattaja and Kalajoki.
A vegetationless, active slipface now remains only on the transgressive ridge of Tahkokorvanpakka (Kalajoki) and in places on the dunes bordering the open sand field of Vattajanhietikko. These dunes are quite stable nowadays, but at the beginning of this century they were advancing an average of one metre a year. The Tahkokorvanpakka dune has advanced about 200 m in all across the forested beach ridge topography (Heikkinen & Tikkanen 1987: 264), having apparently begun its migration after forest fires. The Laakainperänpakka dune at Vattajanhietikko (Tarkastajanpakka on the Basic Maps) has advanced about 150 m since the 1750's (Alestalo 1971: 116). The sand surfaces of these dunes are still largely bare. Leymus, Hieracium, Rumex acetosella and Festuca ovina are the characteristic species of the F. ovina - Leymus communities on their windward slopes, and Ceratodon purpureus, Polytrichum piliferum, Deschampsia flexuosa, Empetrum and lichens are trying to invade the steep slipfaces (12-33o). Empetrum - Pleurozium communities are frequent on the forested leeward slopes.
There are residuals of old dunes in the foreground of the transgressive dunes at both Kalajoki and Vattaja. Tall pines, young spruces, bird-cherry trees, alders, rowans and junipers are growing on these island-like dune hills. Deflation has exposed the root systems of the trees on the windward slopes and some trees have fallen as their substrate has been eroded away. Both species of open dune meadows (such as Leymus, Hieracium, Deschampsia, Calamagrostis epigejos, Festuca ovina and Poa pratensis) and ones typical of the forest flora (such as Empetrum, Vaccinium vitis-idaea, Trientalis europaea and Linnaea borealis) occur together on these dune remnants. Polytrichum species, Ceratodon, Dicranum and Pleurozium are found in the moss layer.
Of the older dunes, only Tahkokorvanpakka in Kalajoki has retained its connection with the modern beach zone, as it is continuously receiving new sand from the deflation surface and the beach. Its vegetative cover bears similarities to that of the incipient foredunes. The crest of this transgressive ridge is covered by lyme-grass, although of a stunted aspect compared with that growing near the beach. The soil acidity of the high crest of Tahkokorvanpakka (pH 5.4) indicates effective leaching, and there is very little organic material in it. Aeolian transport has eliminated the biotic factors which are harmful to lyme-grass and make the bare sand surfaces of more stabilized dunes water repellent, which causes slope erosion through increased overland flow.
Because of land uplift, the Tarkastajanpakka and Kommelipakka dunes of Vattajanhietikko are now already located one kilometre away from the shoreline, and it is unlikely that they could receive sand from the beach any longer over the intervening deflation surfaces that are partly covered by meadow and partly by forest. Even the colour of the dune sand has reverted to a yellowish shade. In the Monäs dune field the older dune has lost its connection with the beach zone only during recent decades, as pine saplings have grown to cover the deflation surface dotted by Racomitrium canescens. This deflation surface is 400 m across, and the old, winding dune 3 m high has been formed on the distal side of it. Empetrum binds the bluffs on its windward slope and its crest is covered by juniper thickets in places.
The highest dune at Yyteri has not moved much from its place of formation, and nowadays the younger dunes trap the aeolian sand. In spite of trampling, even the windward slope of the older dune is mostly covered by vegetation. A more widely spaced Deschampsia - Honkenya - Ceratodon community dominates in places, and the flora of the windward slopes also includes Pinus sylvestris, Betula pubescens, Alnus incana, Sorbus aucuparia, Juniperus communis, Rubus idaeus, Ribes alpinum, Leymus arenarius, Festuca rubra, Calamagrostis epigejos, Poa pratensis, Hieracium umbellatum, Rumex acetosella, Achillea millefolium, Sedum telephium, Equisetum arvense, Tanacetum vulgare, Artemisia vulgaris, Epilobium angustifolium, Stellaria graminea, lichens and mosses, even Pleurozium schreberi.
The vegetation cover of the very steep (40o in places) and high (17-18 m in places) leeward slope is a luxuriant, dense patch of mesic, herb-rich forest (Fig. 15). Prunus padus, Salix caprea and Picea abies are present in the tree layer, while Silene dioica, Milium effusum, Oxalis acetosella, Athyrium filix-femina and Vaccinium myrtillus in the herb layer indicate soil fertility. Besides Pleurozium, Hylocomium splendens is also abundant in the moss layer and the rotting tree trunks are covered by Brachythecium species. The Deschampsia - Pleurozium community occurring on the leeward slopes of the old dunes, at least to the south of Pietarsaari, includes Trientalis europaea, Linnaea borealis, Maianthemum bifolium and Luzula pilosa. This fertility of the leeward slopes can be partly explained by the moisture distribution on dunes. Both rains and melting snow provide more water for the leeward slope than for the windward one, while evapotranspiration is much more powerful on the sunny windward slopes. It should be noted that all the leeward dune slopes covered with mesic, herb-rich forest types consist of finer grain-sizes than the slopes that are covered with dry pine forest. A finer sand has a better capacity for absorbing both moisture and nutrients, the litter of the deciduous trees is continuously fertilizing the habitat, and the tree canopies are able to alter the microclimate. According to Sepponen (1985), the finer grain-size of the sorted forest soils in northern Finland increases the amount of nutrients.
The ecological succession of the Finnish coastal dune fields usually proceeds as follows: The Honkenya peploides communities in the upper part of the geolittoral zone are followed by Leymus arenarius foredunes. Hieracium umbellatum, Rumex acetosella, Festuca species, mosses and lichens are in the process of invading the intermediate dunes. On the more stabilized surfaces the Festuca species are replaced by Deschampsia flexuosa. As the pedological processes proceed, the dune meadows change into Empetrum hermaphroditum heaths, and then, with the growth of pine saplings, into dry Vaccinium or Empetrum - Vaccinium pine forest. Reindeer mosses are often abundant in the forest edge ecotone.
The succession has been clearly perceptible even during this research. On the other hand, Christensen's (1989) results based on sample squares studied for 45 years in Denmark do not indicate any successional trends in the vegetation, so that he claims that the zonation of the vegetation is not related to the age of the substrate but probably to ecological parameters related to the distance from the sea. The most important factors affecting the ecological and geomorphological succession at the present sites are the changes observed in soil characteristics: leaching and the accumulation of organic matter as the sand surfaces stabilize. Moreno-Casasola (1986) proposes that sand movement should be considered among the most important factors that affect the distribution of plant communities on sand dunes. It must be remembered, however, that before the recent forest invasion, the dune coasts of Finland, too, remained open for over one hundred years, and yet clear successional trends in the vegetation can be detected during this period (Figs. 31, 57) and many sand surfaces have stabilized. It was proposed earlier that competition and the age of the plant population are important factors explaining the distribution of the flora in the pine-dominated forests of Finland (Tonteri et al. 1990).
Fig. 57. A photograph of the Yyterinsannat dune field (Pori) taken by Lemberg (1933:app.2, fig. 4) in July 1927. Honkenya peploides and Festuca rubra are abundant on the smooth sand field. Compare with Figures 9 and 51.
The material deposited on the beaches by storms confuses the successional zonation. The flora preceding the storm reacts, but partly survives, and the water repellency of the sand surfaces on the intermediate dunes is also retained. This may be due to the rapid spread of fungal hyphae in the sand.
While comparing the ecological succession of the Finnish dune coasts with data from other European countries collected during the European sand dune inventory (Doody 1991), it can be seen how Ammophila arenaria becomes more frequent in the south. Corynephorus canescens is also a southern species on dune meadows, but it does not occur in Finland, being replaced by Deschampsia flexuosa. Neither do we have coastal heather heaths, as Calluna vulgaris is abundant only in the forest vegetation and on inland heaths. Empetrum hermaphroditum is the main dwarf shrub of the windy coastal heaths in Finland, and heather grows in any quantity only in the southern part of the Herrainpäivät area of Yyteri, which is now rapidly being invaded by pine forest.
In the boreal zone, Sweden and Norway, the plant communities on coastal dunes are quite similar to those in Finland (Doody 1991). Generally speaking, it is the climate which in the first instance determines the boundaries of the distribution of individual species and plant communities. When considering all the coasts of Finland, geographical location in a north-south direction is of obvious significance. The climatic factors which coincide most closely with the vegetational zone boundaries of Finland are length of the growing season, temperature sum during the growing season (threshold +5 oC) and potential evapotranspiration Tuhkanen (1980: 88).