The sea has a major influence on the climate of adjacent coasts. In particular, it reduces annual temperature fluctuations so that many frost-tender plants, especially annuals and biennials, can survive where inland conditions are too harsh. Because of this, small climatic changes which appear to have little effect inland may produce striking changes in the coastal flora. Perring (1974) notes that many of the recent changes (mostly declines) in our seashore flora are probably attributable to climatic trends rather than to human impact, even though the latter is often assumed to be the cause.
Although the less hardy strandline and dune annuals are summer-growing and winter-dormant, many dune plants grow during the cooler, moister months and pass the hot summer as seed, germinating the next autumn for winter growth. Such species are especially typical of open dunes and areas of short turf subject to moderate grazing or trampling. Because of this, these areas are most sensitive to pressure during the summer, just when it is likely to be greatest.
Coastal microclimate is important mainly as it affects plant zonation (see below). In general, there is a simple climatic gradient from more to less marine in a direction away from the sea (whether inland or, in the case of cliffs, upward). Dunes and slacks are exceptional in that they show many microclimatic variations depending on factors such as aspect, slope, wind effects and the water table. This helps produce a complex mosaic pattern of vegetation and prevents simple zonation. An example is shown by creeping willow (Salix repens), which may come into leaf a fortnight earlier on hot dry dunes than in cooler, damper slacks only yards away. This is equivalent to the difference in leafing time for this species between south-western England and the north of Scotland (Ranwell, 1970, p10).
As previously described, the amount of tidal range between high and low tide affects the amount of beach exposed, and thus the sand supply to dunes. This tidal range is determined by the depth of the ocean, and the coastal configuration, thus shallow estuarine waters experience the greatest tidal range. The highest tides, called spring tides, occur when the sun and moon are in line with the earth, creating the greatest gravitational pull. This occurs every 29 days. The combination of the highest spring tides, at the spring and autumn equinoxes, and storm winds or low atmospheric pressure can produce conditions in which damage and erosion is likely to occur to frontal dunes. If damage occurs to sand-trap or access control fencing on the backshore, any repair that is possible should be done as soon as possible. Otherwise the next high tide is likely to completely destroy the damaged fencing.
Tides have a vital role to play in the formation of embryo dunes, by depositing tidal litter. This may be brought to shore in large amounts when autumn and winter storms tear seaweed from intertidal and subtidal surfaces. The litter is piled up on the backshore by the high tides of the spring equinox, and helps trap sand and provide nutrients for colonising strandline plants.
Soil water is of great importance on beaches and dunes. Pure shingle holds very little water between the stones, but with even a slight admixture of sand the ‘field capacity’ (ability to retain water) increases. This may mean the difference between unvegetated shingle and areas where plants find a roothold. With a ‘mulch’ of surface shingle or of organic material the field capacity rises further, making it much easier for plants to germinate and survive (Fuller, 1975).
In dune systems, the soil water level is the main determinant of the pattern of plant cover. Ranwell (1972) divides dune habitats into four categories based on this factor:
- Semiaquatic, with the water table never more than 500m (1″) below the soil surface. Autumn-spring floods and frequent water-logging make this habitat favourable to aquatic and marsh plants such as shoreweed (Littorella uniflora), amphibious bistort (Polygonum amphibium) and common water crowfoot (Ranunculus aquatilis).
- Wet slack, with the water table never more than 1m (3′) below the surface. Moisture is always adequate. Bryophytes (mosses and liverworts) are common and the flora is characterised by species with intermediate water requirements, with few grasses.
- Dry slack, with the water table between 1-2m (3-6′) below the surface at all seasons. In this habitat shallower-rooted species are uninfluenced by the water table but deeper-rooted plants can benefit from it in drought. Plants with deep tap-roots and grasses are especially abundant and lichens mav be locally abundant where there is rabbit grazing.
- Dune, with the water table never nearer than 2m (6′) from the surface. Most plant growth is independent of ground water and dependent on seasonal rainfall. Plants with low water requirements are common and vegetative cover tends to stay open.
Minor slack habitats include wet flushes, man-made pools and turf or peat cuttings, low hummocks, mole and ant hills and rabbit-disturbed ground. In wet slacks, banded communities often develop at slightly different levels according to the local water table and the intensity of wind erosion at the time of formation. In dry slacks, water relations are less important and community boundaries are most often determined by biological factors such as beetle attack or rabbit burrowing.
Many dune systems face a lowering of their water tables, due to the extraction of ground water for public use. This can mean the end of damp slack communities which may have developed over many years – perhaps up to a century. In Britain, manipulation of dune water regimes for conservation purposes has been limited to the digging of breeding pools for natterjack toads, but in the Netherlands there have been a number of experiments in damp slack creation and maintenance. Boorman (1977) summarises the main conclusions:
- Ground water lost to extraction can be replaced by infiltration. The infiltration water should be as mineral-poor as possible (especially in nitrogen and phosphorus) and it should be cleaned of fine sediments, otherwise it is likely to enrich dune soils with a resultant decrease in floristic diversity. Average water levels should be kept as constant as possible but with regular fluctuations in each year. Vegetation should be mown regularly in autumn and the cuttings removed to help keep the habitat mineral-poor.
- If the water table cannot be restored to its original level, damp slacks can be created by excavation or by allowing blowouts to develop naturally. The use of controlled blowouts has several advantages. The wind excavates the sand automatically to the correct depth (the ground water level). Fluctuations in this level produce local erosion and redeposition which increases natural microrelief and habitat diversity. Blowouts develop over a considerable period so that habitats of different ages result. Blowouts also increase the diversity of adjoining dry areas due to the juxtaposition of stable and unstable habitats.
- Where natural blowouts are unlikely to develop or where instability cannot be tolerated, duneslacks can be excavated by hand or machine. For botanical diversity, most of the surface should be between the average summer groundwater level and a level 700-800mm (27-31″) above the winter water level. Slopes should be very gentle (between 1:30 and 1:80) in the area up to 250mm (10″) above winter water level. Areas higher than this should have steeper slopes (1:3 to 1:5). A number of small, irregular slacks is better than a few large ones, since isolation tends to produce diversity. In a large slack, isolation can be achieved by creating low dune ridges within the slack.
Beach and dune soils tend to hold increasing amounts of moisture as they mature through the gradual buildup of humus and more extensive plant cover. But even in well-vegetated backdune communities, seedlings are very dependent on rainfall since the top inch or so of soil quickly heats up and dries out in hot weather. This poses great problems on mobile dunes and can severely limit the success of artificial seeding programmes unless mulching is included in the treatment.
Dune soils are based on sand blown inshore. Their chemical makeup depends on the material from which the sand is derived. At the extremes are the ‘white’ sands of mainly shell origin and the ‘yellow’ sands formed almost entirely of quartz grains more or less coloured by iron salts. White sands are characteristic of the rocky coasts of the west of Britain, since these provide good shellfish habitats. Shell sands can build up into steeper dunes than quartz sands, having a maximum angle of rest of about 42° as compared to 25° (Ritchie, 1972, p22). With a good growth of marram (Ammophila arenaria) the slope can be held at over 45° whatever the sand type (Nature Conservancy, 1969), although this may not be advisable from a management viewpoint.
On dunes, salts are quickly washed out of the soil. The situation is one of general mineral deficiency except on shell sands which are highly calcareous in origin and where calcium levels may remain high. Yellow sands are mineral deficient to begin with, and intermixed sands, which occur in every grade, tend to become deficient quite early in their development.
The nutritional impact of salt spray is clearly visible at the strandline and foredunes or where, as on the Isles of Scilly, low cliffs of acid soil develop an acid heath vegetation except in the spray zone where a narrow band of grassland predominates.
Beach soils may also be salty, but it is the lack of humus rather than salinity which causes real problems. At first the only organic matter consists of sea drift, mainly seaweeds thrown up by storm tides. Once plants establish themselves, their dead remains help build up the soil’s organic content. In places favoured by terns or other socially ground-nesting birds, their excrement, waste food and dead chicks provide the nutrients for a more luxuriant vegetation.
Beaches and dunes are low in productivity. This is true even where the soil is calcareous, since base-rich soils are not necessarily nutrient rich, and nutrients essential for plant growth are easily leached out of dune soils. Nutrient poverty in dunes favours a great diversity of plant species since it restricts the growth of coarse, tall plants and allows many smaller species to flourish.
The general low level of nutrients in dune soils does not, on the whole, affect management. But it does seem that calcareous dune vegetation is rather more resistant to trampling than that of acid dunes. In some cases it may help to mulch and fertilise dune soils to raise nutrient levels and improve water retention and texture (see Chapter 7 – Vegetation Establishment). This process occurs naturally as the soil ages, although the buildup of organic material tends to be restricted to the soil litter layer, below which leaching continues. Where old dunes weather, the nutrient-rich surface breaks down and is mixed with the subsurface sand to counteract, locally, the progression towards more impoverished conditions.
One important side-effect of artificially fertilising dune soils is that it may lead to the dominance of those plant species which can best respond to higher nutrient levels, so that the diversity of species decreases. In addition, local nutrient variations, which are important for species and habitat diversity, are evened out by any overall increase in soil fertility (Boorman, 1977, pp171-2).
Nutrients which leach from the dunes tend to collect in the intervening slacks. Where the slack soil is well aerated, plants can make use of the nutrients but where waterlogging occurs, peat builds up and acid bog vegetation takes over. If the dune then erodes to dry slack level, you may find a mixed situation in. which shallow-rooted plants of acid soil grow side by side with deep-rooted plants tapping the more base-rich soil below (Ranwell, 1972, p162).
Ecological succession is the process by which plant communities tend to create conditions which eventually allow new species to invade. The dynamic nature of a sand dune system means that many if not all the stages of the succession can be seen at any one time, and changes can be seen to take place in only a few years.
Succession on dunes is mainly the result of the buildup of soil humus under increasingly sheltered conditions, combined with leaching of nutrients. Dune succession has such important effects on dune physiography, and on management, that it is worth describing in some detail.
Dune succession may be initiated by strandline plants which trap small amounts of sand during the growing season.
Strandline plants may include sea rocket (Cakile maritima), common orache (Atriplex patula) and prickly saltwort (Salsola kali). But such tiny dunelets are usually overswept and destroyed by winter storms. The formation of dunes which are relatively long lived and capable of growth and extension depends on the invasion of the strandline flora by one of the perennial grasses capable of trapping wind-blown sand and of growing vigorously through it.
Sand couchgrass (Elymus farctus syn. Agropyron junceiforme) usually initiates foredunes or ’embryo dunes. It is well fitted for its pioneer role because it withstands short immersions in salt water and can thus grow within reach of high spring tides. Sea sandwort (Honkenya peploides) is often associated and sometimes produces miniature dunes by itself.
Sand couch dunes tend to remain low and flat-topped. Once the sand is raised above the reach of even the highest tides, marram grass (Ammophila arenaria) usually invades and becomes the most important dune-forming species. Its dunes tend to be high and dome-shaped. Where sand couch is absent, marram may form the foredunes as well as main dune ridges. Where marram is absent, as in some of the sandy bays of the west coast of Ireland, sand couch continues to dominate and the dunes remain low.
Sea lyme grass (Leymus arenarius syn. Elymus arenarius), the third British dune-building species, is important in the North. All three species may be used in dune stabilisation programmes, although marram is most versatile and usually most common. These plants are described further here.
Once marram or, more rarely, sea lyme grass initiates main dune., formation, other species begin to colonise the loose dry sand between grass clumps. The community remains open, however, and as the grasses do little to fix the sand surface the dune remains unstable if exposed to strong winds. The dunes are known as ‘vellow’ or ‘white’ dunes, from the colour of the unvegetated areas ot sand. Typical species of these mobile dunes include sand sedge (Carex arenaria), ragwort (Senerio jacobaea), hawkweed (Hieracium umbellatum), creeping thistle (Cirsium arvense), sand fescue (Festuca rubra var arenaria), sea holly (Eryngium maritimum) and sea bindweed (Calystegia soldanella).
Where new dunes form in front of an existing dune, it becomes less subject to wind erosion. At this stage sand-fixing as opposed to sand-trapping plants invade and cover the surface in a vegetative mat which resists further disruption by the wind. Mosses and lichens raise the humus content of the soil and create a carpet through which sand is unable to escape. It is the Cladonia lichens which give fixed dunes their characteristic grey appearance, and the name of ‘grey’ dunes. Sand sedge often replaces marram as the dominant species, since marram loses its vigour and degenerates once it is cut off from fresh supplies of blown sand. Sand sedge may grow thickly enough to form a loose turf, accompanied by such species as sand fescue, wild thyme (Thymus serpyllum) and birdsfoot trefoil (Lotus corniculatus). In the very wet climate of the west of Ireland, where the main dune-builders are often absent, any creeping plant which roots at the nodes can fix sand on a limited scale to help form a sand plain. Under these conditions, creeping buttercup (Ranunculus repens), creeping thistle and silverweed (Potentilla anserina) are among the most prevalent sand binders.
Additional plants then colonise, depending largely on the dune’s soil chemistry. Sand comprised mainly of sea-shell fragments and other calcareous material tends to result in a species-rich grassland, similar to that on chalk or limestone. Species may include restharrow (Ononis repens) and viper’s bugloss (Echium vulgare), with marsh helleborine (Epipactis palustris), wintergreen (Pyrola rotundifolia) and creeping willow (Salix repens) in the damper dune slacks.
In contrast, sand dunes made of silica sand, or very old dunes where the lime has been leached from the surface, result in acidic conditions, with heather (Calluna vulgaris) often dominant.
Where dunes are fairly stable, ranker grasses and eventually scrub species may seed in, shading out much of the interesting low-growing vegetation. Some of the invading scrub species, notably sea buckthorn (Hippophae rhamnoides), are capable of fixing nitrogen in their roots so that they leave a much-enriched soil if they are later cut down. This means that nitrogen-demanding plants such as nettle (Urtica dioica) and elder (Sambucus nigra) may spring up in the clearings instead of the earlier more diverse dune communities.
At the landward edge of the dunes, succession is determined as much by management practices as by changes in soil composition. Where grazing predominates, dune grassland may persist. Where the soil is very acid, dune heath develops, often dominated by heather. Orchids are an interesting feature of backdune areas. Scrub or woodland is typical where the climate is not too wet and where grazing does not prevent the establishment of seedlings. The effects of wind and salt spray may remain noticeable, especially on trees which may show extreme wind pruning.
This picture of dune succession may be affected at any time by the renewal of erosion and the exposure of bare sand. On some dune systems, cyclic alternations of erosion and accretion may effectively prevent unbroken succession for a very long time (see above).
The presence of slacks between dune ridges further complicates matters since in these areas succession is determined largely by whether or not the soil remains waterlogged and develops increasing acidity. If it does, then rushes (Juncus spp) and bog-moss (Sphagnum spp) are liable to dominate and acid bog conditions will result. Where the water table is lower and the soil remains at least seasonally aerated, shrubs such as creeping willow are likely to seed in, often spreading to surrounding dunes where they grow up through moderate amounts of blown sand.
At the early stage in the development of vegetation on the foredunes, animal species are not noticeably abundant, though butterflies, hoverflies and moths may be seen feeding on the nectar of flowers. Spiders and insects are the dominant animal species, and many are highly mobile and roam over large areas. Characteristic species include the sandwasp (Ammophila) which prey on caterpillars with which to stock their ‘larder’, a hole dug in the sand. Grasshoppers may be heard chirping in the tall marram grass, with shield bugs on the open sandy south facing slopes. Away from the foredunes, the number of species increases greatly. Dragonflies and damselflies may be found in the damp slacks, and earwigs live in the leaf litter of the mature dunes. Beetles are abundant, including the ‘click’ beetles, which can be found in June and July on the mature dunes. The ‘click’ is the sound made as they fling themselves in the air as a protective device to escape from predators. Sometimes dunes are subject to plagues of insects which have migrated or been blown over large stretches of water.
Although dunes are too open and unstable to provide food, shelter or nest sites for many birds, in the summer months skylarks, meadow pipits, linnets and stonechats may be abundant. Likewise, mammals are restricted by the lack of cover, although the soft sandy soils and short turf are suited to rabbits. Although they can cause damage by burrowing, their grazing is an important factor in maintaining the short turf and consequent diversity of plant species. Their populations vary from year to year and place to place according to the mortality from myxomatosis, the disease introduced in 1954 to control their population. Where rabbits exist, predators such as foxes, weasels and stoats may be attracted onto the dunes.
In wooded dunes, grey or red squirrels may be resident, with voles, woodmice, house mice and common shrews where there is suitable cover. The soils are mostly too thin and dry for earthworms, so moles are not abundant.
Sand dunes are nationally important sites for reptiles, notably sand lizards and natterjack toads, for which special management may be required. These species are described here.