Older dunes may become colonised by a wider range of species, including several bryophytes. Later on, grassland or heathland may develop, further stabilising the dunes.

Dune slacks are shallow depressions amongst sand dunes which bear a characteristically different vegetation from the dunes themselves. They are usually wet during the winter months, but the degreeof wetness varies from site to site as well as from year to year. Some slacks are merely damp during the winter months, with their surface lying close to the water table. Others can hold standing water (as in the picture) throughout the winter and early spring.

More than 100 bryophyte species have been recorded in sand-dunes and dune-slacks in Wales. Some are listed below.

Some of the bryophytes found on sand-dunes in Wales are :

Mosses

Brachythecium albicans

Somewhat similar in appearance to the previous species, this one is smaller and tends to grow in somewhat less calcareous conditions. Its leaves are broad, but contracted abruptly to a fine point.

Bryum algavicum var. rutheanum

This is one of a very large genus of mosses, the members of which are often difficult to tell apart without the presence of capsules. They tend to be small, with an erect, unbranched stem, and leaves which are broadest at their middle. The cells of the leaves are usually large and can be distinguished with a hand-lens. This particular species is generally only a few millimetres tall, has a stem covered in tomentum below. The leaves are ovate, reddish-tinged at heir bases, and with strong nerves which project slightly from the tip of the leaf. It is probably the commonest species of Bryum on sand-dunes, where it can help stabilise relatively young dunes.

Climacium dendroides

This moss is one of the few British species with a dendroid structure - i.e. it has the appearance of a small tree (Latin: dendros - a tree), with short erect stem with branches clustered together towards the tip. It occurs in a wide range of habitats, including dunes, willow carr, and wet grassland - always where the conditions are at least mildly basic. The sparse stem leaves are broad and rounded at their tip. The branch leaves are narrower, and coarsely toothed at their apices. Both stem and branch leaves have a nerve to above mid-leaf.

Homalothecium lutescens

This is another common species on dunes in Wales, especially where they are calcareous in nature. It has a glossy golden-yellow appearance, with branched stems mostly lying prostrate, and long, pointed leaves.

Hypnum cupressiforme var. lacunosum

This is a variety of the most common moss species in Wales and Britain. It is a fairly robust form with a deep golden-brown to bronze colour. The leaves are without nerves (or with very short double nerves), and curved at the tips.

 

Rhytidiadelphus triquetrus

This is a relatively large, bushy-looking moss, conspicuously pale-green in colour. The reddish stems grow erect and have frequent short branches. The leaves are broadly triangular, with a double-nerve. A widespread moss, it grows abundantly in certain types of broad-leaf woodland as well as in dunes.

Syntrichia ruraliformis

A highly-characteristic and common species of dunes in Wales. Its leaves are arranged radially around an upright stem and have a long white hair-point at their tips. When moist it appears a bright golden-green in colour but when dry it has quite a different appearance - the leaves become twisted around the stem with the hair-points clustered together tightly, and the predominant colour is brown. It is most abundant on the unstable "yellow" dunes - those which have not yet developed a continuous carpet of vegetation.

Liverworts

Lophocolea bidentata

This is a common liverwort in many situations, of which sand-dunes are only one. It is a leafy liverwort with two rows of larger leaves on each side of the stem, and one of very small underleaves on the underside of the stem.

Scapania aspera

This is a fairly uncommon leafy liverwort in Welsh dunes, where it occurs usually amongst short-turf in stable dunes. As in other Scapania species, each of the lateral leaves is folded so that at first site the plant appears to have 2 rows of leaves on either side of the stem.

Some of the bryophytes found in dune-slacks in Wales are:

Mosses

Barbula recurvirostra

This is a rather diminutive moss with a characteristic rust-red colour to its lower stem, which contrasts with the bright green of the rest of the plant. The leaves are about 3mm long, tapering to a point, and with a continuous nerve. This species often forms smooth lawns in dune-slacks (though it also grows on walls, rock ledges, and soil.

Bryum pseudotriquetrum

This is a fairly robust member of the Bryum genus, with stems commonly 4-10cm in length. The stems are covered with a brown tomentum (rhizoids woven in a dense mat). It)occurs in a variety of wet habitats, including marshes,, fens, flushes, by streams and pools, and on dripping rocks, as well as dune-slacks. It avoids extremely acid habitats, demands at least a little mineral content to the substrate, and is tolerant of lime.

Calliergon cuspidatum

A common moss occurring in a wide variety of habitats, usually where there is some calcareous influence. In dune-slacks it often forms continuous carpets, frequently associated with willow, rushes or coarse grasses. A characteristic feature is that the tips of the shoots are shaped rather like spear-heads. The leaves taper from a wide base to a blunt, rounded tip, have no nerve, (other species of Calliergon do have a nerve) and have distinct auricles at their basal corners.

Cratoneuron filicinum

When well-grown this is a distinctive moss, with a golden-green colour, pinnately-branched stem often matted with reddish-brown rhizoids, and falcate (sickle-shaped) leaves. Often though it is poorly-grown and easily overlooked. It is common in a variety of damp, base-rich situations, including calcareous dune-slacks.

Drepanocladus aduncus

This is a rather sprawling moss, irregularly-branched and with a soft texture. Its leaves are broadest just above their base, and then taper to a fine point. They have a nerve running for about three-quarters their length. This is one of several species of Drepanocladus which may occur in dune-slacks, but is always found in conditions where there is some calcareous or basic influence.

Liverworts

Aneura pinguis

This is a thalloid liverwort that occurs in dune-slacks, fens and flushes, and other moderately base-rich habitats. Bright or deepgreen in colour, it is irregularly branched with branches concave and some 2-3 cm in length and 2-6 mm wide. Branches are thickened in the middle, with thinner margins, which may be somewhat wavy in outline. It tends to be brittle, and have a rather greasy appearance. May be confused with Pellia species, but the tips of the lobes in Aneura are entire, whilst in Pellia they are emarginate.

 

Moerckia hibernica

Another thallose liverwort, consisting of a thickened midrib bordered by delicate "wings" a single cell thick. These wings are folded to give a "rouched" appearance to the plant, which is a bright-green in colour. This is a Nationally Scarce species in the UK, and in Wales is almost exclusively found in dune-slacks (it is known from one inland site in sewage filtration beds at a a water-treatment plant.)

Petalophyllum ralfsii

This beautiful and delicate liverwort has a somewhat similar structure to Moerckia hibernica, consisting as it does of a central midrib with folded wings. Here the wings have a more translucent appearance, and are folded almost like a fan. It is found in calcareous dune slacks around the Welsh coast. It is best seen in the autumn and early spring: at many sites it is submerged in winter due to flooding of the dune slacks, and in summer it's above ground parts disappear completely. It is a rare plant in Britain and Europe, and is specially protected under the British Wildlife and Countryside Act, and under the European Habitats and Species Directive.

Preissia quadrata

This is thalloid liverwort, generally a pale green in colour with red-brown or purplish margins, and its branches up to 1cm wide. The upper surface of the thallus has hexagonal markings and at the centre of each hexagon is a small pore. This liverwort is a strong calcicole and is found on a wide variety of basic substrates, including soil in calcareous fens and flushes, stream-banks, moist or dry rocks and cliffs, on the crumbling mortar of walls, as well as in dune-slacks.

Riccardia chamedryfolia

This is a more slender thallose liverwort than the two previous species, with thallus lobes 0.3-1.2mm wide. In colour it is pale green. It is a widespread species and in addition to dune slacks it occurs on wet rocks and damp soil in a variety of habitats.