This section is from the book "British Wild Flowers - In Their Natural Haunts Vol5-6", by A. R. Horwood. Also available from Amazon: A British Wild Flowers In Their Natural Haunts.
The plants which are found on heaths and moors are those that require humus in a sour (or acid) and free state. The soil is barren and not rich in nitrogen that can be readily assimilated, and consequently unsuited for cultivation. The soil is not rich in lime, and in this respect differs from some marshes, but is similar to that of bogs, which indeed are usually interspersed amongst heaths and moors. But there are in some cases upland marshes which are comparatively rich in lime and peat. The soil of a bog or fen on deep peat, however, is acid, not alkaline in reaction.
The raw humus covers a sterile subsoil of diverse character. Of ericetal species there are about 80, of which we describe 29 here. More than all other plants, even on wet soils, heath plants are xero-philous, and we see in them adaptations to physiologically dry conditions. Heaths are exposed to the wind like halophytic vegetation, and few trees grow on such tracts, except in the low moors. Many marsh plants grow there also. The adaptations to xerophytic conditions include a felt of hairs on the under surface of the leaf, as in the Creeping Willow, and this serves the purpose of keeping open the pores or stomata on the leaves, and at the same time depresses transpiration. Papillae surround or project over the pores for the same reason in sedges. A coating of wax covers the leaf in the Rosemary and Cranberry (see last section). Many stems and leaves are exceptionally thick, and the above are also sclerophyllous. Filiform or threadlike leaves are developed. Heaths, Ling, and Cotton Grasses are comparatively all but leafless. Some leaves present their edges to the light, as in Iris and Asphodel.
The adaptation to dry conditions is connected with the soil characters. The soil is physiologically dry. There are, however, some genera which include marsh or bog species, and others that require a medium supply of moisture, that do not grow in dry places, with the broadest leaves, which is not what one would expect. Galium palustre has narrower leaves than G. Aparine, a mesophilous species. Moreover, some heath plants can grow on dry warm soil and on cold wet soil, as Ling. Crowberry, etc. This suggests that there is some correspondence between the two types of soil, and that some of the factors of life in the case of marsh plants necessitate an economy in the use of water. There is a transpiration optimum, and marsh plants may have to depress their transpiration.
1 Of. Section X, where moors are dealt with in some detail in discussing bogs and marshes also. It is the wetter types of moors that are specially referred to there.
A wet soil is cold, the roots can absorb no water if the temperature of the soil sinks below a certain degree, and the soil is thus what is called physiologically dry. This is seen in the later character of heath and moor or marsh plants, and in the fact that the flowers are in bloom much later than on dry soils. The plants are clothed with hairs, therefore, to prevent an excess of transpiration over absorption, and this is accelerated by high winds. Respiration is affected owing to the soil being badly aerated or lacking in oxygen, so that the activity of the aerial parts must be the less, owing to the less amount of oxygen absorbed by the roots by marsh plants. Hence the ability of Heaths that grow on heaths and dry warm soil to grow on moors, as a heath is just as badly aerated, with often periodically soaked raw humus or dry peat. Peat retains water more than other soils except clay. The cause of physiological drought here may also be due to the abundance of humous acids, etc., in moor soil or peat, which affect the roots and prevent or deter absorption, so that the plants wilt if transpiration is rapid.
Though many plants of heaths and moors exhibit the above xero-philous characteristics, others are hydrophilous in character, and so arc the adaptations to which they give rise in the flora.
Transitional from marsh associations is the low-moor formation, which is characterized by humous acids in the soil containing vegetable accumulations, forming peat with Reeds, with much nitrogen. The water contains calcium and potassium compounds, and is thus like a marsh, often enclosing a marsh or adjoining it.
Sedges grow in tufts, giving rise to sedgemoor, and other plants are Cotton Grass, Rushes, Arrowhead, Helleborine, Angelica, Bog-bean, Marsh Bedstraw, Marsh Willowherb, Grass of Parnassus, and Willow, Birch, Alder, Heaths, etc.
Several associations can be recognized, as amblystegiata (from a moss), cariceta, eriophoreta, molinieta, junceta, etc. There are usually herbaceous perennials and a ground flora of mosses forming two layers below the former. Few are ligneous, and only a few are annuals, as Rhinanthus. The flowers bloom late because of the cold atmosphere. Most plants are dense, tall, and tufted, a few are runners, such as Sedges and Bogbean, and Mosses may predominate.
Grass-heath may be formed by Matgrass, Molinia, Sedges, Sweet Vernal Grass, Bents, and Ling. The soil is dry and not deep.
The high-moor or heather-moor formation is characterized by the occurrence of bog mosses, and it is here that bogs are distributed amid the drier-soil Oxylophytes. The soil is moist and the air damp, and the moisture of the sphagnum moor is derived largely from this last and dew. The high-moor may follow a low-moor formation. Like some bogs the high moor is poor in lime salts, and the peat contains little assimilable nitrogen, phosphorus, or potassium compounds, Sphagnum not being calciphilous.
A sphagnum moor contributes to the steady descent of water, the mass exhibiting great capillarity. The plants die and fresh layers arise above it, forming a thick layer of spongy peat below, acid in nature. Peat is encouraged only by the presence of moisture. The moss rises high in the middle, and gradually grows at the margin.
The soil formed is loose, and the species it nourishes have travelling shoots. Typical of this formation are Sedges, Cotton Grass, Silvery Hair Grass, Bog Asphodel, Arrowhead, Whortleberry, Cranberry, Rosemary, Cross-leaved Heath, Ling, Cloudberry, Sundew, Red Rattle, Dogwood, Bog Myrtle, Creeping Willow, and Sedges, Cotton Grass, Ling, Birch, etc, form special associations.
The plants that produce peat are chiefly Bog-moss, Hair-moss, Bulrushes, Cotton Grass, Heath, Ling, etc.
Forest moors originated from pools or lakes in forest regions, and exist now where the forests have disappeared as high-moor formations, and contain clay formed during the Ice Age.
Following a tundra formation open forest arose, and moors were formed, with Birch and Pines, and Oak came to form high forest, then Beech.
There are notable differences between the low and high moors. The former have the surface covered with water, in the latter the plant subsists on moist soil or above water. Low moors have a flat, high moors a convex surface. The former is characterized by Sedges, Grasses, Rushes, Hypna; the latter by Bog-moss and Heath. A low moor is relatively rich in lime, a high moor is poor in lime. The peat of the low moor is black, and one cannot recognize the included remains; that of a high moor is light, and animals and plants in it are well preserved. While the peat in the former is heavy, rich in mineral salts, the latter is light, with few mineral salts. Low-moor peat is greasy and wet; high-moor peat is dry, and conducts acid salts well. On a low moor the soil is rich in flood material, on a high moor it is poor; and there are few fungi in the first, many in the latter. Myco-rhiza and carnivorous plants are rare on low moor and common on high moor. The high moor is obliged to depend for moisture on the atmosphere.
A sort of lichen-heath develops in which Empetrum, Birch, Heath, Juniper, etc., occur, and Carex, Hair-grass, Mat-grass, and Rushes also occur, and in their absence Lichens.
The dwarf-shrub heath is treeless, with dwarf evergreen shrubs, mainly ericaceous and small-leaved, stunted, and xerophilous. The temperature is usually low, and the atmosphere not dry. The soil is a quartz-sand mainly reconstructed during the Ice Age. Over this there is a layer of humus where Ling and Whortleberry form a thick dense scrub of not more than 1 ft. high, forming heather-peat or raw humus, acid and inimical to ordinary forest mild humus plants. Heather is the dominant plant, and is associated with lack of nutrient substances and a low temperature.
Many plants are decumbent or prostrate, being frequently windswept, and the shoots are curved and brittle. Ling, Empetrum, Cross-leaved Heath, Broad-leaved Whortleberry, Thyme, etc., and of the pinoid type Juniper (deciduous, thin-leaved plants), Broom, Furze, are typical. These plants form the chief food of game, whose distribution is regulated by their occurrence.
Several associations are made up by the dominant species, as Callunetum, Ericetum Ericae Tetralicis, Myricetum, Myrtilletum.
On the heaths and moors we find the pretty deep-blue Milkwort growing in tufts, as radiant as some alpine flower. Grassy Stitchwort links its life with Furze or Broom. Tormentil, with its straggling flowers, forms tufts amid the tussocks. Spread over the turfy sward the creamy flowers of Heath Bedstraw betray the sterile soil. Cat's Foot is rare, and found on the lonely heath. Wall Hawkweed, with many another of this group (of which there are some 200 species), grows on the bare patches near the woods. Sheep's Bit Scabious on rocky heaths or hills chooses a hollow, in which its blue tassel-like blooms, with the graceful pendent bells of the Hare-bell, reflect the colour of the sky. The heaths, too, are clad in a wide mantle of Whortleberry, Cross-leaved Heath, Crimson Heath, and Ling. Where Furze grows the Dodder trails on it, sponging as a parasite, as do Eyebright and Red Rattle, and Common Sylvan Cow-wheat on roots of grass. In the hollows and moist places on the heaths a strong aroma is scattered by Pennyroyal. Creeping Willow, like the Heath and Ling, spreads far and wide. Wood-rush sometimes strays here from the woods. All over the moors and heaths the nodding heads of the Meadow Rush wave in the vernal breeze. Here the skylark is singing overhead, the sky is blue or grey, and all conspires to raise the soul to the tune of Heaven's high caroller. The early Sedge, with its scent of musk, and green-ribbed Sedge, Small Bent Grass, Heath Hair Grass, all wave their slender, graceful panicles backwards and forwards with the infinite rhythm of the wind, and here and there dense tufts are formed by the Matweed.