Читать книгу Mountains and Moorlands - W. Pearsall H. - Страница 18

What has just been said about waterlogged soils serves as a useful preface to the further consideration of this subject. These soils are distinct in being anaerobic or devoid of oxygen, and this is reflected in their “sad” appearance and in the characteristic blue-grey colours of the mineral matter due to ferrous iron salts. Such mineral soils are usually called gley soils, and, in the British uplands, they almost always possess another characteristic. The absence of oxygen prevents the decay of the organic matter derived from the plants growing on the surface. Consequently, upland waterlogged soils are also normally covered by layers of peat, and these are deep wherever waterlogging has been long continued, while they are usually shallower where this condition is of more recent origin. As a result of these accumulations of peat, the mineral soil below may be stained and mottled by peaty material.

The waterlogged peats are of two main types: (i) bog-peats and (ii) flush-peats. The bog-peats are widespread, covering the majority of stable upland soils and characteristic of those of slight slope (see Pl.s 22a and 24). They fall into two topographic types: those found on concave lowland forms, valley bottoms or lake basins, which have sometimes been distinguished as basin-peats, and, in contrast, those on long slopes and gentle ridges, for which Dr. H. Godwin coined the name blanket-bog, a term expressive of the way in which the peat covers all stable features of the original surface. Strictly speaking, basin peats are part of the blanket-bog in the uplands and it is only useful to separate them because they have, at times, a somewhat different and longer history as well as differences in present vegetation.

Flush-peats are also topographically conditioned, occurring only where water from a higher level impinges on the bog surface and brings to it a distinctive supply of minerals in solution. The three main types of dissolved materials (see here) give rise to the flush types: (a) lime-rich, (b) iron-rich, (c) peaty, but the first of these types is not nowadays common in our uplands, except where the bed-rock includes limestone. It is usually marked by the presence of certain mosses, and of gasteropod shells, elsewhere absent from the upland zone. Iron-rich flushes are, however, frequent, not only in the upper woodlands, but also around the bog margins. They are usually indicated by the presence of ferric-iron deposits, either on or in the surface layers of soil or peat. Moreover, if this peat is exposed to air-drying, the red-brown colour frequently becomes widespread. Certain types of vegetation (see here) are characteristic of these iron flushes. Peaty flushes are topographically distinct within the bog area but generally only show variants of the general bog vegetation.

While the properties of the soil in wet flushes are determined largely by the inflowing water in bog soils, certain other properties commonly exist to which attention may now be directed. The development of a peat-covering not only marks a stage in the soil development but it also modifies subsequent development by acting as a blanket which insulates, as it were, the mineral soil from the plants growing on the peat surface. At first these plants are rooted and are drawing mineral matter from the soil, but they get less and less dependent on it as time goes on and the peat gets deeper, and the soil water becomes more and more that derived from rain. As a general rule, then, the vegetation might be expected to show a transition in its mineral salt requirements from eutrophic, with high demands, to oligotrophic, with low salt requirements. Two things result from this: first, a succession of vegetation types, and secondly, a resultant succession of peat types. We shall see later that these facts help us in the analysis both of moorland vegetation and of the history of moorland areas. For the moment, however, we are more simply concerned with its effect on the properties of the soils. There will clearly be a change in composition throughout the peat profile, and the amount of mineral matter present in the peat will decrease as the level rises above the mineral base. This is apparently a general rule in upland peats, though, locally, flush effects may disturb the normal sequence. It should be noted, however, that it does not usually apply to the actual surface peats. Moor-burning is an almost universal practice nowadays, and its effect is to destroy the existing vegetation, leaving the mineral matter it contains to enrich the surface peat. Similarly, any form of oxidation of the surface peat, due, for example, to drainage, must have a similar effect, for the oxidation products of the organic matter are mainly carbon-dioxide and ash—the former of which escapes to the air, leaving the ash to increase the amount of mineral matter in the residual peat. Thus the surface peat, where moor-burning is practised, commonly contains more ash than the layers below it. The table opposite gives illustrative figures from peat-profiles in different British areas.

There are, of course, other effects which appear to be associated with this distribution of ash. Thus the acidity of the peat almost always increases from the lowest levels upwards—showing a general correlation with the decreasing ash content.

It will be seen from Table 5, and it follows from the arguments used above, that typical upland peats are remarkable for the small amount of ash they contain, and when we seek to define the term bog, it is usual to regard it as referring to peat of this type supporting an extremely oligotrophic vegetation. In this use of the term a bog is mainly dependent on atmospheric water (i.e. rain) and uninfluenced by ground-water. The term bog contrasts in usage with the term fen—derived from the extensive peat deposits in East Anglia. In terms of this usage, fen-peat is characterised by its high mineral content and hence by its dependence on ground water. It usually shows signs of an abundance of lime and is always lime-saturated peat with a luxuriant and eutrophic vegetation of tall reeds and small trees, willows and alders. Peats of this type are almost non-existent in the British uplands to-day, although long ago they existed in some of the hollows where lime-rich waters accumulated in small ponds and lakes. These now often show their former character by an underlying bed of marl. Almost all these areas are now deeply buried beneath bog-peat, and only small areas of flush peat remain round the bog margins to illustrate the effects of this type of peat on the vegetation. Where the basal peats were originally calcareous the succession of peat types above usually shows a much more gradual decrease in mineral content, and the sequence of vegetation types was often different.

Table 5 ASH CONTENTS (AS PER CENT OF THE DRY WEIGHT) OF PEAT SAMPLES AT DIFFERENT DEPTHS

Finally, where a peat-profile generally shows signs of differences in botanical composition at different levels, it also usually shows differences in physical structure. The changes are due partly to alterations in the composition of the vegetation from which the peat was formed, and sometimes they may be due to changes in the conditions (of humidity or temperature or drainage perhaps) under which the peat was formed. As a general rule, however, the most important progressive change in the peat must be associated with its decomposition as it gets older. Two sorts of change are possible: partial oxidation, which occurs particularly in the surface layers, and the slower changes which can ensue in water-saturated peat from which oxygen is absent. We assume that these are mainly hydrolytic—that is, caused by the slow action of water on the organic materials present. These are the changes which are generally implied when we say that a peat is humified or that it is undergoing humification. They are thought to result in the plant-remains gradually becoming gelatinous, so that, although the peat appears to retain visible structure, it escapes as a jelly through the fingers when squeezed in the hand. In contrast, the more recent peats usually retain a firm and fibrous structure. Even when a peat bed appears, on first opening it up, to consist of more or less uniform material, the bottom layers will normally differ in the degree of humification from those at the top. One result of this is that if such a bed is cut and a profile exposed to the air, it will soon appear to consist of two different types of peat. The fibres quickly become prominent and some are exposed by weathering, while the upper layers which contain them are often more quickly oxidised and so may become darker in colour. The lower layers, however, remain as a damp gelatinous mass and show less alteration on exposure. After a period of exposure they may thus appear to be of quite different composition, although the apparent difference is really one mainly of different physical and chemical condition.