скачать книгу бесплатно
The principal biological effect of temperature is that it greatly affects the rate of biological processes. Thus a lowering of temperature such as would be experienced at a higher level would retard growth and development so that there would be less likelihood of a given developmental process being completed within the shorter period available in a montane summer. Some upland organisms do in fact appear to take longer over a given process of development. A well-known case is that of a moth, the northern eggar (Lasiocampa callunae), which spends two years in the larval stage instead of the one characteristic of the original woodland race, the oak eggar (L. quercus). It is unlikely, however, that the difference is due to the lower temperature of the upland habitat. To double the period of development, or to halve the rate of development, would require a reduction of temperature of about 7·5° C. or 14° F., equivalent to an increase in altitude of about 4,500 ft.! The lengthened larval period may be just too long to fit into one growing season, but it seems more likely that the change in the length of the life cycle is either genetical or mainly due to nutritional differences imposed by the moorland habitat.
There are, of course, other ways in which lower temperatures may affect distribution. Where two organisms are dependent on one another for success, but possess life-cycles of different duration, an alteration in temperature may put the two life-cycles “out of step” with one another, as it were. A case which might involve something of this nature is one in which an insect mined or fed on a plant organ at some particular stage in development, as in an example discussed later in this chapter.
Lastly, of course, alterations in temperature may produce qualitative effects on plant and animal metabolism (in the widest sense), and it is perhaps in this direction that we have to seek an explanation of the tendency of certain insects to be represented by short-winged races at higher altitudes (see here (#litres_trial_promo)). In plants, the effects of temperatures approaching the freezing point are often to induce the conversion of insoluble food-reserves like starch to soluble sugars. To this type of change has been ascribed the immunity of some evergreen plants from frost injury, which is attributed to the difficulty of freezing cells containing a high sugar-concentration. Undoubtedly the presence of these sugar solutions does confer on plant tissues a certain immunity from frost injury and the effect may easily help to account for the over-wintering of arctic and montane plants, just as it would undoubtedly be advantageous in helping to promote the rapid growth and early flowering observed in arctic climates. Dr. Scott Russell has verified the existence of high sugar-concentrations in spring in arctic plants collected on Jan Mayen Island and in the Karakorum mountains.
The only clear effect of this general type I know of in animal tissues is the very characteristic production of orange-coloured and fat-soluble pigments in certain aquatic copepods during the winter months and commonly also in cold, high-level tarns.
When one goes on to consider the ecological effects of these factors in nature, it is generally difficult to dissociate the effects of temperature and humidity. Thus the presence on mountain-tops of certain spiders usually found in damp cellars might plausibly be attributed either to high humidity or low temperature. A clearer example of the influence of temperature on animal distribution is that of the alpine flatworm, Planaria alpina, for this lives in water and is not therefore subject to the great variations in humidity which may effect mountain-top habitats. Planaria alpina is a small creature about a quarter of an inch long, resembling a somewhat flattened grey slug. It is a carrion feeder, living under stones in the margins of streams and in mountain runnels. In this country, these little water-courses usually contain a second, much darker species of flatworm, Polycelis nigra. The two species are always distributed in the same way, P. alpina at the higher levels, certainly at least to 2,000 ft., and P. nigra in the lower reaches of the water-course. This distribution is mainly a matter of temperature. Numerous observations in Britain and on the Continent have shown that P. alpina is never found in nature where the temperature exceeds 14° C., while P. nigra may be found where the water reaches as much as 20° G. Further, prolonged observations on the animals under controlled conditions by Mr. R. S. A. Beauchamp have shown that P. alpina cannot long survive temperatures exceeding 12° C. Thus in nature it occupies the high-level runnels and cold springs, occurring at high levels in mountain districts. There are reasons for believing that other animals confined to high-level streams and soils owe their distribution to similar effects, particularly perhaps certain insect larvae.
It is less easy to point to instances in which similar effects are produced on plant distribution, though they doubtless exist. Plants are not able to change their positions readily, and most of the high-level species are perennials, which means that the effects of the environment if not immediately lethal are likely to be the integration of the prolonged effects of the given habitat factor or factors. In some cases, perhaps especially in grasses, a given species is represented in the montane zone by separate races, often it may be not very distinct in form, but possessing some ability to live under the especial montane conditions. The common upland grass, the sheep’s fescue (Festuca ovina) is thus represented in the montane zone by an allied highland species (F. vivipara) which has the ability to produce young plants in place of the floral structures. This feature is much accelerated by, if not wholly dependent on, the existence of humid conditions, and this is probably the reason why the viviparous form of this plant is found at low altitudes along the seaward margins of Western Britain.
It seems that in order to get some idea of how climatic factors affected upland plants one would have to consider the influence of whole seasons upon the growth of a chosen plant. After making observations upon a number of plants it became clear that there were good practical reasons for using a relatively common plant like the moor-rush (Juncus squarrosus) as material for estimating these effects of altitude. This plant has certain practical advantages for work of this type. It occurs at almost every altitude in Britain and it prefers the wet and base-deficient peaty soils which predominate in the uplands.
The plant consists of a rosette of rather fibrous leaves just above ground level with a long flower-stalk bearing an upper group of brownish flowers or fruits Pl. IX (#litres_trial_promo). The latter contain numerous small seeds. At ground level there is a woody stem having numerous roots. The flower-stalks are numerous, they are tough and so can be collected rapidly and transported for subsequent measurement. The fruits, small brown capsules about a sixth of an inch long, are also tough and numerous enough to give suitable numerical measurements. The inflorescence is laid down as part of a bud in summer. It develops the following year, and its length may be taken as a partial expression of the conditions favourable to growth in the preceding summer and
FIG. 15.—Effect of altitude on moor-rush, Juncus squarrosus: L, Length of flower-stalk; N, Number of flowers produced; R, Number of mature capsules.
also in the summer in which it has developed. These conditions affect reproduction in addition by controlling the number of flowers and, later, of fruits and seeds. The only method by which the plant is distributed is by the numerous small seeds.
If one studies the performance of such a common moorland plant at different altitudes, it is apparent that the amount of growth and the production of flowers, or better still, of fruits and seeds, both diminish as the altitude increases (see Fig. 15). But fruit production is affected far more than growth in length, so that a point is reached, generally about an altitude of 2,500 ft. to 2,700 ft., above which fertile fruits are not usually produced, although the plants may form inflorescences of considerable size and in other ways be capable of making satisfactory vegetative growth.
This effect is evidently due mainly to the retardation of the development of the flowers and fruits. Thus in the Lake District in 1942, flowering was completed during June at 700 ft., but it had not begun at the end of July at 2,000 ft., and, at 2,500 ft. to 3,000 ft., it was not complete by the end of August. Thus at these highest levels there was little or no chance of most of the fruits becoming mature and they did not in fact do so. Again, in late September, 1943, only one mature capsule per 20 plants was found on the summit of Ingleborough (2,373 ft.). These and similar facts thus suggest that viable seeds are not usually formed above about 2,500 ft. to 2,700 ft., although large and healthy plants can be found up to at least a thousand feet higher. Until 1947, viable seeds had not been collected from above 2,700 ft., but the exceptionally long and warm summer of that year led to very abundant seed production—so much so that viable seeds were obtained from 3,400 ft., on Ben Wyvis.
In view of the infrequency with which such seeds are formed at high levels, the presence of moor-rush plants at 2,700 ft. and upwards is interesting. They are certainly very long-lived (twenty years or more) and possibly originally due mainly to transported seeds. It is noticeable on some mountains that the plants are not only sporadic but also are often collected in colonies, suggesting a group of individuals centred round a parent plant which has fruited only at rare intervals. The fruits are, perhaps, distributed in the wet wool of sheep, for, as far as is known, no mammals eat the inflorescences although snow-buntings habitually eat the dry fruits in winter and so may help to disperse seeds. The rush is commonest on sheep-infested mountains, and although it occurs to at least 3,700 ft., I have looked for it in vain on the high and grassy Scotch summits where deer habitually graze.
However, it seems certain that the effects of altitude are differential, affecting the seed-production most, flower-production less and vegetative growth least. The analysis of these effects shows that they vary little as between districts receiving great differences in rainfall, and they can thus be attributed mainly to the diminution of mean temperature with increasing altitude. Thus temperature, though it actually operates by controlling the relative rates of development, affects the distribution mechanism.
It is interesting to carry this problem a little further by considering how these things affect a little rush-moth, Coleophora caespititiella (see Pl. 30 (#litres_trial_promo)), that lives in association with the moor-rush and also with the common rush. Its life-history is not very well known, but moths are mature and the eggs are apparently laid in June–July, on or near the flowers of the rush. The larvae then feed on the growing seeds inside the developing fruit. By about the end of August, the infection of a fruit capsule becomes noticeable because of the presence of the larval case, a small cylindrical and white papery object in which the larva may live (see Pl. XI (#litres_trial_promo)). The larvae, possibly usually with the case, leave the rush-heads in late autumn and hide in the surrounding vegetation until the following summer. With certain obvious precautions, the presence or absence of the white larval cases can be used to study in an approximate way the extent to which the population of heath-rush is infected by the moth. The data also give a picture of the altitudinal distribution of the moth. This is much more restricted than is that of the rush on which it lives. In the central Lake District, in 1942, the frequency of the larvae decreased rapidly from a maximum infection of about 40 per cent of the capsules at 700 ft. and no signs of the moth were seen above 1,800 ft., although in that district the moor-rush goes up to 3,000 ft. Now at first it was thought that the larval cases might become more frequent at a higher level later in the year. In fact larval cases were never seen above this level except in the abnormal summer of 1947, when some were found at 2,000 ft. on the south-facing slopes of Saddleback.
It seemed obvious at first that at higher altitudes the lower temperatures would retard the development both of rush-flowers and of the moth growth-cycle, for both last a year. When no infection was found above 1,800 ft. it was thought that the lower average temperatures might so retard the development of the larvae from the egg to the case stages, that the cases were not produced at higher levels even although there was infection. In this case the larvae might fail to over-winter or the whole growth-cycle might take two seasons. However, no evidence of a later infection at higher levels could be found.
A possible alternative explanation was that, as suggested earlier, the whole growth-cycle of the moth might get “out of step” with that of the rush, so that mature moths and “infectable” rush-flowers (i.e. in the young stage when they are infected) might not coincide in time.
This does, in fact, happen, though not quite in the manner expected. It was found, in samples from the higher levels, that only the early maturing fruits were infected by Coleophora. It followed that there was normally no infection above 1,800 ft. because no rush-flowers were normally open in July above that altitude (1944 and 1945). Even in the abnormal summer of 1947, no sign of infection was seen above 2,000 ft. (and this on a south slope) in the Lake District, and in the Eastern Highlands (Ben Wyvis and Rothiemurchus district) none was noted above 1,400 ft. On the whole, then, it seems as though the main population of mature Coleophora individuals comes out at one time, about June–July. It may then infect any rush-flowers which are then open. This severely limits its altitudinal range, for as we have already seen, the high-level flowers are not mature at these early dates. One difficulty about these findings is that there seems no reason why the cycle of development of the moth should not be retarded somewhat at the higher levels just as that of the rush-flowers is. If this were the case, a small number of late-maturing individuals should appear at higher levels. No individuals of this type have been seen, nor has it been possible to find signs of rushes which might have been infected in this manner. It seems to be only possible to explain this apparent absence of the mature moths at higher levels by assuming also a temperature bar to their development such as we have already encountered in the flatworm Planaria alpina.
There are many further observations that could usefully be made on this matter. It appears that Coleophora is generally confined to lower altitudes on the Eastern Highlands as compared with the Lake District, and, at first, it seemed that lower mean temperatures might explain this. However, I have not seen this moth at all in the Western Highlands or Islands—perhaps because I have generally been too early or too late for the moth and too early for the larval cases. Certainly the creature seems to be much less common in this area of high rainfall, a result that could not be easily explained on the grounds of temperature alone.
The brief summary of upland climates and the analysis of their possible effects on animals and plants suggests that temperature has much to do with the zonation of plants and animals we observe on ascending a mountain. It controls the distribution of some organisms because they are not able to live in the higher average temperatures of the lowlands. In other cases, it seems that the low montane temperatures so lengthen the life-cycle that it cannot be completed in the short mountain summer. Perhaps more often low temperature retards some part of the developmental cycle, so that we get short-winged insects (see here (#litres_trial_promo)), or plants unable to produce flowers and fruit. For these reasons, some zonation of organisms is inevitable as altitude rises and temperature falls.
In practice, the most widespread influence of altitude is the change in the character of the prevailing plant communities, with all that it implies in its effect on animal habitats. Most noticeable is the disappearance of woodlands and trees with their varied faunas and ground floras. As this commonly takes place at about 2,000 ft. and as the restricted montane species appear above that level, we may take it as a convenient altitudinal separation of montane and sub-montane zones.
Within the limits thus defined by temperature other factors must play their part. Every naturalist knows that shelter from wind is often vitally important, so that here and there among the mountains there are oases in which the frequency of plant and animal life is altogether different from that found on the exposed and wind-swept faces. Within the limits imposed by temperature, humidity also exerts its restrictions, not only by presenting a range of habitats running from pool or rivulet to desiccated rock, but by influencing the character of the soil. It is to the consideration of these soil conditions that we must now turn.
CHAPTER 4 (#u86e587f5-cda5-55d2-886b-26760e4e243b)
SOILS
THE second important group of factors in upland habitats is the nature of the soil covering—or perhaps more strictly, of the surfaces available for plant growth. Geologically, as we have seen, these surfaces may be classified either as stable or unstable, depending on whether they are still subject to active erosion or not. As habitats for plants there is a more profound difference between these two classes. Most of the unstable surfaces are rocky or are covered by rock fragments in various stages of disintegration, and even their physical properties differ greatly from those of fertile lowland soils. They are, in fact, soils in the making, and it is one characteristic of upland areas that they exhibit in profusion all the varied stages of soil-formation. We see the native rock breaking down under the action of frost and other weathering agents to rock fragments, which become progressively finer as the process is longer continued, ultimately to yield the small mineral particles which form the basal material of most soils. The weathered material may remain in situ, covering the original rock surface, or it may be removed by erosion and redeposited elsewhere by streams and rivers as banks of silt or alluvial plains, by solifluction or rain-wash, or formerly in Britain by widespread glacial movements.
The raw mineral material is, however, comparatively sterile. It is converted into what we call a soil partly by chemical modifications resulting from the presence of water, often charged with carbon dioxide or humic acid, and partly resulting from the gradual accumulation of organic materials derived from plant remains. This latter material is called humus, and is particularly important because it forms a medium upon which can grow various micro-organisms, mainly bacteria, moulds and protozoa. With the accumulation of humus and the gradual colonisation of the material by these organisms comes a final stage, when it is usual to imagine that the original particles of mineral substance have become covered by a jelly-like mass of colloidal material—in part gelatinised minerals but also including humus—on and in which the population of soil micro-organisms lives.
It will be evident from this brief summary that upland soils can usefully be considered as belonging to a developmental series. But it is true of any soil that one of its outstanding characteristics is its capacity for change. Soils are inherently dynamic systems even when they are developed in physically stable situations, and to a far greater extent is this true of mountain soils, most of which are of geologically recent origin, even if not physically unstable.
Five types of environmental factor control the development of a soil mantle. First comes the nature of the rock or other parent material, from which soil is formed by physical and chemical weathering. Climate also exerts a marked effect on the weathering process, affecting both its physical and chemical parts, and, in particular, determining the amount of rain-water percolating through the soil in any season, a process known as leaching, which is responsible for the removal of soluble substances, bases like lime as well as plant nutrients like nitrates. Relief influences the lateral movement of percolating water down a slope, the degree of drainage and the stability, and thus affects the degree of leaching. But none of these effects is instantaneous and so there is a time-factor to be considered. Lastly, there are the obvious biological factors, of which the action of vegetation is most significant. Vegetation derives part of its sustenance from the soil and so incorporates a portion of the soil material which is returned to the soil on the decay of the plant tissues. The fertility of a soil is the result of this cyclic exchange. An efficient type of plant which draws heavily on the soil nutrients keeps them in a form of biological circulation which mitigates the losses due to leaching. Thus there is a natural mechanism for maintaining soil fertility, which, by drawing on the deep layers of the soil, is capable even of increasing the fertility of the surface layers provided the leaching factor is not too intense. Further, in any environment where the climatic factors have remained reasonably stable for a long time, it is possible for the soil-vegetation system to achieve a measure of temporary stability. In upland Britain, however, the soils are generally in dynamic states moving along definite trends of soil development. The trends due to a severe climate are particularly marked, and they operate during the different stages of soil development in the following manner.
SKELETAL AND IMMATURE SOILS
The initial stages of soil development in which rock fragments predominate are what we can only call skeletal soils. They are found principally where the surface is unstable or where further development is retarded by hard rocks or low temperatures. Of these factors, the low temperatures have also distinct qualitative effects, because while they greatly retard chemical modifications, the associated physical disintegrations caused by frost and solifluction are especially vigorous. There may thus be much physical commination of rock fragments with little chemical change. Thus the soils, even if finely divided, are immature in the developmental sense because of the deficiencies in their chemical and biological equipment. Soils of this general type occur on mountain-tops, where they are found under the mountain-top detritus except, perhaps, where it is especially coarse and deep. Generally, however, the detritus seems to be a superficial layer of stones extruded from below during the frost-caused or solifluction movements of the materials. Beneath the stones there is commonly a sandy loam, generally brown in colour and little leached. When vegetation is present this merges at the top into the almost black humus that collects among the surface detritus. The depth of the soil varies with the nature of the rock and the degree of erosion, but it is usually between one and three feet, and then comes disintegrating rock.
Parallel to these summit soils in general quality may be the scree-slopes of finely-divided material which occur lower down a mountain, often approaching stability but still subject to soil-wash and soil-creep, and so often distinguished as creep-soils. These show great variability in detail, but, like the mountain-top detritus, they often show coarse material at the surface and finer below. As they approach stability, they merge into the woodland soils described below, but in the earlier “gravel-slide” stages, a vertical section usually reveals a sequence of more or less alternating sandy or stony layers parallel to the slope (see Fig. 8). All soils of these types are alike in possessing a high base-status because they consist mainly of rock particles as yet not greatly modified by chemical change.
In all upland habitats there are in addition the overall trends caused by continual washing by rain, and as a result every exposed and porous surface will be more or less leached. Wherever leaching has taken place there must be corresponding areas that receive the products of leaching. The water that carries away lime or other bases from the higher upland surfaces must produce elsewhere lime-rich or base-rich habitats. Areas of this latter type may be distinguished as flushed or enriched habitats to distinguish them from the leached or impoverished ones.
FLUSHED SOILS
In general, of course, leaching will preponderate in upland regions and enriched soils will be commoner in lowland regions. Nevertheless enriched soils are always to be found occupying characteristic localities in mountain areas. Thus there is a flushed area around every springhead and around every rivulet. However, the water need not emerge as a separate spring but may perfuse the surface soil—a type of flush that can be recognised by a zone of greener vegetation. The various types of “damp flush” may be associated with a soil of almost any physical category. Enrichment by water from a higher level is greatest when such water has penetrated into the rock by means of structural fissures, and permeated the rock strata on its way down; a mere receiving area for surface run-off from acid upland soils is often as severely leached and as acid as the upland soil itself.
Parallel with enrichment by water (“damp flushes”), there is enrichment by presence of freshly-weathered rock particles, and areas of this type might be called “dry flushes.” The lower part of any steep slope is constantly enriched by such particles washed down from above. Screes and gravel-slides, in which the breakdown of new rock by weathering continually yields a supply of bases, could thus be considered among the enriched or flushed habitats. In this category also comes any unstable surface, crag, gullies and the like, where new rock surfaces are being exposed by erosion.
It will be observed that the flushed habitats are determined by a diversity of factors producing enrichment and have thus few physical characteristics in common. They tend to fall technically into four categories:
1 Bare rock or oversteepened slopes with soil particles washed away or present only in narrow fissures. Enrichment by continual weathering of freshly exposed rock surfaces.
2 Block scree in which leaching tends to preponderate over weathering, although the latter nevertheless does continually refurnish some of the bases lost, especially in the case of more rapidly weathering and base-rich rocks.
3 Unstable scree-slopes and solifluction areas with movement and accumulation of weathered soil particles, often below the surface layer of coarse detritus; enrichment both by weathering and particle accumulation.
4 Accumulation areas—nearly always showing fine and deep soils, with enrichment mainly by accumulation from above.
Upwelling of base-rich waters may occur in conjunction with any of the four categories above, although it is rarest in a and commonest in c and d. In both c and d the total “flush” effects normally counteract losses by leaching unless the soils are deriving from base-poor rocks.
LEACHED SOILS
On the whole, the unstable surfaces are areas of enrichment, and as such show distinctive types of vegetation. When they become stable enough to permit a complete vegetation cover, they inevitably tend to become leached—a tendency that is in some slight measure accelerated or retarded according to the nature of the plant cover. Because heavy rainfall is the rule and because plant remains tend to accumulate on the soil surface, it is characteristic of upland soils, once soil-creep and the forces of erosion are sufficiently arrested, that they rapidly develop the stratification or “profile” indicative of the more advanced (or “mature”) stages of soil development.
The extreme form of stratification found is that of the soil type known as a podsol, in which, under a surface layer of peaty humus and of humus-stained soil, there is a grey soil-layer from which almost all of the available bases (especially lime and iron) have been removed by leaching. With them also have gone the finest particles of clay and the humus colloids. These accumulate at a lower level, commonly two or three feet below the surface, as a dark-brown layer of “humus pan,” while immediately below this can usually be seen a red- or orange-brown precipitate of iron compounds (“iron pan”). Still lower is the little altered parent material. Thus these highly leached soils show a characteristic soil profile, with the following layers:
A. a Surface peaty humus and “litter”
b peat-stained inorganic soil
c leached grey ” ”
B. d humus accumulation zone (“pan”)
e iron ” ” (“pan”)
C. f little altered parent material
In continental areas, podsols characterise the cold temperate climates on stable and porous substrata receiving moderate rainfall. They are there associated with the northern evergreen forests of coniferous trees (firs) and with an abundance of small shrubs like the heathers.
In the British uplands, podsols are most characteristic of the eastern regions where the annual rainfall is relatively low (say about 35–40 in.) and the parent material is often sandy or gravelly morainic material. They are often now associated with heather-moor and pine forest, and originally this association may have been still more widespread. In the wetter western areas, although podsolic features in the soils are frequent, good examples of podsols are infrequent. Often this is because of great irregularity or diversity in the composition of the parent material. In particular, the deposits of iron or of peaty colloidal matter which suggest the appearance of a podsol are often due to lateral seepage from higher up the slope and are not necessarily derived from the soil-layers immediately and vertically above them. Varying porosity, in particular, leads to very irregular local accumulations of the humus and iron colloidal layers, which may appear in blotches along the lines of seepage and at a varying distance below the surface. But it is probable that most of what were once free-draining forest soils have now been transformed to bog (see below and Chapter 10) (#litres_trial_promo), and that those which are left are but transitional stages.
Between the extreme podsols and the young or slightly leached soils, there is a range of soil profiles usually classified together as “brown earths” from their ochreous brown colour, and in the lowlands characteristic of forests of deciduous trees, oak, beech and the like. The surface layers have been somewhat leached of bases but retain a brown colour along with a base-status sufficient to give a moderate fertility. This is the characteristic soil-type of lowland Britain. In the uplands, however, this condition can only be long maintained, where the original rock fragments are base-rich, where flushing of some sort maintains the base-supply, or where the vegetation is such as to renew the base-supply in the surface layers. The latter condition would be favoured, for example, by oak woodlands rather than by a covering of birch or of pine, for the lime contents of the leaves of these trees differ considerably: that of oak approaches 3 per cent, while those of birch and of pine are only about 1·5 and 1·0 per cent respectively. Thus by absorbing more lime from the lower layers of soil and returning it to the surface, oaks would maintain the base supply of the surface soil-layers for a longer time and so would tend to retard the effect of leaching.
The upland soils of brown earth type are usually recently stabilised “creep soils,” flushed glacial drifts, or soils derived from base-rich rocks. Almost always they were until recently under woodland, normally of oak though often with much ash. Now they are almost always cleared of trees and are covered by grasslands of various types. The removal of the original tree-cover was often followed by destruction of the surface humus through its oxidation and by the removal of the surface layers by rain-wash. Thus many upland soils of this general type, like the “frydd” soils in Wales, have been considered now to show “truncated profiles,” the top strata having been removed, often by erosion. In some cases, however, it is also probable that the profiles are immature and that the soils owe to this their comparatively high base-status.
It is quite clear that generally in the uplands the process of leaching can only be retarded and not completely stayed. The high summer rainfall in particular ensues that leaching will continue under the most favourable conditions of temperature. In lowland climates, in contrast, there is in summer an excess of evaporation and drying in the surface layers of soil so that base-rich water ascends from below by capillarity. This opportunity for replenishment is lacking even in a well-drained upland soil so that the soils as a whole must tend towards the leached condition.
Thus it seems inevitable that any porous soil, once stabilised, must ultimately develop towards a podsolic stage. In the majority of cases it seems that the process has not ceased at this stage. The downward movement of fine particles of clay and humus which characterises podsol development leads to the formation of impermeable pan-layers which impede drainage. Thus under conditions of high rainfall the upper layers of soil become waterlogged, seasonally if not permanently. This leads to peat accumulation, which in turn accentuates both leaching and poor drainage. Thus in mountainous Britain, podsols have almost always tended to become peat-covered bog soils, such as are described below, and it seems probable that the characteristic podsolic profile becomes modified when this change takes place, and ultimately disappears.
These trends of soil development favourable to bog formation are undoubtedly much accentuated by the topographic relations of the mountain soils. Once bog conditions have been established at any point on a hillside—a process which in most cases clearly must have taken place at an early stage in the physiographic history of the area—there must inevitably be a tendency for bog seepage to extend downward and to produce a general degeneration in the soils below it. The greater the area of peat accumulation in the upper bog zone, the more rapidly would this influence extend downwards. Moreover, because a mountain is a gathering ground with a considerable excess of rainfall over evaporation, it seems likely that the lowest slopes will often also show a marked trend towards the establishment of bog, the degree to which this happens being controlled by the geological structure of the mountain and the amount of mineral bases (lime, potash and so on) the mountain yields to the seeping water. So soon as the mountain-side becomes stable enough to show signs of leaching these trends are strongly in favour of it passing over to bog. Thus the upland soil problems are very complex, affected not only by the immediate characters of the soil but by lateral movements, by adjacent relief, rock and vegetation.
CHEMICAL STAGES IN LEACHING
The chemical stages which can be distinguished during the leaching process can now be outlined. In the first instance, most British soils are principally calcium soils, or, putting it in another way, they have lime as their principal base, and are lime-saturated. Agriculturally and ecologically, soils of this type possess high fertility. As leaching goes on, most of the lime and other bases are removed, being replaced by hydrogen, so that the soil finally becomes acid and sour (e.g. to the taste). For ecological purposes there are, however, two intermediate stages in this process which can usefully be distinguished: a state of partial lime-deficiency and one of higher lime-deficiency. Most British soils also contain a good deal of iron oxide—to which their colour is largely due—and under conditions of good aeration this is removed much more slowly than lime. The following four types of soil can conveniently be recognised as stages in the leaching process:
1 Lime-saturated;
2 Lime-deficient;
3 Base-deficient, iron oxide becoming mobile and relatively more important than lime;
4 Acid, with podsolic profiles in stable soils, often masked by peat accumulation.
Of these b and c represent the stages usually referred to as brown earths. The ecological value of this series lies in the fact that very decided transitions in vegetation occur at a point between b and c (which is also approximately half-way between a and d)—that is, a point of half-saturation with bases.
The technical methods of distinguishing these soil-types depend on the methods of measuring the percentage of base-saturation, which decreases from a to d, or of measuring the increasing acidity. The latter is perhaps a mode of expression more familiar to biologists. Estimates are made of the hydrogen-ion concentration and expressed in the following way. Concentrations, varying, for example, as 1/100, 1/1000 or 1/10,000 gm. per litre of hydrogen ions can be written either 1/10
, 1/10
or 1/10
, or 1 x 10
, 1 x 10
or 1 x 10
g/l, and for convenience these are termed pH 2, 3 or 4 respectively. The notation extends over a range of 1 to 14. In terms of this notation, pure water has a pH value of approximately pH 7, lime-saturated soils have a somewhat similar pH value, of above 6, while natural soils which are about half-saturated with bases have a pH value of approximately 5. The characteristic acid soils in the ecological sense lie below pH 3·8.
These soil types can, however, often be distinguished by their appearance and biological characters. The grey and leached zone in a well-developed podsol is likely to be mainly a hydrogen soil, as is the humus-stained layer on the surface. In many upland soils, the leached but still brown layer of inorganic materials below the surface humus has a characteristic orange-brown colour—not grey as in a proper podsol. This condition is associated with the removal of most of the lime and the mobilisation of iron, at first perhaps dissolved from the soil minerals near the surface by humus compounds, but then reoxidised on the surface of the soil particles in a state which accounts for the characteristic colour. This type of soil is probably very definitely associated with periods of waterlogging, such as are frequent in upland areas, and it normally has a lower base-status than the more typical soils of brown-earth type.
In these and in other ways, therefore, the characteristic appearances of soil profiles give a good deal of information about the base-status of the soils. It is perhaps worth emphasising also at this stage that two factors in particular are especially effective in removing iron and other bases during the final stages of the leaching process.
One of these is the increased acidity and especially the effects of acids derived from plants like oxalic and citric acids, in which iron salts are especially soluble. The second is the establishment or development of waterlogging, which, by eliminating the oxidising effects of air or oxygen, permits the reduction of iron to the ferrous state, in which it is very much more easily soluble, as well as more readily replaced by the process known as base-exchange. In aerated soils, however, waterlogging can only be temporary even though it must be frequent in winter in all upland soils. When it occurs permanently the soil commonly acquires a blue-grey appearance which we associate with the presence of ferrous iron compounds, and this contrasts very noticeably with the reds and browns of the ferric salts in air-containing soils.
Of course, where there are extensive areas of waterlogged soil and especially where peat is abundant, large amounts of ferrous salts may be present in solution in the soil-water. Wherever this becomes exposed to air it becomes oxidised either to metallic iron or to ferric salts and so considerable amounts of ferric substances may be precipitated. The orange-brown or metallic films due to this process are familiar objects round any peaty spring, and, long continued on a large scale, it has in the past been responsible for the production of deposits of bog iron ore. The same process continues in any peaty flush soil which receives drainage from waterlogged surfaces and into which air can, at times, penetrate. To the ecologist, the colours due to iron compounds are of great importance as visual evidence of the existence and progress of leaching or accumulation and also as useful clues to the presence or absence of aerated conditions in the soil. There are, of course, many chemical tests which can be used to confirm and extend these visible signs.
WATERLOGGED SOILS AND PEATS
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 (#litres_trial_promo) and 24 (#litres_trial_promo)). 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 (#ulink_bb50f962-7cd1-5432-9723-e7dd0722279c)) 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 (#litres_trial_promo)) 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.
THE BIOLOGICAL CHARACTERS OF SOILS
The peaty nature of upland soils is one of their most easily observed attributes. It is by no means confined to waterlogged areas, but it is equally noticeable on leached podsolic soils and even in the early stages of soil-formation on mountain-top detritus. This clearly indicates that the climatic effects characteristic of these three extreme types of habitat—waterlogging, leaching and low temperatures—are alike in leading to humus or peat accumulation in the soils affected. They do so in a generally similar way by reducing the activity of the soil micro-organisms.
Their effects differ in some respects and it will therefore be convenient to consider them separately, although actually they overlap in nature to a considerable degree. The effects produced on and by the soil organisms in their turn affect the vegetation of larger plants, and hence, as later chapters will show, have much effect on the larger animals.
Numerically, the soil flora mainly consists of vast numbers of bacteria and fungi. These are colourless plants, mostly of microscopic size in the soil, that obtain their nutriment by chemically changing the remains of dead plants and animals. They are responsible for what we usually call decay, although the chemical processes involved are analogous to and, indeed, often identical with, the processes of food digestion and utilisation in animals. The breakdown of plant organic matter in soil is, however, very generally initiated by small invertebrate animals, sometimes by the larvae of flies but particularly, as Charles Darwin showed, by earthworms. These break down the cellular structure of plant-remains and partly transform it, making it suitable for further transformation by fungi and bacteria. Most soils also contain single-celled animals, protozoa, which browse on the fungi and probably serve to keep them in check. There are also insects, such as springtails and fly larvae, whose role in soil economy is usually less well known.
This large soil-population requires air, or rather oxygen from the air, for breathing and it is consequently said to be aerobic in character.
In a normal soil the chemical materials produced during these transformations by the soil organisms are substances containing oxygen, carbon dioxide, which escapes to the air, nitrates, which contain the nitrogen which was present in the animal and plant proteins, and other substances such as sulphates and phosphates. Of these, nitrates are generally regarded as most important because they are quantitatively the materials a normal plant requires from the soil in largest amounts. Thus the fertility of a soil is usually determined very largely by the rate at which it can produce nitrates, i.e. the rate at which the nitrogen locked up in the decaying organic materials can be released in a form available to plants. In actual fact, plants can also use ammonia, but the amounts of ammonia in a natural soil are normally small. This substance is the first simple substance to be formed by bacteria and fungi during the soil decompositions. It is converted by other soil organisms, the nitrifying bacteria, into nitrate. These processes of ammonia production and nitrate formation seem to be particularly sensitive to adverse soil conditions. So also are the parallel processes of nitrogen fixation by which a fertile soil is usually able to increase its nitrogen content (to an extent which balances leaching losses) by fixing the gaseous nitrogen present in the air. This process is brought about in most soils by bacteria, as well as by the nodule-forming organisms which are found living in the root-nodules of leguminous plants like peas, beans and clover. Adverse soil conditions almost always produce their effects by retarding these processes as well as the numerous other processes, e.g. of decomposition, going on in the soil. Special effects are produced by the different adverse factors.
AEROBIC AND ANAEROBIC—OXIDISING AND REDUCING SOILS
The principal result of a soil becoming saturated with water is that the amount of oxygen in the soil is reduced to vanishing point. Consequently as most of the soil organisms are aerobic, requiring oxygen, the soil population is reduced to the minimum, and there can remain active only a few anaerobic organisms with specialised methods of maintaining their existence without oxygen. The products of the decompositions going on in the soil also change in character. Instead of the formation of carbon dioxide, nitrates, sulphates and phosphates, all containing oxygen, there may be produced instead marsh-gas (or methane, CH
), ammonia (NH
), sulphuretted hydrogen (H
S) or other sulphides, and sometimes phosphine (PH
), a series of compounds devoid of oxygen. All of these products are associated with the activities of anaerobic types of moulds or bacteria, the latter usually being most abundant. The microflora of waterlogged soils is thus specialised in character as well as poor in numbers, while the products of anaerobic composition include substances, in addition to those mentioned above, which may be toxic to the larger rooted plants. Some of the products are also responsible for other manifestations peculiar to boggy soils, such as the “will o’ the wisp” and “corpse-light,” these being attributed to the burning of the highly inflammable marsh and phosphine gases.
In effect, in contrasting waterlogged and aerated soils in this way, we are contrasting two sorts of micro-biological activity—oxidising and reducing—depending on whether the organisms can form chemically oxidised products like carbon dioxide and nitrates or chemically reduced substances like marsh-gas and ammonia.
The particular value of being able to recognise these possibilities is because they give us information as to the effect of the soil conditions on the action of living organisms, and we may infer that the conditions which affect the soil flora will also affect its fauna (see here (#litres_trial_promo)) as well as the larger plants. Moreover, the level of oxygen content which produces these biological effects is low and it is not one which can be detected in the field with any certainty, if at all, by measurements of soil oxygen. For our present purpose, therefore, it is useful to think of upland soils as belonging to the two types named above.
It is useful also to realise that a large proportion of wet soils may be oxidising in drier periods and reducing in wet.
MULL AND MOR
