Читать книгу Mountains and Moorlands - W. Pearsall H. - Страница 11

The influence of the high humidity that is characteristic of British hills is not easily assessed. Atmospheric humidity undoubtedly has a considerable direct effect on plant and animal life, so that most biologists would be able to point to facts of distribution, such as the greater abundance of mosses and lichens in the western hills, which can reasonably be attributed to greater air humidity. But climatic humidity expresses itself not only through its direct effects on the distribution of living organisms but indirectly by affecting the character of the soil, and in the British Isles these indirect effects are extremely important. The only climatic data available for examining them on a sufficiently extensive scale are the rainfall data, and these we must now consider to see how far it is possible to use them in defining climatic limits. In doing this it will be necessary to adopt the following rather rough method of analysing climatic effects.

In the southern part of the Pennines, and probably generally among their eastern foothills, the average annual loss of water by evaporation is equivalent to a rainfall of about 18 in. This figure has been obtained partly as the estimate of the average amount of water lost by evaporation from a 6–ft. Standard tank, and the figures given in Table 4 are actually monthly estimates of the average losses so obtained (as inches of rain). But a similar annual figure (about 18 in.) can be obtained by comparing the rainfall over a given river basin with the “run-off” down the river, and access to much unpublished data has shown that this figure is fairly representative for the eastern Pennines. The difference between rainfall and run-off (assuming no loss into the ground or taking an average over many years) gives the net amount lost by evaporation, and we shall assume it to be distributed seasonally as in the figures given. It may be noted in passing that the problem of estimating evaporation losses may be considerably more complex than this. Empirical formulae have been worked out for estimating these losses in which it is usually assumed that they increase with increasing rainfall as well as with rising temperature.

Table 4 gives, in addition to the monthly figures for evaporation, the average monthly rainfall, also in inches, for two adjacent stations. One of them, Doncaster, lying in the Plain of York and at an altitude of 25 ft., represents a typical lowland station in Eastern England, with an average rainfall of about 25 in. per annum. The other, Woodhead, lies among the high Pennines and is surrounded by “cotton-grass” moors. It therefore represents fairly well the climate of these high moorlands, with an annual average rainfall of about 50 in. The actual figure on the hills is probably more, rather than less, than this, say 55 in.

Table 4 MONTHLY EVAPORATION AND RAINFALL IN INCHES AT DONCASTER AND WOODHEAD

The figures show very plainly that there is no month in the year when the average rainfall at Woodhead does not exceed the evaporation. In contrast, at Doncaster, there are five months when evaporation approximately equals or exceeds rainfall (the rainfall figures in Table 4 are italicised when this is the case). Consider the implication of these facts, and particularly their effects on soil conditions. During the summer, at a station like Doncaster, the soil gradually dries out. This means that the water in the soil interspaces is replaced by air. The drains cease to run until the autumn, when rainfall once more exceeds evaporation and the water-level begins to rise in the soil.

At a station like Woodhead, on the other hand, the same filling of the soil interspaces will take place in winter, but the soils will have no opportunity of recovering and of drying out in summer, for any evaporation will be balanced by the higher rainfall. It follows, therefore, that as a whole, soils will usually be waterlogged in a rainfall of the Woodhead type and only those on considerable slopes will have a chance of becoming drained and well aerated. We may thus recognise that in a rainfall of this type and magnitude there will be a strong tendency towards bog-formation, and it may perhaps be useful to note that in Britain a rainfall of 50 to 55 in. (that is, about three times the evaporation figures) will apparently suffice to give conditions favourable to bog-formation. This is a useful measure, even if a rough one, of the effective humidity of an upland climate.

The influence of high rainfall is exerted in another manner also. When rain falls on soil and percolates through it, the water naturally carries away in solution and into the drainage system any soluble mineral salts present in the soil. These will include most of the substances valuable as plant food as well as the lime which prevents a soil from becoming sour. The process is called leaching, and the rate of leaching will obviously depend very largely on the rainfall. When this only just equals the evaporation losses there will be little or no leaching, but the higher the rainfall becomes in comparison with evaporation, the more rapid leaching will be. Very roughly, then, we shall expect little or no leaching when the rainfall is about 18 in. per annum, but where the annual rainfall is 54 in. we may expect leaching to proceed at about twice the rate expected under a rainfall of 36 in. It will be realised that these rough comparisons as to leaching apply only to porous soils through which water can freely percolate and there is obviously no need to stress the numerical comparison, although it serves to emphasise the high rate of leaching found in upland areas, where a rainfall exceeding 54 in. per annum is common.

The analysis carried out in the preceding paragraphs gives us one method of obtaining a significant boundary of humidity which must have pronounced biological effects. It is perhaps worth noting that a similar figure, an annual rainfall of about 55 in., has been obtained by noting the rainfall at upland sites where reclamation of moorland has proved just possible or has failed. If allowance is made for the nature and porosity of the underlying rock, this is roughly the altitude at which habitation ceases, and in the northern Pennines and eastern Cumberland it is stated to lie very near to the point at which rainfall exceeds 55 in. Of course, this is an extremely indirect method of approaching such a problem, for the result must be greatly affected by the nature of the prevailing occupations in the district examined; nevertheless in this particular instance the relation is clearly one which operates through soil effects, so that it agrees with the conclusion already reached in suggesting that the rainfall indicated is one of distinct biological significance.

The examples already quoted indicate that high rainfall and high altitudes are associated in a general manner, much depending on slopes and topography. No hard-and-fast rule can be given as to the increase of rainfall in relation to altitude, but it is useful to note what is commonly observed in different parts of upland Britain. Along the western margins an annual rainfall of 35 in. is generally found near sea-level, while one of 55 in. would occur at 500 ft. or even less. Where the slopes rise fairly uniformly, as on the west of the Bowland Forest area, the rainfall rises steadily as the height increases, as shown in Fig. 11. The curve given in the figure is contrasted with similar data for the eastern Pennines, showing the much lower rainfall at corresponding heights in the east. The gradient of increase in rainfall

FIG. 11.—Rainfall and altitude on a western slope, B (Bowland Forest), and on the corresponding eastern slope of the Pennines, E.

FIG. 12.—Altitude in Great Britain. Altitudes over 800 feet shown in black.

with altitude rises much more steeply elsewhere, however. Thus, for example, an average rainfall of 150 in. per annum may be assumed at 2,800 to 3,000 ft. in the Central Lake District, in Western Wales and in parts of Western Scotland. In contrast, the rapid decline in the rainfall on the eastern slopes of mountainous Britain is equally striking, for there a rainfall of 55 in. would not be found much below 2,500 ft., and indeed so high a figure is often not reached. A rainfall of 35 in. is not often found below about 700 ft. There is thus a marked difference between the westerly and easterly aspects of British uplands, a point worthy of emphasis because the change-over in the effective climatic conditions often takes place very rapidly in passing in an easterly direction from a watershed.

Moreover, there are indeed large areas in the eastern uplands where a maximum rainfall of between 45 and 50 in. is reached at about 1,500 ft. and no greater rainfall is observed at higher levels. For practical purposes, then, we may say that the western uplands above 500 ft. lie almost wholly above the rainfall limits of the bog-forming climate, while a large proportion of the eastern uplands is below these limits.

The general truth of this statement can be illustrated by a comparison of the maps in Figs. 12 and 14, which show that the zone of high rainfall by no means corresponds with any particular altitude. Further, if the map (Fig. 13) showing moorland and waste lands be compared with that of rainfall, it will be found that a considerable part of the eastern moorlands lies outside the zone possessing a “bog” climate. The distinction is particularly clear in the Scottish Highlands. The importance of this type of relation has hardly received the emphasis it deserves, perhaps because the climatic index is not one it is easy to employ in the field. Indeed, average annual rainfall alone cannot be a reliable guide to the distribution of this type of climate, for the essential feature is the normal absence of soil-drying in summer, and this must depend on evaporation rate and hence on other factors such as mean temperatures, cloudiness and air humidity as well as on local topography. But the field ecologist learns to recognise the certain signs of the existence of local variations in rainfall, of which the most valuable is usually the local distribution of cloud. Some areas are persistently under cloud, while others not far away may be as frequently cloud-free. Generally, rain-showers show a similar distribution, and these are both things which can be noted even in a brief visit.

Left, FIG. 13a.—Moorlands in the British Isles.

Right, FIG. 13b.—Distribution of Rainfall. Areas with over 50 inches of rainfall per annum shown in black.

Left, FIG. 14a.—Distribution of Palaeozoic rocks in Great Britain.

Right, FIG. 14b.—Distribution of sheep in the British Isles.

Very good examples of considerable local variations in climate which can thus be detected are to be found in the eastern Pennines—particularly in the Teesdale-Baldersdale-Stainmoor district just south of Mickle Fell. Stainmoor itself is a well-known bog area (see here) which has a rainfall near to 55 in.; but this rainfall decreases very rapidly towards Lune Forest and Baldersdale on the north and east respectively, where other very different types of moorland vegetation hold sway. Very striking is the frequency of cloud-cover or showers over the Stainmoor bogs in contrast to the clearer skies of the drier and more easterly areas.

On a far grander scale, similar contrasts may very often be seen in the central Scottish Highlands. The eastern mountains, and perhaps especially the Cairngorms, may stand out cloudless or with small fair-weather clouds when the big western Bens are sunk in mist or dwarfed by rain-clouds. The contrast seems to become noticeable about a line drawn north and south through Loch Ericht or Dalwhinnie.