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.
ALTITUDE AND ORGANISM
The influence of climate on upland organisms has so far only been considered in the most general way. We have observed that there is a correlation in distribution between certain types of soil condition and certain types of climate. Thus we assume that the bogs of the Western Highlands are associated with the wet climate. In a similar manner we may observe that there are some plants and animals found only at high levels, the special montane species, and we assume that they are there because they are in some way more suited to the severe climate existing at high altitudes. We have little evidence as to how the climatic factors are effective and it will be useful accordingly to discuss this matter a little more fully.
The distribution of plants is obviously a very important factor in animal distribution, not only for grazing mammals but also for the insects which live on and in plants. In such cases the influence of altitude may be indirect, and there are, as we shall see, instances of the distribution of the animal following that of the plant. If we are to consider plants, the influence of the soil needs to be taken into account, and we have already seen reason to believe that the wet climate may be effective through its influence on soil conditions. But climatic humidity varies greatly in different parts of the country—being high in the west and lower in the east. If this were the effective montane influence then we should expect to find a richer montane fauna and flora in the west. It is well known that on the whole there are on the eastern mountains more of the species restricted to high mountain life; so that in one aspect at least humidity cannot determine the altitudinal zonation. However, the fauna and flora of upland country as a whole is very different from that of the lowlands, in proportionate representation if not always in the individual species, and a large part of this upland fauna and flora is associated with the ill-drained and wet soils. What humidity does do is to give great areas dominated by a limited fauna and flora of this type, which is upland rather than montane and which is evidently related to the soil conditions induced by humidity.
The more common view and one which has been referred to and used already in this chapter, is that temperature largely controls the altitudinal zonation, and we may look at this problem as something which would repay attention from naturalists and as a subject which requires little in the way of special equipment.
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
