This section is from the book "British Wild Flowers - In Their Natural Haunts Vol2-4", by A. R. Horwood. Also available from Amazon: A British Wild Flowers In Their Natural Haunts.
For every successive rise in altitude of 300 ft. the temperature decreases by 1° F. Hence the effect of altitude upon plants in the first place is to drive the southern species downward and the northern species upward. A point is reached above 2000 ft. when Arctic plants begin to become dominant, and at the highest elevations in the British Isles the vegetation is distinctly Alpine, consisting of such groups of plants as Saxifrages, Pinks, and Willows. Another effect of altitude is to make the conditions as a whole moister, owing to the fact that mountains act as condensers of moisture.
As a whole eastern, and a great part of southern, England has a rainfall of not much more than 20 in. per annum, and these are the lowland areas. The rainfall of the north and west and south-west is much greater, and in places amounts to 100 in., whilst in parts of the Lake district it may be as much as 200 in. Though the rainfall is so great in hilly districts the ground is not necessarily so saturated, for water finds its own level very quickly, and it is in the lowlands where it lies longest. Only in the hollows, or on some hill-tops, does the hilly tract develop boggy conditions. Elsewhere the conditions, especially the slopes and rocky summits with shallow soil, are suitable for dry-soil forms, and a large bulk of the plants are adapted to this state. Owing to altitude, again, especially above the deciduous-tree zone (1000 ft.), the ground flora is much more exposed to wind.
A further effect of altitude (or the existence of hills) is to determine the prevalent winds, and to regulate the distribution of valleys and rivers. Another feature is the exposure of plants to mist and fog. The clouding of the sky on hills has an influence on the light conditions.
The general conditions in the lowlands make for uniformity. It is true that wide associations occur also upon hills, as those formed by Heather, Cotton Grass, Sedges, Rushes, and some grass types, but these are, even when widespread, more or less discontinuous owing to physical structure, slope, etc., whereas in the lowlands these conditions are less variable. Moreover, lowland tracts vary little in altitude, so that the temperature and rainfall are more or less uniform. Thus we find in the lowlands wide expanses of forests, pasture, heath, arable, lowland moors, and aquatic vegetation on a grand scale, as formerly in the Fens. Hence there is a preponderance of southern types of plants. Clay plants in particular are characteristic of the lowlands, and those found upon marls and loams, or in other words, the vegetation of the Agrarian Zone.
The uplands, on the other hand, exhibit the opposite characteristics. The associations are not so homogeneous as in the lowlands. The sloping sides of hills are often unstable, the talus and debris moving downwards, and the soil is also of a more barren character, owing to the exposure of the surface to denuding agencies and the slight opportunity afforded for soil formation or retention. Hence not only climatic but also physical factors cause the lowlands and the uplands to present entirely dissimilar types of vegetation, as may be recognized at once by comparing, for instance, the flora of the Welsh hills with that of Bedfordshire.