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.
The direction of the hills is an important factor, especially in relation to the prevailing winds, causing different effects upon different types of vegetation. Plants with the grass habit are less affected than those having the heath habit, for instance, the latter forming crescentic patches under certain conditions. The vegetation on the lee side of a hill is much more luxuriant than that upon the windward side. But the existence of a series of winds blowing from different directions is liable to obscure the influence of prevailing winds. The effect of the direction of the range is again seen in the greater exposure of plants to the sun on the south and west sides, and their xeromorphic conditions of growth, less noticeable upon other aspects.
As one ascends a hill from the surrounding lowlands, one can readily notice a difference in the characters of many common types of plants. Such a plant as the Common Dandelion, in the valleys, has a tall and thick scape with a large flowerhead. The radical leaves are long and broad, with few lobes, or lobes not deeply cut back. The achenes are green as a rule. The root is thick and deep-reaching. The whole plant is, in other words, suited to conditions where there is a uniform and considerable supply of moisture, and a thick or deep soil. At 500-600 ft., the normal type of Dandelion is replaced by a form with a short narrow scape, a smaller flowerhead, and narrower, shorter, and more deeply-divided leaves. The fruits are reddish in colour. The root is short, often dividing at the extremity. The leaves are usually prostrate upon the surface, rarely becoming caespi-tose or ascending.
The types to be met with upon hills differ to a great extent from those of plants found in the lowlands. There is an increase in the members of the grass type, the rosette habit, etc.
It is estimated that a southerly aspect has the effect of an increase of 2° to 3° C. near the surface, and this is proportionally more at higher altitudes. Slope, moreover, and its direction have an effect upon the temperature of the soil; for radiation is more rapid upon a sloping surface than upon a flat surface. Hence the beneficial effect of a southerly aspect, especially in hilly regions.
As a rule the western and southern sides of a hill or mountain are more suitable for plant growth, as the first is more uniformly moist, while the second has a higher average temperature, and the thermal constants of plants on such aspects are greater. Aspect thus has an effect upon the dispersal of plants by natural selection, the sun-loving plants seeking the sunnier aspects, the cold-lovers being found upon the north and east.
One result of the existence of hills is to delimit the areas given up to aquatic vegetation. For such vegetation is more or less entirely confined to plains, valleys, or lowlands generally.
The hygrophilous types, such as those that form part of the moorlands, peat-bogs, cotton-grass associations, and others formed by Carices, Rushes, some Grasses, etc, are not aquatic types in the strict sense.
Some exceptions must, however, be made to this general rule, for though lakes as a rule are lowland, yet there are a number of lakes, e.g. in Wales and the Lake District, that are distinctly of upland type. And there are many lochs and tarns in Scotland which have a characteristic vegetation.