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 physical features of the habitat in the case of hills and dry pastures are as important as the other factors, such as climate, altitude, and soil. As a whole the tree type is less dominant, though Birch along with the sandy-soil form of the Oak, the Sessile Oak, the Pine, and the Yew are characteristic montane trees. The Box locally is also a hillside type.
Scrub, however, is frequent upon the hills on certain soils. The Juniper, for instance, rises to a high elevation, and though dwarfed when exposed is otherwise suited to the upland generally. The Ordinary and Dwarf Furze form wide associations, as does the Broom. Other members are the Sloe, Hawthorn, Spindlewood, Cornel, Buckthorn, Rowan, the last abundant in the doughs of the Lake district.
The undershrubs, such as Whortleberry, Ling, Heather, Crowberry, Cranberry, etc, form also wide associations with a typical habit or growth form, the heath habit. The adaptation of these plants to the special conditions of wind force is highly important, and should be studied in detail.
The dry-soil conditions in a large measure give rise to numerous groups with the rosette habit, such as Hawkweeds and Dyer's Weed, etc.
Many trailers are found in this type of habitat, such as the Rock Rose, Thyme, etc. These like the undershrubs are specially adapted to the particular wind conditions. The grass habit is largely represented, being well suited to the exposed character of hills. All these adaptations are in the main induced by the physical features of the hills and dry pastures.
It was remarked in Section IV that the upper limit of the tree zone in woodlands is largely influenced by the wind. This fact is shown by the manner in which trees are dwarfed in exposed upland situations, or are even (as again by the sea-coast, where wind is the cause also) bent in the direction of the wind, the branches spreading out horizontally in the opposite direction to the prevailing wind; so that not only is height influenced, but also direction of growth. The scrub is similarly affected when growing on hillsides, shrubs such as Hawthorn or Sloe being reduced in height.
The trailing plants that grow on hills are also more prostrate than when growing in the lowlands. The Grasses are as a rule less tall than the lowland types, and amongst them there are allied species of lowland distribution that are normally more lofty, as Fescue Grass or Heath Hair Grass.