Many writers take the growth of grain as the characteristic of the mountain region; but so many varieties of all the common
species are in cultivation, and these have such different
climatal requirements, that they do not afford a factory
criterion. A more natural limit is afforded by the presence
of the chief deciduous trees -- oak, beech, ash and sycamore.
These do not reach exactly to the same elevation, nor are they
often found growing together; but their upper limit corresponds
accurately enough to the change from a temperate to a colder
climate that is further proved by a change in the wild herbaceous
vegetation. This limit usually lies about 4000 feet above
the sea on the north side of the Alps, but on the southern
slopes it often rises to 5000 feet, sometimes even to 5500
feet It must not be supposed that this region is always
marked by the presence of the characteristic trees. The
interference of man has in many districts almost extirpated
them, and, excepting the beech forests of the Austrian Alps, a
considerable wood of deciduous trees is scarcely anywhere to be
found. In many districts where such woods once existed, their
place has been occupied by the Scottish pine and spruce, which
suffer less from the ravages of goats, the worst enemies of tree
vegetation. The mean annual temperature of this region differs
little from that of the British Islands; but the climatal
conditions are widely different. Here snow usually lies for
several months, till it gives place to a spring and summer
considerably warmer than the average of British seasons.